JP2017076955A - Image display apparatus and color conversion apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique capable of suppressing variation in perceptible brightness when converting an input color.SOLUTION: An image display apparatus according to the present invention includes: a setting means to perform setting related to the display luminance of the image display apparatus; and a correcting means to reduce the saturation of the input color and increase lightness of the input color. The correcting means determines an increase amount of the lightness of the input color on the basis of the display luminance, which has been set by the setting means.SELECTED DRAWING: Figure 5

Description

  The present invention relates to an image display device and a color conversion device.

  As a method of converting a color outside a predetermined color gamut into a color within a predetermined color gamut, a method of reducing (compressing) saturation while maintaining hue and lightness is known (Patent Document 1). In the method described in Patent Document 1, the brightness is increased according to the degree of reduction in saturation.

  Further, a visual effect called a Helmholtz-Colelausch effect (HK effect) is known. The HK effect is a visual effect in which a high-saturated color is felt bright and a low-saturated color is felt dark.

  When colors are converted by a method of reducing the saturation while maintaining the hue and brightness, the perceived brightness (apparent brightness) may be reduced by the HK effect. As described above, in the method described in Patent Document 1, the brightness is increased according to the degree of reduction in saturation. However, the degree of the HK effect (the difference between the physically defined brightness and the perceived brightness) does not depend solely on the saturation. Therefore, the perceptual lightness cannot be maintained with high accuracy by the method of increasing the lightness according to the degree of reduction in saturation. In the conventional method, the difference between the perceived lightness of the color before conversion and the perceived lightness of the color after conversion depends on the color before conversion. Therefore, by converting the color of each pixel of the image data, the balance of perceived lightness between the pixels may be lost.

JP 2014-33273 A

  An object of the present invention is to provide a technique capable of suppressing changes in perceived brightness when converting an input color.

The first aspect of the present invention is:
A setting means for setting the display brightness of the image display device;
Correction means for reducing the saturation of the input color and increasing the brightness of the input color;
With
The correction unit is an image display device that determines an increase in brightness of an input color based on the display luminance set by the setting unit.

The second aspect of the present invention is:
Obtaining means for obtaining information on display brightness set in the image display device;
Correction means for reducing the saturation of the input color and increasing the brightness of the input color;
With
The correction unit is a color conversion device that determines an increase in lightness of an input color based on display luminance set in the image display device.

The third aspect of the present invention is:
An acquisition step of acquiring information about display brightness set in the image display device;
A correction step for reducing the saturation of the input color and increasing the brightness of the input color;
Have
In the correction step, the amount of increase in lightness of the input color is determined based on display luminance set in the image display device.

  A fourth aspect of the present invention is a program for causing a computer to execute each step of the color conversion method described above.

  According to the present invention, it is possible to suppress changes in perceived brightness when converting an input color.

1 is a block diagram illustrating an example of a configuration of an image display apparatus according to a first embodiment. The figure which shows an example of the LUV conversion table which concerns on Example 1. FIG. 7 is a flowchart illustrating an example of a processing flow of the image display apparatus according to the first embodiment. The figure which shows an example of the equivalent perceptual lightness conversion table which concerns on Example 1. FIG. FIG. 6 is a diagram for explaining a specific example of the operation of the image display apparatus according to the first embodiment. FIG. 9 is a block diagram illustrating an example of a configuration of an image display device according to a second embodiment. FIG. 6 is a block diagram illustrating an example of the configuration of an image display device according to a third embodiment. FIG. 10 is a diagram for explaining a specific example of the operation of the image display apparatus according to the third embodiment. FIG. 6 is a block diagram illustrating an example of a configuration of an image display device according to a fourth embodiment. FIG. 7 is a block diagram illustrating an example of a configuration of an image display device according to a fifth embodiment. FIG. 10 is a block diagram showing an example of the configuration of a computer according to a sixth embodiment Flowchart showing an example of a processing flow of software according to the sixth embodiment 10 is a flowchart illustrating an example of a processing flow of an image display apparatus according to the seventh embodiment. FIG. 10 is a diagram for explaining a specific example of the operation of the image display apparatus according to the seventh embodiment.

<Example 1>
Embodiment 1 of the present invention will be described below. The color conversion apparatus according to the present embodiment determines an output color that is a color after conversion with respect to an input color that is a color before conversion. Hereinafter, an example will be described in which an image processing apparatus includes the color conversion apparatus according to the present embodiment, and the image display apparatus includes the image processing apparatus. The color conversion device may be a separate device from the image processing device. The image processing apparatus may be a separate apparatus from the image display apparatus.

  FIG. 1 is a block diagram illustrating an example of the configuration of the image display apparatus 100 according to the present embodiment. The image display device 100 includes an image processing device 200 and a display unit 50. The image processing device 200 includes a color conversion device 300 and a color conversion unit 40. The color conversion apparatus 300 includes a table holding unit 10, a display luminance setting unit 20, and an equivalent perceptual lightness conversion table generation unit 30.

The input image data 1 is image data that is input to the image display device 100 via an input terminal (not shown). In this embodiment, the input image data 1 is image data having RGB values as pixel values. Each of the R value, the G value, and the B value is an 8-bit value (0 to 255). Note that the pixel values of the image data are not limited to RGB values. For example, the pixel value of the image data may be a YCbCr value. The number of bits of the gradation value may be more or less than 8 bits. The input image data 1 may be input to the image display device 100 using a cable, or may be input to the image display device 100 by wireless communication.

  The table holding unit 10 is a storage unit that holds the LUV conversion table 11. As the table holding unit 10, for example, a semiconductor memory, a magnetic disk, an optical disk, or the like can be used. In this embodiment, a ROM is used as the table holding unit 10. An example of the LUV conversion table 11 is shown in FIG. The LUV conversion table 11 is a three-dimensional lookup table (3DLUT) showing a correspondence relationship between L * u * v * values (L * u * v * color space values) and RGB values. Therefore, by using the LUV conversion table 11, it is possible to convert L * u * v * values to RGB values, or to convert RGB values to L * u * v * values.

  In the LUV conversion table 11, RGB values indicating colors that can be displayed on the display unit 50 (displayable colors) are associated with L * u * v * values indicating a suspicious color outside the visible range. The “visible range” is a range of colors that can be perceived by humans. In the LUV conversion table 11, RGB values indicating displayable colors are also associated with L * u * v * values indicating colors that cannot be displayed on the display unit 50. Further, an inappropriate color flag F = 0 is associated with an L * u * v * value indicating a displayable color within the visible range. An inappropriate color flag F = 1 corresponds to an L * u * v * value indicating a suspicious color outside the visible range and an L * u * v * value indicating a color that cannot be displayed on the display unit 50. It is attached. Therefore, by referring to the inappropriate color flag F = 1, it can be determined whether or not the color indicated by the L * u * v * value is an inappropriate color. In the present embodiment, the inappropriate color is a suspicious color outside the visible range and a color that cannot be displayed on the display unit 50.

  The configuration of the LUV conversion table 11 is not limited to the above configuration. For example, only the L * u * v * values of appropriate colors (colors other than inappropriate colors) may be indicated by the LUV conversion table 11. In such a case, L * u * v * values not indicated by the LUV conversion table 11 can be detected as L * u * v * values of inappropriate colors. The inappropriate color may be one of a suspicious color outside the visible range and a color that cannot be displayed on the display unit 50. For example, only colors that cannot be displayed on the display unit 50 may be used as inappropriate colors. Instead of the LUV conversion table 11, a function indicating the correspondence between L * u * v * values and RGB values may be used.

  The display brightness setting unit 20 sets the reference display brightness 21 in accordance with a user operation performed on the image display apparatus 100 using a user interface (not shown). The reference display brightness 21 is a reference value of display brightness (screen brightness). The reference display luminance 21 can be said to be “an upper limit value of display luminance” or “a parameter for determining luminance of an image displayed on the display unit 50”. In this embodiment, the reference display luminance 21 is the luminance of a pixel displayed when the pixel value of the input image data 1 is a 100% white pixel value. When an 8-bit RGB value (R value, G value, B value) is used as the pixel value of the input image data 1, for example, a pixel value of 100% white has an RGB value (255, 255, 255). It is.

The unit of the reference display brightness 21 is not particularly limited. In this embodiment, [cd / m ² ] is used as a unit of the reference display luminance 21. For example, if the value of the reference display brightness 21 is 100, 100% white is displayed at 100 [cd / m ² ] on the display unit 50. The reference display luminance 21 may be a predetermined fixed value, or automatically according to the type of the input image data 1, the operation mode of the image display device 100, the usage environment of the image display device 100, and the like. It may be determined.

The equivalent perceptual lightness conversion table generation unit 30 generates the equivalent perceptual lightness conversion table 31 based on the LUV conversion table 11 and the Helmholtz-Colelausch effect (HK effect). The generation of the equivalent perceptual lightness conversion table 31 is performed every time the reference display luminance 21 is changed. The equivalent perceptual lightness conversion table 31 is updated each time the reference display brightness 21 is changed. The equivalent perceptual lightness conversion table 31 is color conversion information for converting an input color that is a color before conversion into an output color that is a color after conversion. The output color is a color obtained by converting the input color in consideration of the HK effect. The configuration of the equivalent perceptual lightness conversion table 31, the generation method of the equivalent perceptual lightness conversion table 31, and the like will be described in detail later.

  The color conversion unit 40 generates display image data 41 by converting the color of each pixel of the input image data 1 based on the equivalent perceptual lightness conversion table 31 (color conversion processing). Specifically, the color conversion unit 40 generates display image data 41 by converting each pixel value of the input image data 1 based on the equivalent perceptual lightness conversion table 31. Note that when the display image data 41 is generated, image processing other than color conversion processing may be further performed.

  The display unit 50 displays an image based on the display image data 41. In this embodiment, an image is displayed on the screen based on the display image data 41 and the reference display brightness 21. The display unit 50 includes, for example, a light emitting unit and a modulation panel that displays an image on a screen by modulating light from the light emitting unit. In this embodiment, the display unit 50 is a transmissive display unit having a backlight module and a liquid crystal panel. The backlight module irradiates light on the back surface of the liquid crystal panel. The liquid crystal panel forms a transmittance pattern according to the image data input to the display unit 50. The light from the backlight module is transmitted through the liquid crystal panel at a transmittance corresponding to the image data input to the display unit 50, whereby an image is displayed on the screen. The backlight module emits light with a light emission luminance (light emission intensity) corresponding to the reference display luminance 21. Thereby, the luminance (display luminance) in the screen area in which the pixel value of the display image data 41 is a pixel value of 100% white substantially matches the luminance specified by the reference display luminance 21. “Substantially match” includes “perfect match”.

  The configuration of the display unit 50 is not particularly limited. For example, a reflective display unit that displays an image on a screen by reflecting light irradiated on the front surface may be used as the display unit 50. The reflective display unit forms a reflectance pattern according to image data input to the display unit 50, for example. A MEMS shutter type display unit using a MEMS (Micro Electro Mechanical System) shutter instead of the liquid crystal element of the liquid crystal panel may be used as the display unit 50. An organic EL display panel, a plasma display panel, or the like may be used as the display unit 50.

  Next, an example of a method for generating the equivalent perceptual lightness conversion table 31 and an example of the configuration of the equivalent perceptual lightness conversion table 31 will be described in detail. FIG. 3 is a flowchart showing an example of a method for generating the equivalent perceptual lightness conversion table 31. A part of the processing shown in the flowchart of FIG. 3 may be performed by a functional unit different from the equivalent perceptual lightness conversion table generation unit 30.

  First, in S301, the equivalent perceptual lightness conversion table generation unit 30 starts loop processing. The loop processing in S301 is a repetition of processing for selecting a color within a predetermined color gamut as an input color. By the loop processing of S301, each color within a predetermined color gamut is selected in order as an input color. The predetermined color gamut is not particularly limited. In this embodiment, the predetermined color gamut is a color range corresponding to a range of pixel values that the input image data 1 can take. In this embodiment, each pixel value that can be taken by the input image data 1 is sequentially selected as pixel values (R value, G value, B value) = (iR, iG, iB) of the input color. Specifically, in S301, a loop process that gradually changes the R value from 0 to 255, a loop process that gradually changes the G value from 0 to 255, and a loop that gradually changes the B value from 0 to 255. A triple loop process is performed. By this triple loop processing, each pixel value that can be taken by the input image data 1 is sequentially selected as the pixel value (iR, iG, iB) of the input color.

  Next, in S302, the equivalent perceptual lightness conversion table generation unit 30 converts the pixel values (iR, iG, iB) selected in S301 into L * u * v * values (L * values, u * values, v *). Value) = (L0, U0, V0).

  The process of S302 will be described in detail.

  First, red point XYZ values (X value, Y value, Z value) = (RX, RY, RZ), green point XYZ values (GX, GY, GZ), blue point XYZ values (BX, BY, BZ) ) And XYZ values (WX, WY, WZ) of the white point are acquired by a profile acquisition unit (not shown). The red point can be referred to as “R origin”, the green point can be referred to as “G origin”, and the blue point can be referred to as “B origin”. These XYZ values may be prepared in advance.

Next, the equivalent perceptual lightness conversion table generation unit 30 calculates an XYZ value (iX, iY, iZ) of the input color according to the following equation 1 defined by CIE (Commission Internationale de l'Eclairage).

Then, the equivalent perceptual lightness conversion table generation unit 30 performs u′v ′ color coordinate value (u ′, v ′) = (iu ′) of the input color according to the following formulas 2-1 and 2-2 defined by the CIE. , Iv ′).

iu ′ = 4 × iX / (iX + 15 × iY + 3 × iZ) (Formula 2-1)
iv ′ = 9 × iY / (iX + 15 × iY + 3 × iZ) (Formula 2-2)

Next, the equivalent perceptual lightness conversion table generation unit 30 calculates the u′v ′ color coordinate values (Wu ′, Wv ′) of the white point according to the following expressions 3-1 and 3-2 defined by the CIE. .

Wu ′ = 4 × WX / (WX + 15 × WY + 3 × WZ) (Formula 3-1)
Wv ′ = 9 × WY / (WX + 15 × WY + 3 × WZ) (Formula 3-2)

Then, the equivalent perceptual lightness conversion table generation unit 30 calculates L * u * v * values (L0, U0, V0) of the input color according to the following equations 4-1 to 4-3 defined by the CIE.

L0 = 116 × (iY / WY) ^1/3 −16 (Formula 4-1)
U0 = 13 × L0 × (iu′−Wu ′) (Formula 4-2)
V0 = 13 × L0 × (iv′−Wv ′) (Formula 4-3)

  Returning to the description of FIG. Following S302, in S303, the equivalent perceptual lightness conversion table generating unit 30 initializes the reduction coefficient R to 1.0. The reduction coefficient R is a coefficient (ratio) for reducing the saturation. The reduction coefficient R = 1.0 is a reduction rate that does not reduce the saturation. The reduction coefficient R is a value of 1.0 or less. The smaller the reduction coefficient R, the higher the reduction rate when reducing the saturation.

Next, in S304, the equivalent perceptual lightness conversion table generation unit 30 acquires corrected saturation by reducing the saturation of the input color (first correction processing). The corrected saturation is the saturation of the input color after reduction. Specifically, the equivalent perceptual lightness conversion table generation unit 30 performs L * of the color (first correction color) with reduced saturation of the input color according to the following formulas 5-1 and 5-2 defined by the CIE. u * v * values (L0, U, V) are calculated. When the reduction coefficient R = 1.0, the first correction color matches the input color.

U = U0 × R (Formula 5-1)
V = V0 × R (Formula 5-2)

  In S305 to S306, based on the hue of the input color, the saturation of the input color before the reduction, the corrected saturation, and the reference display luminance 21, the perceived lightness is reduced due to the reduction of the saturation of the input color. The brightness of the input color is increased (second correction process). Thereby, the corrected lightness is acquired. The corrected lightness is the lightness of the input color after the increase. Here, consider a case where the reference display luminance 21 is a fixed value. In such a case, the second correction process is “reduction in perceived lightness due to the reduction of the saturation of the input color based on the hue of the input color, the saturation of the input color before the reduction, and the corrected saturation. It can be said that “the corrected lightness is obtained by increasing the lightness of the input color so as to be reduced (suppressed)”.

  In S305, the equivalent perceptual lightness conversion table generation unit 30 calculates the HK effect value G of the first correction color from the reference display luminance 21 and the L * u * v * values (L0, U, V) of the first correction color. Is calculated. The HK effect value G indicates the magnitude of reduction in perceived lightness due to the HK effect.

  The process of S305 will be described in detail.

First, the equivalent perceptual lightness conversion table generation unit 30 calculates the hue angle θ using the following Expression 6. The hue angle θ is the hue angle of the first correction color and the hue angle of the input color.

θ = tan ⁻¹ (V / U) (Formula 6)

Next, the equivalent perceptual lightness conversion table generation unit 30 calculates the HK effect factor Q corresponding to the hue angle θ using the following Expression 7.

Q = −0.01585-0.03017 cos θ−0.04556 cos 2θ
−0.02667 cos 3θ−0.00295 cos 4θ
+0.14592 sin θ + 0.05082 sin 2θ
-0.01900sin3θ-0.00764sin4θ
... (Formula 7)

Then, the equivalent perceptual lightness conversion table generation unit 30 calculates the HK effect factor K corresponding to the reference display luminance 21 using the following Expression 8. In the present embodiment, the luminance constant La in Expression 8 is the reference display luminance 21. Originally, the observer's adaptive luminance should be used as the luminance constant La used when calculating the HK effect factor K. However, the image display device cannot accurately measure the observer's adaptation brightness. Therefore, in this embodiment, the reference display luminance 21 is used instead of the adaptation luminance. On the screen of the image display device, 100% white is displayed with the reference display luminance 21. If it is assumed that the viewer is gazing at only the screen, the luminance of 100% white displayed on the screen may be considered as the adaptation luminance of the viewer.

K = 0.2717 * ((6.649 + 6.362 * ^La0.4995 ) / (6.649 + ^La0.4995 ))
... (Formula 8)

Next, the equivalent perceptual lightness conversion table generation unit 30 calculates the HK effect factor S corresponding to the saturation of the first correction color using the following Expression 9.

S = (U ² + V ² ) ^1/2 / L0 (formula 9)

Then, the equivalent perceptual lightness conversion table generation unit 30 calculates the HK effect value G of the first correction color using the following Expression 10.

G = 0.4462 × (1 + (− 0.8660 × Q + 0.0872 × K) × S
+0.3086) ³
... (Formula 10)

  In S3051, the equivalent perceptual lightness conversion table generation unit 30 determines whether or not the reduction coefficient R is 1.0. When the reduction coefficient R = 1.0, the HK effect value G of the input color is obtained in S305. If the reduction coefficient R = 1.0, the process proceeds to S3052, and if the reduction coefficient R ≠ 1.0, the process proceeds to S306.

  In S3052, the equivalent perceptual lightness conversion table generation unit 30 stores the HK effect value G obtained by the process in S305 as the initial effect value G0. Thereafter, the process proceeds to S306.

  In S306, the equivalent perceptual lightness conversion table generation unit 30 calculates the corrected lightness L using the HK effect value. Then, the equivalent perceptual lightness conversion table generation unit 30 determines a color having the hue of the input color, the corrected saturation, and the corrected lightness (second corrected color) as an output color obtained by converting the input color. Specifically, the equivalent perceptual lightness conversion table generation unit 30 determines the L * u * v * value (L, U, V) as the L * u * v * value of the output color.

  The process of S306 will be described in detail.

First, the equivalent perceptual lightness conversion table generation unit 30 calculates the HK effect ratio GR using the following Expression 11. The HK effect ratio GR is the ratio of the perceived lightness of the first correction color to the perceived lightness of the input color.

GR = G / G0 (Formula 11)

Next, the equivalent perceptual lightness conversion table generation unit 30 calculates the corrected lightness L using the following Expression 12.

L = L0 / GR (Formula 12)

According to the above method, as the difference between the input color lightness L0 before the increase and the corrected lightness L (the increase amount of the input color lightness), the difference between the saturation of the input color before the decrease and the corrected saturation is The larger the value, the larger the value. Further, as the difference between the lightness L0 and the corrected lightness L, a larger value is obtained as the reference display luminance 21 is larger.

  In this embodiment, the correction saturation and the correction lightness are updated so that a color within a color gamut narrower than the color gamut of the input color (the predetermined color gamut described above) is determined as the output color. In the present embodiment, the corrected saturation and the corrected lightness are updated so that a color (displayable color) that can be displayed on the display unit 50 is determined as an output color. Specifically, the lightness of the input color is repeatedly increased while the saturation of the input color is gradually reduced until the displayable color is determined as the output color by the processing of S304 to S309.

  In addition, the acquisition method of correction | amendment saturation and correction | amendment brightness is not restricted to the said method. For example, saturation reduction may be performed a predetermined number of times. The saturation reduction may be performed only once or a plurality of times.

  Returning to the description of FIG. Next to S306, in S307, the equivalent perceptual lightness conversion table generation unit 30 refers to the LUV conversion table 11 and converts the L * u * v * values (L, U, V) obtained in S306 into RGB values. Convert to (oR, oG, oB). Specifically, an RGB value associated with the same L * u * v * value as the L * u * v * value (L, U, V) is converted into an RGB value (oR, oG, oB) as an LUV conversion table. 11 is obtained. At this time, the equivalent perceptual lightness conversion table generation unit 30 determines that the LUV conversion table 11 has an inappropriate color associated with the same L * u * v * value as the L * u * v * value (L, U, V). The flag F is acquired from the LUV conversion table 11. Here, the inappropriate color flag F = 0 means that the color of the L * u * v * value associated with the inappropriate color flag F is a displayable color, and the inappropriate color flag F = 1 This means that the color of the L * u * v * value associated with the inappropriate color flag F is an undisplayable color. The non-displayable color is a color that cannot be displayed on the display unit 50.

  Next, in S308, the equivalent perceptual lightness conversion table generation unit 30 determines whether or not the color of the L * u * v * value (L, U, V) is a displayable color. Specifically, it is determined whether the value of the inappropriate color flag F acquired in S307 is 0 or 1. If the inappropriate color flag F = 0, it is determined that the color of the L * u * v * value (L, U, V) is a displayable color, and the process proceeds to S310. If the inappropriate color flag F = 1, it is determined that the color of the L * u * v * value (L, U, V) is not a displayable color, and the process proceeds to S309.

In S <b> 309, the equivalent perceptual lightness conversion table generation unit 30 reduces the reduction coefficient R using Expression 13 below. That is, the reduction rate when reducing the saturation is increased. In Expression 13, R1 is a reduction coefficient before reduction, R2 is a reduction coefficient after reduction, and dR is an offset value for reducing the reduction coefficient R. In this embodiment, 0.01 is used as the offset value dR.

R2 = R1-dR (Formula 13)

Any value that is greater than 0 and less than 1 may be used as the offset value dR. As the offset value dR is smaller, the saturation can be reduced with higher accuracy. Therefore, the output color can be determined with higher accuracy as the offset value dR is smaller. However, the decrease in the offset value dR increases the processing load (the number of repetitions of the processes in S304 to S309). On the other hand, the output color can be determined with a smaller processing load as the offset value dR is larger. However, an increase in the offset value dR causes overcorrection that reduces the saturation more than necessary. That is, an increase in the offset value dR causes a decrease in the accuracy of the process for reducing the saturation, and a decrease in the accuracy of the process for determining the output color. Therefore, it is preferable to determine the offset value dR in consideration of processing accuracy and processing load. The offset value dR may be a predetermined fixed value or a value that can be changed by the user. The offset value dR may be automatically determined according to the type of input image data, the operation mode of the image display device 100, the usage environment of the image display device 100, and the like.

  In S310, the equivalent perceptual lightness conversion table generation unit 30 generates the RGB values (iR, iG, iB) of the input color selected in S301, and the RGB values (oR, oG, oB) of the output color obtained in S307. Are associated with each other and written to the equivalent perceptual lightness conversion table 31.

  In S311, the equivalent perceptual lightness conversion table generation unit 30 determines whether or not the processing of S301 to S310 has been performed for all input colors. If there is an input color not selected in S301, the process returns to S301. And the process of S301-S311 is repeated until the process of S301-S310 is performed about all the input colors. When the processing of S301 to S310 is performed for all input colors, this flowchart is ended.

  An example of the equivalent perceptual lightness conversion table 31 generated by the flowchart of FIG. 3 is shown in FIG. In the example of FIG. 4, the equivalent perceptual lightness conversion table 31 is a 3DLUT indicating a correspondence relationship between RGB values (iR, iG, iB) of input colors and RGB values (oR, oG, oB) of output colors. The correspondence between the RGB values (iR, iG, iB) and the RGB values (oR, oG, oB) is a one-to-one correspondence. Therefore, by using the equivalent perceptual lightness conversion table 31, the RGB values (iR, iG, iB) can be converted into RGB values (oR, oG, oB).

  The configuration of the equivalent perceptual lightness conversion table 31 is not limited to the above configuration. For example, a table indicating the correspondence between the L * u * v * value of the input color and the L * u * v * value of the output color may be used as the equivalent perceptual lightness conversion table 31. A table indicating the correspondence between the RGB value of the input color and the L * u * v * value of the output color may be used as the equivalent perceptual lightness conversion table 31. A table indicating the correspondence between the L * u * v * values of the input colors and the RGB values of the output colors may be used as the equivalent perceptual lightness conversion table 31. The color conversion information is not limited to the equivalent perceptual lightness conversion table 31. For example, a function indicating the correspondence between the input color and the output color may be used as the color conversion information.

  Next, a specific example of the operation of the image display apparatus 100 will be described. 5A to 5C are diagrams for explaining a specific example of the operation of the image display apparatus 100. FIG. FIG. 5A shows the hue plane of the L * u * v * color space. The vertical axis in FIG. 5A indicates the lightness, and the horizontal axis in FIG. 5A indicates the saturation. A region surrounded by a thick solid line is a color gamut (color gamut limit) of displayable colors. Pixel values can be plotted on any of the hue planes.

  First, consider a case where an input pixel value (pixel value of input image data 1) is plotted at point A in FIG. The color of point A is a displayable color. Therefore, in the image display device 100, the same pixel value as the input pixel value is used as the display pixel value (the pixel value of the display image data 41) without changing the color of the input pixel value. As a result, the same color as the input pixel value is displayed.

  Next, consider a case where the input pixel value is plotted at point B in FIG. The color of point B is not a displayable color. Therefore, the input pixel value is converted into the display pixel value of the displayable color. In the conventional method, the saturation is reduced while maintaining the lightness. The display pixel value obtained by such a conventional method is plotted at point C in FIG. However, due to the HK effect, the color at point C is perceived as a darker color than the color at point B.

In this embodiment, the saturation is reduced in consideration of the HK effect. Specifically, the saturation is reduced along the equal perceptual lightness line (equivalent color perceptual lightness line) in FIG. The iso-perceptual lightness line indicates a plurality of colors having the same perceived lightness. As a result, in this embodiment, the pixel value plotted at the point D in FIG. 5A is obtained as the display pixel value. The brightness at point D is higher than the brightness at point B. However, due to the HK effect, the color at point D is perceived with the same brightness as the color at point B. Thus, according to the present embodiment, it is possible to convert colors while maintaining the perceived lightness of each pixel. As a result, an image can be displayed without breaking the balance of perceived lightness between pixels.

  As described above, the amount of the HK effect varies depending on the brightness, hue, saturation, and reference display luminance. Specifically, the higher the lightness, the smaller the HK effect, the smaller the HK effect in the hue near yellow, the larger the HK effect in reddish purple to blue-green hues, and the higher the saturation, the larger the HK effect and the reference display brightness. The higher the value, the greater the HK effect. For example, as shown in FIG. 5B, the HK effect amount G increases as the reference display luminance La increases.

  For this reason, as shown in FIG. 5C, the iso-perceptual brightness line varies depending on the size of the reference display brightness, and the brightness correction result changes depending on the reference display brightness La. That is, even when the input pixel values that are not displayable colors are the same (point B), the reference display brightness is higher than the brightness of the corrected color (point D) when the reference display brightness is low. The brightness of the corrected color (D ′ point) becomes high.

  As described above, according to the present embodiment, not only the saturation of the input color is reduced, but also the brightness of the input color is increased. Thereby, the color can be converted while maintaining the perceived lightness with high accuracy.

  In the present embodiment, an example in which brightness and saturation are corrected using L * u * v * values has been described. However, values in a color space different from the L * u * v * color space are used. Also good. For example, L * a * b * color space values may be used.

  As a model for the HK effect, a simpler model than the model used in the present embodiment may be used. For example, as a calculation formula regarding the HK effect, a calculation formula simpler than the calculation formula described above may be used.

  In the example of FIG. 5A, the non-displayable color is converted into the boundary color between the non-displayable color and the displayable color. However, the converted color may not be the boundary color. For example, even if the non-displayable color is converted to the displayable color so that the saturation of the output color smoothly decreases from the saturation of the boundary as the saturation of the input color that is an undisplayable color decreases. Good.

  Note that instead of selecting each color within a predetermined color gamut as an input color, the color of each pixel of the input image data is selected as the input color, and the output color corresponding to the selected input color is determined by the method described above. Good. Then, the color of each pixel of the input image data may be converted into the determined output color. When the color conversion device is a separate device from the image processing device, the image processing device only needs to have a storage unit that stores color conversion information. As the storage unit, a magnetic disk, a semiconductor memory, an optical disk, or the like can be used.

  In this embodiment, the example in which the saturation of the input color is reduced and the lightness of the input color is increased in that order is not limited to this. For example, the brightness of the input color may be increased prior to the reduction of the saturation of the input color. The reduction of the saturation of the input color and the increase of the brightness of the input color may be performed at a time. Specifically, information (table or function) indicating the equivalent perceptual lightness line in FIG. 5A may be prepared in advance, and the corrected saturation and the corrected lightness may be determined simultaneously based on the information. It is only necessary to reduce the saturation of the input color and increase the lightness of the input color so that the perceived lightness of the input color is maintained.

<Example 2>
Embodiment 2 of the present invention will be described below. In the first embodiment, the example in which the equivalent perceptual lightness conversion table 31 is generated every time the reference display luminance 21 is changed has been described. In this embodiment, an example in which an equivalent perceptual lightness conversion table is prepared in advance will be described. In the following, configurations and processes different from those in the first embodiment will be described in detail, and descriptions of the same configurations and processes as those in the first embodiment will be omitted.

  FIG. 6 is a block diagram illustrating an example of the configuration of the image display apparatus 400 according to the present embodiment. In FIG. 6, the same functional parts as those in the first embodiment (FIG. 1) are denoted by the same reference numerals as those in the first embodiment. The image display apparatus 400 includes a table holding unit A310, a table holding unit B320, a table holding unit C330, a table selection unit 60, a display luminance setting unit 20, a color conversion unit 40, and a display unit 50.

  The table holding unit A310, the table holding unit B320, and the table holding unit C330 store a plurality of equivalent perceptual lightness conversion tables respectively corresponding to a plurality of reference display luminances. The equivalent perceptual lightness conversion table is generated in advance by the same method as in the first embodiment. As each of the table holding unit A310, the table holding unit B320, and the table holding unit C330, for example, a semiconductor memory, a magnetic disk, an optical disk, or the like can be used. In this embodiment, ROM is used as each of the table holding unit A310, the table holding unit B320, and the table holding unit C330.

  In the table holding unit A310, an equivalent perceptual lightness conversion table 311 corresponding to the reference display luminance 21 = 100 is recorded in advance. In the table holding unit B320, an equivalent perceptual lightness conversion table 321 corresponding to the reference display luminance 21 = 200 is recorded in advance. In the table holding unit C330, an equivalent perceptual lightness conversion table 331 corresponding to the reference display luminance 21 = 300 is recorded in advance.

  The table selection unit 60 selects any one of the equal perceptual lightness conversion table 311, the equal perceptual lightness conversion table 321, and the equal perceptual lightness conversion table 331 according to the reference display brightness 21 output from the display brightness setting unit 20. To do. Specifically, when the reference display brightness 21 = 100 is output from the display brightness setting unit 20, the equivalent perceptual lightness conversion table 311 is selected. When the reference display brightness 21 = 200 is output from the display brightness setting unit 20, the equivalent perceptual lightness conversion table 321 is selected. When the reference display brightness 21 = 300 is output from the display brightness setting unit 20, the equivalent perceptual lightness conversion table 331 is selected. Then, the table selection unit 60 outputs the selected equivalent perceptual lightness conversion table 61 to the color conversion unit 40. The color conversion unit 40 performs color conversion processing using the equivalent perceptual lightness conversion table 61.

  When the reference display brightness 21 output from the display brightness setting unit 20 is not any of 100, 200, and 300, the table selection unit 60 uses the equivalent perceptual lightness conversion tables 311, 321, and 331. Perform interpolation processing. As a result, the equivalent perceptual lightness conversion table corresponding to the reference display brightness 21 output from the display brightness setting unit 20 is generated as the equal perceptual lightness conversion table 61.

  As described above, according to the present embodiment, the equivalent perceptual lightness conversion table is prepared in advance. Then, color conversion processing is performed using an equivalent perceptual lightness conversion table prepared in advance. Accordingly, it is possible to convert the color while maintaining the perceptual lightness with high accuracy, and it is possible to reduce the processing load on the image display device and the image processing device.

Note that the interpolation process may not be performed. For example, even when an equivalent perceptual lightness conversion table corresponding to the reference display brightness output from the display brightness setting unit is not prepared in advance, one of a plurality of prepared equal perceptual lightness conversion tables may be selected. . Specifically, even if one equivalent perceptual lightness conversion table corresponding to the reference display brightness closest to the reference display brightness output from the display brightness setting unit is selected from a plurality of previously prepared equal perceptual lightness conversion tables. Good. Then, color conversion processing using the selected equivalent perceptual lightness conversion table may be performed. According to such a configuration, it is possible to perform color conversion processing that roughly considers the HK effect. Therefore, it is possible to convert colors while maintaining the perceived lightness with high accuracy. Furthermore, the processing load on the image display device and the image processing device can be further reduced.

  Note that the number of equivalent perceptual lightness conversion tables prepared in advance may be one. Then, the color conversion process using the equivalent perceptual lightness conversion table prepared in advance may be always performed regardless of the reference display luminance 21 output from the display luminance setting unit 20. According to such a configuration, the color can be converted while maintaining the perceptual lightness with high accuracy, and the processing load on the image display device and the image processing device can be further reduced. Furthermore, by reducing the number of isoperceptual lightness conversion tables generated in advance, it is possible to reduce the processing load when the color conversion device generates the isoperceptual lightness conversion tables in advance.

<Example 3>
Embodiment 3 of the present invention will be described below. In the first embodiment, the example of generating the perceptual lightness conversion table 31 that corrects both the color gamut and the lightness has been described. In the present embodiment, an example will be described in which color gamut correction and brightness correction are performed individually. In the following, configurations and processes different from those in the first embodiment will be described in detail, and descriptions of the same configurations and processes as those in the first embodiment will be omitted.

  FIG. 7 is a block diagram illustrating an example of the configuration of the image display apparatus 500 according to the present embodiment. 7, the same reference numerals as those in the first embodiment are assigned to the same functional units as those in the first embodiment (FIG. 1). The image display apparatus 500 includes a correction coefficient determination unit 70, a luminance correction unit 80, an RGB table holding unit 90, a display luminance setting unit 20, a color conversion unit 40, and a display unit 50.

  The RGB table holding unit 90 is a storage unit that holds the RGB table 91. As the RGB table holding unit 90, for example, a semiconductor memory, a magnetic disk, an optical disk, or the like can be used. In this embodiment, a ROM is used as the RGB table holding unit 90. The RGB table 91 shows the correspondence between the RGB values (iR, iG, iB) of the input color and the RGB values (m1R, m1G, m1B) of the color (first correction color) with reduced saturation of the input color. 3DLUT. Therefore, the corrected saturation can be acquired by using the RGB table 91. Specifically, the RGB value (iR, iG, iB) of the input color can be converted into the RGB value (m1R, m1G, m1B) of the first correction color. Instead of the RGB table 91, a function indicating a correspondence relationship between the RGB values (iR, iG, iB) of the input color and the RGB values (m1R, m1G, m1B) of the first correction color may be used. In this embodiment, the first correction color is a color in a color gamut narrower than the color gamut of the input color. Specifically, the first correction color is an RGB value of a displayable color.

  The color conversion unit 40 generates display image data 41 from the input image data 1 by performing color conversion processing using the RGB table 91 instead of the equivalent perceptual lightness conversion table 31. The display image data 41 of the present embodiment is different from the display image data 41 of the first embodiment. Specifically, the color of the display image data 41 of the first embodiment is the second correction color (the color in which saturation and lightness are corrected), whereas the color of the display image data 41 of the present embodiment is the first color. It is a correction color (a color in which only saturation is corrected).

The correction coefficient determination unit 70 determines the lightness correction coefficient 71 based on the input image data 1, the display image data 41, and the reference display luminance 21. First, the correction coefficient determination unit 70 performs an initial effect from the RGB values (iR, iG, iB) of the input image data 1 by performing the same processing as the processing of S302 and S305 of the first embodiment (FIG. 3). The value G0 is calculated. Next, the correction coefficient determination unit 70 calculates the HK effect value G from the RGB values (m1R, m1G, m1B) of the display image data 41 by performing the same processing as the processing of S302 and S305 of FIG. To do. Then, the correction coefficient determination unit 70 calculates the HK effect ratio GR by dividing the HK effect value G by the initial effect value G0. The correction coefficient determination unit 70 outputs the calculated HK effect ratio GR as the brightness correction coefficient 71. These processes are performed for each pixel of the input image data 1 (display image data 41).

The brightness correction unit 80 acquires the corrected brightness based on the display image data 41 and the brightness correction coefficient 71. Then, the luminance correction unit 80 converts the hue of the input color (the hue of the input image data 1 and the display image data 41), the corrected saturation (the saturation of the display image data 41), and the color having the acquired corrected brightness. Determine as output color. Specifically, the luminance correction unit 80 converts the RGB values (m1R, m1G, m1B) of the display image data 41 into the RGB values (m2R, m2G) of the output color using the following equations 14-1 to 14-3. , M2B). This process is performed for each pixel of the display image data 41. Thereby, corrected display image data 81 is generated. The luminance correction unit 80 outputs the generated corrected display image data 81 to the display unit 50.

m2R = m1R × GR (Formula 14-1)
m2G = m1G × GR (Formula 14-2)
m2B = m1B × GR (Formula 14-3)

  Next, a specific example of the operation of the image display apparatus 500 will be described. FIG. 8 is a diagram for explaining a specific example of the operation of the image display apparatus 500. FIG. 8 shows the hue plane of the L * u * v * color space. The vertical axis in FIG. 8 indicates lightness, and the horizontal axis in FIG. 8 indicates saturation. A region surrounded by a thick solid line is a color gamut of displayable colors. Pixel values can be plotted on any of the hue planes. Hereinafter, a case where the input pixel value is plotted at the point B in FIG. 8 will be described.

  First, the color conversion unit 40 reduces the saturation of the input pixel value while maintaining the brightness of the input pixel value. Thereby, the input pixel value is converted into a pixel value of a displayable color. Specifically, the input pixel value at point B is converted to the pixel value at point C. The pixel value at point C is output from the color conversion unit 40 as the pixel value of the display image data 41.

  Next, the correction coefficient determination unit 70 calculates the ratio of the perceived lightness of point C to the perceived lightness of point B as the lightness correction coefficient 71.

  Then, the brightness correction unit 80 increases the brightness of the point C using the brightness correction coefficient 71. Specifically, the pixel value at point C is converted to the pixel value at point D. The pixel value at the point D is output from the luminance correction unit 80 as the pixel value of the corrected display image data 81.

  As described above, according to the present embodiment, not only the saturation of the input color is reduced, but also the brightness of the input color is increased. Thereby, the color can be converted while maintaining the perceived lightness with high accuracy. Furthermore, according to the present embodiment, the output color is determined by a simple method of “increasing the brightness of the input color after reducing the saturation of the input color so that the input color is converted into a displayable color”. . Thereby, the repetition of the processing of S304 to S309 in FIG. 3 can be omitted, and the output color can be determined with a smaller processing load than in the first embodiment.

<Example 4>
Embodiment 4 of the present invention will be described below. In the first embodiment, the example in which the reference display brightness set by the display brightness setting unit is used as the upper limit perceptual lightness (assumed upper limit value of perceptual lightness) has been described. In order to display an image with a wider luminance dynamic range, luminance information that is information about the luminance of each pixel may be used. In this embodiment, an example in which the upper limit perceived lightness is determined from the reference display brightness and the brightness information will be described. In the following, configurations and processes different from those in the first embodiment will be described in detail, and descriptions of the same configurations and processes as those in the first embodiment will be omitted.

  FIG. 9 is a block diagram illustrating an example of the configuration of the image display apparatus 600 according to the present embodiment. In FIG. 9, the same reference numerals as those in the third embodiment are assigned to the same functional units as those in the third embodiment (FIG. 7). The image display device 500 includes an HDR display unit 55, a correction coefficient determination unit 70, a luminance correction unit 85, an RGB table holding unit 90, a display luminance setting unit 20, and a color conversion unit 40.

  The input luminance information 2 is luminance information input to the image display device 500 via an input terminal (not shown). The input luminance information 2 is information regarding the luminance of each pixel of the input image data 1. Therefore, it can be said that the input luminance information 2 is “information about the brightness of each pixel of the input image data 1”. A pixel whose value of the input luminance information 2 is 10 is a pixel to be displayed with a luminance 10 times the luminance corresponding to the input pixel value (the pixel value of the input image data 1). The terminal to which the input image data 1 is input may be the same as the terminal to which the input luminance information 2 is input, or may be different from the terminal to which the input luminance information 2 is input. The input luminance information 2 may be added to the input image data 1 as metadata.

The correction coefficient determination unit 70 determines the lightness correction coefficient 71 based on the input image data 1, the input luminance information 2, the display image data 41, and the reference display luminance 21. The brightness correction coefficient 71 is determined by the same method as that of the third embodiment. However, in the present embodiment, the HK effect factor K is calculated using the following Expression 15. In Expression 15, Yg is the value of the input luminance information 2. That is, in this embodiment, La × Yg is used as the upper limit perceived lightness.

K = 0.2717 * ((6.649 + 6.362 * (La * Yg) ^0.4995 ) / (6.649+ (La * Yg) ^0.4995 )) ... (Formula 15)

  The brightness correction unit 85 acquires the corrected brightness based on the input brightness information 2 and the brightness correction coefficient 71. Then, the hue of the input color (the hue of the input image data 1 and the display image data 41), the corrected saturation (the saturation of the display image data 41), and the acquired corrected lightness by the color conversion unit 40 and the luminance correction unit 85. Is determined as the output color. Specifically, the luminance correction unit 85 calculates the value of the corrected luminance information 82 by multiplying the value of the input luminance information 2 by the brightness correction coefficient 71. This process is performed for each pixel of the display image data 41. As a result, information regarding the corrected brightness of each pixel is generated as corrected luminance information 82. The output color is expressed by the corrected luminance information 82 and the display image data 41. The luminance correction unit 85 outputs the generated corrected luminance information 82 to the HDR display unit 55. As in the third embodiment, the display image data 41 may be corrected using the brightness correction coefficient 71.

The HDR display unit 55 displays an image on the screen based on the reference display brightness 21, the corrected brightness information 82, and the display image data 41. In this embodiment, the HDR display unit 55 is a transmissive display unit having a backlight module and a liquid crystal panel. The backlight module irradiates light on the back surface of the liquid crystal panel. The liquid crystal panel forms a transmittance pattern according to the image data input to the HDR display unit 55. The light emission luminance in each of the plurality of regions of the backlight module can be individually controlled. The backlight module emits light with light emission luminance (light emission intensity) based on the reference display luminance 21 and the corrected luminance information 82. Thereby, the luminance (display luminance) in the screen area in which the pixel value of the display image data 41 is a pixel value of 100% white and the value of the corrected luminance information 82 is 1 is designated by the reference display luminance 21. It almost matches the brightness. Further, the luminance in the screen area in which the pixel value of the display image data 41 is a pixel value of 100% white and the value of the corrected luminance information 82 is 10 is 10 times the luminance specified by the reference display luminance 21. It almost matches the brightness.

  As described above, according to the present embodiment, in the configuration in which luminance information is used, the upper limit perceived lightness is acquired in consideration of the luminance information. Then, using such upper limit perceived lightness, not only the saturation of the input color is reduced, but also the lightness of the input color is increased. Thereby, in a configuration in which luminance information is used, it is possible to convert colors while maintaining the perceived lightness with high accuracy.

<Example 5>
Embodiment 5 of the present invention will be described below. The upper limit value of the display brightness may be limited for reasons such as preventing a failure of the display unit. In this embodiment, an example will be described in which the upper limit perceived lightness is determined from the reference display brightness set by the display brightness setting unit and the limit upper limit value (the limit value of the limited display brightness). In the following, configurations and processes different from those in the first embodiment will be described in detail, and descriptions of the same configurations and processes as those in the first embodiment will be omitted.

  FIG. 10 is a block diagram illustrating an example of the configuration of the image display apparatus 700 according to the present embodiment. 10, the same reference numerals as those in the first embodiment are assigned to the same functional units as those in the first embodiment (FIG. 1). The image display device 700 includes a table holding unit 10, a display luminance setting unit 20, an equivalent perceptual lightness conversion table generation unit 30, a color conversion unit 40, and a display unit 50.

The display unit 50 performs the same processing as the display unit 50 of the first embodiment. However, the display unit 50 of this embodiment limits the display luminance based on information such as the detection value of the current sensor and the detection value of the thermal sensor. For example, even if the value of the reference display brightness 21 is 1000 [cd / m ² ], if the internal temperature of the image display device 700 rises to a very high temperature due to continuous display for a long time, the reference display brightness The value of 21 is limited to 1000 [cd / m ² ] to 800 [cd / m ² ]. As a result, the display brightness is reduced. Then, the display unit 50 outputs the currently used reference display brightness as the actual display brightness 51. When the display brightness is not limited, the same value as the reference display brightness 21 output from the display brightness setting unit 20 is output as the actual display brightness 51. When the display brightness is limited, a value smaller than the reference display brightness 21 output from the display brightness setting unit 20 is output as the actual display brightness 51. In other words, when the display brightness is limited, the limit upper limit value is output as the actual display brightness 51.

  The equivalent perceptual lightness conversion table generation unit 30 performs the same processing as the equivalent perceptual lightness conversion table generation unit 30 of the first embodiment. However, in the equivalent perceptual lightness conversion table generation unit 30 of this embodiment, the actual display luminance 51 output from the display unit 50 is used instead of the reference display luminance 21 output from the display luminance setting unit 20. Further, the equal perceptual lightness conversion table generation unit 30 of the present embodiment generates (updates) the equal perceptual lightness conversion table 31 every time the actual display luminance 51 is changed.

  The present invention can also be applied to the case of performing display luminance control for limiting the maximum power consumption of the image display device, as in the case of thermal limitation. For example, an average image level (APL: average picture level) of the input image data 1 is calculated, and when the APL exceeds a predetermined threshold, the reference display luminance according to the degree (difference between the APL and the predetermined threshold) The actual display brightness 51 is calculated with 21 being limited. Other configurations and operations are the same as those of the fifth embodiment described above. In this way, even when the display screen is darkened by automatically performing display luminance control according to the luminance feature amount of the input image data 1, the perceptual brightness balance in the screen is maintained. The image can be displayed.

As described above, according to the present embodiment, in the configuration in which the upper limit value of display luminance is limited, the reference display luminance is acquired in consideration of the upper limit limit value. Then, using such reference display brightness, not only the saturation of the input color is reduced, but also the brightness of the input color is increased. Thereby, in the configuration in which the upper limit value of the display luminance is limited, it is possible to convert the color while maintaining the perceived lightness with high accuracy.

<Example 6>
Embodiment 6 of the present invention will be described below. In this embodiment, an example in which the processing described in the other embodiments is realized by software will be described.

  FIG. 11 is a block diagram illustrating an example of the configuration of a computer 800 that executes software according to the present embodiment. The computer 800 includes a processor 601 and a storage unit 602. An external storage device 603 is connected to the computer 800.

  The storage unit 602 stores various data. As the storage unit 602, a semiconductor memory, a magnetic disk, an optical disk, or the like can be used. In this embodiment, a software program for realizing the processing described in the other embodiments is recorded in the storage unit 602. The storage unit 602 is also used as a work memory that temporarily stores data used in the processing of the processor 601.

  The processor 601 performs control of each functional unit included in the computer 800, control of each device connected to the computer 800, various information processing, and the like. For example, the processor 601 reads a program from the storage unit 602 and executes the read program. Thereby, the processing described in the other embodiments is realized.

  The external storage device 603 stores various data. As the external storage device 603, a semiconductor memory, a magnetic disk, an optical disk, or the like can be used. In this embodiment, the external storage device 603 stores an input image file 604 that is input image data in a file format. Further, the processor 601 records an output image file 605 that is output image data in a file format in the external storage device 603. The input image data is image data before color conversion processing, and the output image data is image data after color conversion processing. The external storage device 603 is detachable from the computer 800. Instead of the external storage device 603, a storage unit built in the computer 800 may be used.

  Next, an example of a software processing flow according to the present embodiment will be described. FIG. 12 is a flowchart illustrating an example of a processing flow of software according to the present embodiment.

  First, in S <b> 600, the processor 601 reads the input image file 604 from the external storage device 603, and stores the read input image file 604 data (input image data) in the storage unit 602.

  Next, in S601, the processor 601 starts loop processing. The loop process of S601 is a repetition of the process of selecting the pixel color of the input image data acquired in S600 as the input color. The color of each pixel of the input image data is sequentially selected as the input color by the loop processing of S601. In this embodiment, in S601, each pixel value of the input image data is sequentially selected as a pixel value (iR, iG, iB) of the input color.

  The processing from S602 to S609 is the same as the processing from S302 to S309 in the first embodiment (FIG. 3). The reference display brightness used in S602 to S609 may be a value input to the computer 800 by the user, a predetermined fixed value, or automatically determined by the computer 800. It may be a value.

  In S610, the processor 601 stores the pixel value (oR, oG, oB) obtained in S607 as the pixel value of the output image data at the same position as the pixel value (iR, iG, iB) selected in S601. The data is stored in the unit 602.

  Next, in S611, the processor 601 determines whether or not the processing of S601 to S610 has been performed for all the pixel values of the input image data. If there is a pixel value not selected in S601, the process returns to S601. And the process of S601-S611 is repeated until the process of S601-S610 is performed about all the pixel values. When the processing of S601 to S610 is performed for all pixel values, the processing proceeds to S612.

  In S612, the processor 601 reads out each pixel value of the output image data from the storage unit 602, generates an image file (output image file 605) based on the read pixel value, and generates the generated output image file 605 as an external storage device. Write to 603.

  As described above, according to the present embodiment, processing similar to that of the other embodiments is performed by software. Thereby, the same effects as those of the other embodiments can be obtained by software.

<Example 7>
Hereinafter, Example 7 of the present invention will be described. In the first embodiment, the example in which the lightness of the input color is increased while reducing the saturation of the input color has been described. In the present embodiment, an example will be described in which the saturation of the input color is reduced after the brightness of the input color is converted into a perceived brightness that takes into account the HK effect. In the following, configurations and processes different from those in the first embodiment will be described in detail, and descriptions of the same configurations and processes as those in the first embodiment will be omitted.

  The configuration of the image display apparatus according to the present embodiment is the same as that of the first embodiment. However, the operation of the equivalent perceptual lightness conversion table generation unit 30 according to the present embodiment is different from that of the first embodiment. An example of the operation of the equivalent perceptual lightness conversion table generation unit 30 according to the present embodiment will be described. FIG. 13 is a flowchart illustrating an example of the operation of the equivalent perceptual lightness conversion table generation unit 30 according to the present embodiment.

  The process of S701 is the same as the process of S301 of the first embodiment (FIG. 3), and the process of S702 is the same as the process of S302 of FIG.

Following S702, in S703, the equivalent perceptual lightness conversion table generating unit 30 calculates the HK effect value (initial effect value) G0 corresponding to the L * u * v * values (L0, U0, V0), and the lightness. L0 is converted into perceptual lightness L0hk considering the HK effect. Thereby, perceptual color coordinate values (L0hk, U0, V0) are obtained. The initial effect value G0 is calculated by the same method as in the first embodiment (the process of S305 in FIG. 3). The perceived lightness L0hk is calculated using the following Expression 16.

L0hk = L0 × G0 (Equation 16)

  In step S <b> 704, the equivalent perceptual lightness conversion table generation unit 30 initializes the reduction coefficient R to 1.0.

In step S <b> 705, the equivalent perceptual lightness conversion table generation unit 30 performs the perceptual color coordinate value (first correction color) of the color (first correction color) with reduced saturation according to the following equations 17-1 and 17-2. L0hk, U, V) is calculated.

U = U0 × R (Formula 17-1)
V = V0 × R (Formula 17-2)

In step S <b> 706, the equal perceptual lightness conversion table generation unit 30 calculates the HK effect value G corresponding to the perceptual color coordinate value (L0hk, U, V), and converts the perceptual lightness L0hk into the corrected lightness L. Thereby, the L * u * v * values (L, U, V) of the output color are obtained. The HK effect value G is calculated by the same method as in the first embodiment (the process of S305 in FIG. 3) using the perceived lightness L0hk as the lightness. The corrected lightness L is calculated using Equation 18 below.

L = L0hk / G (Formula 18)

  The processing of S707 to S711 is the same as the processing of S307 to S311 in FIG.

  Next, a specific example of the operation of the image display apparatus according to the present embodiment will be described. 14A and 14B are diagrams for describing a specific example of the operation of the image display apparatus according to the present embodiment. FIG. 14A shows the hue plane of the L * u * v * color space, and FIG. 14B shows the hue plane of the perceptual color space. In FIG. 14A, the vertical axis represents lightness, and in FIG. 14A, the horizontal axis represents saturation. The vertical axis in FIG. 14B indicates the perceived lightness, and the horizontal axis in FIG. 14B indicates the saturation. A region surrounded by a thick solid line is a color gamut of displayable colors. Pixel values can be plotted on any of the hue planes. Hereinafter, a case where the L * u * v * values (L0, U0, V0) of the input color are plotted at the point B in FIG.

  First, the lightness L0 of the input color is converted into the perceptual lightness L0hk, and the L * u * v * values (L0, U0, V0) are converted into the perceptual color coordinate values (L0hk, U0, V0). Specifically, the coordinate value of point B is converted to the coordinate value of point b. That is, point B in the L * u * v * color space is mapped to the perceptual color space.

  Next, the saturation of the perceptual color coordinate values (L0hk, U0, V0) is reduced, and the perceptual color coordinate values (L0hk, U0, V0) are converted into perceptual color coordinate values (L0hk, U, V). Specifically, the coordinate value of point b is converted to the coordinate value of point d.

  Then, the perceived lightness L0hk is converted into the corrected lightness L, and the perceptual color coordinate values (L0hk, U, V) are converted into L * u * v * values (L, U, V). Specifically, the coordinate value of point d is converted to the coordinate value of point D. That is, the point d in the perceptual color space is mapped to the L * u * v * color space.

  As described above, also in this embodiment, not only the saturation of the input color is reduced, but also the brightness of the input color is increased. Thereby, the color can be converted while maintaining the perceived lightness with high accuracy.

<Example 8>
An image display device (High Dynamic Range (HDR) display device) that greatly expands the dynamic range of luminance that can be handled as image data in order to faithfully reproduce (represent) a specular reflector or a self-luminous subject. There is. The present invention can also be applied to an HDR display device.

When displaying an image conforming to HDR (the above-described expanded dynamic range), the HDR display device can perform display with much higher luminance than a conventional display device. However, such very high luminance is limitedly used mainly for specular reflectors and self-luminous subjects. And the observer is not adapted to such very high brightness. Therefore, when the present invention is applied to an HDR display device, a more appropriate value should be used as the luminance constant La in calculating the HK effect factor K.

Here, consider a case where EOTF (Electro-Optical Transfer Function) of the input image data 1 is a ratio to the reference white. Specifically, consider a case where the EOTF of the input image data 1 is a standard signal defined in the range of 0% to 1000%. In this case, it is preferable that the luminance at which 100% reference white is displayed be the luminance constant La. For example, when the maximum value of the EOTF of the input image data 1 is 1000% and the image display device is set to display 1000% of pixels at 2000 [cd / m ² ], 200 [cd / M ² ] is the value of the luminance constant La. If the setting is such as to display similarly to 1000% of the pixels 500 cd / ^{m 2} is a 50 cd / ^{m 2} with the value of the luminance constant La.

Also, when displaying a high-luminance pixel by limiting the luminance to a predetermined luminance, it is preferable to determine the luminance constant La based on the same concept. For example, within the range of 0% to 1000%, to display the pixels from 0% to 400% in luminance of ^{0cd / m 2 ~400cd / m 2} , to restrict all 400% or more pixels to 400 cd / ^{m 2} May be displayed. In that case, since 100% of the pixels are displayed at 100 cd / m ² , the luminance constant La is set to 100 cd / m ² .

Further, even when the EOTF of the input image data 1 is a standard signal defined by the absolute value of luminance, the luminance constant La can be determined based on the same concept. In this case, it is necessary to separately supplement how many cd / m ^{2 the} 100% reference white color is determined. For example, when 100% white luminance is defined as metadata associated with the input image data 1, it is preferable to use the value (defined luminance) as the luminance constant La. When there is no metadata, it is advisable to use an appropriate luminance as the luminance constant La as a luminance arbitrarily set by the user or a default value unique to the apparatus. In that case, it is preferable to use the luminance within the range of 100 [cd / m ² ] to 500 [cd / m ² ] as the luminance constant La. The reference display luminance may be estimated by analyzing the input image data 1. For example, the APL of the image may be calculated, and the maximum APL (maximum value of APL) of content including a plurality of images (for example, a moving image including a plurality of frames) may be estimated as the reference display luminance. Then, the estimated reference display luminance may be used as the luminance constant La.

  Although the present invention has been described in detail based on preferred embodiments thereof, the present invention is not limited to these specific embodiments, and various forms within the scope of the present invention are also included in the present invention. included. A part of the above-described embodiments may be appropriately combined.

<Other examples>
The present invention supplies a program that realizes one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program This process can be realized. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

30: Equal perceptual lightness conversion table generation unit 40: Color conversion unit 50: Display unit 55: HDR display unit 60: Table selection unit 70: Correction coefficient determination unit 80, 85: Brightness correction unit 90: RGB table holding unit 100, 400 , 500, 600, 700: Image display device 200: Image processing device 300: Color conversion device 601: Processor 602: Storage unit 800: Computer 310: Table holding unit A
320: Table holding unit B 330: Table holding unit C

Claims (20)

A setting means for setting the display brightness of the image display device;
Correction means for reducing the saturation of the input color and increasing the brightness of the input color;
With
The image display apparatus according to claim 1, wherein the correction unit determines an increase amount of lightness of the input color based on the display luminance set by the setting unit. The image display apparatus according to claim 1, wherein the correction unit increases the amount of increase in lightness of the input color as the display luminance set by the setting unit is higher. The correction means includes
First correction means for acquiring corrected saturation, which is the saturation of the input color after reduction, by reducing the saturation of the input color;
Based on the saturation of the input color before reduction, the corrected saturation, and the display brightness set by the setting unit, the brightness of the input color is increased by increasing the brightness of the input color. Second correction means to be acquired;
The image display apparatus according to claim 1, further comprising: 4. The image according to claim 3, wherein the difference between the saturation of the input color before reduction and the corrected saturation is larger, the difference between the brightness of the input color before reduction and the corrected brightness is larger. 5. Display device. The setting means sets a display luminance reference value of the image display device;
5. The image display according to claim 1, wherein the correction unit determines the amount of increase in lightness of the input color based on the reference value set by the setting unit. apparatus. The setting means sets an upper limit value of display brightness of the image display device;
5. The image display device according to claim 1, wherein the correction unit determines an increase amount of lightness of the input color based on the upper limit value set by the setting unit. . The image display apparatus according to claim 1, wherein the correction unit increases the brightness of the input color based on the Helmholtz-Colelausch effect. The correction means reduces the saturation of the input color and increases the brightness of the input color so that a color within a color gamut narrower than the color gamut of the input color is determined as the output color. The image display apparatus of any one of Claims 1-7. The correction means repeatedly increases the brightness of the input color while gradually reducing the saturation of the input color until a color within a color gamut narrower than the color gamut of the input color is determined as the output color. The image display device according to claim 8. The image display apparatus according to claim 8, wherein the correction unit reduces the saturation of the input color so that the input color is converted into a color in a color gamut narrower than the color gamut of the input color. The image display apparatus according to claim 1, wherein the correction unit determines an increase in lightness of the input color based further on a hue of the input color. The image display device according to claim 1, further comprising a selection unit that selects each color in a predetermined color gamut as an input color. Selection means for selecting the color of each pixel of the input image data as an input color;
Conversion means for converting the color of each pixel of the input image data into an output color;
The image display apparatus according to claim 1, further comprising: Storage means for storing color conversion information for converting an input color to an output color;
Conversion means for converting the color of each pixel of the input image data based on the color conversion information;
The image display apparatus according to claim 1, further comprising: The image display device according to claim 1, wherein the setting unit performs a setting related to display brightness of the image display device in accordance with a user operation. The image display device according to claim 1, wherein the setting unit performs setting related to display brightness of the image display device according to a temperature of the image display device. The said setting means performs the setting regarding the display brightness of the said image display apparatus according to the brightness | luminance feature-value of the image displayed on the said image display apparatus, The any one of Claims 1-14 characterized by the above-mentioned. Image display device. Obtaining means for obtaining information on display brightness set in the image display device;
Correction means for reducing the saturation of the input color and increasing the brightness of the input color;
With
The color conversion apparatus according to claim 1, wherein the correction unit determines an increase in lightness of the input color based on display luminance set in the image display apparatus. An acquisition step of acquiring information about display brightness set in the image display device;
A correction step for reducing the saturation of the input color and increasing the brightness of the input color;
Have
In the correction step, the amount of increase in lightness of the input color is determined based on display luminance set in the image display device.   A program causing a computer to execute each step of the color conversion method according to claim 19.

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