JP7252400B2 - Image processing device - Google Patents

Description

The present invention relates to an image processing apparatus that performs color processing on digital image data.

Conventionally, image data expressed in the YCbCr color space such as digital video is converted into the RGB color space on the output side. However, since the YCbCr color space has a wider color space than the RGB color space, the values after conversion sometimes fall outside the color gamut of the RGB color space. In this case, if the value outside the color gamut is simply stored in the RGB color space, the RGB ratio will change, causing changes in the brightness and hue of the image data, resulting in image quality deterioration such as bleeding.

Therefore, in order to reduce the change in the brightness and hue of the image data that may occur during such conversion processing, while fixing the Y value (luminance value), There is disclosed an image processing apparatus that can be stored without changing the ratio of (that is, lowering the saturation while maintaining the same hue) (see, for example, Patent Document 1).

Also, in a pixel of interest whose color coordinates are outside the reproducible color gamut in the color space of the output image, a plurality of color difference signals are generated while maintaining the luminance signal and maintaining the ratio of the plurality of color difference signals. An image processing apparatus that adjusts saturation while maintaining the hue of a pixel of interest is also disclosed (see, for example, Patent Document 2).

JP-A-2005-252393 Patent No. 6318497

In the conventional image processing apparatus as disclosed in the above-mentioned patent document, it is true that even colors outside the color gamut of the color space on the output side in conversion processing can be appropriately reproduced without changing the luminance or hue. However, in conventional conversion processing, there is a problem in that calculations for correcting pixel values outside the color gamut on the output side to pixel values within the color gamut are redundant and require processing time.

For example, the calculation method of the conversion processing in the above-mentioned Patent Document 1 will be described. In the image processing apparatus disclosed in Patent Document 1, a vertical line is drawn from the position of the pixel value outside the color gamut of the RGB color space on the output side after conversion to the Y axis (luminance axis), and a range corresponding to the RGB color space is drawn. A correction coefficient α is obtained from the pixel value at the point of intersection with . Then, based on this correction coefficient α, a calculation is performed to replace the corrected pixel value. However, when obtaining this correction coefficient α, the differences Δr, Δg, and Δb between each color component value (r, g, b) after RGB conversion and the y value (luminance value) are obtained, and these Δr, Δg, and Δb Six saturation correction coefficients αR0, αR1, αG0, αG1, αB0, and αB1 (six boundaries) for each color component are obtained based on the equations derived by transforming the equations from . Furthermore, from these candidates for saturation correction coefficients αR0, αR1, αG0, αG1, αB0, and αB1, those less than 0.0 or greater than 1.0 are excluded, and among the remaining saturation correction coefficients, the maximum and determine it as the correction factor α. Finally, pixel values outside the color gamut of the RGB color space are corrected to within the color gamut based on the determined correction coefficient α. Therefore, it must be said that the calculation for correcting the pixel value within the color gamut is redundant.

The present invention has been made in view of the above problems, and provides an image processing apparatus that performs conversion processing to fit colors outside the color gamut in the output color space within the color space without changing the lightness and hue, It is an object of the present invention to provide an image processing apparatus capable of realizing conversion processing at a higher speed by suppressing the amount of calculation.

In order to achieve the above object, the present invention converts image data represented by a first color space into image data represented by a second color space by fixing the ratio of lightness and color difference and correcting the color difference. An image processing device that performs conversion processing, wherein the signal values of the image data indicated in the first color space are converted into the signal values of the image data indicated in the second color space based on a predetermined calculation. and calculating a correction coefficient for correcting the signal value when the signal value after conversion by the converting means exists outside the color gamut corresponding to the second color space in the first color space. correction coefficient calculation means; and correction means for correcting signal values represented in the second color space using the correction coefficients calculated by the correction coefficient calculation means to obtain post-correction signal values, A correction coefficient calculation means obtains a value farthest from the upper limit among signal values existing above the upper limit of the color gamut and a value farthest from the lower limit among signal values existing below the lower limit of the color gamut, The correction factor is determined from one of these values.

In this image processing device, the first color space is a YCbCr color space, the second color space is an RGB color space, and the conversion means converts signal values (y, cb, cr) in the YCbCr color space to a pixel value (r, g, b) in the RGB color space, where y is the luminance component, cb and cr are the chrominance components, r is the luminance value of the R channel, g is the luminance of the G channel The value b is the brightness value of the B channel, and the correction coefficient calculation means detects when any of the pixel values (r, g, b) exists outside the color gamut corresponding to the RGB color space in the YCbCr color space. In pixel values (r, g, b) existing outside the color gamut, the maximum absolute value of the difference from the upper limit of the color gamut is ΔP, and the absolute value of the difference from the lower limit of the color gamut is It is preferable that the largest value among them be ΔN, and the correction coefficient be calculated based on the pixel value of the color component having the largest value of ΔP or (1−y)ΔN/y.

In this image processing apparatus, the correction coefficient α calculated by the correction coefficient calculation means is α=(1−y )/(ΔP+1−y), and α=y/(ΔN+y) when calculating from the largest absolute value ΔN of the difference from the lower limit of the color gamut.

In order to achieve the above object, the present invention converts image data represented by a first color space into image data represented by a second color space by fixing the ratio of lightness and color difference and correcting the color difference. An image processing device that performs conversion processing, wherein the signal values of the image data indicated in the first color space are converted into the signal values of the image data indicated in the second color space based on a predetermined calculation. and calculating a correction coefficient for correcting the signal value when the signal value after conversion by the converting means exists outside the color gamut corresponding to the second color space in the first color space. correction coefficient calculation means; and correction means for correcting signal values represented in the second color space using the correction coefficients calculated by the correction coefficient calculation means to obtain post-correction signal values, A correction coefficient calculation means obtains a value farthest from the upper limit among signal values existing above the upper limit of the color gamut and a value farthest from the lower limit among signal values existing below the lower limit of the color gamut, determining the correction factor from one of these values, the first color space being the YCbCr color space, the second color space being the RGB color space, and the transforming means converting the signal in the YCbCr color space Convert the value (y,cb,cr) to a pixel value (r,g,b) in the RGB color space, where y is the luminance component, cb and cr are the chrominance components, and r is the luminance of the R channel. g is the luminance value of the G channel, and b is the luminance value of the B channel. When the upper limit of the color gamut is exceeded, the correction coefficient α = (1-y)/((maximum value of any pixel value of (r, g, b) exceeding the upper limit) - y) , when any pixel value of the pixel values (r, g, b) is less than the lower limit of the color gamut, the correction coefficient α=y/(y−(exceeding the lower limit (r, g, b ) is the minimum value among the pixel values of any one of )).

In this image processing apparatus, it is preferable that, when a plurality of correction coefficients are derived, the correction coefficient calculation means selects the smallest one from among the plurality of correction coefficients as the correction coefficient.

Further, in order to achieve the above object, the present invention converts image data represented in a first color space into an image represented in a second color space by fixing the ratio of lightness and color difference and correcting the color difference. A program that functions in an image processing device that performs data conversion processing, wherein the signal values of the image data represented by the first color space are transformed into the image data represented by the second color space based on a predetermined calculation. a converting step of converting into a signal value; and correcting the signal value when the converted signal value in the converting step exists outside the color gamut corresponding to the second color space in the first color space. and a correction step of obtaining a post-correction signal value by correcting the signal value represented by the second color space using the correction coefficient calculated in the correction coefficient calculation step. and, in the correction coefficient calculation step, among the signal values existing above the upper limit of the color gamut, the value farthest from the upper limit, and among the signal values existing below the lower limit of the color gamut, the lowest and determining the correction factor from one of these values.

Further, in order to achieve the above object, the present invention converts image data represented in a first color space into an image represented in a second color space by fixing the ratio of lightness and color difference and correcting the color difference. An image processing method for converting data, wherein the signal values of the image data represented by the first color space are transformed into the signal values of the image data represented by the second color space based on a predetermined calculation. and a correction coefficient for correcting the signal value after conversion in the converting step when the signal value after conversion exists outside the color gamut corresponding to the second color space in the first color space. and a correction step of correcting the signal value represented in the second color space using the correction coefficient calculated in the correction coefficient calculation step to obtain the signal value after correction. , in the correction coefficient calculating step, the value farthest from the upper limit among the signal values existing above the upper limit of the color gamut and the value farthest from the lower limit among the signal values existing below the lower limit of the color gamut and determining the correction factor from one of these values.

An image processing apparatus according to the present invention includes a converter that converts a signal value of image data represented by the YCbCr color space into a pixel value of image data represented by the RGB color space based on a predetermined calculation; If it exists outside the color gamut corresponding to the second color space in the color space, a correction coefficient calculation unit that obtains a correction coefficient for correcting the pixel value, and a second color space using the correction coefficient and a correction unit that corrects the indicated pixel value to obtain a corrected pixel value. A correction coefficient calculation unit obtains a value farthest from the upper limit among signal values existing above the upper limit of the color gamut and a value farthest from the lower limit among signal values existing below the lower limit of the color gamut, and calculates these values. The correction factor is determined from one of the values. With this configuration, in the present invention, especially when converting the YCbCr color space to the RGB color space, the amount of calculation for correction for preventing changes in the brightness and hue of image data can be suppressed, and the conversion can be performed at a higher speed. processing can be realized.

1 is a block diagram showing the configuration of an image processing device according to an embodiment of the present invention; FIG. 4 is a flow chart showing an operation procedure of the same image processing apparatus; 9 is a flow chart showing the operation procedure of the image processing device according to the modified example of the embodiment of the present invention;

(Embodiment)
An image processing apparatus according to an embodiment of the present invention will be described with reference to FIGS. 1 to 3. FIG. Note that the image processing apparatus according to the present embodiment mainly corrects values outside the color gamut to within the color gamut of the RGB space when video data whose color space in digital video is the YCbCr format is converted into the RGB color space. It performs calculations.

First, each processing unit provided in the image processing apparatus 1 will be described with reference to FIG. The image processing apparatus 1 includes a control section 10, an image processing section 11, a storage section 12, a communication section 13, a display section 14, an operation section 15 and a reading section 16, as shown in FIG. Although the image processing apparatus 1 and the operation of the image processing apparatus 1 will be described below assuming one computer, the processing may be distributed among a plurality of computers.

The control unit 10 uses a processor such as a CPU and a memory to control components of the image processing apparatus 1 to realize various functions. In this embodiment, the control unit 10 performs image correction processing (color conversion processing) by executing an image processing program 1P stored in the storage unit 12, for example. That is, the control unit 10 performs a predetermined conversion on input target image data (target data), and performs calculations for correcting conversion values outside the color gamut.

The image processing unit 11 uses a processor such as a GPU or a dedicated circuit and a memory, and executes image processing according to control instructions from the control unit 10 . Note that the control unit 10 and the image processing unit 11 are configured as one piece of hardware (SoC: System on a Chip) in which a processor such as a CPU or GPU, a memory, a storage unit 12 and a communication unit 13 are integrated. good too.

The storage unit 12 uses a hard disk or flash memory. The storage unit 12 stores an image processing program 1P, image data 1D to be displayed on the display unit 14, and the like. This image data 1D includes digital video expressed in the YCbCr color space, and is image data obtained from a network via the communication section 13 or image data read from the reading section 16, for example.

The communication unit 13 is a communication module that realizes communication connection to a communication network such as the Internet. For the communication unit 13, for example, a network card, a wireless communication device, or a carrier communication module is used.

The display unit 14 uses a liquid crystal panel, an organic EL (Electro Luminescence) display, or the like. The display unit 14 can display an image by processing in the image processing unit 11 according to instructions from the control unit 10 . The display unit 14 displays image data in accordance with general RGB format data for each pixel forming the screen. Normally, when a digital image in the YCbCr color space is displayed on the display unit 14, it is displayed after being converted into the sRGB (standard RGB) color space. This is because an image displayed on the display unit 14 of a general information display device such as a personal computer can only be displayed in RGB format data.

Digital image data in the RGB color space is expressed as a collection of pixels (picture elements) partitioned by coordinates, and each pixel is given a pixel value (three values of color information RGB). Here, a pixel value is represented by (r, g, b) where r is the luminance value of the R channel, g is the luminance value of the G channel, and b is the luminance value of the B channel at a certain pixel position. . Normally, each (r, g, b) value is represented by 8 bits (256 steps), etc., but in this embodiment, for convenience, each (r, g, b) can take a range of 0.0 to 1.0 Description will be made by normalizing to the range of (upper limit 1.0 and lower limit 0.0).

The operating unit 15 includes a user interface such as a keyboard or mouse. A physical button provided on the housing may be used, or a software button or the like displayed on the display unit 14 may be used. The operation unit 15 notifies the control unit 10 of operation information by the user.

The reading unit 16 uses, for example, a disk drive, and can read the image processing program 2P stored in the recording medium 2 (optical disk, magnetic disk, magneto-optical disk, etc.) using an optical disk or the like. Further, the reading unit 16 may be used when reading image data of a moving image or a still image of a color image to be displayed on the display unit 14 from the recording medium 2 or the like. The image processing program 1P stored in the storage unit 12 is acquired from the network via the communication unit 13, or the image processing program 2P read from the recording medium 2 by the reading unit 16 is read by the control unit 10 from the storage unit. 12 may be replicated.

Next, correction calculations in the image processing apparatus 1 when converting image data expressed in the YCbCr color space into the RGB color space will be described. Normally, image data composed of YCbCr signals can reproduce a wider range than the output color space (sRGB color space). Therefore, depending on the content of the image data to be displayed on the display unit 14, the color coordinate values after conversion may be outside the color gamut of the RGB color space (that is, outside the range of 0.0 to 1.0). Therefore, the image processing apparatus 1 obtains a correction coefficient α for correcting the color coordinates outside the color gamut to within the color gamut, and performs correction processing.

Specifically, the control section 10 of the image processing apparatus 1 includes a conversion section (conversion means) 10a, a correction coefficient calculation section (correction coefficient calculation means) 10b, and a correction section (correction means) 10c. The conversion unit 10a converts signal values of image data represented by the YCbCr color space (first color space) into pixel values of image data represented by the RGB color space (second color space) based on a predetermined calculation. Convert. The correction coefficient calculation unit 10b obtains a correction coefficient α for correcting the pixel value when the pixel value converted by the conversion unit 10a exists outside the color gamut corresponding to the RGB color space in the YCbCr color space. A method for obtaining the correction coefficient α will be described later. The correction unit 10c corrects the pixel values represented in the RGB color space using the correction coefficient α calculated by the correction coefficient calculation unit 10b, and obtains the corrected pixel values. Here, the correction coefficient calculation unit 10b calculates the difference between the pixel value existing above the upper limit of the color gamut and the upper limit of the color gamut, and the difference between the pixel value existing below the lower limit of the color gamut and the lower limit of the color gamut. Differences are calculated, and the correction coefficient α is calculated based on the largest difference among these differences. From this, in the image processing device 1, when the YCbCr space is converted into the RGB space, even if the pixel value in the converted RGB color space does not fall within the range of 0.0 to 1.0, the correction process is performed to Perform processing to fit in 1.0.

なお、ここでは、変換前の元の画像データのYCbCr色空間での信号値を（ｙ、ｃｂ、ｃｒ）（ここで、ｙは輝度成分、ｃｂ及びｃｒは色差成分である）、式１を用いてRGB色空間へと変換したときの画素値を（ｒ，ｇ，ｂ）（ここでｒはＲチャネルの輝度値、ｇはＧチャネルの輝度値、ｂはＢチャネルの輝度値）とする。また、YCbCr色空間での信号値ｙは0.0≦ｙ≦1.0、式差信号ｃｂ、ｃｒは-0.5≦ｃｂ、ｃｒ≦0.5の範囲をとる。 Next, an example of derivation of correction coefficients in the image processing apparatus 1 will be described. First, upon receiving a display command of image data from the user via the operation unit 15 or the like to the display unit 14, the conversion unit 10a converts the designated image data from the YCbCr color space to the RGB color space. At this time, the conversion unit 10a performs, for example, the calculation shown in Equation 1 below.

Here, the signal values in the YCbCr color space of the original image data before conversion are (y, cb, cr) (where y is the luminance component, cb and cr are the color difference components), and Equation 1 is Let (r, g, b) be the pixel value when converted to the RGB color space using . The signal value y in the YCbCr color space is in the range of 0.0≤y≤1.0, the difference signals cb and cr are in the range of -0.5≤cb and cr≤0.5.

During conversion by the conversion unit 10a, among the signal values (y, cb, cr) of the image data composed of YCbCr signals, the pixel values (r, g, b) after RGB conversion are outside the range of 0.0 to 1.0 (that is, the RGB color outside the color gamut of the space). In order to allow (r, g, b) after conversion to fall outside the range of 0.0 to 1.0, a number of bits exceeding the original number of bits may be assigned to each color component.

Next, the correction coefficient calculation unit 10b performs correction processing when there is a pixel value outside the color gamut. Specifically, the signal values (y, cb, cr) of the input pixel are divided into a range corresponding to the RGB space by drawing a perpendicular line to the Y axis (luminance axis) of the YCbCr color space using the correction coefficient α. If the corrected signal value at the crossing point is defined as (y', cb', cr'), the following equation 2 holds.
y' = y
cb′=α×cb
cr′=α×cr Expression 2
Here, y′=y indicates that the lightness is fixed, and cb′=α×cb and cr′=α×cr indicate that the hue (that is, color difference ratio) is fixed.

Based on the corrected signal values (y', cb', cr'), the corrected RGB values obtained by applying Equation 1, which is an inverse transformation formula, are (r', g', b') Then, the following formula 3 holds.
r'=y+α×(ry)
g′=y+α×(gy)
b′=y+α×(by) …Equation 3

When any of (r, g, b) after conversion is outside the color gamut corresponding to the RGB color space in the YCbCr color space, the correction coefficient calculation unit 10b corrects the color gamut in this (r, g, b). Let ΔP be the largest absolute value of the difference from the upper limit (1.0 here), and let ΔN be the largest absolute value of the difference from the lower limit of the color gamut (0.0 here). Further, the correction coefficient calculation unit 10b determines that the pixel value having the largest value of ΔP or (1−y)ΔN/y is the most out of the range of 0.0 to 1.0, and clips at the position of this pixel value. is necessary, and based on that, the correction coefficient α is obtained from Equation 3. For example, if the ΔP corresponding to r is the largest among the pixel values (r, g, b) after conversion, the correction coefficient α is obtained from the above r'=y+α×(ry). It will happen. When the luminance component y is 0 or 1, (r, g, b) are all 0 or 1, so the correction coefficient α need not be obtained.

The reason why the correction coefficient calculator 10b calculates ΔP and (1−y)ΔN/y will now be described.
First, when the upper limit is set to 1.0 (that is, when the correction coefficient α is obtained from the largest value ΔP among the absolute values of the difference from the upper limit of the color gamut), the correction coefficient α is obtained from the above equation 3 by the following equation 4. .
1=y+α(ry)=y+α(1+ΔP-y)
∴α=(1−y)/(ΔP+1−y) Equation 4
Note that although r is used in the description here, a color component that is most out of the color gamut may be used, and g or b other than r may be used. Note that 0≦correction coefficient α≦1, luminance value y is a constant between 0 and 1, and ΔP is the amount exceeding 1 (difference from upper limit 1.0). Also, it can be seen from the equation 4 that the larger the ΔP, the smaller the correction coefficient α.

Next, when the lower limit is set to 0.0 (that is, when the correction coefficient α is obtained from the largest value ΔN among the absolute values of the difference from the lower limit of the color gamut), the correction coefficient α is obtained from the above equation 3 by the following equation 5. .
0=y+α(ry)=y+α(-ΔN-y)
∴α=y/(ΔN+y) Equation 5
Although r is also used in the description here, the color component that is most out of the color gamut may be used, and g or b other than r may be used. ΔN is less than 0.0 (difference from the lower limit of 0.0). Also, it can be seen from Equation 4 that the larger ΔN is, the smaller the correction coefficient α is.

Considering the case where α in Equations 4 and 5 are the same, Equation 6 below is obtained.
(1−y)/(ΔP+1−y)=y/(ΔN+y) Equation 6
If the denominator and the numerator are inverted here, the following equation 7 is derived.
(ΔP+1−y)/(1−y)=(ΔN+y)/y
ΔP/(1−y)+1=ΔN/y+1
ΔP/(1−y)=ΔN/y
ΔP=(1−y)/y×ΔN Equation 7
Then, it can be seen from Equation 7 that if ΔN is multiplied by (1−y)/y, it becomes equivalent to ΔP, and ΔP and ΔN can be compared. That is, the correction coefficient calculation unit 10b selects the pixel value having the largest value of ΔP or (1−y)ΔN/y among (r, g, b), thereby Pixel values that deviate greatly can be determined.

Finally, the correction unit 10c calculates (r', g', b') after clipping from the determined correction coefficient α and Equation (3). Note that the values of (r', g', b') after clipping obtained using Equation 3 are in the range of 0.0 to 1.0, and any one value (that is, the pixel value at the clipped position) always satisfies the condition of 1.0 or 0.0. Thus, the image processing apparatus 1 according to the present embodiment can obtain the correction coefficient α by a very simple calculation. Next, the pixel values (r′, g′, b′) after correction are calculated based on the determined correction coefficient α and Equation 3 to further suppress the amount of calculation, and eventually color processing conversion can be performed at a higher speed. can be realized.

Next, the operation procedure of the image processing apparatus 1 according to this embodiment will be described with reference to the flowchart shown in FIG. First, when the user selects image data to be displayed on the display unit 14 via the operation unit 15 or the like, the conversion unit 10a of the control unit 10 converts the image data from the YCbCr color space to the RGB color space. (S21). For example, when the image data is represented by RGB color information of 1920×1080 pixels, this conversion process is executed for each pixel.

Next, the correction coefficient calculator 10b determines whether any of the pixel values (r, g, b) after conversion processing is outside the range of 0.0 to 1.0 (S22). Then, if it is outside the range of 0.0 to 1.0 (that is, the color coordinate value after conversion is outside the color gamut of the RGB color space) (Yes in S22), ΔP and (1−y)ΔN/y are the largest. It is determined that clipping is necessary for any pixel value of the (r, g, b) color components having values, and based on that, the correction coefficient α is obtained from Equation 3 (S23).

Then, the correction unit 10c obtains pixel values (r', g', b') within the color gamut using the correction coefficient α calculated by the correction coefficient calculation unit 10b and Equation 3 (S24). At this time, the pixel value at the clipped position becomes 0 or 1 without the need to calculate from α as described above. On the other hand, when the RBG pixel values (r, g, b) after conversion processing are within the range of 0.0 to 1.0 (No in S22), the correction coefficient calculation unit 10b does not require correction processing. S22 and S23, which is the processing in the correction unit 10c, are skipped.

(Modification)
A modified example of the image processing apparatus 1 according to the embodiment of the present invention will be described. In this modified example, the correction coefficient calculator 10b does not calculate ΔP and (1−y)ΔN/y in a different calculation method from the above-described embodiment. That is, the correction coefficient calculation unit 10b according to the present modification calculates the largest value exceeding the upper limit value or the smallest value exceeding the lower limit value among the pixel values (r, g, b) after conversion processing. are calculated, and the smaller value of either one is adopted. That is, in this modified example, there is no need to calculate the pixel value with the largest value of ΔP or (1−y)ΔN/y as in the above embodiment.

Specifically, if any pixel value of the converted pixel values (r, g, b) exceeds the upper limit (1.0) of the color gamut, the correction coefficient calculation unit 10b according to the present modification uses the above formula 4, the correction coefficient α=(1−y)/((maximum value of any pixel value of (r, g, b) exceeding the upper limit)−y). On the other hand, if any of the pixel values (r, g, b) after conversion is less than the lower limit (0.0) of the color gamut, the correction coefficient α = y/(y-(lower limit It is assumed that the minimum value among the pixel values exceeding (r, g, b))).

When a plurality of correction coefficients α are derived from (r, g, b) after conversion processing, the correction coefficient calculation unit 10b selects the smallest one from among the plurality of correction coefficients α. can be a correction coefficient α.

Next, the operation procedure of the image processing apparatus 1 according to this modified example will be described with reference to the flowchart shown in FIG. First, when the user selects image data to be displayed on the display unit 14 via the operation unit 15 or the like, the conversion unit 10a of the control unit 10 converts the image data from the YCbCr color space to the RGB color space. (S31).

Next, the correction coefficient calculator 10b determines whether any of the pixel values (r, g, b) after conversion processing exceeds the upper limit of 1.0 (S32). Then, if the upper limit is exceeded (Yes in S32), it is determined whether or not it is less than the lower limit (S33). If it is less than the lower limit (Yes in S33), as described above, the correction coefficient α=(1-y)/((one of the pixel values (r, g, b) exceeding the upper limit) (maximum value)-y) and correction coefficient α = y/(y-(minimum value of any pixel value of (r, g, b) exceeding the lower limit))), and the smallest one from these is selected as the correction coefficient α (S34). On the other hand, if it is equal to or higher than the lower limit (No in S33), the correction coefficient α=(1-y)/((maximum value of any pixel value (r, g, b) exceeding the upper limit) -y) (S35).

Further, if any of the pixel values (r, g, b) after conversion processing does not exceed the upper limit (No in S32), it is determined whether or not it is less than the lower limit (S36). If so (Yes in S36), the correction coefficient α=y/(y-(minimum value of any pixel value of (r, g, b) exceeding the lower limit)) is obtained (S37). Next, after S34, S35 and S37, the correction unit 10c corrects the pixel values (r', g', b') within the color gamut using the derived correction coefficient α (S38). On the other hand, if all of the RBG pixel values (r, g, b) after the conversion process are within the range of 0.0 to 1.0 (No in S36), the correction coefficient calculator 10b particularly obtains the correction coefficient α. Since there is no need, S38, which is the processing in the correction unit 10c, is skipped.

As described above, in the present embodiment, the image data represented in the first color space (YCbCr color space) is converted to the second color space by fixing the lightness and the color difference ratio and correcting the color difference. (RGB color space), the signal values of the image data represented in the first color space are represented in the second color space based on a predetermined calculation. A conversion unit 10a that converts into signal values of image data, and if the signal values after conversion in the conversion unit 10a are outside the color gamut corresponding to the second color space in the first color space, the signal values are converted to A correction coefficient calculation unit 10b for obtaining a correction coefficient for correction, and a signal value after correction is obtained by correcting the signal value represented by the second color space using the correction coefficient calculated by the correction coefficient calculation unit 10b. and a correction unit 10c. The correction coefficient calculation unit 10b obtains the value farthest from the upper limit among the signal values existing above the upper limit of the color gamut and the value farthest from the lower limit among the signal values existing below the lower limit of the color gamut. The correction factor is determined from one of the values of . With this configuration, the image processing apparatus 1 performs conversion processing to fit colors outside the color gamut in the output color space into the color space without changing the lightness and hue (i.e., fixing the lightness and hue). In the image processing device, it is possible to reduce the amount of calculation and realize conversion processing at a higher speed.

The present invention is not limited to the configuration of the above-described embodiment, and various modifications are possible without changing the gist of the invention. Further, in order to achieve the object of the present invention, the present invention is implemented as an image processing method having steps of characteristic constituent means included in the image processing apparatus 1, or as a program including those characteristic steps. can also The program can be distributed not only in a ROM or the like, but also via a recording medium such as a USB memory or a communication network.

In the above description, the case of converting the YCbCr color space into the RGB color space as the conversion processing of the digital image was explained, but the first color space is not limited to the YCbCr color space, and other color spaces (for example, YPbPr space or YUV color space) may also be used. Also, in the description of the above embodiment, an example in which each function of the image processing apparatus 1 is realized by a program was described, but it may be realized by hardware.

Furthermore, the present invention can also be implemented as a computer system that transmits input data to an image processing device or computer program and receives and uses output data from the image processing device or computer program. A device used in this system is an image processing device having a display unit and a communication unit or an information processing device capable of transmitting and receiving information to and from a computer.

1 image processing device 10 control section 10a conversion section (conversion means)
10b correction coefficient calculation unit (correction coefficient calculation means)
10c correction unit (correction means)
REFERENCE SIGNS LIST 11 image processing unit 12 storage unit 13 communication unit 14 display unit 15 operation unit 16 reading unit

Claims (7)

An image processing device for converting image data represented by a first color space into image data represented by a second color space by fixing the ratio of lightness and color difference and correcting the color difference ,
conversion means for converting the signal values of the image data represented by the first color space into the signal values of the image data represented by the second color space based on a predetermined calculation;
correction coefficient calculation for obtaining a correction coefficient for correcting the signal value when the signal value after conversion by the conversion means exists outside the color gamut corresponding to the second color space in the first color space means and
a correction means for correcting the signal value indicated in the second color space using the correction coefficient calculated by the correction coefficient calculation means to obtain a signal value after correction;
The correction coefficient calculation means obtains a value farthest from the upper limit among signal values existing above the upper limit of the color gamut and a value farthest from the lower limit among signal values existing below the lower limit of the color gamut. , determining said correction factor from one of these values. the first color space is the YCbCr color space and the second color space is the RGB color space;
The converting means converts signal values (y, cb, cr) in the YCbCr color space into pixel values (r, g, b) in the RGB color space,
where y is the luminance component, cb and cr are the color difference components, r is the luminance value of the R channel, g is the luminance value of the G channel, b is the luminance value of the B channel,
If any of the pixel values (r, g, b) exists outside the color gamut corresponding to the RGB color space in the YCbCr color space, the correction coefficient calculation means calculates the pixel value outside the color gamut ( r, g, b), let ΔP be the largest absolute value of the difference from the upper limit of the color gamut, ΔN be the largest absolute value of the difference from the lower limit of the color gamut, and ΔP or 2. The image processing apparatus according to claim 1, wherein the correction coefficient is calculated based on the pixel value of the color component having the largest value of (1-y)ΔN/y. The correction coefficient α calculated by the correction coefficient calculation means is
When calculating from the largest value ΔP among the absolute values of the difference from the upper limit of the color gamut,
α = (1-y) / (ΔP + 1-y)
When calculating from the largest value ΔN among the absolute values of the difference from the lower limit of the color gamut,
α=y/(ΔN+y)
3. The image processing apparatus according to claim 2, characterized in that it is calculated by: An image processing device for converting image data represented by a first color space into image data represented by a second color space by fixing the ratio of lightness and color difference and correcting the color difference ,
conversion means for converting the signal values of the image data represented by the first color space into the signal values of the image data represented by the second color space based on a predetermined calculation;
correction coefficient calculation for obtaining a correction coefficient for correcting the signal value when the signal value after conversion by the conversion means exists outside the color gamut corresponding to the second color space in the first color space means and
a correction means for correcting the signal value indicated in the second color space using the correction coefficient calculated by the correction coefficient calculation means to obtain a signal value after correction;
The correction coefficient calculation means obtains a value farthest from the upper limit among signal values existing above the upper limit of the color gamut and a value farthest from the lower limit among signal values existing below the lower limit of the color gamut. , determining said correction factor from one of these values;
the first color space is the YCbCr color space and the second color space is the RGB color space;
The converting means converts signal values (y, cb, cr) in the YCbCr color space into pixel values (r, g, b) in the RGB color space,
where y is the luminance component, cb and cr are the color difference components, r is the luminance value of the R channel, g is the luminance value of the G channel, b is the luminance value of the B channel,
The correction coefficient calculation means calculates a correction coefficient α=(1−y) when any one of the pixel values (r, g, b) after conversion in the conversion means exceeds the upper limit of the color gamut. / ((maximum value of any pixel value of (r, g, b) exceeding the upper limit) - y),
If any of the pixel values (r, g, b) is less than the lower limit of the color gamut, the correction coefficient α = y/(y-(over the lower limit (r, g, b) and the minimum value of any one of the pixel values))). 1. The correction coefficient calculating means further selects the smallest one from among the plurality of correction coefficients and uses it as the correction coefficient when a plurality of correction coefficients are derived. 5. The image processing device according to any one of 4. A program that functions in an image processing device that converts image data represented by a first color space into image data represented by a second color space by fixing the ratio of lightness and color difference and correcting the color difference. and
a conversion step of converting the signal values of the image data represented by the first color space into the signal values of the image data represented by the second color space based on a predetermined calculation;
Correction coefficient calculation for obtaining a correction coefficient for correcting the signal value after conversion in the conversion step when the signal value after conversion exists outside the color gamut corresponding to the second color space in the first color space a step;
a correction step of obtaining a post-correction signal value by correcting the signal value indicated in the second color space using the correction coefficient calculated in the correction coefficient calculation step;
In the correction coefficient calculating step, a value farthest from the upper limit among signal values existing above the upper limit of the color gamut and a value farthest from the lower limit among signal values existing below the lower limit of the color gamut are calculated. and determining said correction factor from one of these values. An image processing method for converting image data represented by a first color space into image data represented by a second color space by fixing the ratio of lightness and color difference and correcting the color difference ,
a conversion step of converting the signal values of the image data represented by the first color space into the signal values of the image data represented by the second color space based on a predetermined calculation;
Correction coefficient calculation for obtaining a correction coefficient for correcting the signal value after conversion in the conversion step when the signal value after conversion exists outside the color gamut corresponding to the second color space in the first color space a step;
a correction step of obtaining a post-correction signal value by correcting the signal value indicated in the second color space using the correction coefficient calculated in the correction coefficient calculation step;
In the correction coefficient calculating step, a value farthest from the upper limit among signal values existing above the upper limit of the color gamut and a value farthest from the lower limit among signal values existing below the lower limit of the color gamut are calculated. and determining said correction factor from one of these values.

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