US20070132784A1 - Circuit and method for modifying a region of an encoded image - Google Patents
Circuit and method for modifying a region of an encoded image Download PDFInfo
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- US20070132784A1 US20070132784A1 US11/674,121 US67412107A US2007132784A1 US 20070132784 A1 US20070132784 A1 US 20070132784A1 US 67412107 A US67412107 A US 67412107A US 2007132784 A1 US2007132784 A1 US 2007132784A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/85—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression
- H04N19/89—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression involving methods or arrangements for detection of transmission errors at the decoder
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/90—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using coding techniques not provided for in groups H04N19/10-H04N19/85, e.g. fractals
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/20—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using video object coding
- H04N19/27—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using video object coding involving both synthetic and natural picture components, e.g. synthetic natural hybrid coding [SNHC]
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/40—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using video transcoding, i.e. partial or full decoding of a coded input stream followed by re-encoding of the decoded output stream
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/48—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using compressed domain processing techniques other than decoding, e.g. modification of transform coefficients, variable length coding [VLC] data or run-length data
Definitions
- the invention relates generally to image-processing techniques, and more particularly to a circuit and method for modifying a region of a encoded image.
- the invention allows one to blend an electronic program guide (EPG) with a region of an encoded video frame without decoding the entire frame.
- EPG electronic program guide
- FIG. 1 is a video image 6 , which is a video frame that includes a first region 7 and a second region 8 . Although described as a video frame for example purposes, the image 6 may also be a video field. Furthermore, although shown as two rectangles in a top-bottom arrangement, the number, shape, and respective locations of these regions is arbitrary.
- another image i.e., an overlay image
- EPG electronic program guide
- the overlay image is typically a partial frame that is the same size as or that is smaller than the frame region that it overlays, although the overlay frame can overlay an entire video frame.
- FIG. 2 is a block diagram of a conventional television receiver system 10 , which includes a set-top box 11 such as a cable TV (CATV) or satellite TV box, a remote control 12 , and a digital video display 13 .
- the box 11 allows one to view overlay images—such as those that compose an EPG—in the respective regions 7 of a sequence of video frames 6 ( FIG. 1 ).
- the box 11 includes a processing circuit 14 , which receives an encoded, multiplexed broadcast video signal on an input terminal 15 , receives command signals from the remote control 12 on a command terminal 16 , and generates a video display signal on an output terminal 17 .
- the broadcast video signal includes one or more broadcast channels and one or more overlay frames such as the frames that compose an EPG, and is encoded according to a compression standard such as the Moving Pictures Experts Group (MPEG) standard (discussed below).
- MPEG Moving Pictures Experts Group
- the circuit 14 blends the video frames from the selected channel with the appropriate overlay frame or frames and generates the display signal as a sequence of these blended video frames.
- the display 13 receives the display signal from the terminal 17 and decodes and displays the sequence of blended video frames.
- the processing circuit 14 includes a command decoder 18 , which decodes the commands from the remote control 12 and generates corresponding control signals, such as an overlay signal, that control other portions of the processing circuit 14 .
- a channel selector 20 receives the broadcast signal from the terminal 15 and, in response to a channel-select signal from the command decoder 18 , demultiplexes the selected channel from the broadcast signal. In response to an overlay signal from the decoder 18 , the selector 20 also demultiplexes the selected overlay frames from the broadcast signal. For example, the selector 20 may demultiplex the EPG that corresponds to the selected channel.
- a video decoder 22 decodes the video frames of the selected channel into pixel-domain frames, i.e., frames of pixel luminance and chromanance values. In response to the overlay signal, the video decoder 22 also decodes the selected overlay frames into the pixel domain, and an overlay/video combiner 24 blends the decoded video frames with the decoded overlay frames. Conversely, if the command decoder 18 does not generate an overlay signal, then the selector 20 does not demultiplex the overlay frames, and thus the combiner 24 merely passes through the decoded video frames from the decoder 22 . In one embodiment, the output terminal of the combiner 24 is coupled directly to the output terminal 17 .
- the circuit 14 includes an optional re-encoder 26 , which re-encodes the decoded video frames from the combiner 24 before providing them to the display 13 .
- the processing circuit 14 mav include one or more processors that perform the functions of the above-described circuit blocks 18 , 20 , 22 , 24 , and 26 .
- the remote control 12 in operation during a period when a viewer does not want to view an overlay frame, he selects a channel with the remote control 12 , which generates a corresponding control signal.
- the control terminal 16 which is typically an infrared detector, receives the control signal and couples it to the command decoder 18 .
- the decoder 18 In response to the control signal, the decoder 18 generates the channel-select signal, which causes the channel selector 20 to recover the encoded video signal of the selected channel by demultiplexing the broadcast signal.
- the video decoder 22 decodes the recovered video signal into frames of pixel values, and the combiner 24 passes these frames to the optional re-encoder 26 , which re-encodes the frames and provides a re-encoded video signal to the display 13 . If, however, the re-encoder 26 is omitted, then the combiner 24 passes the decoded frames directly to the display 13 .
- the viewer In operation during a period when the viewer wants to view an overlay frame, he selects a channel as described above and also selects an overlay frame or a series of overlay frames, such as an EPG, with the remote control 12 .
- the decoder 18 generates the channel-select signal and an overlay signal, which together cause the channel selector 20 to recover both the encoded video signal of the selected channel and the encoded video signal containing the overlay frame or frames.
- the overlay signal causes the video decoder 22 to decode the recovered channel and 20 overlay video signals from the channel selector 20 into respective sequences of frames, and causes the combiner 24 to blend the overlay frames with the channel frames to generate blended frames.
- the optional re-encoder 26 re-encodes these blended frames and provides them to the display 13 , which decodes the re-encoded blended frames. If, however, the re-encoder 26 is omitted, then the combiner 24 provides the blended frames directly to the display 13 .
- the set-top box 11 cannot utilize the decoding ability of the display 13 , and thus includes its own redundant decoding circuitry, which often adds significant size and cost to the box 11 .
- the display 13 includes channel-select and full decoding circuitry respectively similar to the channel selector 20 and the decoder 22 of the box 11 .
- the display 13 typically can directly receive the encoded, multiplexed broadcast video signal, recover the encoded video signal of the selected channel, and decode and display the video frames of the recovered video signal.
- the display 13 typically cannot blend overlay frames with the video frames. Therefore, to allow such blending, the box 11 includes the same decoding capability (the decoder 22 ) as the display 13 .
- the processing circuitry also includes the re-encoder 26 , which adds even more size and expense to the box 11 !
- HDTV High-Definition-Television
- an image may be compressed before being sent over the Internet.
- an image may be compressed before being stored thereon.
- MPEG Moving Pictures Experts Group
- JPEG Joint Photographic Experts Group
- each video frame is divided into subregions called macro blocks, which each include one or more pixels.
- FIG. 3A is a 16-pixel-by-16-pixel macro block 30 having 256 pixels 32 (not drawn to scale).
- a macro block is always 16 ⁇ 16 pixels, although other compression standards may use macro blocks having other dimensions.
- each pixel 32 has a respective luminance value Y and a respective pair of color-, i.e., chroma-, difference values C B and C R .
- the digital luminance (Y) and chroma-difference (C B and C R ) values that will be used for compression are generated from the original Y, C B , and C R values of the original frame.
- the pre-compression Y values are the same as the original Y values.
- each pixel 32 merely retains its original luminance value Y.
- the MPEG 4:2:0 format allows only one pre-compression C B value and one pre-compression C R value for each group 34 of four pixels 32 .
- Each of these pre-compression C B and C R values are respectively derived from the original C B and C R values of the four pixels 32 in the respective group 34 .
- a pre-compression C B value may equal the average of the original C B values of the four pixels 32 in the respective group 34 .
- the pre-compression Y, C B , and C R values generated for the macro block 10 are arranged as one 16 ⁇ 16 matrix 36 of pre-compression Y values (equal to the original Y value for each pixel 32 ), one 8 ⁇ 8 matrix 38 of pre-compression C B values (equal to one derived C B value for each group 34 of four pixels 32 ), and one 8 ⁇ 8 matrix 40 of pre-compression C R values (equal to one derived C R value for each group 34 of four pixels 32 ).
- the matrices 36 , 38 , and 40 are often called “blocks” of values.
- the block 36 of pre-compression Y values is subdivided into four 8 ⁇ 8 blocks 42 a - 42 d , which respectively correspond to the 8 ⁇ 8 blocks A-D of pixels in the macro block 30 .
- eight 8 ⁇ 8 blocks of pre-compression pixel data are generated for each macro block 30 : four 8 ⁇ 8 blocks 42 a - 42 d of pre-compression Y values, one 8 ⁇ 8 block 38 of pre-compression C B values, and one 8 ⁇ 8 block 40 of pre-compression C R values.
- FIG. 4 is a block diagram of an MPEG compressor 50 , which is more commonly called an encoder.
- the encoder 50 converts the pre-compression data for a frame or sequence of frames into encoded data that represent the same frame or frames with significantly fewer data bits than the pre-compression data. To perform this conversion, the encoder 50 reduces or eliminates redundancies in the pre-compression data and reformats the remaining data using efficient transform and coding techniques.
- the encoder 50 includes a frame-reorder buffer 52 , which receives the pre-compression data for a sequence of one or more frames and reorders the frames in an appropriate sequence for encoding.
- the reordered sequence is often different than the sequence in which the frames are generated and will be displayed.
- the encoder 50 assigns each of the stored frames to a respective group, called a Group Of Pictures (GOP), and labels each frame as either an intra (I) frame or a non-intra (non-I) frame.
- GOP Group Of Pictures
- each GOP may include three I frames and 12 non-I frames for a total of fifteen frames.
- the encoder 50 always encodes an I frame without reference to another frame, but can and often does encode a non-I frame with reference to one or more of the other frames in the GOP.
- the encoder 50 does not, however, encode a non-I frame with reference to a frame in a different GOP.
- the 8 ⁇ 8 blocks ( FIGS. 3B-3D ) of the pre-compression Y, C B , and C R values that represent the I frame pass through a summer 54 to a Discrete Cosine Transformer (DCT) 56 , which transforms these blocks of values into respective 8 ⁇ 8 blocks of one DC (zero frequency) coefficient and sixty-three AC (non-zero frequency) coefficients. That is, the summer 54 is not needed when the encoder 50 encodes an I frame, and thus the pre-compression values pass through the summer 54 without being summed with any other values. As discussed below, however, the summer 54 is often needed when the encoder 50 encodes a non-I frame.
- DCT Discrete Cosine Transformer
- a quantizer 58 limits each of the coefficients to a respective maximum value, and provides the quantized AC and DC coefficients on respective paths 60 and 62 .
- a prediction encoder 64 predictively encodes the DC coefficients, and a variable-length coder 66 converts the quantized AC coefficients and the quantized and predictively encoded DC coefficients into variable-length codes, such as Huffman codes. These codes form the encoded data that represent the pixel values of the encoded I frame.
- a transmit buffer 68 then temporarily stores these codes to allow synchronized transmission of the encoded data to a decoder (discussed below in conjunction with FIG. 6 ).
- the coder 66 may provide the variable-length codes directly to a storage medium such as a CD-ROM.
- the encoder 50 If the I frame will be used as a reference (as it often will be) for one or more non-I frames in the GOP, then, for the following reasons, the encoder 50 generates a corresponding reference frame by decoding the encoded I frame with a decoding technique that is similar or identical to the decoding technique used by the decoder ( FIG. 6 ). When decoding non-I frames that are referenced to the I frame, the decoder has no option but to use the decoded I frame as a reference frame. Because MPEG encoding and decoding are lossy—some information is lost due to quantization of the AC and DC transform coefficients—the pixel values of the decoded I frame will often be different than the pre-compression pixel values of the I frame.
- pre-compression I frame may cause additional artifacts in the decoded non-I frame because the reference frame used for decoding (decoded I frame) would be different than the reference frame used for encoding (pre-compression I frame).
- the encoder 50 includes a dequantizer 70 and an inverse DCT 72 , which are designed to mimic the dequantizer and inverse DCT of the decoder ( FIG. 6 ).
- the dequantizer 70 dequantizes the quantized DOT coefficients from the quantizer 58
- the inverse DCT 72 transforms these dequantized DOT coefficients into corresponding 8 ⁇ 8 blocks of decoded Y, C B , and C R pixel values, which compose the reference frame.
- decoded pixel values may be different than their corresponding pre-compression pixel values, and thus the reference frame may be different than its corresponding pre-compression frame as discussed above.
- the decoded pixel values then pass through a summer 74 (used when generating a reference frame from a non-I frame as discussed below) to a reference-frame buffer 76 , which stores the reference frame,
- the encoder 50 initially encodes each macro-block of the non-I frame in at least two ways: in the manner discussed above for I frames, and using motion prediction, which is discussed below. The encoder 50 then saves and transmits the resulting code having the fewest bits. This technique insures that the macro blocks of the non-I frames are encoded using the fewest bits.
- an object in a frame exhibits motion if its relative position changes in the succeeding frames. For example, a horse exhibits relative motion if it gallops across the screen. Or, if the camera follows the horse, then the background exhibits relative motion with respect to the horse.
- each of the succeeding frames in which the object appears contains at least some of the same macro blocks of pixels as the preceding frames. But, such matching macro blocks in a succeeding frame often occupy respective frame locations that are different than the respective frame locations they occupy in the preceding frames.
- a macro block that includes a portion of a stationary object (e.g., tree) or background scene (e.g., sky) may occupy the same frame location in each of a succession of frames, and thus exhibit “zero motion”.
- the macro blocks R and Z of frame I non-I frame
- This “statement” is encoded as a motion vector. For a relatively fast moving object, the location values of the motion vectors are relatively large. Conversely, for a stationary or relatively slow-moving object or background scene, the location values of the motion vectors are relatively small or equal to zero.
- FIG. 5 illustrates the concept of motion vectors with reference to the non-I frame 1 and the I frame 0 discussed above.
- a motion vector MVR indicates that a match for the macro block in the location R of frame I can be found in the location S of frame 0 .
- MVR has three components. The first component, here 0, indicates the frame (here frame 0) in which the matching macro block can be found. The next two components, X R and Y R , together comprise the two-dimensional location value that indicates where in the frame 0 the matching macro block can be found.
- a motion predictor 78 compares the pre-compression Y values (the C B and C R values are not used during motion prediction) of the macro blocks in the non-I frame to the decoded Y values of the respective macro blocks in the reference frame and identifies matching macro blocks. For each macro block in the non-I frame for which a match is found in the reference frame, the motion predictor 78 generates a motion vector that identifies the reference frame and the location of the matching macro block within the reference frame.
- the decoder uses the motion vectors to obtain the pixel values of the motion-encoded macro blocks from the matching macro blocks in the reference frame.
- the prediction encoder 64 predictively encodes the motion vectors, and the coder 66 generates respective codes for the encoded motion vectors and provides these codes to the transmit buffer 48 .
- the encoder 50 encodes these differences along the with motion vector so that the decoder can account for them. More specifically, the motion predictor 78 provides the decoded Y values of the matching macro block of the reference frame to the summer 54 , which effectively subtracts, on a pixel-by-pixel basis, these Y values from the pre-compression Y values of the matching macro block of the non-I frame.
- residuals are arranged in 8 ⁇ 8 blocks and are processed by the DOT 56 , the quantizer 58 , the coder 66 , and the buffer 68 in a manner similar to that discussed above, except that the quantized DC coefficients of the residual blocks are coupled directly to the coder 66 via the line 60 , and thus are not predictively encoded by the prediction encoder 44 .
- a non-I frame as a reference frame.
- the quantized residuals from the quantizer 58 are respectively dequantized and inverse transformed by the dequantizer 70 and the inverse DCT 72 so that this non-I reference frame will be the same as the one used by the decoder for the reasons discussed above.
- the motion predictor 78 provides to the summer 74 the decoded Y values of the I reference frame from which the residuals were generated.
- the summer 74 adds the respective residuals from the circuit 72 to these decoded Y values of the I reference frame to generate the respective Y values of the non-I reference frame.
- the reference frame buffer 76 then stores the non-I reference frame along with the I reference frame for use in encoding subsequent non-I frames.
- the encoder 50 also includes a rate controller 80 to insure that the transmit buffer 68 , which typically transmits the encoded frame data at a fixed rate, never overflows or empties, i.e., underflows, If either of these conditions occurs, errors may be introduced into the encoded data stream. For example, if the buffer 68 overflows, data from the coder 66 is lost.
- the rate controller 80 uses feed back to adjust the quantization scaling factors used by the quantizer 58 based on the degree of fullness of the transmit buffer 68 . The fuller the buffer 68 , the larger the controller 80 makes the scale factors, and the fewer data bits the coder 66 generates. Conversely, the more empty the buffer 68 , the smaller the controller 80 makes the scale factors, and the more data bits the coder 66 generates. This continuous adjustment insures that the buffer 68 neither overflows nor underflows.
- FIG. 6 is a block diagram of a conventional MPEG decompresser 82 , which is commonly called a decoder and which can decode frames that are encoded by the encoder 60 of FIG. 4 .
- variable-length decoder 84 decodes the variable-length codes received from the encoder 50 .
- a prediction decoder 86 decodes the predictively encoded DC coefficients
- a dequantizer 87 which is similar or identical to the dequantizer 70 of FIG. 4 , dequantizes the decoded AC and DC transform coefficients.
- An inverse DOT 88 which is similar or identical to the inverse DCT 72 of FIG. 4 , transforms the dequantized coefficients into pixel values.
- the decoded pixel values pass through a summer 90 —which is used during the decoding of motion-predicted macro blocks of non-I frames as discussed below—into a frame-reorder buffer 92 , which stores the decoded frames and arranges them in a proper order for display on a video display unit 94 . If a decoded I frame is used as a reference frame, it is also stored in the reference-frame buffer 96 .
- the decoder 84 For motion-predicted macro blocks of non-I frames, the decoder 84 , dequantizer 87 , and inverse DOT 88 process the residuals as discussed above in conjunction with FIG. 4 .
- the prediction decoder 86 decodes the motion vectors, and a motion interpolator 98 provides to the summer 90 the pixel values from the reference-frame macro blocks that the motion vectors point to.
- the summer 90 adds these reference pixel values to the residuals to generate the pixel values of the decoded macro blocks, and provides these decoded pixel values to the frame-reorder buffer 92 . If a decoded non-I frame is used as a reference frame, it is stored in the reference-frame buffer 96 .
- the encoder 50 and the decoder 82 may be implemented in hardware, software, or a combination of both.
- the encoder 50 and the decoder 82 are often implemented by a respective one or more processors that perform the respective functions of the circuit blocks.
- a video processing circuit includes a processor that receives an encoded image having first and second regions, decodes the first region of the image, modifies the decoded first region, and re-encodes the modified first region.
- Such a circuit allows one to modify a region of an image by decoding and re-encoding only that region instead of the entire image. For example, if one wishes to overlay an EPG on a bottom portion of a video frame, then the circuit can decode only the EPG and the bottom portion of the frame, overlay the decoded EPG on the bottom frame portion, and re-encode the overlaid bottom frame portion. Therefore, this technique often reduces the processing time, and thus the cost and complexity of the processing circuit, as compared to a circuit that decodes and re-en codes the entire frame during an image overlay process.
- FIG. 1 a diagram of a video frame that includes a conventional overlay region.
- FIG. 2 is a block diagram of a conventional television receiving-and-display system that includes a set-top box for blending an overlay image with overlay regions of respective video frames.
- FIG. 3A is a diagram of a conventional macro block of pixels in an image.
- FIG. 3B is a diagram of a conventional block of pre-compression luminance values that respectively correspond to the pixels in the macro block of FIG. 3A .
- FIGS. 30 and 3 D are diagrams of conventional blocks of pre-compression chroma values that respectively correspond to the pixel groups in the macro block of FIG. 3A .
- FIG. 4 is a block diagram of a conventional MPEG encoder.
- FIG. 5 is a diagram that illustrates the concept of conventional motion vectors.
- FIG. 6 is a block diagram of a conventional MPEG decoder.
- FIG. 7 is a block diagram of a video-frame formatter according to an embodiment of the invention.
- FIG. 8 is a diagram of a group of multi-region video frames generated by the formatter of FIG. 7 .
- FIG. 9 is a block diagram of a set-top box according to an embodiment of the invention.
- FIG. 10 is a block diagram of a set-top box according to another embodiment of the invention.
- FIG. 7 is a block diagram of a video-frame formatter 100 according to an embodiment of the invention.
- the formatter 100 includes video processing circuits 102 1 - 102 N for respectively formatting encoded video-channel signals 1 -N received from respective broadcaster networks (e.g., FOX, CNN). Specifically, the processing circuits 102 format the frames of each respective channel signal such that the frames each have multiple independent regions. The processing circuits 102 then provide the respective processed channel signals 1 -N to a multiplexer 104 , which combines all of the processed channel signals into an encoded multiplexed broadcast video signal. Although described as formatting video frames, the formatter 100 can be designed to modify video fields or still images.
- the processing circuit 102 1 includes a decoder 108 1 for decoding the channel signal 1 , which in one embodiment is encoded according to a compression standard that is the same as or is similar to the MPEG compression standard described in conjunction with FIGS. 3-6 .
- a motion-vector analyzer 110 1 receives the decoded channel signal and identifies macro blocks having motion vectors that point from one region of a frame to a different region of the same or another frame.
- the analyzer 110 1 identifies a macro block in a region of a frame if the macro block has a motion vector that points to a reference macro block in a different region of the same or another frame.
- a multi-region frame re-encoder 112 1 re-encodes the identified macro blocks such that no identified macro block has a motion vector pointing to a region of a frame that is different from the region containing the identified macro block.
- the re-encoder 112 1 re-encodes an identified macro block in a region of a frame such that the macro block has a motion vector that points to a reference macro block in the same region of another frame.
- the re-encoder 112 re-encodes an identified macro block as an I block such that it has no motion vector.
- FIG. 8 shows a group of three video frames 114 , 116 , and 118 .
- the frame 114 is an I frame and the frames 116 and 118 are non-I frames.
- the circuit 102 1 formats each of the frames 114 , 116 , and 118 into multiple regions, here two regions 120 a and 122 a , 120 b and 122 b , and 120 c and 122 c , respectively.
- the first regions 120 a - 120 c (unshaded) occupy the same respective upper areas of the frames 114 , 116 , and 118
- the second regions 122 a - 122 c (shaded) occupy the same respective lower areas of the frames 114 , 116 , and 118 .
- Both sets of regions 120 a - 120 c and 122 a - 122 c contain respective macro blocks 123 .
- the decoder 108 1 decodes the video frames, motion vectors, and other components of the channel I signal.
- the decoder 108 1 conventionally decodes each macro block in every frame down to its respective pixel values, i.e., down to the pixel domain. But as discussed below, because the decoder 108 1 does not decode the channel I signal for display, it may conventionally decode the macro blocks down only to their DOT coefficients, i.e., down to the transform domain. Alternatively, as discussed below, the decoder 108 1 may decode only some of the macro blocks in a frame. Such partial decoding often reduces the cost, complexity, and decoding time of the decoder 108 1 as compared to known decoders.
- the motion-vector analyzer 110 1 then examines the decoded motion vectors from the decoder 108 1 and identifies the macro blocks having motion vectors that point from one of the first regions 120 a - 120 c to one of the second regions 122 a - 122 c and vice versa. For example, the analyzer 110 1 identifies the decoded motion vectors 124 and 126 (shown in solid line) as being “boundary-crossing” vectors. Specifically, the analyzer 110 1 determines that the decoded motion vector 124 points from a macro block 123 a in the region 120 b of the frame 116 to a reference macro block 123 b in the region 122 a of the frame 114 .
- the analyzer 110 1 determines that the decoded motion vector 126 points from a macro block 123 c in the region 122 c of the frame 118 to a reference macro block l 23 d in the region 120 b of the frame 116 .
- the analyzer 110 1 identifies the motion vectors 124 and 126 as crossing the “boundary” between the first regions 120 a - 120 c and the second regions 122 a - 122 c , and informs the re-encoder 112 1 that the macro blocks 123 a and 123 c have boundary-crossing motion vectors.
- the re-encoder 112 1 generates substitute motion vectors for the macro blocks having boundary-crossing motion vectors.
- the re-encoder 112 1 generates substitute motion vectors 128 and 130 (shown in dashed line) for the macro blocks 123 a and 123 d , respectively.
- the substitute motion vector 128 points from its macro block 123 a in the region 120 b to a reference macro block 123 e in the region 120 a
- the substitute motion vector 130 points from its macro block 123 c in the region 122 c to a reference macro block 123 f in the region 122 b .
- neither of the substitute motion vectors 128 and 130 crosses the boundary between the first regions 120 a - 120 c and the second regions 122 a - 122 c .
- the re-encoder 112 c re-encodes the regions 120 a - 120 c so that they are independent of the regions 122 a - 122 c . That is, the first regions 120 a - 120 c are encoded with no reference to the second regions 122 a - 122 c , and the second regions 122 a - 122 c are encoded with no reference to the first regions 120 a - 120 c .
- Such independent regions are often called “slices” in MPEG terminology.
- first and second regions 120 a - 120 c and 122 a - 1 22 c allow one to modify one set of the regions, for example the regions 122 a - 122 c , without having to decode the other set or sets of regions, for example the regions 120 a - 120 c .
- the substitute motion vectors 128 and 130 are shown as having nonzero and zero location values respectively, both may have either nonzero or zero location values in other examples.
- the substitute motion vectors 128 and 130 are shown pointing to the same frames 114 and 116 as the boundary-crossing motion vectors 124 and 126 , respectively, they may point to different frames in other examples.
- the re-encoder 112 1 generates the substitute motion vectors by conventionally scanning the appropriate frame regions for the best reference macro blocks and then motion encoding the original macro blocks using the new reference macro blocks. For example, to generate the substitute motion vector 128 , the re-encoder 112 1 scans the region 120 a and determines that the macro block 123 e is the best reference macro block in the region 120 a . In one embodiment, the re-encoder 1121 starts scanning at the macro block 123 g , which is the macro block in the region 120 a that is closest to the original reference block 123 b . Thus, the reference block 123 e replaces the original reference block 123 b . Then, as discussed above in conjunction with FIG. 4 , the re-encoder 112 1 motion encodes the block 123 a using the substitute reference block 123 e instead of the reference block 123 b .
- the decoder 108 1 decodes at least the identified macro blocks, the reference macro blocks, and the macro blocks in the scan regions such as the region 120 a . For example, if the re-encoder 112 1 is constructed to generate substitute motion vectors that point to the same respective frames as the original motion vectors, then the decoder 108 1 can be constructed to decode only the identified macro blocks and the frames to which the identified motion vectors point. Alternatively, the decoder 108 1 can be constructed to decode only the identified macro blocks, the reference macro blocks, and the regions of the pointed-to frames that correspond to the regions containing the identified macro blocks.
- the decoder 108 1 can be constructed to recognize that the substitute motion vector 128 will point to the region 120 a , which corresponds to the region 120 b of the identified macro block 123 a . Based upon this recognition, the decoder 108 1 decodes the macro blocks 123 a and 123 b and all the macro blocks in the region 120 a , which is the region to be scanned by the re-encoder 112 1 . Of course the decoder 108 1 can be constructed to decode all of the frames in their entireties.
- the decoder 108 1 and analyzer 110 1 interact in the following manner. First, the decoder 108 1 decodes the motion vectors for a frame. Then, the analyzer 110 1 determines the cross-bolmdary motion vectors, the macro blocks to which they belong, the reference macro blocks to which they point, and the frame scan regions that contain the reference macro blocks.
- the motion analyzer 110 1 provides the decoder 108 1 with the identifies of the macro blocks having cross-boundary motion vectors, the reference macro blocks, and the macro blocks in the scan regions that the re-encoder 112 1 will scan during motion re-encoding.
- the decoder 108 1 then decodes these identified macro blocks and provides the decoded macro blocks to the re-encoder 112 1 via the analyzer 110 1 .
- the re-encoder 112 1 can be constructed to generate all the substitute motion vectors having location values of zero. This eliminates the need to scan a frame region because the substitute reference macro blocks are inherently known. For example, to generate the substitute motion vector 130 having a location value of zero as shown in FIG. 8 , the re-encoder 112 1 need not scan the region 122 b for the best substitute reference macro block because it is constrained to use the macro block 123 f as the substitute reference block. That is, the block 123 f is the only reference block that will give the vector 130 a location value of zero.
- the re-encoder 112 1 selects the block 123 f as the substitute reference block and then motion encodes the macro block 123 c using the substitute reference block 123 f instead of the original reference block 123 d .
- scanless encoding is faster and less complex than the scan encoding described above, scanless encoding is often less efficient, i.e., generates more bits per pixel, than scan encoding.
- the re-encoder 112 1 is constructed to encode the identified macro blocks as I-encoded blocks such that they have no motion vectors.
- encoding is faster and less complex than the motion (non-I) encoding techniques discussed above, it is often less efficient than motion encoding.
- the re-encoder 112 1 re-encodes the substitute motion vectors and any other decoded portions of the channel 1 signal and provides the re-encoded channel 1 signal, which includes the re-encoded video frames formatted into independent regions, to the multiplexer 104 .
- the decoder 108 1 can be constructed to decode down to either the transform domain or to the pixel domain. Because the DOT is a linear transform, the re-encoder 112 1 can scan, motion encode, and I encode using DOT coefficients as well as using pixel valves.
- the processing circuit 102 1 may include one or more respective processors that perform the functions of these circuit blocks in hardware, software or a combination of hardware and software. Additionally, the above-described functions may be performed in an order other than that described above.
- the processing circuit 102 1 includes the region dimensions and locations in the re-encoded channel signal. As discussed below in conjunction with FIG. 9 , a set-top box can use this dimension and location information when blending an overlay frame with a video-frame region.
- the increased cost for the processing power that allows the video processing circuits 102 1 - 102 N to format video frames into multiple independent regions will be more than offset by the reduced complexity and cost of the set-top box ( FIG. 9 ).
- the overall cost savings is actually very significant, because for every frame formatter 100 , which will typically be installed in a cable-company office, there will be hundreds, thousands, or even millions of set-top boxes. Thus, the combined cost savings for the set-top boxes will likely be much greater than the increased cost of installing one formatter 100 in a cable-company office.
- FIG. 9 is a block diagram of a television receiver system 130 according to an embodiment of the invention.
- the system 130 includes a remote control 132 and an HDTV receiver/display 134 , which are respectively similar to the remote control 12 and the receiver/display 13 of FIG. 2 .
- the system also includes a set-top box 136 , which blends overlay frames with regions of respective video frames.
- the structure and operation of the box 136 is discussed with reference to the frames 114 , 116 , and 118 of FIG. 8 and to an EPG as the overlay frame. It is understood, however, that this discussion is applicable to video frames having a different formatting and to different overlay frames.
- the box 136 blends the EPG with the regions 122 a - 122 c of the frames 114 , 116 , and 118 . Because the regions 122 a - 122 c are encoded independently of the regions 120 a - 120 c , the box 136 need only decode and re-encode the regions 122 a - 122 c . Therefore, because the box 136 need not decode or re-encode the regions 120 a - 120 c , it can have significantly less processing power than, and thus can be significantly less expensive than, the conventional set-top box 11 ( FIG. 2 ), which must decode and re-encode the frames in their entirety to perform this blending.
- the set-top box 136 of the system 130 includes a processing circuit 138 , which receives an encoded, multiplexed broadcast video signal from a cable or satellite company. In response to a channel-select signal from a command decoder 140 , a channel selector 142 demultiplexes the broadcast signal and provides a selected video-channel signal to an overlay-region decoder 144 .
- the decoder 144 decodes the overlay frames and the overlay frame regions with which the overlay frames will be blended. For example, referring to FIG. 8 , the decoder 144 decodes the EPG and the overlay regions 122 a - 122 c of the frames 114 , 116 , and 118 , respectively. For reasons discussed above in conjunction with FIG. 7 , the decoder 144 decodes the EPG and the overlay regions 122 a - 122 c down to the transform domain or all the way down to the pixel domain.
- the decoder 144 does not, however, decode the non-overlay frame regions 120 a - 120 c . Therefore, the decoder 144 provides the decoded EPG and overlay frame regions 122 a - 122 c on an output terminal 145 , and passes through the undecoded non-overlay regions 120 a - 120 c to an output terminal 146 .
- An overlay/region combiner 147 is coupled to the output terminal 145 of the decoder 144 and blends the decoded overlay frames with the decoded frame overlay regions in a manner described below.
- the decoder 144 blends the decoded EPG with the decoded overlay frame regions 122 a - 122 c .
- An overlay region re-encoder 148 re-encodes the blended overlay frame regions using conventional encoding techniques similar to those discussed above in conjunction with FIG. 4 .
- the re-encoder 148 re-encodes the blended overlay frame regions 122 a - 122 c.
- a frame buffer 150 receives the re-encoded overlay frame regions from the re-encoder 148 and receives the undecoded non-overlay frame regions that the overlay region decoder 144 passes through via the output terminal 146 .
- the frame buffer 150 stores the undecoded and re-encoded frame regions in respective buffer sections to “reassemble” the video frames.
- the buffer 150 stores the undecoded non-overlay regions 120 a - 120 c in a first set of buffer sections, and stores the re-encoded overlay regions 122 a - 122 c in a second set of buffer sections (buffer sections not shown in FIG. 9 ).
- the buffer 150 then provides the contents of the first and seconds sets of buffer sections to the display 134 such that the regions 120 a and 122 a are provided as a single frame, and so on.
- a rate controller 154 which is similar to the rate controller 80 of FIG. 4 , monitors the fullness of the buffer 150 and prevents the buffer 150 from overflowing or under flowing by controlling the degree of quantization used by the re-encoder 148 .
- the display 134 decodes and displays the reassembled video frames from the set-top box 136 .
- the display 134 displays the frames 114 , 116 , and 118 having the EPG in the regions 122 a , 122 b , and 122 c , respectively.
- the processing circuit 138 can be much less complex, and thus much less expensive, than processing circuits that decode and re-encode the frames in their entirety.
- the channel signal includes the locations (within the respective frames) and the dimensions of the regions 122 a - 122 c , and the processing circuit 138 uses this information to properly configure the decoder 144 , combiner 147 , re-encoder 148 , and frame buffer 150 .
- This allows the cable or satellite operator to change the dimensions or location of the regions 122 a - 122 c without requiring hardware modifications to the set-top box 136 .
- the processing circuit 138 does not process the encoded video frames from the channel selector 142 , but instead passes them through to the frame buffer 150 .
- the remote control 132 does not generate an overlay command, and thus the command decoder 140 does not generate the overlay signal.
- the overlay region decoder 144 passes through the previously encoded video frames in there entirety from the channel selector 142 to the output terminal 146 .
- the decoder 144 performs no decoding, and thus passes the undecoded frame regions 120 a - 120 c and 122 a - 122 c to the output terminal 146 .
- the decoder 144 does not pass through the EPG to either of the output terminals 145 and 146 . That is, the decoder 144 prevents the processing circuit 138 from providing the EPG to the display 134 .
- the frame buffer 150 receives the undecoded frames 114 , 116 , and 118 in their entirety from the output terminal 146 of the decoder 144 , and provides these undecoded frames to the display 134 .
- the overlay/frame-region combiner 147 blends the overlay frame with the overlay frame regions on a pixel-by-pixel basis using conventional alpha blending.
- the combiner 147 can perform it in the transform domain or in the pixel domain.
- the overlay region decoder 144 can decode the macro blocks of the regions 122 a - 122 c ( FIG. 8 ) down to the transform domain or the pixel domain.
- the re-encoder 148 provides a relatively high number of quantization levels to preserve the higher frequencies in the overlay frame regions. For example, referring to FIG. 8 , this low degree of quantization sharpens the edges of the EPG characters (not shown in FIG. 8 ) in the overlay regions 122 a - 122 c . In some cases, this may cause the regions 120 a - 120 c to have different quantization levels than the regions 122 a - 122 c . But because the regions 120 a - 120 c and 122 a - 122 c are MPEG slices as discussed above in conjunction with FIGS.
- the display 134 can easily change its dequantization levels as it moves from one slice to the other—for example from the region 120 a to the region 122 a —during the decoding of the frame 114 . That is, the display 134 uses one set of dequantization levels to decode the regions 120 a - 120 c and another set of dequantization levels to decode the regions 122 a - 122 c .
- the respective dequantization levels needed to decode the regions 120 a - 120 c are conventionally generated and included in the channel signal by the re-encoder 112 1 of FIG. 7 and are passed through to the display 134 by the processing circuit 138 .
- the dequantization levels needed to decode the regions 122 a - 122 c are generated and included with the re-encoded overlay frame regions by the re-encoder 148 of FIG. 9 and are passed to the display 134 by the buffer 150 .
- the re-encoder 148 can re-encode the macro blocks of the blended overlay frame regions in a number of ways. For example, referring to FIG. 8 , according to a first technique, the re-encoder 148 re-encodes all the macro blocks of the regions 122 a - 122 c as I blocks. Or, according to a second technique, the re-encoder 148 re-encodes at least some of these macro blocks so that they have motion vectors with location values of 0. This is particularly efficient if the EPG is stationary and opaque, since all of the macro blocks in the regions 122 a - 122 c will be the same from frame to frame.
- the re-encoder 148 re-encodes at least some of the macro blocks to have motion vectors with constant, nonzero location values based on the scroll rate. That is, if the scroll rate is constant, then the re-encoder 148 can calculate from the scroll rate how far a macro block in the region 122 of one frame has moved in the region 122 of the next frame. If the EPG scrolls in a vertical direction, then the motion-vector location values will be (0, Y), where the value of Y is based on the vertical scroll rate.
- the motion-vector location values will be (X, 0), where the value of X is based on the horizontal scroll rate.
- This re-encoding technique is particularly efficient if the EPG is opaque. Sometimes, however, the EPG is not opaque. That is, the EPG characters are opaque but the spaces between the characters are transparent to show the original scene. In this situation, therefore, this re-encoding technique is often inefficient because of relatively large residuals.
- the re-encoder 148 re-encodes the blended overlay frame regions using full-scale motion encoding as discussed above in conjunction with FIG. 4 .
- the re-encoder 148 re-encodes the region 122 b by scanning the region 122 a for reference macro blocks and then generating corresponding motion vectors and residuals.
- the first technique requires the least processing power but is the least efficient
- the fourth technique requires the most processing power but is the most efficient
- the second and third techniques fall in between. Because the rate controller 152 ensures that there is no overflow of the buffer 150 , the lower the efficiency of a re-encoding technique, the greater the losses incurred by the re-encoding process. These losses may show up as undesirable visual artifacts in the overlay frame regions when an overlay frame like the EPG is viewed.
- the processor 138 can implement an I-frame-only mode during which it “skips” the non-overlay frame information to reduce the number of bits in the re-encoded channel signal.
- This technique which is often called “skipping the background”, virtually insures that the frame buffer 150 wilt not overflow when the re-encoder 148 uses a low degree of quantization to generate high-quality overlay frame regions. For example, referring to FIG.
- the combiner 147 combines the EPG with the regions 122 of the I frames (like region 122 a of the frame 114 ) and stores only these modified I frames in the frame buffer 150 .
- the buffer 150 provides these I frames to the display 134 by repeatedly sending the most recent, i.e., current, I frame (or by instructing the display 134 to repeatedly display the current I frame) until the combiner 147 and re-encoder 148 process the next I frame. Because the ratio of I frames to non-I frames in a GOP is relatively low, for example 1/15, the I-frame-only mode may cause jerky motion in the non-EPG frame regions 120 .
- EPG is not opaque, then such jerky motion might also be visible in the background of the EPG regions 122 as well. But because such jerky motion is visible only while the display 134 displays the EPG, because a viewer typically displays the EPG for only a relatively small fraction of his/her total viewing time, and because the viewer is more likely to concentrate on the EPG than the background, the jerky motion during EPG viewing is often unnoticeable, or is at least acceptable, to the viewer.
- a modification of the I-frame-only technique is to skip the background only in the non-overlay regions.
- the overlay decoder 144 passes through only the encoded non-overlay regions 120 of the I frames to the buffer 150 , but continues to provide the EPG and the overlay regions 122 of all the frames to the combiner 147 .
- the combiner 147 and the re-encoder 148 operate as described above such that the blended regions 122 have normal motion.
- the buffer 150 then combines the region 120 of the current I frame with the regions 122 of the current I frame and the subsequent non-I frames, respectively.
- the display 134 displays the blended video frames having normal motion in the regions 122 and jerky motion only in the regions 120 .
- the processing circuit 138 can also be designed to blend the EPG with the overlay regions 122 such that the processed frames have normal motion in the regions 120 and jerky motion in the regions 122 .
- FIG. 10 is a block diagram of a set-top box 160 according to another embodiment of the invention.
- the box 160 is similar to the box 136 of FIG.
- the box 160 includes a processing circuit 164 , which is similar to the processing circuit 138 of FIG. 9 except that it includes a conventional decoder 166 and a conventional HDTV-to-STV converter 168 .
- the converter 168 reduces the number of pixels, and thus the resolution, of each of the frames that the box 160 provides to the display 162 . As is known, the converter 168 can perform this reduction in either the transform or pixel domains. Accordingly, the decoder 166 decodes the video frames of the selected channel signal to whichever domain the converter 168 calls for. Furthermore, because the decoder 166 decodes each frame in its entirety to allow the converter 168 to down convert the frames, the circuit 164 includes a re-encoder 170 that re-encodes both the overlay and non-overlay frame regions.
- the command decoder 140 When a viewer does not want to view an overlay frame, the command decoder 140 generates no overlay signal. In the absence of the overlay signal, the converter 168 down converts the decoded video frames from the decoder 166 and provides the down-converted frames in their respective entireties to the re-encoder 170 via a line 172 . The re-encoder 170 re-encodes the down-converted frames and provides them to the frame buffer 150 . The display 162 decodes and displays the re-encoded frames from the buffer 150 .
- the command decoder 140 When the viewer wants to view an overlay frame, the command decoder 140 generates the overlay signal in response to a command from the remote control 132 .
- the converter 168 down converts the decoded frames and the decoded EPG from the decoder 166 , provides the down-converted frame regions 120 a - 120 c to the re-encoder 170 via the line 172 , and provides the down-converted frame regions 122 a - 122 c and the down-converted EPG to the overlay/frame combiner 147 via a line 174 .
- the combiner 147 blends the EPG and the regions 122 a - 122 c as discussed above in conjunction with FIG. 9 .
- the re-encoder 170 re-encodes the decoded regions 120 a - 120 c and the blended regions 122 a - 122 c in a manner similar to that discussed above in conjunction with FIG. 9 .
- the re-encoder 170 provides these re-encoded regions to the buffer 150 , which stores these regions as respective re-encoded frames for display on the display 162 .
- the rate controller 154 prevents the buffer 150 from overflowing or under flowing as discussed above in conjunction with FIG. 9 .
- the processor circuit 164 may include one or more processors that perform the functions of these circuit blocks in hardware or in software. Additionally, the above-described functions may be performed in an order other than that described above. Furthermore, although shown as part of the processing circuit 164 , the command decoder 140 and the channel selector 142 may be separate from the circuit 164 .
Abstract
Description
- This application is a divisional of pending U.S. patent application Ser. No. 09/390,241, filed on Sep. 3, 1999, the disclosure of which is incorporated herein in its entirety by reference.
- The invention relates generally to image-processing techniques, and more particularly to a circuit and method for modifying a region of a encoded image. For example, the invention allows one to blend an electronic program guide (EPG) with a region of an encoded video frame without decoding the entire frame.
-
FIG. 1 is a video image 6, which is a video frame that includes a first region 7 and asecond region 8. Although described as a video frame for example purposes, the image 6 may also be a video field. Furthermore, although shown as two rectangles in a top-bottom arrangement, the number, shape, and respective locations of these regions is arbitrary. - Typically, one views a sequence of video frames 6 in their respective entireties. But one may sometimes wish to view another image, i.e., an overlay image, in one of the
regions 7 and 8. For example, one may wish to view an electronic program guide (EPG) in theregion 8 while he/she is watching a program in the region 7 (and also in theregion 8 if the EPG is transparent). Or, one may wish to view an internet order menu in theregion 8 while he/she is viewing merchandise for sale in the region 7 (and also in theregion 8 if the menu is transparent). Thus, the overlay image is typically a partial frame that is the same size as or that is smaller than the frame region that it overlays, although the overlay frame can overlay an entire video frame. But for simplicity, both partial and full overlay frames are referred to as “overlay frames”.FIG. 2 is a block diagram of a conventionaltelevision receiver system 10, which includes a set-top box 11 such as a cable TV (CATV) or satellite TV box, aremote control 12, and adigital video display 13. Generally, the box 11 allows one to view overlay images—such as those that compose an EPG—in the respective regions 7 of a sequence of video frames 6 (FIG. 1 ). The box 11 includes aprocessing circuit 14, which receives an encoded, multiplexed broadcast video signal on aninput terminal 15, receives command signals from theremote control 12 on acommand terminal 16, and generates a video display signal on anoutput terminal 17. The broadcast video signal includes one or more broadcast channels and one or more overlay frames such as the frames that compose an EPG, and is encoded according to a compression standard such as the Moving Pictures Experts Group (MPEG) standard (discussed below). In response to channel-select and overlay commands from theremote control 12, thecircuit 14 blends the video frames from the selected channel with the appropriate overlay frame or frames and generates the display signal as a sequence of these blended video frames. Thedisplay 13 receives the display signal from theterminal 17 and decodes and displays the sequence of blended video frames. - More specifically, the
processing circuit 14 includes acommand decoder 18, which decodes the commands from theremote control 12 and generates corresponding control signals, such as an overlay signal, that control other portions of theprocessing circuit 14. Achannel selector 20 receives the broadcast signal from theterminal 15 and, in response to a channel-select signal from thecommand decoder 18, demultiplexes the selected channel from the broadcast signal. In response to an overlay signal from thedecoder 18, theselector 20 also demultiplexes the selected overlay frames from the broadcast signal. For example, theselector 20 may demultiplex the EPG that corresponds to the selected channel. Avideo decoder 22 decodes the video frames of the selected channel into pixel-domain frames, i.e., frames of pixel luminance and chromanance values. In response to the overlay signal, thevideo decoder 22 also decodes the selected overlay frames into the pixel domain, and an overlay/video combiner 24 blends the decoded video frames with the decoded overlay frames. Conversely, if thecommand decoder 18 does not generate an overlay signal, then theselector 20 does not demultiplex the overlay frames, and thus thecombiner 24 merely passes through the decoded video frames from thedecoder 22. In one embodiment, the output terminal of thecombiner 24 is coupled directly to theoutput terminal 17. But because it is sometimes undesirable to couple decoded video frames (blended or unblended) directly to thedisplay 13, in another embodiment thecircuit 14 includes anoptional re-encoder 26, which re-encodes the decoded video frames from thecombiner 24 before providing them to thedisplay 13. Although shown as including a number of separate circuit blocks, theprocessing circuit 14 mav include one or more processors that perform the functions of the above-describedcircuit blocks - Still referring to
FIG. 2 , in operation during a period when a viewer does not want to view an overlay frame, he selects a channel with theremote control 12, which generates a corresponding control signal. Thecontrol terminal 16, which is typically an infrared detector, receives the control signal and couples it to thecommand decoder 18. In response to the control signal, thedecoder 18 generates the channel-select signal, which causes thechannel selector 20 to recover the encoded video signal of the selected channel by demultiplexing the broadcast signal. Thevideo decoder 22 decodes the recovered video signal into frames of pixel values, and thecombiner 24 passes these frames to theoptional re-encoder 26, which re-encodes the frames and provides a re-encoded video signal to thedisplay 13. If, however, there-encoder 26 is omitted, then thecombiner 24 passes the decoded frames directly to thedisplay 13. - In operation during a period when the viewer wants to view an overlay frame, he selects a channel as described above and also selects an overlay frame or a series of overlay frames, such as an EPG, with the
remote control 12. Thedecoder 18 generates the channel-select signal and an overlay signal, which together cause thechannel selector 20 to recover both the encoded video signal of the selected channel and the encoded video signal containing the overlay frame or frames. The overlay signal causes thevideo decoder 22 to decode the recovered channel and 20 overlay video signals from thechannel selector 20 into respective sequences of frames, and causes thecombiner 24 to blend the overlay frames with the channel frames to generate blended frames. Theoptional re-encoder 26 re-encodes these blended frames and provides them to thedisplay 13, which decodes the re-encoded blended frames. If, however, there-encoder 26 is omitted, then thecombiner 24 provides the blended frames directly to thedisplay 13. - Unfortunately, the set-top box 11 cannot utilize the decoding ability of the
display 13, and thus includes its own redundant decoding circuitry, which often adds significant size and cost to the box 11. Typically, thedisplay 13 includes channel-select and full decoding circuitry respectively similar to thechannel selector 20 and thedecoder 22 of the box 11. Thus, thedisplay 13 typically can directly receive the encoded, multiplexed broadcast video signal, recover the encoded video signal of the selected channel, and decode and display the video frames of the recovered video signal. But thedisplay 13 typically cannot blend overlay frames with the video frames. Therefore, to allow such blending, the box 11 includes the same decoding capability (the decoder 22) as thedisplay 13. The viewer, however, typically requests the display of overlay frames for only a small portion of the time that he/she spends watching a program. Therefore, because the blending abilities of the box 11 are needed only a small part of the time, the decoding abilities of the box 11 are redundant to those of thedisplay 13 most of the time. That is, the viewer paid for two full decoders when one decoder will do the job the vast majority of the time! Furthermore, where it is desired to provide thedisplay 13 with an encoded video signal, the processing circuitry also includes there-encoder 26, which adds even more size and expense to the box 11! - To help the reader more easily understand the concepts discussed below in the description of the invention, following is a basic overview of conventional video-compression techniques.
- To electronically transmit a relatively high-resolution image over a relatively low-band-width channel, or to electronically store such an image in a relatively small memory space, it is often necessary to compress the digital data that represents the image. Such image compression typically involves reducing the number of data bits necessary to represent an image. For example, High-Definition-Television (HDTV) video images are compressed to allow their transmission over existing television channels. Without compression, HDTV video images would require transmission channels having bandwidths much greater than the bandwidths of existing television channels. Furthermore, to reduce data traffic and transmission time to acceptable levels, an image may be compressed before being sent over the Internet. Or, to increase the image-storage capacity of a CD-ROM or server, an image may be compressed before being stored thereon.
- Referring to
FIGS. 3-6 , the basics of the popular block-based Moving Pictures Experts Group (MPEG) compression standards, which include MPEG-1 and MPEG-2, are discussed. For purposes of illustration, the discussion is based on using an MPEG 4:2:0 format to compress video images represented in a Y, CB, CR color space. However, the discussed concepts also apply to other MPEG formats, to images that are represented in other color spaces, and to other block-based compression standards such as the Joint Photographic Experts Group (JPEG) standard, which is often used to compress still images. Furthermore, although many details of the MPEG standards and the Y, CB, CR color space are omitted for brevity, these details are well-known and are disclosed in a large number of available references. - Still referring to
FIGS. 3-6 , the MPEG standards are often used to compress temporal sequences of images—video frames for purposes of this discussion—such as found in a television broadcast. Each video frame is divided into subregions called macro blocks, which each include one or more pixels.FIG. 3A is a 16-pixel-by-16-pixel macro block 30 having 256 pixels 32 (not drawn to scale). In the MPEG standards, a macro block is always 16×16 pixels, although other compression standards may use macro blocks having other dimensions. In the original video frame, i.e., the frame before compression, eachpixel 32 has a respective luminance value Y and a respective pair of color-, i.e., chroma-, difference values CB and CR. - Referring to
FIGS. 3A-3D , before compression of the frame, the digital luminance (Y) and chroma-difference (CB and CR) values that will be used for compression, i.e., the pre-compression values, are generated from the original Y, CB, and CR values of the original frame. In the MPEG 4:2:0 format, the pre-compression Y values are the same as the original Y values. Thus, eachpixel 32 merely retains its original luminance value Y. But to reduce the amount of data to be compressed, the MPEG 4:2:0 format allows only one pre-compression CB value and one pre-compression CR value for eachgroup 34 of fourpixels 32. Each of these pre-compression CB and CR values are respectively derived from the original CB and CR values of the fourpixels 32 in therespective group 34. For example, a pre-compression CB value may equal the average of the original CB values of the fourpixels 32 in therespective group 34. Thus, referring toFIGS. 3B-3D , the pre-compression Y, CB, and CR values generated for themacro block 10 are arranged as one 16×16matrix 36 of pre-compression Y values (equal to the original Y value for each pixel 32), one 8×8matrix 38 of pre-compression CB values (equal to one derived CB value for eachgroup 34 of four pixels 32), and one 8×8matrix 40 of pre-compression CR values (equal to one derived CR value for eachgroup 34 of four pixels 32). Thematrices block 36 of pre-compression Y values is subdivided into four 8×8 blocks 42 a-42 d, which respectively correspond to the 8×8 blocks A-D of pixels in themacro block 30. Thus, referring toFIGS. 3A-3D , six 8×8 blocks of pre-compression pixel data are generated for each macro block 30: four 8×8 blocks 42 a-42 d of pre-compression Y values, one 8×8block 38 of pre-compression CB values, and one 8×8block 40 of pre-compression CR values. -
FIG. 4 is a block diagram of anMPEG compressor 50, which is more commonly called an encoder. Generally, theencoder 50 converts the pre-compression data for a frame or sequence of frames into encoded data that represent the same frame or frames with significantly fewer data bits than the pre-compression data. To perform this conversion, theencoder 50 reduces or eliminates redundancies in the pre-compression data and reformats the remaining data using efficient transform and coding techniques. - More specifically, the
encoder 50 includes a frame-reorder buffer 52, which receives the pre-compression data for a sequence of one or more frames and reorders the frames in an appropriate sequence for encoding. Thus, the reordered sequence is often different than the sequence in which the frames are generated and will be displayed. Theencoder 50 assigns each of the stored frames to a respective group, called a Group Of Pictures (GOP), and labels each frame as either an intra (I) frame or a non-intra (non-I) frame. For example, each GOP may include three I frames and 12 non-I frames for a total of fifteen frames. Theencoder 50 always encodes an I frame without reference to another frame, but can and often does encode a non-I frame with reference to one or more of the other frames in the GOP. Theencoder 50 does not, however, encode a non-I frame with reference to a frame in a different GOP. - During the encoding of an I frame, the 8×8 blocks (
FIGS. 3B-3D ) of the pre-compression Y, CB, and CR values that represent the I frame pass through asummer 54 to a Discrete Cosine Transformer (DCT) 56, which transforms these blocks of values into respective 8×8 blocks of one DC (zero frequency) coefficient and sixty-three AC (non-zero frequency) coefficients. That is, thesummer 54 is not needed when theencoder 50 encodes an I frame, and thus the pre-compression values pass through thesummer 54 without being summed with any other values. As discussed below, however, thesummer 54 is often needed when theencoder 50 encodes a non-I frame. Aquantizer 58 limits each of the coefficients to a respective maximum value, and provides the quantized AC and DC coefficients onrespective paths prediction encoder 64 predictively encodes the DC coefficients, and a variable-length coder 66 converts the quantized AC coefficients and the quantized and predictively encoded DC coefficients into variable-length codes, such as Huffman codes. These codes form the encoded data that represent the pixel values of the encoded I frame. A transmitbuffer 68 then temporarily stores these codes to allow synchronized transmission of the encoded data to a decoder (discussed below in conjunction withFIG. 6 ). Alternatively, if the encoded data is to be stored instead of transmitted, thecoder 66 may provide the variable-length codes directly to a storage medium such as a CD-ROM. - If the I frame will be used as a reference (as it often will be) for one or more non-I frames in the GOP, then, for the following reasons, the
encoder 50 generates a corresponding reference frame by decoding the encoded I frame with a decoding technique that is similar or identical to the decoding technique used by the decoder (FIG. 6 ). When decoding non-I frames that are referenced to the I frame, the decoder has no option but to use the decoded I frame as a reference frame. Because MPEG encoding and decoding are lossy—some information is lost due to quantization of the AC and DC transform coefficients—the pixel values of the decoded I frame will often be different than the pre-compression pixel values of the I frame. Therefore, using the pre-compression I frame as a reference frame during encoding may cause additional artifacts in the decoded non-I frame because the reference frame used for decoding (decoded I frame) would be different than the reference frame used for encoding (pre-compression I frame). - Therefore, to generate a reference frame for the encoder that will be similar to or the same as the reference frame for the decoder, the
encoder 50 includes adequantizer 70 and aninverse DCT 72, which are designed to mimic the dequantizer and inverse DCT of the decoder (FIG. 6 ). Thedequantizer 70 dequantizes the quantized DOT coefficients from thequantizer 58, and theinverse DCT 72 transforms these dequantized DOT coefficients into corresponding 8×8 blocks of decoded Y, CB, and CR pixel values, which compose the reference frame. Because of the losses incurred during quantization, however, some or all of these decoded pixel values may be different than their corresponding pre-compression pixel values, and thus the reference frame may be different than its corresponding pre-compression frame as discussed above. The decoded pixel values then pass through a summer 74 (used when generating a reference frame from a non-I frame as discussed below) to a reference-frame buffer 76, which stores the reference frame, - During the encoding of a non-I frame, the
encoder 50 initially encodes each macro-block of the non-I frame in at least two ways: in the manner discussed above for I frames, and using motion prediction, which is discussed below. Theencoder 50 then saves and transmits the resulting code having the fewest bits. This technique insures that the macro blocks of the non-I frames are encoded using the fewest bits. - With respect to motion prediction, an object in a frame exhibits motion if its relative position changes in the succeeding frames. For example, a horse exhibits relative motion if it gallops across the screen. Or, if the camera follows the horse, then the background exhibits relative motion with respect to the horse. Generally, each of the succeeding frames in which the object appears contains at least some of the same macro blocks of pixels as the preceding frames. But, such matching macro blocks in a succeeding frame often occupy respective frame locations that are different than the respective frame locations they occupy in the preceding frames. Alternatively, a macro block that includes a portion of a stationary object (e.g., tree) or background scene (e.g., sky) may occupy the same frame location in each of a succession of frames, and thus exhibit “zero motion”. In either case, instead of encoding each frame independently, it takes fewer data bits to tell the decoder “the macro blocks R and Z of frame I (non-I frame) are the same as the macro blocks that are in the locations S and T, respectively, of frame 0 (I frame).” This “statement” is encoded as a motion vector. For a relatively fast moving object, the location values of the motion vectors are relatively large. Conversely, for a stationary or relatively slow-moving object or background scene, the location values of the motion vectors are relatively small or equal to zero.
-
FIG. 5 illustrates the concept of motion vectors with reference to thenon-I frame 1 and theI frame 0 discussed above. A motion vector MVR indicates that a match for the macro block in the location R of frame I can be found in the location S offrame 0. MVR has three components. The first component, here 0, indicates the frame (here frame 0) in which the matching macro block can be found. The next two components, XR and YR, together comprise the two-dimensional location value that indicates where in theframe 0 the matching macro block can be found. Thus, in this example, because the location S of theframe 0 has the same X,Y coordinates as the location R in theframe 1, XR=YR=0. Conversely, the macro block in the location T matches the macro block in the location Z, which has different X,Y coordinates than the location T. Therefore, XZ and YZ represent the location T with respect to the location Z. For example, suppose that the location T is ten pixels to the left of (negative X direction) and seven pixels down from (negative Y direction) the location Z. Therefore, MVz=(0, −10, −7). Although there are many other motion-vector schemes available, they are all based on the same general concept. - Reierring again to
FIG. 4 , motion prediction is now discussed in detail. During the encoding of a non-I frame, amotion predictor 78 compares the pre-compression Y values (the CB and CR values are not used during motion prediction) of the macro blocks in the non-I frame to the decoded Y values of the respective macro blocks in the reference frame and identifies matching macro blocks. For each macro block in the non-I frame for which a match is found in the reference frame, themotion predictor 78 generates a motion vector that identifies the reference frame and the location of the matching macro block within the reference frame. Thus, as discussed below in conjunction withFIG. 6 , during decoding of these motion-encoded macro blocks of the non-I frame, the decoder uses the motion vectors to obtain the pixel values of the motion-encoded macro blocks from the matching macro blocks in the reference frame. Theprediction encoder 64 predictively encodes the motion vectors, and thecoder 66 generates respective codes for the encoded motion vectors and provides these codes to the transmit buffer 48. - Furthermore, because a macro block in the non-I frame and a matching macro block in the reference frame are often similar but not identical, the
encoder 50 encodes these differences along the with motion vector so that the decoder can account for them. More specifically, themotion predictor 78 provides the decoded Y values of the matching macro block of the reference frame to thesummer 54, which effectively subtracts, on a pixel-by-pixel basis, these Y values from the pre-compression Y values of the matching macro block of the non-I frame. These differences, which are called residuals, are arranged in 8×8 blocks and are processed by theDOT 56, thequantizer 58, thecoder 66, and thebuffer 68 in a manner similar to that discussed above, except that the quantized DC coefficients of the residual blocks are coupled directly to thecoder 66 via theline 60, and thus are not predictively encoded by the prediction encoder 44. - Additionally, it is possible to use a non-I frame as a reference frame. When a non-I frame will used as a reference frame, the quantized residuals from the
quantizer 58 are respectively dequantized and inverse transformed by the dequantizer 70 and theinverse DCT 72 so that this non-I reference frame will be the same as the one used by the decoder for the reasons discussed above. Themotion predictor 78 provides to thesummer 74 the decoded Y values of the I reference frame from which the residuals were generated. Thesummer 74 adds the respective residuals from thecircuit 72 to these decoded Y values of the I reference frame to generate the respective Y values of the non-I reference frame. Thereference frame buffer 76 then stores the non-I reference frame along with the I reference frame for use in encoding subsequent non-I frames. - Still referring to
FIG. 4 , theencoder 50 also includes arate controller 80 to insure that the transmitbuffer 68, which typically transmits the encoded frame data at a fixed rate, never overflows or empties, i.e., underflows, If either of these conditions occurs, errors may be introduced into the encoded data stream. For example, if thebuffer 68 overflows, data from thecoder 66 is lost. Thus, therate controller 80 uses feed back to adjust the quantization scaling factors used by thequantizer 58 based on the degree of fullness of the transmitbuffer 68. The fuller thebuffer 68, the larger thecontroller 80 makes the scale factors, and the fewer data bits thecoder 66 generates. Conversely, the more empty thebuffer 68, the smaller thecontroller 80 makes the scale factors, and the more data bits thecoder 66 generates. This continuous adjustment insures that thebuffer 68 neither overflows nor underflows. -
FIG. 6 is a block diagram of aconventional MPEG decompresser 82, which is commonly called a decoder and which can decode frames that are encoded by theencoder 60 ofFIG. 4 . - For I frames and macro blocks of non-I frames that are not motion predicted, a variable-
length decoder 84 decodes the variable-length codes received from theencoder 50. Aprediction decoder 86 decodes the predictively encoded DC coefficients, and adequantizer 87, which is similar or identical to thedequantizer 70 ofFIG. 4 , dequantizes the decoded AC and DC transform coefficients. Aninverse DOT 88, which is similar or identical to theinverse DCT 72 ofFIG. 4 , transforms the dequantized coefficients into pixel values. The decoded pixel values pass through asummer 90—which is used during the decoding of motion-predicted macro blocks of non-I frames as discussed below—into a frame-reorder buffer 92, which stores the decoded frames and arranges them in a proper order for display on avideo display unit 94. If a decoded I frame is used as a reference frame, it is also stored in the reference-frame buffer 96. - For motion-predicted macro blocks of non-I frames, the
decoder 84,dequantizer 87, andinverse DOT 88 process the residuals as discussed above in conjunction withFIG. 4 . Theprediction decoder 86 decodes the motion vectors, and amotion interpolator 98 provides to thesummer 90 the pixel values from the reference-frame macro blocks that the motion vectors point to. Thesummer 90 adds these reference pixel values to the residuals to generate the pixel values of the decoded macro blocks, and provides these decoded pixel values to the frame-reorder buffer 92. If a decoded non-I frame is used as a reference frame, it is stored in the reference-frame buffer 96. - Referring to
FIGS. 4 and 6 , although described as including multiple functional circuit blocks, theencoder 50 and thedecoder 82 may be implemented in hardware, software, or a combination of both. For example, theencoder 50 and thedecoder 82 are often implemented by a respective one or more processors that perform the respective functions of the circuit blocks. - More detailed discussions of the
MPEG encoder 50 anddecoder 82 ofFIGS. 4 and 6 , respectively, and of the MPEG standard in general are available in many publications including “Video Compression” by Peter D. Symes, McGraw-Hill, 1998, which is incorporated by reference. Furthermore, there are other well-lnown block-based compression techniques for encoding and decoding images. - In one aspect of the invention, a video processing circuit includes a processor that receives an encoded image having first and second regions, decodes the first region of the image, modifies the decoded first region, and re-encodes the modified first region.
- Such a circuit allows one to modify a region of an image by decoding and re-encoding only that region instead of the entire image. For example, if one wishes to overlay an EPG on a bottom portion of a video frame, then the circuit can decode only the EPG and the bottom portion of the frame, overlay the decoded EPG on the bottom frame portion, and re-encode the overlaid bottom frame portion. Therefore, this technique often reduces the processing time, and thus the cost and complexity of the processing circuit, as compared to a circuit that decodes and re-en codes the entire frame during an image overlay process.
-
FIG. 1 a diagram of a video frame that includes a conventional overlay region. -
FIG. 2 is a block diagram of a conventional television receiving-and-display system that includes a set-top box for blending an overlay image with overlay regions of respective video frames. -
FIG. 3A is a diagram of a conventional macro block of pixels in an image. -
FIG. 3B is a diagram of a conventional block of pre-compression luminance values that respectively correspond to the pixels in the macro block ofFIG. 3A . -
FIGS. 30 and 3 D are diagrams of conventional blocks of pre-compression chroma values that respectively correspond to the pixel groups in the macro block ofFIG. 3A . -
FIG. 4 is a block diagram of a conventional MPEG encoder. -
FIG. 5 is a diagram that illustrates the concept of conventional motion vectors. -
FIG. 6 is a block diagram of a conventional MPEG decoder. -
FIG. 7 is a block diagram of a video-frame formatter according to an embodiment of the invention. -
FIG. 8 is a diagram of a group of multi-region video frames generated by the formatter ofFIG. 7 . -
FIG. 9 is a block diagram of a set-top box according to an embodiment of the invention. -
FIG. 10 is a block diagram of a set-top box according to another embodiment of the invention. - Formatting Encoded Video Images Into Respective Multiple Independent Regions
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FIG. 7 is a block diagram of a video-frame formatter 100 according to an embodiment of the invention. Theformatter 100 includes video processing circuits 102 1-102 N for respectively formatting encoded video-channel signals 1-N received from respective broadcaster networks (e.g., FOX, CNN). Specifically, the processing circuits 102 format the frames of each respective channel signal such that the frames each have multiple independent regions. The processing circuits 102 then provide the respective processed channel signals 1-N to amultiplexer 104, which combines all of the processed channel signals into an encoded multiplexed broadcast video signal. Although described as formatting video frames, theformatter 100 can be designed to modify video fields or still images. - For example purposes, the structure and operation of the video processing circuit 102 1 is discussed in detail, it being understood that the processing circuits 102 2-102 N are similar. The processing circuit 102 1, includes a decoder 108 1 for decoding the
channel signal 1, which in one embodiment is encoded according to a compression standard that is the same as or is similar to the MPEG compression standard described in conjunction withFIGS. 3-6 . A motion-vector analyzer 110 1 receives the decoded channel signal and identifies macro blocks having motion vectors that point from one region of a frame to a different region of the same or another frame. That is, the analyzer 110 1 identifies a macro block in a region of a frame if the macro block has a motion vector that points to a reference macro block in a different region of the same or another frame. A multi-region frame re-encoder 112 1 re-encodes the identified macro blocks such that no identified macro block has a motion vector pointing to a region of a frame that is different from the region containing the identified macro block. In one embodiment, the re-encoder 112 1 re-encodes an identified macro block in a region of a frame such that the macro block has a motion vector that points to a reference macro block in the same region of another frame. For example, the encoder 112 1 may encode the macro block to have a motion vector with a location value =(0,0). In another embodiment, the re-encoder 112 re-encodes an identified macro block as an I block such that it has no motion vector. - Referring to
FIGS. 7-8 , the operation of theprocessing circuit 1021 is discussed.FIG. 8 shows a group of threevideo frames frame 114 is an I frame and theframes frames regions frames frames - The decoder 108 1 decodes the video frames, motion vectors, and other components of the channel I signal. In one embodiment, the decoder 108 1, conventionally decodes each macro block in every frame down to its respective pixel values, i.e., down to the pixel domain. But as discussed below, because the decoder 108 1 does not decode the channel I signal for display, it may conventionally decode the macro blocks down only to their DOT coefficients, i.e., down to the transform domain. Alternatively, as discussed below, the decoder 108 1 may decode only some of the macro blocks in a frame. Such partial decoding often reduces the cost, complexity, and decoding time of the decoder 108 1 as compared to known decoders.
- The motion-vector analyzer 110 1 then examines the decoded motion vectors from the decoder 108 1 and identifies the macro blocks having motion vectors that point from one of the first regions 120 a-120 c to one of the second regions 122 a-122 c and vice versa. For example, the analyzer 110 1 identifies the decoded
motion vectors 124 and 126 (shown in solid line) as being “boundary-crossing” vectors. Specifically, the analyzer 110 1 determines that the decodedmotion vector 124 points from a macro block 123 a in theregion 120 b of theframe 116 to a reference macro block 123 b in the region 122 a of theframe 114. Likewise, the analyzer 110 1 determines that the decodedmotion vector 126 points from amacro block 123 c in theregion 122 c of theframe 118 to a reference macro block l23 d in theregion 120 b of theframe 116. (The differences in the relative frame locations between themacro blocks 122 a and 122 c and the referencemacro blocks 122 b and 122 d, respectively, are exaggerated for purposes of illustration.) Thus, the analyzer 110 1 identifies themotion vectors macro blocks 123 a and 123 c have boundary-crossing motion vectors. - In one embodiment, the re-encoder 112 1 generates substitute motion vectors for the macro blocks having boundary-crossing motion vectors. For example, the re-encoder 112 1 generates
substitute motion vectors 128 and 130 (shown in dashed line) for themacro blocks 123 a and 123 d, respectively. Specifically, thesubstitute motion vector 128 points from its macro block 123 a in theregion 120 b to a referencemacro block 123 e in the region 120 a, and thesubstitute motion vector 130 points from itsmacro block 123 c in theregion 122 c to a referencemacro block 123 f in theregion 122 b. Thus, neither of thesubstitute motion vectors motion vectors FIG. 9 , such independently encoded first and second regions 120 a-120 c and 122 a-1 22 c allow one to modify one set of the regions, for example the regions 122 a-122 c, without having to decode the other set or sets of regions, for example the regions 120 a-120 c. Furthermore, as discussed below, although thesubstitute motion vectors substitute motion vectors same frames crossing motion vectors - In one embodiment, the re-encoder 112 1 generates the substitute motion vectors by conventionally scanning the appropriate frame regions for the best reference macro blocks and then motion encoding the original macro blocks using the new reference macro blocks. For example, to generate the
substitute motion vector 128, the re-encoder 112 1 scans the region 120 a and determines that themacro block 123 e is the best reference macro block in the region 120 a. In one embodiment, the re-encoder 1121 starts scanning at themacro block 123 g, which is the macro block in the region 120 a that is closest to the original reference block 123 b. Thus, thereference block 123 e replaces the original reference block 123 b. Then, as discussed above in conjunction withFIG. 4 , the re-encoder 112 1 motion encodes the block 123 a using thesubstitute reference block 123 e instead of the reference block 123 b. - To allow the re-encoder 112 1 to perform such scanning and motion encoding, the decoder 108 1 decodes at least the identified macro blocks, the reference macro blocks, and the macro blocks in the scan regions such as the region 120 a. For example, if the re-encoder 112 1 is constructed to generate substitute motion vectors that point to the same respective frames as the original motion vectors, then the decoder 108 1 can be constructed to decode only the identified macro blocks and the frames to which the identified motion vectors point. Alternatively, the decoder 108 1 can be constructed to decode only the identified macro blocks, the reference macro blocks, and the regions of the pointed-to frames that correspond to the regions containing the identified macro blocks. For example, the decoder 108 1 can be constructed to recognize that the
substitute motion vector 128 will point to the region 120 a, which corresponds to theregion 120 b of the identified macro block 123 a. Based upon this recognition, the decoder 108 1 decodes the macro blocks 123 a and 123 b and all the macro blocks in the region 120 a, which is the region to be scanned by the re-encoder 112 1. Of course the decoder 108 1 can be constructed to decode all of the frames in their entireties. - To allow the decoder 108 1 to determine which macro blocks to decode, in one embodiment the decoder 108 1 and analyzer 110 1 interact in the following manner. First, the decoder 108 1 decodes the motion vectors for a frame. Then, the analyzer 110 1 determines the cross-bolmdary motion vectors, the macro blocks to which they belong, the reference macro blocks to which they point, and the frame scan regions that contain the reference macro blocks.
- Next, the motion analyzer 110 1 provides the decoder 108 1 with the identifies of the macro blocks having cross-boundary motion vectors, the reference macro blocks, and the macro blocks in the scan regions that the re-encoder 112 1 will scan during motion re-encoding. The decoder 108 1 then decodes these identified macro blocks and provides the decoded macro blocks to the re-encoder 112 1 via the analyzer 110 1.
- Alternatively, to decrease its complexity and encoding time, the re-encoder 112 1 can be constructed to generate all the substitute motion vectors having location values of zero. This eliminates the need to scan a frame region because the substitute reference macro blocks are inherently known. For example, to generate the
substitute motion vector 130 having a location value of zero as shown inFIG. 8 , the re-encoder 112 1 need not scan theregion 122 b for the best substitute reference macro block because it is constrained to use themacro block 123 f as the substitute reference block. That is, theblock 123 f is the only reference block that will give the vector 130 a location value of zero. Therefore, without scanning theregion 122 b, the re-encoder 112 1 selects theblock 123 f as the substitute reference block and then motion encodes themacro block 123 c using thesubstitute reference block 123 f instead of theoriginal reference block 123 d. Although this scanless encoding is faster and less complex than the scan encoding described above, scanless encoding is often less efficient, i.e., generates more bits per pixel, than scan encoding. - In another embodiment, to further reduce its complexity and encoding time, the re-encoder 112 1 is constructed to encode the identified macro blocks as I-encoded blocks such that they have no motion vectors. Although encoding is faster and less complex than the motion (non-I) encoding techniques discussed above, it is often less efficient than motion encoding.
- In addition to re-encoding the identified macro blocks, the re-encoder 112 1 re-encodes the substitute motion vectors and any other decoded portions of the
channel 1 signal and provides there-encoded channel 1 signal, which includes the re-encoded video frames formatted into independent regions, to themultiplexer 104. - For any of the above described frame-formatting embodiments, the decoder 108 1 can be constructed to decode down to either the transform domain or to the pixel domain. Because the DOT is a linear transform, the re-encoder 112 1 can scan, motion encode, and I encode using DOT coefficients as well as using pixel valves.
- Furthermore, although shown as including separate circuit blocks 108 1, 110 1 and 112 1 the processing circuit 102 1, may include one or more respective processors that perform the functions of these circuit blocks in hardware, software or a combination of hardware and software. Additionally, the above-described functions may be performed in an order other than that described above.
- Additionally, although shown as rectangles at the tops and bottoms of the
frames FIG. 9 , a set-top box can use this dimension and location information when blending an overlay frame with a video-frame region. - As discussed below in conjunction with
FIG. 9 , the increased cost for the processing power that allows the video processing circuits 102 1-102 N to format video frames into multiple independent regions will be more than offset by the reduced complexity and cost of the set-top box (FIG. 9 ). The overall cost savings is actually very significant, because for everyframe formatter 100, which will typically be installed in a cable-company office, there will be hundreds, thousands, or even millions of set-top boxes. Thus, the combined cost savings for the set-top boxes will likely be much greater than the increased cost of installing oneformatter 100 in a cable-company office. - Modifying An Image Region
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FIG. 9 is a block diagram of atelevision receiver system 130 according to an embodiment of the invention. Thesystem 130 includes aremote control 132 and an HDTV receiver/display 134, which are respectively similar to theremote control 12 and the receiver/display 13 ofFIG. 2 . The system also includes a set-top box 136, which blends overlay frames with regions of respective video frames. For example purposes, the structure and operation of thebox 136 is discussed with reference to theframes FIG. 8 and to an EPG as the overlay frame. It is understood, however, that this discussion is applicable to video frames having a different formatting and to different overlay frames. Generally, in the discussed example, thebox 136 blends the EPG with the regions 122 a-122 c of theframes box 136 need only decode and re-encode the regions 122 a-122 c. Therefore, because thebox 136 need not decode or re-encode the regions 120 a-120 c, it can have significantly less processing power than, and thus can be significantly less expensive than, the conventional set-top box 11 (FIG. 2 ), which must decode and re-encode the frames in their entirety to perform this blending. - The set-
top box 136 of thesystem 130 includes aprocessing circuit 138, which receives an encoded, multiplexed broadcast video signal from a cable or satellite company. In response to a channel-select signal from acommand decoder 140, achannel selector 142 demultiplexes the broadcast signal and provides a selected video-channel signal to an overlay-region decoder 144. - When the viewer wants to view an overlay frame such as an EPG, he manipulates the
remote control 132 to generate an overlay command. Thecommand decoder 140 generates an overlay signal in response to the overlay command. In response to the overlay signal, thedecoder 144 decodes the overlay frames and the overlay frame regions with which the overlay frames will be blended. For example, referring toFIG. 8 , thedecoder 144 decodes the EPG and the overlay regions 122 a-122 c of theframes FIG. 7 , thedecoder 144 decodes the EPG and the overlay regions 122 a-122 c down to the transform domain or all the way down to the pixel domain. Thedecoder 144 does not, however, decode the non-overlay frame regions 120 a-120 c. Therefore, thedecoder 144 provides the decoded EPG and overlay frame regions 122 a-122 c on anoutput terminal 145, and passes through the undecoded non-overlay regions 120 a-120 c to anoutput terminal 146. - An overlay/
region combiner 147 is coupled to theoutput terminal 145 of thedecoder 144 and blends the decoded overlay frames with the decoded frame overlay regions in a manner described below. For example, thedecoder 144 blends the decoded EPG with the decoded overlay frame regions 122 a-122 c. Anoverlay region re-encoder 148 re-encodes the blended overlay frame regions using conventional encoding techniques similar to those discussed above in conjunction withFIG. 4 . For example, the re-encoder 148 re-encodes the blended overlay frame regions 122 a-122 c. - A
frame buffer 150 receives the re-encoded overlay frame regions from the re-encoder 148 and receives the undecoded non-overlay frame regions that theoverlay region decoder 144 passes through via theoutput terminal 146. Theframe buffer 150 stores the undecoded and re-encoded frame regions in respective buffer sections to “reassemble” the video frames. For example, thebuffer 150 stores the undecoded non-overlay regions 120 a-120 c in a first set of buffer sections, and stores the re-encoded overlay regions 122 a-122 c in a second set of buffer sections (buffer sections not shown inFIG. 9 ). Thebuffer 150 then provides the contents of the first and seconds sets of buffer sections to thedisplay 134 such that the regions 120 a and 122 a are provided as a single frame, and so on. - A
rate controller 154, which is similar to therate controller 80 ofFIG. 4 , monitors the fullness of thebuffer 150 and prevents thebuffer 150 from overflowing or under flowing by controlling the degree of quantization used by the re-encoder 148. - The
display 134 decodes and displays the reassembled video frames from the set-top box 136. In the given example, thedisplay 134 displays theframes regions - Therefore, by decoding and re-encoding only the overlay frame regions with which overlay frames will be blended, the
processing circuit 138 can be much less complex, and thus much less expensive, than processing circuits that decode and re-encode the frames in their entirety. - In one embodiment as discussed above in conjunction with
FIG. 7 , the channel signal includes the locations (within the respective frames) and the dimensions of the regions 122 a-122 c, and theprocessing circuit 138 uses this information to properly configure thedecoder 144,combiner 147, re-encoder 148, andframe buffer 150. This allows the cable or satellite operator to change the dimensions or location of the regions 122 a-122 c without requiring hardware modifications to the set-top box 136. When the viewer does not want to view an overlay frame such as the EPG, theprocessing circuit 138 does not process the encoded video frames from thechannel selector 142, but instead passes them through to theframe buffer 150. Specifically, theremote control 132 does not generate an overlay command, and thus thecommand decoder 140 does not generate the overlay signal. In the absence of the overlay signal, theoverlay region decoder 144 passes through the previously encoded video frames in there entirety from thechannel selector 142 to theoutput terminal 146. For example, referring toFIG. 8 , thedecoder 144 performs no decoding, and thus passes the undecoded frame regions 120 a-120 c and 122 a-122 c to theoutput terminal 146. In one embodiment, however, thedecoder 144 does not pass through the EPG to either of theoutput terminals decoder 144 prevents theprocessing circuit 138 from providing the EPG to thedisplay 134. Furthermore, because thedecoder 144 does not provide a decoded overlay frame and decoded frame regions on theoutput terminal 145, thecombiner 147 and re-encoder 148 are effectively inactive. Therefore, theframe buffer 150 receives theundecoded frames output terminal 146 of thedecoder 144, and provides these undecoded frames to thedisplay 134. - Still referring to
FIG. 9 , in one embodiment of the invention, the overlay/frame-region combiner 147 blends the overlay frame with the overlay frame regions on a pixel-by-pixel basis using conventional alpha blending. Alpha blending is defined by the following equation:
Blended pixel value=(α)×(frame pixel value)+(1−α)×(overlay pixel value) (1)
Assuming that the overlay image is an EPG, if a particular EPG pixel (overlay pixel) is opaque, then α=0 such that the EPG pixel completely blocks the coincident overlay-frame-region pixel (frame pixel) from view. That is, the coincident frame pixel is invisible, i.e., transparent, if α=0. Likewise, if the particular EPG pixel is transparent, then α=1 such that coincident frame pixel is opaque. For 0 <α<1, as a increases from 0, the EPG pixel gradually fades out and the coincident frame pixel gradually fades in. Furthermore, because alpha blending is a linear function, thecombiner 147 can perform it in the transform domain or in the pixel domain. Thus, as discussed above, theoverlay region decoder 144 can decode the macro blocks of the regions 122 a-122 c (FIG. 8 ) down to the transform domain or the pixel domain. - In an embodiment where the overlay frame is opaque, the re-encoder 148 provides a relatively high number of quantization levels to preserve the higher frequencies in the overlay frame regions. For example, referring to
FIG. 8 , this low degree of quantization sharpens the edges of the EPG characters (not shown inFIG. 8 ) in the overlay regions 122 a-122 c. In some cases, this may cause the regions 120 a-120 c to have different quantization levels than the regions 122 a-122 c. But because the regions 120 a-120 c and 122 a-122 c are MPEG slices as discussed above in conjunction withFIGS. 7-8 , thedisplay 134 can easily change its dequantization levels as it moves from one slice to the other—for example from the region 120 a to the region 122 a—during the decoding of theframe 114. That is, thedisplay 134 uses one set of dequantization levels to decode the regions 120 a-120 c and another set of dequantization levels to decode the regions 122 a-122 c. In one embodiment, the respective dequantization levels needed to decode the regions 120 a-120 c are conventionally generated and included in the channel signal by the re-encoder 112 1 ofFIG. 7 and are passed through to thedisplay 134 by theprocessing circuit 138. Likewise, the dequantization levels needed to decode the regions 122 a-122 c are generated and included with the re-encoded overlay frame regions by there-encoder 148 ofFIG. 9 and are passed to thedisplay 134 by thebuffer 150. - As discussed above for the re-encoder 112 1 of the video-
frame formatting system 100, the re-encoder 148 can re-encode the macro blocks of the blended overlay frame regions in a number of ways. For example, referring toFIG. 8 , according to a first technique, the re-encoder 148 re-encodes all the macro blocks of the regions 122 a-122 c as I blocks. Or, according to a second technique, the re-encoder 148 re-encodes at least some of these macro blocks so that they have motion vectors with location values of 0. This is particularly efficient if the EPG is stationary and opaque, since all of the macro blocks in the regions 122 a-122 c will be the same from frame to frame. - Alternatively, if the EPG scrolls at a known constant rate, then according to a third technique, the re-encoder 148 re-encodes at least some of the macro blocks to have motion vectors with constant, nonzero location values based on the scroll rate. That is, if the scroll rate is constant, then the re-encoder 148 can calculate from the scroll rate how far a macro block in the region 122 of one frame has moved in the region 122 of the next frame. If the EPG scrolls in a vertical direction, then the motion-vector location values will be (0, Y), where the value of Y is based on the vertical scroll rate. Conversely, if the EPG scrolls in a horizontal direction, then the motion-vector location values will be (X, 0), where the value of X is based on the horizontal scroll rate. This re-encoding technique is particularly efficient if the EPG is opaque. Sometimes, however, the EPG is not opaque. That is, the EPG characters are opaque but the spaces between the characters are transparent to show the original scene. In this situation, therefore, this re-encoding technique is often inefficient because of relatively large residuals.
- According to a fourth technique, the re-encoder 148 re-encodes the blended overlay frame regions using full-scale motion encoding as discussed above in conjunction with
FIG. 4 . - For example, the re-encoder 148 re-encodes the
region 122 b by scanning the region 122 a for reference macro blocks and then generating corresponding motion vectors and residuals. - Generally, the first technique requires the least processing power but is the least efficient, the fourth technique requires the most processing power but is the most efficient, and the second and third techniques fall in between. Because the
rate controller 152 ensures that there is no overflow of thebuffer 150, the lower the efficiency of a re-encoding technique, the greater the losses incurred by the re-encoding process. These losses may show up as undesirable visual artifacts in the overlay frame regions when an overlay frame like the EPG is viewed. - Still referring to
FIG. 9 , in embodiments where it is desired that the overlay frame have a relatively high visual quality or where the overlay frame region is relatively large, theprocessor 138 can implement an I-frame-only mode during which it “skips” the non-overlay frame information to reduce the number of bits in the re-encoded channel signal. This technique, which is often called “skipping the background”, virtually insures that theframe buffer 150 wilt not overflow when the re-encoder 148 uses a low degree of quantization to generate high-quality overlay frame regions. For example, referring toFIG. 8 , thecombiner 147 combines the EPG with the regions 122 of the I frames (like region 122 a of the frame 114) and stores only these modified I frames in theframe buffer 150. Thebuffer 150 provides these I frames to thedisplay 134 by repeatedly sending the most recent, i.e., current, I frame (or by instructing thedisplay 134 to repeatedly display the current I frame) until thecombiner 147 and re-encoder 148 process the next I frame. Because the ratio of I frames to non-I frames in a GOP is relatively low, for example 1/15, the I-frame-only mode may cause jerky motion in the non-EPG frame regions 120. If the EPG is not opaque, then such jerky motion might also be visible in the background of the EPG regions 122 as well. But because such jerky motion is visible only while thedisplay 134 displays the EPG, because a viewer typically displays the EPG for only a relatively small fraction of his/her total viewing time, and because the viewer is more likely to concentrate on the EPG than the background, the jerky motion during EPG viewing is often unnoticeable, or is at least acceptable, to the viewer. - Referring to
FIGS. 8 and 9 , where the overlay frame is not opaque, a modification of the I-frame-only technique is to skip the background only in the non-overlay regions. For example, theoverlay decoder 144 passes through only the encoded non-overlay regions 120 of the I frames to thebuffer 150, but continues to provide the EPG and the overlay regions 122 of all the frames to thecombiner 147. Thecombiner 147 and the re-encoder 148 operate as described above such that the blended regions 122 have normal motion. Thebuffer 150 then combines the region 120 of the current I frame with the regions 122 of the current I frame and the subsequent non-I frames, respectively. Therefore, thedisplay 134 displays the blended video frames having normal motion in the regions 122 and jerky motion only in the regions 120. Theprocessing circuit 138 can also be designed to blend the EPG with the overlay regions 122 such that the processed frames have normal motion in the regions 120 and jerky motion in the regions 122. - Referring to
FIG. 9 , although described as including separate circuit blocks 140, 142, 144, 147, 148, 150, and 152, theprocessor circuit 138 may include one or more processors that perform the functions of these circuit blocks in hardware or software. Additionally, the above-described functions may be performed in an order other than that described above, Furthermore, although shown as part of theprocessing circuit 138, thecommand decoder 140 and thechannel selector 142 may be separate from thecircuit 138.FIG. 10 is a block diagram of a set-top box 160 according to another embodiment of the invention. Thebox 160 is similar to thebox 136 ofFIG. 9 except that it also converts a HDTV video signal down to a standard television (STV), i.e., lower resolution, video signal for display on an STV receiver/display 162. Thebox 160 includes aprocessing circuit 164, which is similar to theprocessing circuit 138 ofFIG. 9 except that it includes aconventional decoder 166 and a conventional HDTV-to-STV converter 168. Theconverter 168 reduces the number of pixels, and thus the resolution, of each of the frames that thebox 160 provides to thedisplay 162. As is known, theconverter 168 can perform this reduction in either the transform or pixel domains. Accordingly, thedecoder 166 decodes the video frames of the selected channel signal to whichever domain theconverter 168 calls for. Furthermore, because thedecoder 166 decodes each frame in its entirety to allow theconverter 168 to down convert the frames, thecircuit 164 includes a re-encoder 170 that re-encodes both the overlay and non-overlay frame regions. - When a viewer does not want to view an overlay frame, the
command decoder 140 generates no overlay signal. In the absence of the overlay signal, theconverter 168 down converts the decoded video frames from thedecoder 166 and provides the down-converted frames in their respective entireties to the re-encoder 170 via aline 172. The re-encoder 170 re-encodes the down-converted frames and provides them to theframe buffer 150. Thedisplay 162 decodes and displays the re-encoded frames from thebuffer 150. - When the viewer wants to view an overlay frame, the
command decoder 140 generates the overlay signal in response to a command from theremote control 132. Referring toFIG. 8 for example purposes, in response to the overlay signal, theconverter 168 down converts the decoded frames and the decoded EPG from thedecoder 166, provides the down-converted frame regions 120 a-120 c to the re-encoder 170 via theline 172, and provides the down-converted frame regions 122 a-122 c and the down-converted EPG to the overlay/frame combiner 147 via aline 174. Thecombiner 147 blends the EPG and the regions 122 a-122 c as discussed above in conjunction withFIG. 9 . The re-encoder 170 re-encodes the decoded regions 120 a-120 c and the blended regions 122 a-122 c in a manner similar to that discussed above in conjunction withFIG. 9 . The re-encoder 170 provides these re-encoded regions to thebuffer 150, which stores these regions as respective re-encoded frames for display on thedisplay 162. Therate controller 154 prevents thebuffer 150 from overflowing or under flowing as discussed above in conjunction withFIG. 9 . - Referring to
FIG. 10 , although described as including separate circuit blocks 140, 142, 147, 150, 154, 166, 168, and 170, theprocessor circuit 164 may include one or more processors that perform the functions of these circuit blocks in hardware or in software. Additionally, the above-described functions may be performed in an order other than that described above. Furthermore, although shown as part of theprocessing circuit 164, thecommand decoder 140 and thechannel selector 142 may be separate from thecircuit 164. - From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.
Claims (37)
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JP2003519940A (en) | 2003-06-24 |
KR20020064771A (en) | 2002-08-09 |
US7194032B1 (en) | 2007-03-20 |
TW496090B (en) | 2002-07-21 |
WO2001019091A3 (en) | 2001-05-10 |
EP1212896A2 (en) | 2002-06-12 |
AU7114000A (en) | 2001-04-10 |
CN1372768A (en) | 2002-10-02 |
WO2001019091A2 (en) | 2001-03-15 |
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