WO1997034423A1 - Image sensor with multiple resolution and compression capability - Google Patents
Image sensor with multiple resolution and compression capability Download PDFInfo
- Publication number
- WO1997034423A1 WO1997034423A1 PCT/US1997/003998 US9703998W WO9734423A1 WO 1997034423 A1 WO1997034423 A1 WO 1997034423A1 US 9703998 W US9703998 W US 9703998W WO 9734423 A1 WO9734423 A1 WO 9734423A1
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- WIPO (PCT)
- Prior art keywords
- image
- pixels
- color
- signals
- still camera
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- 230000006835 compression Effects 0.000 title abstract description 11
- 238000007906 compression Methods 0.000 title abstract description 11
- 238000000034 method Methods 0.000 claims abstract description 32
- 238000012545 processing Methods 0.000 claims description 8
- 238000012935 Averaging Methods 0.000 claims description 5
- 238000001914 filtration Methods 0.000 claims description 4
- 238000003384 imaging method Methods 0.000 claims description 4
- 239000011800 void material Substances 0.000 claims description 4
- 238000000926 separation method Methods 0.000 claims description 3
- 238000004891 communication Methods 0.000 claims 1
- 230000009977 dual effect Effects 0.000 abstract description 3
- 239000003086 colorant Substances 0.000 description 20
- 230000007704 transition Effects 0.000 description 19
- 238000005286 illumination Methods 0.000 description 10
- 238000003491 array Methods 0.000 description 4
- 238000005070 sampling Methods 0.000 description 3
- 230000008859 change Effects 0.000 description 2
- 230000006837 decompression Effects 0.000 description 2
- 230000004075 alteration Effects 0.000 description 1
- 238000013459 approach Methods 0.000 description 1
- 238000009125 cardiac resynchronization therapy Methods 0.000 description 1
- 238000013500 data storage Methods 0.000 description 1
- 238000001454 recorded image Methods 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
Classifications
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
- G06T3/00—Geometric image transformation in the plane of the image
- G06T3/40—Scaling the whole image or part thereof
- G06T3/4015—Demosaicing, e.g. colour filter array [CFA], Bayer pattern
-
- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
- G06T3/00—Geometric image transformation in the plane of the image
- G06T3/40—Scaling the whole image or part thereof
- G06T3/4007—Interpolation-based scaling, e.g. bilinear interpolation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
- H04N23/10—Cameras or camera modules comprising electronic image sensors; Control thereof for generating image signals from different wavelengths
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N2209/00—Details of colour television systems
- H04N2209/04—Picture signal generators
- H04N2209/041—Picture signal generators using solid-state devices
- H04N2209/042—Picture signal generators using solid-state devices having a single pick-up sensor
- H04N2209/045—Picture signal generators using solid-state devices having a single pick-up sensor using mosaic colour filter
- H04N2209/046—Colour interpolation to calculate the missing colour values
Definitions
- the present invention relates generally to an apparatus and method for interpolating and compressing image data and, more particularly, to an apparatus and method for sensing color sub-sampled image data and thereafter interpolating for non-color sub-sampled image data that substantially reduces color fringing and preserves detail while allowing selection of the overall quantity of image data.
- Electronic imaging cameras for recording still images are well known in the art. Such cameras can record a plurality of still images on a single magnetic disk or tape in either analog or digital format for subsequent playback on an output device, such as any well-known cathode ray tube (CRT) viewing device for example. Printers may also be utilized with such cameras in a well-known manner to provide hard copies of the recorded images.
- CTR cathode ray tube
- Such electronic imaging still cameras often utilize two-dimensional image sensing arrays such as charge coupled devices (CCDs) which integrate incident scene light over a predetermined time to provide an electronic information signal corresponding to the scene light intensity incident on the array.
- CCDs charge coupled devices
- Such two- dimensional image sensing arrays comprise a predetermined number of discrete image sensing elements or pixels arranged in a two-dimensional array, in which each image sensing element responds to incident illumination to provide an electronic information signal corresponding to the intensity of the incident of illumination.
- the incident illumination to the two dimensional image sensing array is filtered so that different image sensing elements receive different colored illumination.
- the filters are arranged in well known patterns across the face of an image sensing array, such as a repeating pattern of red, green, and blue stripes.
- individual image sensing elements or pixels across each line may be filtered in a repeating pattern of red, green, blue, and green filters in a two- by-two checkerboard, as is well known in the art. Since each image sensing element can only detect one color of illumination, the color information for the other colors not detected by that image sensing element must be filled in. Filling in the missing color information is generally accomplished by inte ⁇ olating the detected image data for each color to determine color values for all the colors for each image sensing element.
- the missing color inte ⁇ olation also increases the number of pixels by a factor of three from the raw image data sensed by the CCD. Storing the image data locally
- the increased number of pixels means that a file size for storing the image, on local magnetic media for example, leads to a requirement for increased storage capacity.
- the pixels on a CCD can be rectangular in shape particularly when striped color filter arrays are employed.
- outputting the image on an output device that has substantially square pixels distorts the image. Since most conventional output devices such as printers and CRT's require square pixels, it is necessary to form substantially square pixels from the rectangular pixels prior to outputting the image. A result is to get the desired pixels geometry.
- the invention provides, in one aspect, an electronic still camera and a method associated therewith for image processing to form a processed image with reduced color artifacts, where an image is represented by a plurality of image signals in three colors.
- the method is useful with an electronic still camera, and comprises the steps of separating the plurality of image signals into a first color plane, a second color plane, and a third color plane.
- many conventional CCDs use colored stripes formed onto the CCD to filter colors, thereby making individual pixels associated with only one color. These individual pixels are broken into color planes. Therefore, if colored stripes of red, green, and blue (RGB) are attached to the CCD, then the pixels associated with each individual color are broken into a red color plane, a green color plane, and a blue color plane.
- RGB red, green, and blue
- difference signals are created at each pixel location that represent the differences between one channel and a second, and the first channel and the third within the RGB triplet.
- those difference signals can be any combination of the red, the green, or the blue, as long as from those three colors, two different difference signals are created.
- R- G and B-G are used.
- the difference signals are then filtered using a median filter which substantially reduces or removes color artifacts.
- the image is subsequently reconstructed from the original image signals and the filtered difference signals to form a processed image which has substantially preserved detail and reduced color artifacts.
- the improved RGB triplets thus formed from linearly inte ⁇ olated RGB triplets have the same pixel geometry as the original single color pixels. In the case of rectangular pixel geometry, some method of producing pixels of square geometry must be employed for use with any display device that renders images on a raster of square geometry.
- the invention provides dual resolution modes for easily producing square pixels for display.
- one resolution mode neighboring pairs of pixels are averaged in the direction normal to the major axis of the rectangular pixels within each color plane such that one half as many square pixel RGB triplets are formed as there were original single color rectangular pixels, while in the second resolution mode new RGB triplets are interjected by inte ⁇ olation in the direction parallel to the major axis of the rectangular pixels such that twice as many square pixel RGB triplets are formed as there were original single color rectangular pixels.
- multi resolution mode can be supported with more complicated down sampling and up sampling schemes.
- the stored image signals are compressed versions of the two filtered difference signals and a compressed version of the single color signal common to both difference signals reconstructed from the filtered difference signals and the original sampled pixel values.
- compressed, filtered R-G and B-G are used as well as a compressed version of the G signal formed from the filtered R-G and B-G and the original RGB samples.
- the difference signals are highly compressible while the G signal formed from the filtered R-G and B-G and the original RGB samples provides the complete detail signal originally sensed, and the overall image size can be drastically compressed.
- pixels of square geometry can be reconstructed in either of the dual resolution modes.
- Figs. IA and IB show, respectively, pixel locations as found on a CCD, and a representative example;
- Figs. 2A and 2B show the pixel locations of the example of Figs. IA and IB, respectively, after linear inte ⁇ olation is performed;
- Fig. 3 shows the representative example of Figs. IB and 2B during performance of median filtering
- Fig. 4 shows the result of the median filter on the example of Fig. 3;
- Fig. 5 shows reconstruction of pixels of Fig. 3.
- Figs. 6 A, 6B, and 6C show, respectively, the pixel values reconstructed after median filtration of the color difference signals, the pixel values after averaging in the direction normal to the major axis of the rectangular pixels, and the pixel values after inte ⁇ olation along the major axis of the rectangular pixels.
- an image sensing array comprising a predetermined number of discrete image sensing elements or pixels arranged in a two-dimensional array in which the image sensing elements respond to incident illumination to provide an electronic information signal corresponding to the intensity of the incident illumination.
- Such an image sensing arrays may be a CCD of the frame transfer type. It is well known to sense color images using a single two-dimensional CCD array by filtering the illumination incident to the image sensing array so that different groups of the image sensing elements arranged in well-known patterns across the image sensing array receive different wavelengths for colored illumination.
- each color of illumination is sampled by each group of image sensing elements, and thereafter inte ⁇ olated to provide color values corresponding to the other groups of image sensing elements.
- the full color image is therefore estimated or inte ⁇ olated between the different groups of image sensing elements or pixels to fill in all colors for each image sensing element or pixel.
- a typical CCD arrangement includes color filter stripes thereon such that individual stripes of pixels measure an intensity of light for only a single color.
- Figure 1 A shows a typical arrangement of a CCD using color stripes, where individual pixels are marked by R, G, or B representing red, green, and blue colors. Numerical designations associated with the several colors, i.e., Rj Gi and B ⁇ , together are referred to a triplet.
- Fig. IB a light intensity level is shown graphically for a white-to-black transition in an image.
- the abscissa is c _
- pixel number across the CCD is by signal level, often measured in volts.
- Keys to Fig. IB is as follows: the solid line represents the edge transition in the original continuous image of the scene before sampling, the circle represents an original red sample, the triangle represents an original green sample, and the square represents an original blue sample. It can be seen from the illustration that for the original continuous image the falloff during the transition is substantially vertical. This indicates a sha ⁇ transition. A slope in the falloff would be indicative of a more gradual, or less sha ⁇ , transition.
- Fig. 2A again shows a pixel layout of the pixels of Fig. IA, but now the pixels have gone through the step of inte ⁇ olation.
- this is a linear inte ⁇ olation, as will be later hereinafter described.
- the convention used is that a capital letter represents original raw data and a lower case letter represents inte ⁇ olated data. Therefore, the colors inte ⁇ olated between R] and R are shown as r, 1 and ri".
- ' contains 2/3 of the intensity of R and 1/3 of the intensity of R 2
- H" contains 1/3 the intensity of R ( and 2/3 the intensity of R .
- Fig. 2B shows the step of linear inte ⁇ olation on the white-to-black transition shown in Fig. IB.
- the lines indicate paths of the linear inte ⁇ olation where the circles, triangles, and squares represent original red, green, and blue samples and the x's represent inte ⁇ olated values.
- Figure 2B illustrates how the inte ⁇ olation tends to blur the sha ⁇ transition previously seen in the image by inte ⁇ olating where the dashed line represents red, the dotted line represents green, and the solid line represents blue. The slope of the line is indicative of a less sha ⁇ transition, thus blurring the image.
- Fig. IB the lines indicate paths of the linear inte ⁇ olation where the circles, triangles, and squares represent original red, green, and blue samples and the x's represent inte ⁇ olated values.
- Figure 2B illustrates how the inte ⁇ olation tends to blur the sha ⁇ transition previously seen in the image by inte ⁇ olating where the dashed line represents
- FIG. 2B also illustrates how the inte ⁇ olation tends to create color fringe artifacts.
- the red, green, and blue intensity transitioned at the same spatial location while it can be seen in Fig. 2B that the red signal after inte ⁇ olation initiates transition before the green which initiates transition before the blue.
- An image reconstructed from the data represented in Fig. 2B would show a pronounced white-to-cyan-to-pu ⁇ le-to-blue-to-black color fringe.
- a next step is to create two difference signals at each location.
- the color fringe artifact at a white-to-black transition in the scene followed by inte ⁇ olation within color planes would appear as a sudden rise in a color difference signal followed by a corresponding sudden fall or as a sudden fall followed by a sudden rise. It is this rapid increase and decrease in the difference between the colors which is a characteristic of objectionable color fringing, and is not simply a sudden rise in the difference between colors, which is indicative of a change from one color to a different color. Thus, it is unlikely that a real scene would result in the creation of such a color spike, and it is not desirable to create such a color spike as a result of a method of inte ⁇ olation chosen.
- Fig. 3 illustrates the aforementioned color spikes, where in the preferred embodiment, the difference signals are performed by subtracting the intensity of green from the intensity of red and subtracting the intensity of green from the intensity of blue, that is R-G and B-G.
- the difference signals are performed by subtracting the intensity of green from the intensity of red and subtracting the intensity of green from the intensity of blue, that is R-G and B-G.
- the difference signals are performed by subtracting the intensity of green from the intensity of red and subtracting the intensity of green from the intensity of blue, that is R-G and B-G.
- Fig. 3(i) graphically illustrates the results of the difference signal for R-G, for the signal shown in Figure 2B.
- Fig. 3(iii) graphically illustrates B-G.
- a median filter is employed, such as that shown in commonly assigned patents 4,663,655, issued May 5, 1987, 4,774,565, issued September 27, 1988, and 4,724,395, issued February 8, 1988, all to William T. Freeman, which are each inco ⁇ orated herein by reference.
- the median filter takes a series of pixels, such as those shown in Fig. 3(ii), and replaces the pixel value at the center of the filter region with the median value of all the pixels within the region.
- the median filter uses nine pixels and returns the median value of the nine pixels. Therefore, as the filter is worked horizontally across the signal, it can be seen that unless the spike is longer than one half the pixel length, the spike in the signal will be essentially eliminated.
- Figure 4 illustrates the results of passing the median filter across the difference signals, where 4(i) corresponds to the R-G difference signal shown in 3(i), and 4(ii) corresponds to the B-G difference signal shown in 3(iii), where these signals can now be referred to as (R-G) 1 and (B-G)', respectively.
- the differences are now flat and the reconstructed colors will now remain substantially constant with respect to each other before, during and after the color transition. Reference is again made to Figure IB as an example of this.
- G@G G
- B@B B - (B-G)'
- a square pixel In order to display the resulting image on most commercially available CRTs and most commercially available printers, a square pixel must now be attained. Since many CCDs, and specifically that of the preferred embodiment, have rectangular pixels, generally having a 2 to 1 aspect ratio, a way of solving this problem is to average two neighboring pixels, R] and ⁇ , for example, to form a single square pixel at that location.
- Fig. 6A shows pixels values from an RGB striped sensor with 2:1 aspect ratio rectangular pixels after reconstruction from original sampled color values and median filtered color difference signals.
- Figure 6B shows pixel values on a square grid after averaging neighboring values. This would take an image resolution that has one million pixels, for example, and create an image that is displayable to a user having 500,000 three color pixels.
- An alternative method of creating square pixels is optionally used in the preferred embodiment. Rather than averaging horizontally, the pixels are inte ⁇ olated vertically, as is shown in Fig. 6C.
- Fig. 6C(i) an original line of RGB data is shown, and in Fig. 6C(iii) a second line of RGB data is shown, separated by Fig. 6C(ii) which is an inte ⁇ olated line of RGB data.
- the inte ⁇ olation between these lines can be as simple as nearest neighbor inte ⁇ olation, which, in essence, duplicates a previous line to create a new line.
- the preferred methods include linear inte ⁇ olation, bi-cubic convolution inte ⁇ olation, or frequency domain inte ⁇ olation, such as Fourier, DCT, wavelet, et cetera.
- inte ⁇ olated line of Fig. 6C(ii) creates additional pixels available for display.
- two million three color pixels would now be available. That is, if the image was 1600 * 600 * 3 where the three represents the three color planes, inte ⁇ olation would result in 1600 x 1200 x 3 available pixels.
- the method of the invention does not, per se, result in square pixels.
- the pixel geometry has actually not changed.
- the method works since output devices do not actually know the geometry of the pixels coming into the device, but instead simply displays according to an electronic signal. Therefore, rectangular pixels are displayed either separated by an inte ⁇ olated line which carries color information that makes the transition between the rectangular pixels or effectively formed into squares by averaging. This serves to eliminate the distortion that would otherwise result and simulates having a square pixels geometry.
- the resulting increase in pixels bodes well for output image quality but presents a difficulty for data storage. Increasing the image pixel count by two requires an increase in storage capacity for an image by two. Therefore, compression is used to decrease image storage requirements.
- a characteristic that makes data highly compressible is redundancy.
- the actual amount of compression is determined by the compression method chosen.
- DCT compression is used which is described in commonly assigned patent application number 08/441,000 filed May 15, 1995 which is inco ⁇ orated herein by reference.
- Signals that have a high degree of redundancy are the filtered difference signals as can be seen in Figure 4.
- the signals are compressed along with the reconstructed data of one color.
- the one color must be chosen such that the one color can be used to derive the remaining colors with the difference signals. In this case, green is used since both difference signals inco ⁇ orate green. Therefore, the other colors can be derived by the following equations where the superscripted plus indicates that reconstructed data is incorporated therein:
- visually lossless compression is used though lossless compression can be used without detriment to the invention.
- Visually lossless is preferred since higher compression ratios can be obtained, on the order of 10:1, while by definition changes to the reconstructed image are not detected to the human eye.
- generation of RGB pixel values with square geometry in either of two resolutions is accomplished during decompression as described in commonly assigned patent application number 08/441,000.
Abstract
Description
Claims
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
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EP97915944A EP0886974A1 (en) | 1996-03-14 | 1997-03-14 | Image sensor with multiple resolution and compression capability |
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US61538396A | 1996-03-14 | 1996-03-14 | |
US08/615,383 | 1996-03-14 |
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WO1997034423A1 true WO1997034423A1 (en) | 1997-09-18 |
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PCT/US1997/003998 WO1997034423A1 (en) | 1996-03-14 | 1997-03-14 | Image sensor with multiple resolution and compression capability |
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WO (1) | WO1997034423A1 (en) |
Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP1022912A2 (en) | 1999-01-20 | 2000-07-26 | Canon Kabushiki Kaisha | Image sensing apparatus and image processing method therefore |
EP0920221A3 (en) * | 1997-11-25 | 2001-01-17 | Seiko Epson Corporation | Image processing apparatus and method |
DE10313250A1 (en) * | 2003-03-25 | 2004-10-07 | Bts Media Solutions Gmbh | Sensor arrangement for film scanner to provide electrical image signals with sensor made up of parallel lines of sensor pixels |
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1997
- 1997-03-14 EP EP97915944A patent/EP0886974A1/en not_active Withdrawn
- 1997-03-14 WO PCT/US1997/003998 patent/WO1997034423A1/en not_active Application Discontinuation
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Publication number | Priority date | Publication date | Assignee | Title |
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EP0920221A3 (en) * | 1997-11-25 | 2001-01-17 | Seiko Epson Corporation | Image processing apparatus and method |
US6914628B1 (en) | 1997-11-25 | 2005-07-05 | Seiko Epson Corporation | Image processing apparatus and method, and medium containing image processing control program |
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US7929026B2 (en) | 1999-01-20 | 2011-04-19 | Canon Kabushiki Kaisha | Image sensing apparatus and image processing method thereof using color conversion and pseudo color removing |
DE10313250A1 (en) * | 2003-03-25 | 2004-10-07 | Bts Media Solutions Gmbh | Sensor arrangement for film scanner to provide electrical image signals with sensor made up of parallel lines of sensor pixels |
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