JP2007019970A - Device and method for color correction - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and device for color correction with high practicability for achieving highly accurate color correction with specific color in a free manner in a simple configuration using a two-dimensional LUT(look up table) when performing color correction only in a specified area. <P>SOLUTION: The device converts an input image signal into each variable of hue H, lightness V and saturation C by using a predetermined expression, uses the two-dimensional LUT obtained by combining two variables among prepared hue H, lightness V and saturation C to apply color correction to the variables of the hue H, the lightness V and the saturation C converted from the input image signal by a color correcting means 12, and uses a second predetermined expression to color-corrected hue H', lightness V' and saturation C' to generate an output image. Then, it is possible to improve color correction accuracy and make a circuit scale small without lowering an operation processing speed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention converts an image signal input to a color correction device into a color correction signal when performing color correction of a color image, and combines two components of the three components of the color correction signal to obtain a color correction amount. The present invention relates to a color correction method and a color correction apparatus that perform color correction processing using a two-dimensional LUT (Look Up Table) that defines the above.

  A conventional color correction method and color correction apparatus is disclosed in Japanese Patent Laid-Open No. 9-326941. Japanese Patent Laid-Open No. 9-326941 discloses a color correction device that extracts a color within a predetermined range with respect to the reference color from the input image data, and a reference color setting unit that sets a reference color. Representative color calculation means for calculating the representative color, condition setting means for setting a correction condition to be applied to the input image data based on the reference color and the representative color, and correction according to the set condition Correction means for performing the processing on the input image data.

  The color correction procedure is as follows. The RGB image data is approximated to an L * a * b * space using a one-dimensional LUT and matrix operation. The a * b * approximated to the L * a * b * space is converted from the orthogonal coordinate system to the HC polar coordinate system by referring to the two-dimensional LUT, converted to image data in the L * HC space, and color correction is performed. Processing is performed. In the lightness correction process, range correction for L * is performed, and in the color balance correction process, color correction based on RGB data is performed. Of L * HC after color correction, HC data is converted into an a * b * orthogonal coordinate system with reference to a two-dimensional LUT, and then converted into output RGB image data by matrix calculation. The two-dimensional LUT is used only for color space conversion without holding the color correction amount, and the color correction is performed using the one-dimensional LUT.

  Another example is disclosed in Japanese Patent Application Laid-Open No. 2004-112694. The color correction method presented in Japanese Patent Application Laid-Open No. 2004-112694 is characterized in that color correction is performed by partially expanding or contracting a color reproduction region. Since a three-dimensional LUT is used as the color correction means, the calculation speed is slow and not suitable for real-time processing.

JP-A-9-326941 JP 2004-112694 A

  The conventional color correction disclosed in Japanese Patent Laid-Open No. 9-326941 uses a one-dimensional LUT. The one-dimensional LUT can independently perform hue correction for hue H, lightness V, and saturation C. For example, when color correction is performed for lightness, the lightness for all hues and all saturations regardless of hue and saturation. Color correction is made. That is, it is not possible to perform color correction on the brightness of only the blue sky. Therefore, there is a problem that the accuracy of color correction is low because it is not possible to perform color correction only for the lightness of a specific hue.

  Also, the conventional example disclosed in Japanese Patent Application Laid-Open No. 2004-112694 performs color correction by applying a three-dimensional LUT. The three-dimensional LUT can correct a specific lightness of a specific hue, but a color region that is not color-corrected is subjected to a large amount of interpolation calculation by the three-dimensional LUT, and the calculation speed is slow and is not suitable for real-time processing. there were.

  The present invention has been made to solve the above-described problem, and realizes color correction of a specific color freely and with a simple configuration using a two-dimensional LUT when performing color correction only on a specific area. An object of the present invention is to obtain a highly practical color correction method and color correction apparatus.

The color correction apparatus according to the present invention is:
First image signal conversion means for converting an input image signal into variables of hue H, lightness V, and saturation C using a predetermined formula;
Two-dimensional LUT storage means for storing a two-dimensional LUT (Look Up Table) for color correction combining two variables of hue H, lightness V, and saturation C, and hue H obtained by converting the input image signal Color correction means for correcting the lightness V and saturation C variables based on the two-dimensional LUT of the two-dimensional LUT storage means;
And second image signal conversion means for generating an output image signal using a second predetermined formula for hue H ′, lightness V ′, and saturation C ′ after color correction.

Further, the color correction method according to the present invention is as follows:
A first image signal conversion step of converting an input image signal into hue H, brightness V, and saturation C variables using a predetermined formula;
The hue H, lightness V, and saturation C of the input image signal converted based on a two-dimensional LUT (Look Up Table) for color correction combining two variables of hue H, lightness V, and saturation C. A color correction process for correcting any of the colors;
And a second image signal conversion step of generating an output image using the second predetermined formula for hue H ′, lightness V ′, and saturation C ′ after color correction.

A color correction method and a color correction apparatus according to the present invention convert an input image signal into variables of hue H, lightness V, and saturation C using predetermined formulas,
Using a two-dimensional LUT (Look Up Table) that combines two variables of hue H, lightness V, and saturation C created in advance,
Color correction is performed for variables of hue H, brightness V, and saturation C converted from the input image signal,
Since the output image is generated using the second predetermined formula for the hue H ′, the lightness V ′, and the saturation C ′ after color correction, the specific color can be freely selected with high accuracy without reducing the calculation processing speed. Can be realized, and the circuit scale can be further reduced. As a result, the color correction device can be reduced in size and cost.

Embodiment 1 of the present invention will be described below.
Embodiment 1 FIG.
FIG. 1 is a diagram showing an example of the configuration of a color correction apparatus according to Embodiment 1 of the present invention. In FIG. 1, the first image signal converting means 11 uses the input image signal 1 of input red (R), green (G), and blue (B) as hue H, brightness V, and saturation C as parameters. Convert to color space and output HVC variable. The color correction unit 12 uses the two-dimensional LUT that holds the color correction amount stored in the two-dimensional LUT storage unit 14 to perform color correction on the HVC variable by interpolation, and sets the H′V′C ′ variable after color correction. Output. The second image signal conversion means 13 converts the input color-corrected H′V′C ′ variable into an output image signal 5 of R′G′B ′ and outputs it.

  Next, the operation of the color correction method will be described. The first image signal conversion means 11 receives the RGB signal as the first image signal, converts it into chromaticity coordinates such as YCbCr color space or L * a * b * color space, and then the human visual characteristics. Are converted into HVC variables of hue H, lightness V, and saturation C having The conversion method to the HVC variable may be directly converted from the RGB signal which is the first image signal without converting into the chromaticity coordinates.

  The color correction unit 12 uses the input HVC variable and a two-dimensional LUT stored in the two-dimensional LUT storage unit 14 and created from a preset two-dimensional LUT or a color correction amount set by the user. Then, H′V′C ′ after color correction is calculated and output by interpolation calculation.

  The second image signal conversion means 13 converts the H′V′C ′ variable into the R′G′B ′ signal of the output image signal 5 by the inverse conversion of the predetermined formula and outputs it.

  Next, the operation of the first image signal conversion process 11 will be described. Conversion from the RGB signal to the HVC variable is performed using a known HSI conversion formula or a trigonometric function. Here, the brightness V is normalized so that black is 0 and white is the maximum number of bits of the input image signal 1. The hue H is set so that the six representative colors are red R, yellow Y, green G, cyan C, blue B, and magenta M, and the range between the representative hues is 0 to the maximum number of bits of the input image signal 1. . The hue H holds a hue number in addition to the required hue H. The hue number is expressed using six representative hues of R, Y, G, C, B, and M. Of the representative hues that sandwich the hue H, the representative hue on the right side of the hue H is the first hue, and the hue on the left side is the second hue. These first and second hues are set as hue numbers of the hue H.

  Next, the operation of the color correction unit 12 will be described. FIG. 2 is an example of a two-dimensional LUT when hue and lightness are combined. FIG. 2A shows a two-dimensional LUT before correction. Here, lattice points are created according to the gradation and hue to be corrected from lightness V0 to V5 and hue H0 to H5. FIG. 2B is a two-dimensional LUT including a color correction amount. For example, the color P1 in FIG. 2A with the hue between H1 and H2 and the lightness between V3 and V4 is converted to the color P2 in FIG. 2B. Interpolation of hue between H1 and H2 is linear interpolation. When the LUT shown in FIG. 2 is used, it is possible to perform color correction between H1 and H2 only for colors between V3 and V4 with low brightness. When the one-dimensional LUT is applied, it is possible to adjust the lightness for all hues, but it is not possible to perform color correction only on the specific lightness color of the specific hue as in the case of applying the two-dimensional LUT. . If a three-dimensional LUT is used, the degree of freedom of color correction is high, but it is not suitable for real-time processing because the calculation speed is very high.

  Similarly, for a two-dimensional LUT combining hue and saturation, color correction is performed using the LUT before and after correction.

  In the two-dimensional LUT, the hue H and hue number before correction of the six hues to be corrected, the hue H and hue number after correction, and the six gradations before and after correction for each hue are corrected. Describe.

  Further, in the hue interpolation, nonlinear interpolation may be performed so that the interpolation density decreases as the distance from H2 increases. Further, H0 to H5 and V0 to V5 may be set at equal intervals. In that case, the brightness and saturation interval width are held internally only by the description of the corrected two-dimensional LUT.

  Although FIG. 2 shows an example of 6 steps of hue and 6 steps of lightness, the number of steps is not limited to 6, and may be increased or decreased within a range in which the calculation speed can be handled in real time.

  The combination of the two-dimensional LUTs is not limited to this, and the two-dimensional LUT may be created by combining the lightness and the saturation so that the specific saturation at the specific lightness or the specific lightness at the specific saturation can be corrected.

  As described above, according to the first embodiment, since color correction is performed using the two-dimensional LUT, the color correction accuracy can be improved without reducing the calculation processing speed, and the circuit scale can be reduced. it can. As a result, the color correction device can be reduced in size and cost.

Embodiment 2. FIG.
The second embodiment of the present invention will be described below. The second embodiment is different from the first embodiment in that the color correction unit 12 performs color correction by combining a one-dimensional LUT and a two-dimensional LUT. FIG. 3 shows an example of the configuration of the color correction apparatus according to the second embodiment. In order to adjust the color of the entire image, the color correction unit 12 includes a one-dimensional LUT storage unit 15 that stores a one-dimensional LUT for correcting any one of the hue H, lightness V, and saturation C. Color correction is performed using the two-dimensional LUT storage unit 14 that stores the two-dimensional LUTs in the remaining two variables. The order of color correction may be two-dimensional LUT correction after one-dimensional LUT correction or one-dimensional LUT correction after two-dimensional LUT correction.

  As described above, according to the second embodiment, since the two-dimensional LUT and the one-dimensional LUT are combined to perform color correction on all of the HVC variables, the correction accuracy can be improved.

Embodiment 3 FIG.
The third embodiment of the present invention will be described below. In the third embodiment, when there is a saturation up mode as a function of an input image, for example, a night view, a person, a sport, or an imaging device, a two-dimensional LUT suitable for each of the saturation up modes can be created. Different from the first embodiment.
FIG. 4 is a diagram illustrating an example of the configuration of the color correction apparatus according to the third embodiment. The two-dimensional LUT setting unit 16 selects an LUT corresponding to the mode signal 6 of the input image from several types of LUTs stored in advance in the two-dimensional LUT storage unit 14 and stores it in a register in the two-dimensional LUT setting unit 16. Set and use for color correction means 12 reference

  Next, the operation of the two-dimensional LUT setting unit 16 will be described. For example, when there is a function called “saturation enhancement mode” as a function of an imaging apparatus such as a digital camera or a mobile phone, the saturation enhancement set by the user from a plurality of LUTs stored in advance in the two-dimensional LUT storage means 14 The mode LUT is selected and set in the register in the two-dimensional LUT setting means 16. The color correction unit 12 performs color correction based on the LUT for the saturation increase mode in the two-dimensional LUT setting unit 16 to create an image with increased saturation. At this time, in the saturation up mode, the imaging apparatus may automatically transmit the mode signal 6 to the two-dimensional LUT setting unit 16 during photographing. In addition, the setting mode is not limited to increasing the saturation, and can correspond to various modes such as a night view, a landscape during drizzle, a person, and sports.

  As described above, according to the third embodiment, in order to select a two-dimensional LUT corresponding to the mode of the input image from a plurality of two-dimensional LUTs stored in advance, and to set the two-dimensional LUT setting means, There is an effect that the user can easily perform color correction to a favorite color or color correction according to a shooting situation.

Embodiment 4 FIG.
The fourth embodiment of the present invention will be described below. The fourth embodiment is different from the first embodiment in that the color correcting unit 12 can correct only a specific hue of a specific brightness. FIG. 5 shows a configuration diagram of the two-dimensional LUT in the fourth embodiment. This figure shows an example of a two-dimensional LUT by a combination of hue and lightness as in FIG. In addition to the hue, hue number, and brightness before and after correction, a description of the hue correction amount according to the brightness gradation is added to the two-dimensional LUT. Based on this description, lattice points are interpolated as in the first embodiment. The interpolation may be linear or non-linear. Also, the combination of hue and lightness has been described here, but in the case of the combination of hue and saturation as well, a description of the hue correction amount corresponding to the gradation of saturation is added and interpolation is performed.

  As described above, according to the fourth embodiment, if the hue correction amount corresponding to the lightness gradation or the saturation gradation is described in the two-dimensional LTU, the color of the specific hue of the specific lightness or the specific saturation is described. The color of a specific hue can be corrected.

  As described above, according to the fourth embodiment, a two-dimensional LUT combining at least two variables of hue H, lightness V, and saturation C is based on one of the two variables. Since the function of correcting the color of the remaining variables is provided, there is an effect that the color of the specific hue with the specific brightness or the color of the specific hue with the specific saturation can be corrected.

Embodiment 5. FIG.
The fifth embodiment of the present invention will be described below. The fifth embodiment is different from the first embodiment in that a color gamut compression unit 17 is provided after the second image signal conversion unit 13. FIG. 6 shows an example of the configuration of the color correction apparatus according to the fifth embodiment. In this embodiment, out of the colors converted by the second image signal conversion unit 13 into the output image signal (second image signal) by the color gamut compression unit 17, colors outside the standard color gamut are selected. Performs conversion to a color within the color gamut.

  Next, the operation of the color gamut compression means 17 will be described. The second image signal conversion means 13 performs the second image signal conversion processing from the hue H ′, lightness V ′, and saturation C ′ calculated and color-corrected by the color correction means 12, and R′G′B ′. An image signal is obtained. Here, among the R′G′B ′ image signals, only the color having the bit maximum value or the negative value of any of the second image signals is subjected to color gamut compression using the LUT. The difference between the maximum value of the R′G′B ′ image signal and the maximum bit value of the second image signal, or the difference between the minimum value of the R′G′B ′ image signal and 0 The ratio of compression is obtained by referring to the LUT based on the larger one. The compression ratio is multiplied by the R′G′B ′ image signal to obtain an R ″ G ″ B ″ image signal, which is output as a color after color gamut compression.

  The reference color gamut may be a color gamut in the output image signal 5 or a standard color space such as sRGB.

  As described above, according to the fifth embodiment, when there is a color outside the predetermined color reproduction gamut among the colors after the color correction, the color gamut to be subjected to the color gamut compression processing in the color reproduction gamut. Since the compression means is provided, color gradation can be maintained, and there is an effect of improving reproducibility on the side of high saturation, high brightness, and low brightness.

Embodiment 6 FIG.
The sixth embodiment of the present invention will be described below. Embodiment 6 differs from Embodiment 5 in that color gamut compression processing by color gamut compression means 17 is performed after color correction processing by color correction means 12. FIG. 7 shows an example of the configuration of the color correction apparatus according to the sixth embodiment. The color gamut compression means 17 is a means for converting a color outside the reference color reproduction gamut out of the colors corrected by the color correction means 12 into a color within the color reproduction gamut.

  Next, the operation of the color gamut compression means 17 will be described. One of the lightness V ′ and saturation C ′ calculated and color-corrected by the color correction means 12 is the maximum bit value of the second image signal, or the maximum possible brightness V ′ and saturation C ′. Only a color having a value larger than or negative with the LUT is subjected to color gamut compression using the LUT. The difference between the maximum value of lightness V ′ and saturation C ′ and the maximum bit value of the second image signal or the maximum value that lightness V ′ and saturation C ′ can take, or lightness V ′ and saturation The compression ratio is obtained by referring to the LUT with the larger of the differences between the minimum value of C ′ and 0 as a reference. By multiplying the compression ratio by the lightness V ′ and the saturation C ′, the lightness V ″ and the saturation C ″ are obtained and output as a color after color gamut compression. The color gamut compression method is not limited to this, and may be combined with other color gamut compression methods.

  As described above, according to the sixth embodiment, when there is a color outside the predetermined color reproduction gamut among the colors after the color correction, the color gamut to be subjected to the color gamut compression processing in the color reproduction gamut. Since the compression means is provided, color gradation can be maintained, and there is an effect of improving reproducibility on the side of high saturation, high brightness, and low brightness.

  INDUSTRIAL APPLICABILITY This invention is suitable for application to a color adjustment function of a small-sized device such as a digital color copying machine, a printer, a color display, etc. including color adjustment of a color image as a part of its function, particularly a mobile phone or a DSC (Digital Steal Camera). It is.

It is a figure which shows the structure of the color correction apparatus by Embodiment 1 of this invention. It is a block diagram of the two-dimensional LUT by Embodiment 1 of this invention. It is a figure which shows the structure of the color correction apparatus by Embodiment 2 of this invention. It is a figure which shows the structure of the color correction apparatus by Embodiment 3 of this invention. It is a block diagram of the two-dimensional LUT by Embodiment 4 of this invention. It is a figure which shows the structure of the color correction apparatus by Embodiment 5 of this invention. It is a figure which shows the structure of the color correction apparatus by Embodiment 6 of this invention.

Explanation of symbols

11; First image signal conversion means, 12; Color correction means, 13; Second image signal conversion means, 14; Two-dimensional LUT storage means, 15; One-dimensional LUT storage means, 16; Two-dimensional LUT setting means, 17: Color gamut compression means.

Claims (6)

First image signal conversion means for converting an input image signal into variables of hue H, lightness V, and saturation C using a predetermined formula;
Two-dimensional LUT storage means storing a two-dimensional LUT (Look Up Table) for color correction combining two variables of hue H, lightness V, and saturation C, and hue H obtained by converting the input image signal Color correction means for correcting the lightness V and saturation C variables based on the two-dimensional LUT of the two-dimensional LUT storage means;
And second image signal conversion means for generating an output image signal using a second predetermined expression for hue H ′, lightness V ′, and saturation C ′ after color correction. Color correction device.   The color correction means includes a color correction based on a one-dimensional LUT for correcting any one of hue H, lightness V, and saturation C and a two-dimensional LUT for correcting the other two variables. 2. The color correction apparatus according to claim 1, wherein color correction is performed in combination with color correction based on the color correction.   A two-dimensional LUT setting unit for selecting and setting a two-dimensional LUT corresponding to an input image signal mode from a plurality of two-dimensional LUTs stored in advance in the two-dimensional LUT storing unit; 3. The color correction apparatus according to claim 1, wherein color correction is performed based on the two-dimensional LUT selected by the means.   The two-dimensional LUT in which at least two variables of the hue H, lightness V, and saturation C are combined is a table that corrects the color of the other variables based on one of the two variables. The color correction apparatus according to claim 1.   The color gamut compression means for converting the color into the color gamut when there is a color outside the predetermined color gamut as a result of the color correction by the color correction means. Item 5. The color correction device according to any one of Items 4 to 4. A first image signal conversion step of converting an input image signal into hue H, brightness V, and saturation C variables using a predetermined formula;
The hue H, lightness V, and saturation C of the input image signal converted based on a two-dimensional LUT (Look Up Table) for color correction combining two variables of hue H, lightness V, and saturation C. A color correction process for correcting any of the colors;
And a second image signal conversion step of generating an output image using a second predetermined formula for hue H ′, lightness V ′, and saturation C ′ after color correction. Correction method.

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