JP4757761B2 - Chroma key detection range adjustment method and information processing apparatus - Google Patents

Description

  The present invention relates to a method for adjusting a chroma key detection range for detecting a chroma key used in chroma key composition and an information processing apparatus therefor.

  In the conventional chroma key composition, the first screen is replaced with the second screen by replacing a part of the screen at a predetermined level (for example, red = 0, green = 0, blue = 255) with the first screen. The screen and the second screen are combined.

  FIG. 1 shows a configuration of a system having a conventional chroma key composition function.

  Referring to FIG. 1, a video signal generated by the CPU 102 is output from the CPU 102. This video signal is, for example, an R (red), G (green), and B (blue) signal, and 8 bits are consumed for each color.

  The graphics chip 103 outputs a video signal for displaying graphics by performing predetermined arithmetic processing on the video signal input from the CPU 102. In the video signal from the graphics chip, 6 bits are consumed for each color.

  The 6-bit / 8-bit conversion unit 104 performs 6-bit / 8-bit conversion on the video signal from the graphics chip 103 by complementing the lower 2 bits for each color.

  The frame synchronization unit 107 temporarily adjusts the timing for reading out the video signal from the television 105 stored in the memory 106, whereby the video signal from the 6-bit / 8-bit conversion unit 104 and the video signal from the television 105 are Frame synchronization between them.

The chroma key composition is performed by the comparator 909 and the screen composition unit 108. When the level of the video signal from the 6-bit / 8-bit conversion unit 104 is a predetermined level, the comparator 909 sets the level of the control signal output to the screen synthesis unit 108 to HIGH. The screen composition unit 108 selects the video signal from the 6-bit / 8-bit conversion unit 104 if the control signal is LOW, and selects the video signal from the memory 106 if the control signal is HIGH.
JP 2000-221960 A

  In the above-described conventional system, the comparator compares the video signal from the 6-bit / 8-bit conversion unit 104 with a predetermined chroma key level, so that the screen synthesis unit 108 should select the first video signal. Alternatively, it is determined for each pixel whether the second video signal should be selected, and chroma key composition is performed. The predetermined chroma key level set in the comparator 909 by the CPU 102 is fixed.

  Therefore, even if the chroma key level of the video data output from the CPU 102 is unchanged, if the processing of the graphics chip 103 is changed or replaced with a different one, it is no longer set in the comparator 909 until then. The chroma key level is inaccurate and chroma key composition cannot be performed accurately.

  Therefore, the present invention provides a chroma key detection range adjustment method and chroma key detection range adjustment that enable correct chroma key composition even if chroma key composition cannot be performed correctly normally due to the above reasons. An object is to provide an apparatus.

According to the present invention, when the image processing unit performs color conversion processing related to subtractive color and approximation processing, when the upper chroma key level and the lower chroma key level sandwiching the chroma key level are derived from the chroma key level, An information processing apparatus for detecting an upper chroma key level and a lower chroma key level, wherein a maximum estimated value of the upper chroma key level is set as a first upper threshold, and a minimum estimate of the lower chroma key level Means for setting a value as a first lower threshold, means for setting a second upper threshold with the first upper threshold as an upper limit, the first lower threshold as a lower limit, and , Means for setting a second lower threshold value that is less than or equal to the second upper threshold value, and the number of pixels in the frame that fall within a range between the first upper threshold value and the first lower threshold value. A first detector for detecting a ratio of the number of pixels in the frame that falls within a range between the second upper threshold and the second lower threshold; and a second that maintains the ratio of the number of pixels at a predetermined value or more. A minimum range of the upper limit threshold value and the second lower limit threshold value, and the second upper limit threshold value and the second lower limit threshold value corresponding to the minimum range are set to the upper chroma key level and the lower chroma key level, respectively. An information processing apparatus comprising: a second detection unit .

  Further, according to the present invention, in the case where the image processing unit performs color conversion processing related to color reduction and approximation processing, and the upper chroma key level and the lower chroma key level sandwiching the chroma key level are derived from the chroma key level. An information processing method for detecting the upper chroma key level and the lower chroma key level, wherein a maximum estimated value of the upper chroma key level is set as a first upper threshold value; and Setting a minimum estimated value as a first lower threshold, setting a second upper threshold with the first upper threshold as an upper limit, and setting the first lower threshold as a lower limit And setting a second lower threshold value that is less than or equal to the second upper threshold value and a range that falls within a range between the first upper threshold value and the first lower threshold value. A first detection step of detecting a ratio of the number of pixels in a frame that falls within a range between a second upper threshold and a second lower threshold with respect to the number of pixels in a frame; and the ratio of the number of pixels is predetermined Detecting a minimum range of a second upper limit threshold and a second lower limit threshold that maintain a value equal to or greater than the value, and setting the second upper limit threshold and the second lower limit threshold corresponding to the minimum range to the upper chroma key, respectively. And a second detection step for setting the lower chroma key level. An information processing method is provided.

  Further, according to the present invention, when the image processing unit performs color conversion processing related to subtractive color and approximation processing, when the upper chroma key level and the lower chroma key level sandwiching the chroma key level are derived from the chroma key level. A program for causing a computer to function as a device for detecting the upper chroma key level and the lower chroma key level, wherein the computer sets the maximum estimated value of the upper chroma key level as a first upper threshold value. Means for setting a minimum estimated value of the lower chroma key level as a first lower threshold value, means for setting a second upper threshold value having the first upper threshold value as an upper limit value, Means for setting a first lower threshold value as a lower limit value and setting a second lower threshold value equal to or lower than the second upper threshold value; Detecting the ratio of the number of pixels in a frame falling within the range between the second upper threshold and the second lower threshold to the number of pixels in the frame falling between the value and the first lower threshold A first detection unit that detects a minimum range of a second upper limit threshold and a second lower limit threshold that maintain a ratio of the number of pixels equal to or greater than a predetermined value, and the second upper limit threshold corresponding to the minimum range; There is provided a program for causing the second lower limit threshold to function as second detection means for setting the upper chroma key level and the lower chroma key level, respectively.

  According to the present invention, a chroma key for enabling chroma key composition even when the chroma key level is changed by performing dithering processing, offset processing, rounding processing, or the like in the previous stage of the screen composition unit. The level can be detected.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  In the following description, the description relating to color resolution, color gradation, color variation, or color error is representative for the case of one component color among the color components such as (R, G, B). To explain.

  The graphics chip 103 converts data having 8-bit resolution (having 256 gradations) to data having 6-bit resolution (having 64 gradations). Even when the gradation resolution is reduced in this way, the graphics chip 103 performs a dithering process so that it can be visually recognized as having 256 gradations. Therefore, an error of ± 3 gradations due to the dithering process occurs. Further, an offset error of ± 3 gradations due to a decrease in dynamic range occurs. Furthermore, a rounding error of ± 1 gradation occurs. In this case, an error of ± 7 gradations is generated. Since the level between ± 7 is 15 (0 level is also included in the count), each pixel input to the graphics chip has an uncertainty of 15 gradations in the output.

  Next, each error will be described.

[Error due to dithering]
FIG. 2 is a diagram for explaining the dithering process. The scales attached to the vertical bars shown in FIGS. 2A to 2D represent levels at 8-bit resolution. When the level at 8 bits resolution is dithered to 6 bits, the result is as shown in FIG.

  FIG. 2A shows a state in which a pixel having a level r1 in an 8-bit resolution is converted into four pixels having a level r1 by a dithering process. FIG. 2B shows a state in which a pixel having a level of r1 + 1 in the 8-bit resolution is converted into three pixels having a level of r1 and one pixel having a level of r2 by a dithering process. FIG. 2C shows a state in which a pixel having a level of r1 + 2 is converted into two pixels having a level of r1 and two pixels having a level of r2 by dithering processing in 8-bit resolution. FIG. 2D shows a state in which a pixel having a level of r1 + 3 is converted into one pixel having a level of r1 and three pixels having a level of r2 by dithering processing in 8-bit resolution.

  Since it is uncertain in which case the level of a certain pixel corresponds to FIG. 2 (a) to FIG. 2 (d), the error due to the dithering process is a maximum of ± 3 gradations.

[Offset error]
There is a graphics chip that changes the dynamic range of the input level from 0 to 252 (when dithering to 6-bit resolution: = 63 × 4) and forcibly sets the pixels in the range of 253 to 255 to 252. is there. Further, there is a graphics chip that changes the dynamic range of the input level from 3 to 255 (= 63 × 4 + 3) and forcibly sets the pixels in the range of 0 to 2 to 3. Therefore, when attention is paid to a certain pixel, an offset error of ± 3 gradations occurs.

[Rounding error]
In the graphics chip, calculation processing (conversion processing between 8-bit resolution and 6-bit resolution) in data conversion with different precision is performed, and an approximation process occurs at this time, so that ± 1 gradation Rounding error occurs.

[Purpose of this embodiment]
As described above, each pixel has an uncertainty of 15 gradations in the output of the graphics chip. Therefore, the color selected as the chroma key also has an uncertainty of 15 gradations. Therefore, if all the pixels having a level in the range having the uncertainty of 15 gradations (the range of CPU output gradations ± 7 gradations) are all set as the chroma key level, even the color that is not actually the chroma key color is obtained. Will also be recognized as a chroma key level. Therefore, in this embodiment, the accuracy of the uncertainty of the chroma key level is narrowed down to the minimum range. This minimum range varies depending on conditions, and in some cases it is 5 gradations, and in other cases it is 1 gradation.

[Constitution]
FIG. 3 shows the configuration of a system having a chroma key composition function according to the present invention.

  3 that are the same as those shown in FIG. 1 are marked with the same symbols and descriptions of them will be omitted.

  Comparing the configuration shown in FIG. 3 with the configuration shown in FIG. 1, it can be seen that the comparator 909 is replaced with the first comparator 109 and the second comparator 110 and a control unit 111 is added.

  The first comparator 109, the second comparator 110, and the control unit 111 can be realized by hardware, software, or a combination thereof. Note that “realization by software” means that the computer reads the program and executes the program.

  Until the high-accuracy upper threshold and the high-accuracy lower threshold are obtained, the first comparator 109 is set with the low-accuracy upper threshold and the low-accuracy lower threshold. Then, until the high-accuracy upper threshold value and the high-accuracy lower threshold value are obtained, the screen synthesis unit 108 determines that the signal level from the 6-bit / 8-bit conversion unit 104 is the low-accuracy upper threshold value. A simple chroma key composition is selected by selecting a signal from the memory 106 when it is between the low-precision lower threshold and a signal from the 6-bit / 8-bit conversion unit 104 otherwise. I do.

  If the highly accurate upper threshold and the highly accurate lower threshold are found, the first comparator 109 is set with the highly accurate upper threshold and the highly accurate lower threshold. After that, when the level of the signal from the 6-bit / 8-bit conversion unit 104 is between the high-precision upper threshold and the high-precision lower threshold, the signal from the memory 106 is selected. By selecting a signal from the 6-bit / 8-bit conversion unit 104, high-accuracy chroma key composition is performed.

  The control unit 111 obtains a highly accurate upper threshold and a highly accurate lower threshold in conjunction with the first comparator 109 and the second comparator 110.

[Specific operation example]
FIG. 4 shows a case where KEY1 is used as a high-precision upper threshold when the chroma key level KEY (8) is dithered as shown in FIG. 2B and expressed by KEY1 (6) and KEY2 (6). It is a timing diagram which shows a mode that the level of (6) is calculated | required and KEY2 (6) is calculated | required as a highly accurate lower side threshold value.

In frame F1, HTH (8) is set as the low precision upper threshold of the first comparator 109, and LTH (8) is set as the low precision lower threshold of the first comparator 109. In the frame F1, H1 (= HTH (8)) is set as the variable upper threshold of the second comparator 110, and L1 (= LTH (8)) is set as the variable lower threshold of the second comparator 110. Is set. HTH (8) is a level four gradations higher than the chroma key level KEY (8) output from the CPU 102 for simplification of the drawing, and LTH (8) is a chroma key level KEY output from the CPU 102. Although it is a level 4 gradations lower than (8), it is actually a level 7 gradations higher than the chroma key level KEY (8) output from the CPU 102, and LTH (8) is output from the CPU 102. This is a level 7 gradations below the chroma key level KEY (8).
By the way, in each frame, the first comparator 109 obtains the number of pixels N1 having a level not lower than the lower threshold and not higher than the upper threshold set in the first comparator 109. Similarly, the second comparator 110 obtains the number N2 of pixels having a level not lower than the lower threshold and not higher than the upper threshold set in the second comparator 110. Then, the ratio α = N2 / N1 is obtained.

  In the frame F1, the upper threshold value set in the first comparator 109 is equal to the upper threshold value set in the second comparator 110, and the lower threshold value set in the first comparator 109 is equal to the second threshold value. Therefore, N1 = N2 and therefore α = 100%.

  In frame F2, HTH (8) is set as the low precision upper threshold value of the first comparator 109, and LTH (8) is set as the low precision lower threshold value of the first comparator 109. In the frame F2, H2 (= H1-1) is set as the variable upper threshold value of the second comparator 110, and L1 (= LTH (8)) is set as the variable lower threshold value of the second comparator 110. Is done.

  In the frame F2, since there are some pixels whose level is between H1 and H2, α is, for example, 99%.

  In the frame F3, HTH (8) is set as the low precision upper threshold of the first comparator 109, and LTH (8) is set as the low precision lower threshold of the first comparator 109. In the frame F3, H3 (= H2-1) is set as the variable upper threshold value of the second comparator 110, and L1 (= LTH (8)) is set as the variable lower threshold value of the second comparator 110. Is done.

  In the frame F3, there are some pixels whose level is between H1 and H2, and furthermore, the level of KEY1 (6) is equal to the level H2, so α is 75%, for example. Therefore, it is determined that the level H2 is an upper level for expressing KEY (8) by dithering.

  Therefore, in the frame F4, the level H2 is set as the determined upper threshold value of the second comparator 110.

  In frame F4, HTH (8) is set as the low precision upper threshold value of the first comparator 109, and LTH (8) is set as the low precision lower threshold value of the first comparator 109.

  Further, L2 (= L1 + 1) is set as the variable lower threshold value of the second comparator 110 in the frame F4. Since there are some pixels whose level is between L1 and L2, α is, for example, 98%.

  In the frame F5, HTH (8) is set as the low precision upper threshold of the first comparator 109, and LTH (8) is set as the low precision lower threshold of the first comparator 109. In frame F5, H2 (determined value) is set as the variable upper threshold value of the second comparator 110, and L3 (= L2 + 1) is set as the variable lower threshold value of the second comparator 110. Since there are some pixels whose level is between L2 and L3, α is 97%, for example.

  In the frame F6, HTH (8) is set as the low precision upper threshold of the first comparator 109, and LTH (8) is set as the low precision lower threshold of the first comparator 109. In frame F6, H2 (determined value) is set as the variable upper threshold value of the second comparator 110, and L4 (= L3 + 1) is set as the variable lower threshold value of the second comparator 110. Since there are some pixels whose level is between L3 and L4, α is, for example, 96%.

  In the frame F7, HTH (8) is set as the low precision upper threshold of the first comparator 109, and LTH (8) is set as the low precision lower threshold of the first comparator 109. In frame F7, H2 (determined value) is set as the variable upper threshold value of the second comparator 110, and L5 (= L4 + 1) is set as the variable lower threshold value of the second comparator 110.

  In the frame F7, there are some pixels whose level is between L4 and L5, and furthermore, the level of KEY2 (6) is equal to the level L4, so α is 24%, for example. Therefore, it is determined that the level L4 is a lower level for expressing KEY (8) by dithering.

  After the frame F8, the upper level H2 determined in the frame F3 and the lower level L4 determined in the frame F7 are set in the first comparator 109. Based on the comparison result output from the first comparator 109, the screen composition unit 108 performs chroma key composition.

  In this way, until the high-accuracy threshold is obtained, the first comparator 109 is set with a low-accuracy threshold and can only perform low-accuracy chroma key composition. Once obtained, a high-accuracy threshold is set in the first comparator 109, and high-accuracy chroma key composition can be performed.

  FIG. 5 shows a case where KEY1 (6) is used as a highly accurate upper threshold when the chroma key level KEY (8) is dithered as shown in FIG. 2 (c) and is expressed by KEY1 (6) and KEY2 (6). 6) It is a timing chart showing how KEY2 (6) (= L3) is obtained as a highly accurate lower threshold value by obtaining the level (= H3). The operation principle is the same as in FIG.

  FIG. 6 shows a case where the key value KEY1 (6) is used as a highly accurate upper threshold when the chroma key level KEY (8) is dithered as shown in FIG. 2 (d) and expressed by KEY1 (6) and KEY2 (6). 6) It is a timing chart showing how KEY2 (6) (= L2) is obtained as a highly accurate lower threshold value by obtaining the level (= H4). The operation principle is the same as in FIG.

  FIG. 7 shows a high-precision upper threshold when the chroma key level KEY (8) is dithered as shown in FIG. 2A and expressed by KEY1 (6) (= KEY2 (6)). It is a timing chart which shows a mode that the level of KEY1 (6) (= H5) is calculated | required and KEY2 (6) (= H5) is calculated | required as a highly accurate lower side threshold value. The operation principle is the same as in FIG. In the case of FIG. 7, since the value of α becomes 0% when the variable upper threshold is lowered to H6, it is determined that the dithering as shown in FIG. Both the threshold value and the high precision lower threshold value are set to H5.

  In addition to this, even when an offset error or a rounding error occurs, it is possible to obtain the high accuracy upper threshold and the high accuracy lower threshold by a similar algorithm.

A configuration of a system having a conventional chroma key composition function is shown. It is a figure which shows a mode that the precision of the range for chroma key detection goes up by this invention. It is a figure which shows the structure of the system which has the function of a chroma key composition by embodiment of this invention. FIG. 4 is a first diagram showing how to obtain a highly accurate threshold value for detecting a chroma key level in the system shown in FIG. 3. FIG. 4 is a second diagram showing how to obtain a highly accurate threshold value for detecting a chroma key level in the system shown in FIG. 3. FIG. 4 is a third diagram showing how to obtain a highly accurate threshold value for detecting a chroma key level in the system shown in FIG. 3. FIG. 4 is a fourth diagram illustrating a state in which a highly accurate threshold value for detecting a chroma key level is obtained in the system illustrated in FIG. 3.

Explanation of symbols

101 personal computer 102 CPU
DESCRIPTION OF SYMBOLS 103 Graphics chip 104 6 bit / 8 bit conversion part 105 Television 106 Memory 107 Frame synchronization part 108 Screen composition part 109 1st comparator 110 2nd comparator 111 Control part

Claims (12)

In the image processing unit, color conversion processing related to color reduction and approximation processing is performed, and when the upper chroma key level and the lower chroma key level sandwiching the chroma key level are derived from the chroma key level, the upper chroma key level and the lower chroma key level are derived. An information processing apparatus for detecting a side chroma key level,
Means for setting the maximum estimated value of the upper chroma key level as a first upper threshold;
Means for setting a minimum estimated value of the lower chroma key level as a first lower threshold;
Means for setting a second upper threshold value with the first upper threshold value as an upper limit;
Means for setting the first lower threshold to a lower limit and setting a second lower threshold equal to or lower than the second upper threshold;
The number of pixels in the frame that falls within the range between the second upper threshold and the second lower threshold with respect to the number of pixels in the frame that falls within the range between the first upper threshold and the first lower threshold. First detecting means for detecting a ratio;
Detecting a minimum range of a second upper limit threshold value and a second lower limit threshold value, wherein the ratio of the number of pixels maintains a predetermined value or more, and the second upper limit threshold value and the second lower limit threshold value corresponding to the minimum range; , Second detection means for setting the upper chroma key level and the lower chroma key level, respectively,
The information processing apparatus comprising: a. The information processing apparatus according to claim 1,
The information processing apparatus, wherein the color conversion processing is dither processing, offset processing, rounding processing, or a combination thereof. The information processing apparatus according to claim 1 or 2 ,
In the second detection means, the second upper threshold value is subtracted, and the second upper threshold value and the second upper threshold value for the number of pixels in the frame falling within the range between the first upper threshold value and the first lower threshold value are set. When the amount of change in the ratio of the number of pixels in a frame that falls within a range between two lower thresholds is equal to or greater than a predetermined value, the immediately preceding second upper threshold is set as the upper chroma key level. Information processing device. The information processing apparatus according to any one of claims 1 to 3 ,
In the second detection means, a second lower threshold is increased, and a second upper threshold for the number of pixels in the frame falling within a range between the first upper threshold and the first lower threshold; When the amount of change in the ratio of the number of pixels in the frame that falls within the range between the second lower threshold value is equal to or greater than a predetermined value, the immediately preceding second lower threshold value is set as the lower chroma key level. A characteristic information processing apparatus. In the image processing unit, color conversion processing related to color reduction and approximation processing is performed, and when the upper chroma key level and the lower chroma key level sandwiching the chroma key level are derived from the chroma key level, the upper chroma key level and the lower chroma key level are derived. An information processing method for detecting a side chroma key level,
Setting a maximum estimated value of the upper chroma key level as a first upper threshold;
Setting a minimum estimated value of the lower chroma key level as a first lower threshold;
Setting a second upper threshold value having the first upper threshold value as an upper limit;
Setting the first lower threshold as a lower limit and setting a second lower threshold equal to or lower than the second upper threshold;
The number of pixels in the frame that falls within the range between the second upper threshold and the second lower threshold with respect to the number of pixels in the frame that falls within the range between the first upper threshold and the first lower threshold. A first detection step of detecting a ratio;
Detecting a minimum range of a second upper limit threshold value and a second lower limit threshold value, wherein the ratio of the number of pixels maintains a predetermined value or more, and the second upper limit threshold value and the second lower limit threshold value corresponding to the minimum range; A second detection step for setting the upper chroma key level and the lower chroma key level, respectively,
An information processing method characterized by comprising:   The information processing method according to claim 5,
  The information processing method, wherein the color conversion process is a dither process, an offset process, a rounding process, or a combination thereof.   The information processing method according to claim 5 or 6,
  In the second detection step, the second upper threshold is subtracted, and the second upper threshold and the second upper threshold for the number of pixels in the frame falling within the range between the first upper threshold and the first lower threshold are set. When the amount of change in the ratio of the number of pixels in a frame that falls within a range between two lower thresholds is equal to or greater than a predetermined value, the immediately preceding second upper threshold is set as the upper chroma key level. Information processing method.   The information processing method according to any one of claims 5 to 7,
  In the second detection step, the second lower threshold is increased, and the second upper threshold for the number of pixels in the frame that falls within the range between the first upper threshold and the first lower threshold; When the amount of change in the ratio of the number of pixels in the frame that falls within the range between the second lower threshold value is equal to or greater than a predetermined value, the immediately preceding second lower threshold value is set as the lower chroma key level. A characteristic information processing method.   In the image processing unit, color conversion processing related to color reduction and approximation processing is performed, and when the upper chroma key level and the lower chroma key level sandwiching the chroma key level are derived from the chroma key level, the upper chroma key level and the lower chroma key level are derived. A program for causing a computer to function as a device for detecting a side chroma key level,
  The computer,
  Means for setting the maximum estimated value of the upper chroma key level as a first upper threshold;
  Means for setting a minimum estimated value of the lower chroma key level as a first lower threshold;
  Means for setting a second upper threshold value with the first upper threshold value as an upper limit;
  Means for setting the first lower threshold to a lower limit and setting a second lower threshold equal to or lower than the second upper threshold;
  The number of pixels in the frame that falls within the range between the second upper threshold and the second lower threshold with respect to the number of pixels in the frame that falls within the range between the first upper threshold and the first lower threshold. First detecting means for detecting a ratio;
  Detecting a minimum range of a second upper limit threshold value and a second lower limit threshold value, wherein the ratio of the number of pixels maintains a predetermined value or more, and the second upper limit threshold value and the second lower limit threshold value corresponding to the minimum range; , Second detection means for setting the upper chroma key level and the lower chroma key level, respectively,
  Program to function as.   The program according to claim 9,
  The color conversion process is a program that is a dither process, an offset process, a rounding process, or a combination thereof.   In the program according to claim 9 or 10,
  In the second detection means, the second upper threshold value is subtracted, and the second upper threshold value and the second upper threshold value for the number of pixels in the frame falling within the range between the first upper threshold value and the first lower threshold value are set. When the amount of change in the ratio of the number of pixels in a frame that falls within a range between two lower thresholds is equal to or greater than a predetermined value, the immediately preceding second upper threshold is set as the upper chroma key level. program.   The program according to any one of claims 9 to 11,
  In the second detection means, a second lower threshold is increased, and a second upper threshold for the number of pixels in the frame falling within a range between the first upper threshold and the first lower threshold; When the amount of change in the ratio of the number of pixels in the frame that falls within the range between the second lower threshold value is equal to or greater than a predetermined value, the immediately preceding second lower threshold value is set as the lower chroma key level. A featured program.

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