CA1130447A

CA1130447A - Digital video effects system employing a chroma-key tracking technique

Info

Publication number: CA1130447A
Application number: CA309,421A
Authority: CA
Inventors: Masao Inaba; Kazuo Kashigi
Original assignee: Nippon Electric Co Ltd
Current assignee: NEC Corp
Priority date: 1977-08-17
Filing date: 1978-08-15
Publication date: 1982-08-24
Also published as: FR2400809B1; US4163992A; GB2002989B; JPS6011875B2; DE2835761B2; AU518425B2; DE2835761C3; JPS5432222A; AU3901578A; FR2400809A1; DE2835761A1; GB2002989A

Abstract

ABSTRACT OF THE DISCLOSURE
In a chroma-kev tracking system for compressing and inserting a second television picture into an area corresponding to a chroma-key frame located at a specified position in a first television picture, there is described a tracking system which compresses the second picture to have an aspect ratio correspond-ing to that of the chroma-key frame into which it is to be in-serted. In known systems, the aspect ratio of the inserted picture is not varied to that of the chroma-key frame with the result of awkward-looking pictures from some camera angles. In this invention, a key signal is produced representing the key frame and which is used for producing a position signal represent-ing the position of a circumscribed key frame. From the position signal there is produced an imaginary frame position signal representing an imaginary-frame with the same aspect ratio as the first and second pictures. A compressed second video signal representing the compressed second picture is produced from the second video signal and imaginary-frame position signal to be identical in size with the imaginary frame. The first video signal and compressed second video signal are selectively gated in response to the key signal such that the compressed second picture is inserted into the chroma-key frame of the first pic-ture.

Description

113~)4~7 This invention relates to digital video effects systems, and more particularly to chroma-key tracking systems for electronically compressing and moving a television picture into a position specified by a chroma-key frame in another picture and for inserting the compressed picture into an area corresponding to the chroma-key frame.
This so-called keyed insertion technique by which a part of one television picture is inserted into another picture to produce a special ef-fect is frequently used in television broadcasting. One example of this technique is the chroma-key insertion by which part of a first picture is designated by the chroma-key signal produced from a second picture and the designated part is inserted into the second picture. However, since a chroma-key signal undergoes a change in position and dimension when the tel-evision camera employed for the pickup of the second picture is moved, the shooting angle of the camera must be changed accordingly. This involves ser-ious difficulty. To eliminate it, a method has been proposed in which the picture to be inserted is compressed in accordance with the chroma-key signal amplify (Japanese Patent Publication No. 53-9896 published on April 10, 1978 to Chubu Nippon Broadcasting Corporation). By this technique, the position and dimensions are determined by comparing the chroma-key frame with a stand-ard television frame.
The following description of the keyed insertion technique will be better understood when read in conjunction with the following drawings, in which;
Figures lA to lE show television pictures relating to video signals produced by a conventional system.
Figures 2A to 2D show television pictures for illustrating the principle of the present invention.

1~3~4~'7 Figure 3 is a block diagram of an embodiment of this invention;
Figure 4 is a block diagram of the frame-position signal detecting circuit used in the embodiment shown in Figure 3;
Figures 5A to 5C illustrate the relationships on the full picture among various frames obtained by one embodiment of this invention;
Figure 6 is a flow chart for the arithmetic circuit employed in the embodiment shown in Figure 3; and Figure 7 is a block diagram of the picture compressing circuit employed in the embodiment shown in Figure 3.

_ la -~1304~7 Referring to Figures lA to lE, it is assumed that picture B is compressed into size of a chroma-key frame C to produce a picture D and that the picture D is inserted into a picture A to produce a picture E. In this instance, the chroma-key signal representing the chroma-key frame C serves as the standard signal to designate into what position and what ~ men-sions picture B should be compressed. Generally, the chroma-key signal C is produced by mixing, in appropriate proportions, the blue component, which is the main constituent of the various chromatic components (the red, green and blue components) con-stituting the picture A, with the two other components. It is usual therefore, when producing a chroma-key signal, to compose picture A with the hatched portion left in blue.
Since the aspect ratio (ratio of the width to height of a frame) of a television picture is generally prescribed to be 4:3, that of the chroma-key frame C is naturally set at 4:3.
However, when the television camera for the pickup of the picture A shoots in an oblique direction to the chroma-key frame C, the aspect ratio of the picked up chroma-key frame ceases to be 4:3.
This is especially so when the camera is moved laterally in which case the chroma-key frame C in the picture may deviate substant-ially from the standard aspect ratio as illustrated in Figures 2A and 2B. The result is an awkward picture in which the com-pressed picture inserted into the chroma-key frame does not have an aspect ratio of 4:3. The positional relationship between the chroma-key frame and the picture to be compressed and inserted, in this case, should prefereably be such that the height and width of the picture to be inserted are compressed equally so that no distortion of the aspect ratio occurs~ As illustrated in ~1304~7 Figures 2C and 2D, the picture should look as if it were pasted to the chroma-key frame.
An object of the present invention is therefore to pro-vide a chroma-key tracking system which, even when the chroma-key frame deviates from ihe standard aspect ratio, compresses a picture to be inserted into the chroma-key frame with the pre-scribed aspect ratio.
According to this invention, there is provided a track-ing system for producing special effects on a television picture said television picture comprised of a first picture and a second picture inserted into said first picture, said first and second pictures being represented by first and second video signals, respectively, wherein said first and second video signals have a predetermined aspeet ratio, said system comprising: means for producing a key signal representing a key frame to be positioned on said first picture; means responsive to said key signal for producing a position signal representing a position of a circum-scribed frame of said key frame; means responsive to said position signal for produeing an imaginary-frame position signal, said imaginary-frame position signal representing an imaginary frame and said imaginary frame having said predetermined aspeet ratio;
means responsive to said seeond video signal and said imaginary-frame position signal for produeing a compressed seeond video signal representing a eompressed seeond picture, said compressed seeond pieture being identieal in size to said imaginary frame;
and means responsive to said key signal for seleetively combining said first video signal and said compressed second video signal, whereby said special effects are produced.

~30447 first pioturo.
The invention will now be described in detail with reference to Figures 3 to 7.
With reference to Figure 3, a first video signal A is supplied to first input terminal 1, and a second video signal B is supplied to a second input termanal 2. A chroma-key signal generator 11 generates, in response to the first video signal from the first ir~ut terminal 1, a chroma-key signal. The chroma-key signal is supplied to a gate circuit 12 via a switch 19, and gated by the output of a circumscribed frame signal generator 15 to be deprived of its noise component. The output of the gate ~ 30447 circuit 12 is supplied to a frame-position signal detecting cir-cuit 13 for cletecting four values, i.e., those of the leftmost and rightmost points in the horizontal direction and the upper-most and ]owermost points in the vertical direction of the chro-ma-key frame on the telev;sion picture.
Gn example of the frame position signal detecting cir-cuit 13 is illustrated in Figure L~. A chroma-key signal 31, whose noise component has been removed by the gate circuit 12, is shaped by a wave shaper 33, and fed to horizontal and vertical position detectors 30H and 30~.
In the horizontal position detector 30H, a counter 35, so composed as to be driven by clock pulses 3~ to advance by one per clock pulse, is cleared to zero by a horizontal synchroniz-ing pulse 32H. The counter 35 feeds the counted values to a minimum-value detector 36 and a maximum-value detector 37. The minimum-value detector 36 detects the counted value of the count-er 35 at the leading edge of the chroma-key signal once every horizontal scanning period. The detector 36 holds the detected value~ which is then compared with that for the immediately following horizontal scanning period to hold the smaller one as a new value~ This procedure is repeated once every horizontal scanning period for one vertical scanning period. The last-held value in each vertical scanning period is detected as the mini-mum horizontal value ASH representing the leftmost position of the chroma-key signal in the horizontal direction.
The maximum-value detector 37 holds the counted value of the counter 35 at the trailing edge of the chroma-key signal to detect the last-held value in each horizontal scanning period.
The detector 37 also holds the detected value, which is then ~304~

compared with that for the immediately following horizontal scanning period to hold the greater one as a new value. This procedure is repeated once every horizontal scanning period for one vertical scanning period. The last-held value in each vertical scanning period is detected as the maximum horizontal value i~E~ representing the rightmost position of the chroma-key signal in the horizonta] direction. The detectors 36 and 37 are reset by the vertical synchronizing pulse 3 2V .
The detected minimum and maximum horizontal values are supplied to hysteresis circuits 3~ and 39, respectively, which are for removing the jitters present at the leading and trailing edges of the chroma-key signal. A chroma-key signal, even if the original picture from which it is made is stable, is sus-ceptible to some jitters at its leading and trailing edges.
Accordingly, the outputs of the minimum-value detector 36 and the maximum-value detector 37 are constantly fluctuating in minute degree. The hysteresis circuits 3~ and 39, so composed that their outputs may not vary even if their inputs minutely fluctuate, greatly contribute to stabilization of the function-ing of the system. The stabilized outputs ASH and AEH are fed to an arithmetic circuit 14 and the frame signal generator 150 In the vertical position detector 30V, a counter L~2, so composed as to be driven by the horizontal synchronizing pulse 32H to advance by one per horizontal period, is reset to zero by the vertical synchronizing pulse 32V. The counter 42 feeds the counted values to a minimum-value detector 43 and a maximum-value detector 44. The minimum-value detector 43 detects the counted value of the counter 42 as the minimum vertical value ASv at the leading edge of the vertical component of the chroma-~130~47 key signa] once every vertical scanning period. The minimum vertical value represents the uppermost position of the chroma-key signal in the vertical direction. The maximum-value detect-or 44 holds the counted value of the counter 42 at the trailing edge of the vertical component of the chroma-key signal to de-tect the last-held value in each vertical scanning period. The detected value is the maximum vertical value AEV representing the lowermost position of the chroma-key signal. The detected minimum and maximum vertical values are supplied to hysteresis circuits 45 and 46, respectively to remove jitters. The stabiliz-ed outputs A~V and AEV are supplied to the arithmetic circuit 14 and the frame signal generator 15.
The four values AsH~ AEH' dSV' and AEV ob the frame-position signal detector 13 represent the dimensions and position of the quadrilateral circumscribing around the chroma-key frame. Thus in Figures 5A to 5C, a reference numeral 20 indicates the dimensions of the standard picture, 21 (21A, 21B, 21C) shows the chroma-key frame and the four detected values ASH, AEH, ASv, and AEV correspond to the points of the four corners of the circumscribed frame 22 (22A, 22B, 22C). The four values are fed to the frame signal generator 15 (Figure 3) to generate a frame signal corresponding to the frame 22. This frame signal is supplied to the gate circuit 12 to gate the chroma-key signal to remove the noise componentO The noise component outside the frame 22 is thus eliminated.
The four values are also fed to the arithmetic circuit 14 and so corrected as to make a corrected frame have the pres-cribed aspect ratio of L,:30 In Figures 5A to 5C, reference numerals 23A, 24B, and 24C show the corrected frame, i.e., an 1130~47 imaginary frame corresponding to the corrected four values.
The position signals of the imaginary f~ames 23A, 24~ and

2~C are determined in the following manner: The vertical and horizontal dimensions of the circumscribed frame 22 are first determined, and the ratio~ (l/aspect ratio) between them is calculated. Thus AEV ASV
AEH- ASH
~hen, the aspect ratio of the chroma-key frame on the television picture is 4:3, i.e., the ratio~ is 0.75, the four values ASH, AEH, ASv, and ~EV are fed as it is, to picture compressing cir-cuit 16.
Where ~0.75, a primary correction is applied to the values Asv and AEv, or AsH and i~EH~ IJhen the ratio~ is smaller than 0.75, the value ASv is decreased and AEV is increased by such an equal quantity as wi]l make the ratio ~ 0.750 When the ratio ~ is greater than 0.75, ASH is decreased and llEH is in-creased by an equal quantity to make the ratio ~ 0.75. Figures 5A to 5C show examples of correction in which the ratio ~ is greater than 0.75. In Figures 5~l to 5C, symbols l;'sH and l~'EH
indicate the primarily-corrected values corresponding to the primarily-corrected ~rames 23l~, 23B, and 23C. To enlarge the vertical dimension (when ~ is smaller than 0.75) or the horizon-tal dimension (when ~ is greater than 0.75) by the primary correction means, the correction ls so accomplished that the corrected frame 23 is always greater than the circumscribed frame 22.
ljrhen the primarily-corrected frame 23 is within the _ g _ ~130447 standard frame 20, i~e., when primarily-corrected minimum values ~ ' SH and A'Sv are equal to or greater than hori~ontal and vert-ical minimum values A~Hmin and Avmin of the standclrd frame 20, respectively, and when the primarily-corrected maximum values A'EH and A'EV are equa] to or smaller than horizontal and vert-ical maximum values A~lmax and Avmax of the standar~l frame 20, respectively, as sho~n in Figure 5A, the ~rimarily-corrected es i~ SH ~ A ~ Asv and ~V (for~0. 75 ) or r~S ~ ~
and A'EV (for ~ C0.75) are fed to the picture compressing circuit 16 as the corrected values.
When the primarily-corrected frame 23 extends outside the standard frame 20 as shown in Figures 5B and 5C, a secondary correction is applied to the primarily-corrected values so that the primarily-corrected frames 23B and 23C are laterally trans-ferred off the standard frame 20 until those parts of the prim-arily-corrected frames which are outside the standard frame are doubled in length. Symbol 2~B in Figure 5B indicates the frame after the secondary correction, in which A'SH was modified to A''SH and A'EH to A"EH. If the circumscribed frame 22C is in the peripheral area and in contact ~ith the standard frame, for in-stance if the value of ASH is AHmin the secondary correction is so accomplised that the whole primarily-corrected frame is later-ally transferred until A"E~I e4uals AEH.
In other words, in case where AIsH~AHmin, as shown in Figures 5B and 5C, the primarily-corrected frame is transferred to the left by AHmin-A'sH to provide secondary corrected values ~"SH and A"EH as follows A SH 2A SH Hmi n EH = A EH + A'sH ~ AHmin _ 9 _ In case where A'~"~ max, the primarily-corrected frame is transferred to the right by ~'EH ~ ~ max, whereby the secondary-corrected values are provided as follows:

SH = A ~H + A'~,H ~ AHmaX
li EH = 2A EH i~lmax In case where h'SV< Avmin (~0.75), the primarily-corrected frame is transferred upward by ~vmin-.~'sv, whereby the secondary-corrected values A"Sv and A"EV are provided as follows:

SV 2 SV vmin EV = A EV + A'~V ~ Avmin In case where, A'EV ~ Avmax (~ 0.75), the primarily-corrected frame is transferred downward by A'EV - Avmax, whereby the secondary-corrected values are provided as follows:

SV = A SV ~ A'Ev ~ Avmax A''EV = 2A'RV - Avmax The secondary-corrected values thus obtained are fed to the picture compressing circuit 16.
The arithemetic circuit 14 for achieving the signal processing outlined above may consist of a micro-computer whose operation can be summarized in the form of a flow chart shown in Figure 6.
The arithmetic circuit 14 supplies a picture compress-ing circuit 16 (Figure 3) with various values needed for compres-sing a second video signal to a size corresponding to the imagin-ary frame. The output of the compressing circuit 16 is supplied to the mixer-keyer 17 as one of its inputs. The mixer-keyer 17 is also supplied with the first video signal A and the chroma-key signal, so that the compressed second video signal B is in-serted into and keyed with the first video signal A to give the 11304~7 required output signal 4. The size of the picture compressed by the compressing circuit 16 corresponds to the imaginary frame.
Figure 7 is a schematic diagram of the picture compress-ing circuit 16, in which the second video signal B from the input terminal 3 (Figure 3) is supplied to the analogue/digital convert-er ~1 and converted into a PCM (pulse code modulation) signal.
At the same time, the signal B is also supplied to the write-in clock generator ~2 to generate a continuous wave phase-locked to the color burst signal. This continuous wave is multiplied and sent out as a clock pulse for the analogue/digital converter ~1.
The output PCM signal from the analogue/digital converter ~1 is supplied to the interpolating circuit ~3.
The function of the interpolating circuit ~3 is to alter the number of picture elements in the horizontal direction and that of scanning lines in the vertical direction. When, for in-stance, a picture is to be compressed in the ratio of 1/1.5 in the horizontal direction, the circuit ~3 allows the first of the series of input picture elements to pass therethrough as they are, creating by interpolation between the second and third elements a picture element corresponding exactly to the middle of the two elements and which is delivered as the second of the output pic-ture element. The fourth picture element is allowed to pass there-through so as to constitute the third output picture element. By repeating this process, the number of output picture elements can be reduced to 1/1.5 of the number of input picture elements. This is equivalent to a 1.5-fold expansion of the sample gap in the analogue/digital converter. The output of the interpolating cir-cuit ~3 is written into the memory ~4. ~ccordingly, the compress-ed picture is already written into the memory ~4. These controls ~13~)447 are effected by the signals of the output 93 and 94 ~rom the arithmetic circuit 14. The signal 93 controls the gap between the picture elements newly created by the interpolating circuit g3, and the signal 94 controls the write-in address generator ~5 which generates the address when a signal is written into the memory ~4 in such a manner that the address value is increased by one every time a picture element arrives.
The readout address generator g6 generates the read-out address to be used when a signal is read out of the memory ~.
The switch ~7 is intended to be used for the selection of the write-in address when the signal is to be written in or for the read-out address when it is to be read out. The read-out clock generator 90, using the reference sync signal 92 as input, gener-ates the read-out clock signal to drive the read-out address gen-erator ~6 and the D/A converter ~. The D/A converter ~ converts the read out PCM signal received from the memory ~4 into an an-alogue signal. This analogue signal is fed in to the process amplifier ~9, amplified therein and turned out as the signal 91.
This output signal, in the form of a picture signal compressed into a prescribed position and size, is supplied to the mixer-keyer 17 as one of its inputs.
As explained above, this system, with which it is pos-sible to automatically compress an input picture into the size of a chroma-key signal supplied from outside, is very effective in the production of television programs. Although the chroma-key signal has been referred to in the above description as an example of the key signal, the principle of the present invention is also directly applicable to the wipe key or the like ~rom the wave generator l~. This, even when the chroma-key frame is at an end of the standard frame, the system functions so as to make - 12 _ the compressed picture appear as if it l,~ere pasted on the chroma-key frame, resul.tlng in a very natural-lookin~ output picture.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A tracking system for producing special effects on a television picture said television picture comprised of a first picture and a second picture inserted into said first picture, said first and second pictures being represented by first and second video signals, respectively, wherein said first and second video signals have a predetermined aspect ratio, said system comprising: means for producing a key signal representing a key frame to be positioned on said first picture; means respon-sive to said key signal for producing a position signal represent-ing a position of a circumscribed frame of said key frame; means responsive to said position signal for producing an imaginary-frame position signal, said imaginary-frame position signal representing an imaginary-frame and said imaginary-frame having said predetermined aspect ratio; means responsive to said second video signal and said imaginary-frame position signal for producing a compressed second video signal representing a compressed second picture, said compressed second picture being identical in size to said imaginary frame; and means responsive to said key signal for selectively combining said first video signal and said com-pressed second video signal, whereby said special effects are produced.

2. A tracking system in accordance with claim 1, wherein said imaginary-frame position signal producing means includes means for expanding one of a horizontal and vertical length of said circumscribed frame.

3. A tracking system in accordance with claim 2, wherein said position signal includes two vertical position signal values and two horizontal position signal values, said tracking system further including means responsive to said vertical position signal values and said horizontal position signal values for determining a control ratio, inversely proportional to said pre-determined aspect ratio.

4. A tracking system in accordance with claim 3, wherein said imaginary-frame position signal producing means includes means for increasing selected ones of said vertical and horizontal position signal values in response to said control ratio being less than a predetermined value and for decreasing other selected ones of said vertical and horizontal position signal values in response to said control ratio being greater than a predetermined value.

5. A tracking system in accordance with claim 3, wherein said determining means comprises a microprocessor.