Image correction processing device and program

Abstract

To provide an image correction processing apparatus and a program capable of lowering saturation only in a specific color area according to production intention.SOLUTION: An image correction processing apparatus includes at least a saturation correction processing unit. The saturation correction processing unit obtains a correction factor fcor for correcting the saturation of a pixel having a color that satisfies a predetermined condition in the image, and corrects the saturation of the pixel on the basis of the correction factor fcor. Further, the saturation correction processing unit determines the correction factor fcor that continuously changes in response to changes in the brightness, and also continuously changes according to the change in the hue angle on the basis of at least the brightness and the hue angle of the pixel, and corrects the saturation of the pixel by multiplying the saturation of the pixel by the determined correction factor fcor.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image correction processing device and a program. [Background technology]

[0002] Broadcast programs using HDR video (HDR stands for "High Dynamic Range") are produced with the HDR reference white in mind to minimize variations in brightness between programs. When using the HLG format (HLG stands for "Hybrid Log Gamma"), the HDR reference white is 75% of the HLG video signal level (hereinafter referred to as "75% HLG").

[0003] The level of human facial skin is important when composing an image. Patent Document 1 reports that the level of human facial skin falls within the range of 45% HLG to 55% HLG when taking HDR reference white into consideration. However, when the intention of the video production or the person and lighting conditions change, facial skin may be expressed at a level higher than 55% HLG. In the HLG format, brightness is adjusted so that skin becomes white at 75% HLG, so skin expressed between 55% HLG and 75% HLG maintains its saturation as a skin color.

[0004] On the other hand, Non-Patent Document 1 reports that in programs produced using conventional SDR video (SDR stands for "Standard Dynamic Range"), the facial skin quality of women is higher than that of men. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-025241 [Non-patent literature]

[0006] [Non-Patent Document 1] Report ITU-R BT.2408-3 “Guidance for operational practices in HDR television production”, July 2019, ITU-R (Radiocommunication Sector of ITU). Summary of the Invention [Problem to be solved by the invention]

[0007] In SDR video programs, the higher the level of a woman's facial skin, the more emphasis is placed on gradation expression and the lower the saturation is expressed. When attempting to express this using the saturation correction process described in Patent Document 1, many skin tones have a brightness lower than that equivalent to 75% HLG, and the saturation correction process cannot be applied, resulting in the problem that the skin tones cannot be expressed at low saturation.

[0008] Furthermore, even if the brightness corresponding to 75% HLG, which is the inflection point of the saturation correction process, is reduced to the brightness corresponding to 55% HLG, the saturation is reduced equally at all hue angles, which poses the problem of reducing the saturation of colors other than facial skin.

[0009] One possible solution would be to lower the inflection point of the saturation correction process to a lightness equivalent to 55% HLG only for the skin-color hue angle range. However, when switching from a skin-color hue angle to a non-skin-color hue angle, or vice versa, if the correction process formula results in a mathematically steep conversion, discontinuity in the gradation expression after conversion may occur. Furthermore, when limiting the skin-color range using lightness and hue angle, there is a risk that highly saturated colors that are not perceived as skin colors may also be subject to the correction process, even though they have the same hue angle.

[0010] The present invention was made based on the recognition of the above-mentioned problems, and aims to provide an image correction processing device and program that can lower the saturation of only specific color areas when it is desired to increase the facial skin level in HDR program production according to production intentions, while lowering the saturation as in the expression of facial skin in conventional SDR program production, and further that can prevent discontinuities from appearing in the gradation expression after saturation correction, and prevent colors other than the intended color from being corrected by the correction process. [Means for solving the problem]

[0011] [1] In order to solve the above problem, an image correction processing device according to one aspect of the present invention includes a correction factor f for correcting the saturation of pixels of a color that meets a predetermined condition in an image. cor is calculated, and the saturation of the pixel is adjusted by the correction factor f cor The saturation correction processing unit is configured to correct the saturation based on at least the lightness and hue angle of the pixel, by adjusting the correction factor f cor and the correction factor f is continuously changed in response to the change in the hue angle. cor , and the correction factor f determined for the saturation of the pixel is cor The saturation of the pixel is corrected by multiplying the pixel by . With this configuration, the image correction processing device applies a correction factor f to the saturation of pixels in the image. cor The saturation of the pixel can be corrected by multiplying it by the correction factor f cor Since σ changes continuously in response to changes in lightness and also in response to changes in hue angle, no discontinuous changes in gradation occur in the corrected image.

[0012] [2] Furthermore, in one aspect of the present invention, the image correction processing device further includes a color space conversion processing unit that converts an image included in an input high dynamic range (HDR) video into a signal in a CIELAB color space corresponding to HDR and passes the converted signal to the saturation correction processing unit, and a color space reconversion processing unit that reconverts the CIELAB color space signal corresponding to HDR after processing by the saturation correction processing unit into an HDR video and outputs the HDR video, and the saturation correction processing unit corrects the saturation of the pixel based on the CIELAB color space signal corresponding to HDR passed from the color space conversion processing unit. Here, the color space conversion processing unit converts the image included in the CIELAB color space HDR video into a signal in the CIELAB color space in which 75% HLG scene luminance corresponds to diffuse white (lightness 100). The CIELAB color space in which 75% HLG scene luminance corresponds to diffuse white (lightness 100) is referred to as the "CIELAB color space compatible with HDR." The saturation correction processing unit also corrects the saturation of pixels based on the signal in the CIELAB color space corresponding to the HDR, which is passed from the color space conversion processing unit. As mentioned above, "HDR" stands for "High Dynamic Range."

[0013] [3] In addition, one aspect of the present invention is the above-mentioned image correction processing device, wherein the correction factor f cor has the effect of reducing the saturation of the pixel, and the saturation correction processing unit reduces the saturation C of the pixel before correction. * ab The higher the correction factor f cor The purpose is to reduce the degree of decrease in saturation caused by the

[0014] [4] In addition, in one aspect of the present invention, in the image correction processing device, the saturation correction processing unit is * ab The correction factor f cor The decrease function value C represents the degree of decrease in saturation due to * func and the decreasing function value C *func is the saturation C of the pixel before correction * ab The saturation C of the pixel before correction decreases monotonically with an increase in * ab When is 50 or less, the decreasing function value C * func is 0.95 or more, and the saturation C of the pixel before correction * ab When is 150 or more, the decreasing function value C * func is equal to or less than 0.02, and the saturation correction processing unit reduces the decreasing function value C to a predetermined value determined based on the lightness and hue angle of the pixel. * func By multiplying the correction factor f cor Determine the degree of saturation loss due to

[0015] [5] In addition, in one aspect of the present invention, in the image correction processing device, the saturation correction processing unit is cor is calculated using a predetermined formula. However, the result of the calculation using the formula is f cor If <0, we force f cor = 0. The meanings of the values ​​that appear in the formula are as follows: L * max is the maximum value of lightness. * is the brightness of the pixel (0≦L * ≦L * max ) is L * f is the hue angle h of the pixel ab h1 and h4 are the predetermined inflection point values ​​for the lightness that are determined to change continuously according to the change in hue angle h ab These are parameters (where h1≦h4) that are determined appropriately depending on the region. f is a saturation correction parameter that is determined so as to change continuously in accordance with the change in the brightness of the pixel. * func is the decreasing function value.

[0016] [6] In addition, one aspect of the present invention is a correction factor f for correcting the saturation of pixels of a color that meets a predetermined condition in an image. cor is calculated, and the saturation of the pixel is adjusted by the correction factor f cor and a saturation correction processing unit that corrects the saturation based on at least the lightness and hue angle of the pixel, and the saturation correction processing unit calculates the correction factor f cor and the correction factor f is continuously changed in response to the change in the hue angle. cor , and the correction factor f determined for the saturation of the pixel is cor This is a program for causing a computer to function as an image correction processing device that corrects the saturation of a pixel by multiplying the pixel by [Effects of the Invention]

[0017] According to the image correction processing device of the present invention, it is possible to correct the saturation only in areas that meet specific conditions regarding hue and brightness. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a block diagram showing a schematic functional configuration of an image correction processing device according to an embodiment of the present invention. [Figure 2] 10 is an example of a graph showing the relationship between the hue angle hab and the lightness function value L* f in the processing of the image correction processing device according to the embodiment. [Figure 3] 10 is an example of a graph showing the relationship between the hue angle hab and the saturation correction parameter function value σf in the processing of the image correction processing device according to the embodiment. [Figure 4] 10 is an example of a graph showing the relationship between lightness L* and saturation correction parameter σf in processing of the image correction processing device according to the embodiment. [Figure 5] 10 is a graph showing an example of the relationship between input and output of a decreasing function that has the effect of making the effect of skin color correction processing less effective as saturation increases in processing by the image correction processing device according to the embodiment. [Figure 6] 10 is a flowchart showing a processing procedure of the image correction processing device according to the embodiment. [Figure 7] FIG. 2 is a block diagram showing an example of the internal configuration of the image correction processing device according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] Next, an embodiment of the present invention will be described with reference to the drawings. An image correction processing device 1 according to this embodiment corrects an input HDR (high dynamic range) video signal. Specifically, when a human face's skin level is expressed highly in the HDR video signal, the image correction processing device 1 corrects the skin color in the image by reducing saturation and preserving gradation. Note that in this embodiment, "skin color" refers to a color also known as pale orange, a name for a human skin color commonly seen in Japan and other Far Eastern regions. The video processed by the image correction processing device 1 is a time series of frame images. The video may or may not include audio. When the image correction processing device 1 corrects a video signal, it performs correction on each individual frame image.

[0020] The image correction processing device 1 of this embodiment performs the following overall processing. That is, the image correction processing device 1 inputs an HDR video signal. The image correction processing device 1 then sequentially converts the input HDR video signal into a linear signal and a signal in the CIELAB color space (CIE 1976 (L*, a*, b*) color space) that corresponds to HDR, and then corrects the saturation of only the skin color region. The image correction processing device 1 then converts the signal after the saturation correction into an HDR video signal and outputs the HDR video signal. The functional configuration and processing details of the image correction processing device 1 are explained below.

[0021] FIG. 1 is a block diagram showing a schematic functional configuration of an image correction processing device according to this embodiment. As shown in the figure, the image correction processing device 1 is configured to include a color space conversion processing unit 21, a saturation correction processing unit 22, and a color space reconversion processing unit 23. These functional units can be implemented using electronic circuits. Input signals to the image correction processing device 1 and output signals from the image correction processing device 1 may be electrical signals. At least a portion of the image correction processing device 1 can also be implemented using a computer and a program. Also, at least a portion of the image correction processing device 1 can also be implemented using a dedicated circuit (hardware). Each functional unit has a storage unit as needed. The storage unit is implemented using, for example, a semiconductor memory. Also, non-volatile storage units such as a magnetic hard disk drive or a solid-state drive (SSD) may be used as needed.

[0022] The image correction processing device 1 receives an HDR video signal (a signal standardized under the BT.2020 standard), corrects the signal, and outputs the corrected HDR video signal. Specifically, the image correction processing device 1 is configured to correct the saturation of only areas of the input HDR video signal with a high facial skin level, and then convert the corrected signal back into an HDR video signal for output (a signal standardized under the BT.2020 standard, like the input signal). The detailed functions of each unit are as follows:

[0023] The color space conversion processing unit 21 converts the input HDR input video signal into a signal in the CIELAB color space that corresponds to HDR. That is, the color space conversion processing unit 21 converts the image included in the input HDR video into a signal in the CIELAB color space that corresponds to HDR, and passes the converted image to the saturation correction processing unit 22. The color space conversion processing unit 21 converts an image included in the CIELAB color space HDR video into a signal in the CIELAB color space in which 75% HLG scene luminance corresponds to diffuse white (lightness 100). The CIELAB color space in which 75% HLG scene luminance corresponds to diffuse white (lightness 100) is referred to as an "HDR-compatible CIELAB color space."

[0024] Specifically, the color space conversion processing unit 21 converts the HDR input video signal E′ hin ={R′ HDRin ,G′ HDRin ,B′ HDRin}, the display luminance signal Fd HDRin ={R HDRin ,G HDRin ,B HDRin Furthermore, the color space conversion processing unit 21 obtains the display luminance signal Fd HDRin By matrix calculation, the tristimulus value X HDRin Y HDRin Z HDRin Furthermore, the color space conversion processing unit 21 converts these tristimulus values ​​into signals in the CIELAB color space that corresponds to HDR. By this processing of the color space conversion processing unit 21, the lightness L * and color coordinate a * , b * is calculated.

[0025] In addition, a * is the red direction (a * positive direction) and green direction (a * The value indicates the degree of the negative direction of * is the yellow direction (b * positive direction) and blue direction (b * The value indicates the degree of the color (in the negative direction of the color coordinate a). * , b * Once the value of is determined, the hue angle and saturation are determined.

[0026] During this processing, the color space conversion processing unit 21 may convert to a scene luminance signal using an inverse OETF function conforming to the HDR method to obtain an RGB linear signal, and then convert to a CIELAB color space in which 75% HLG scene luminance corresponds to diffuse white (lightness 100).

[0027] The processing performed by the color space conversion processing unit 21 itself is a conventional technique, and is equivalent to the processing described in Patent Document 1, for example.

[0028] The color space conversion processing unit 21 passes the CIELAB space signal corresponding to HDR obtained by the above processing to the saturation correction processing unit 22.

[0029] The saturation correction processing unit 22 calculates a correction factor f for correcting the saturation of pixels of a color that meets a predetermined condition in the image. cor is calculated, and the saturation of the pixel is adjusted by the correction factor f cor The saturation correction processing unit 22 corrects the saturation based on at least the brightness and hue angle of the pixel, by calculating the correction factor f cor and the correction factor f is continuously changed in response to the change in the hue angle. cor The saturation correction processing unit 22 determines the correction factor f determined for the saturation of the pixel. cor The saturation correction processing unit 22 corrects the saturation of the pixel based on the signal in the CIELAB color space corresponding to the HDR passed from the color space conversion processing unit 21.

[0030] The saturation correction processing unit 22 of this embodiment corrects the saturation of the skin color area. Specifically, the saturation correction processing unit 22 performs processing to correct the saturation of only the skin color area exceeding 55% HLG in the CIELAB space corresponding to HDR by equal lightness / equal hue conversion. When the HDR signal is converted into the CIELAB color space corresponding to HDR, the skin color area exceeding 55% HLG has a lightness of L * min From L * max and hue angle is within the range of h ab,min From h ab,max Here, the brightness L * min is the brightness equivalent to 55% HLG, and when calculated from the display brightness, L * min = 63, calculated from the scene luminance, L * min = 76. Lightness L * maxis the brightness equivalent to the upper limit of the video signal, 109% HLG, and is calculated from the display brightness. * max =224, calculated from scene luminance, L * max = 217. The lower limit of the hue angle range for skin tones in the HDR-compatible CIELAB color space, h ab,min and upper limit h ab,max is, h ab,min =35°, h ab,max = 65°. Here, L * min , L * max , h ab,min , and h ab,max Although the above description assumes that these are fixed values, each of these may be parameters that can be freely set by the user. Also, although the upper limit of the video signal is set to 109% HLG, it may be set to 100% HLG.

[0031] In the saturation correction process by the saturation correction processing unit 22, as shown in the following formula (1), chroma C * ab and hue angle h ab Calculate the hue angle h ab h ab ≧h ab,min Kats H ab ≦h ab,max In the range of luminosity L * L * >L * min The correction factor f for reducing the saturation of skin tones is cor However, as a result of the calculation using formula (1), f cor If <0, we force f cor =0.

[0032] Furthermore, as shown in equation (1), if the above conditions are not met, the correction factor f cor Set the value to 1 (no saturation correction).

[0033]

number

[0034] In formula (1), σ is an arbitrarily adjustable user parameter, and σ ≥ 0.

[0035] When performing correction as in the above formula (1), the hue angle h ab is h ab,min or h ab,max changes such that the saturation rapidly (discontinuously) decreases at the boundary part. Therefore, discontinuity in gradation expression may occur in the converted result. Thus, in the present embodiment, the brightness L * f at the inflection point is functionalized as shown in the following formula (2) so that the brightness at the inflection point changes continuously according to the hue angle.

[0036]

Equation

[0037] Here, L * ref is the brightness corresponding to 75% HLG. Also, L * ip is the brightness corresponding to 55% HLG. Also, each of h1, h2, h3, and h4 is the hue angle at the inflection point on the hue angle side of the brightness at the inflection point. h1, h2, h3, and h4 are user parameters that can be arbitrarily adjusted as long as they satisfy the relationships of h1 ≤ h ab,min ≤ h2, h3 ≤ h ab,max ≤ h4, h1 ≤ h2 < h3 ≤ h4, respectively. In particular, a constraint may be provided to limit to the case of h1 < h2 < h3 < h4, excluding the case where h1 = h2 or h3 = h4. As an example, h1 = 34°, h2 = 38°, h3 = 62°, h4 = 66°, etc. may be used.

[0038] FIG. 2 is a graph showing the relationship between the hue angle h ab and the brightness function value L * f in the case of the above example (h1 = 34°, h2 = 38°, h3 = 62°, h4 =ab and its unit is degrees (°). Hue angle h ab The range of 0≦h ab <360. The vertical axis is the brightness L * f Lightness L * f is the hue angle h ab In this graph, the thick solid line indicates the brightness of the inflection point according to this embodiment (Equation (2)). The thin solid line indicates the brightness of the inflection point when Equation (1) is used. The dashed line indicates the brightness of the inflection point when L * ref (Lightness equivalent to 75% HLG). The dashed line indicates the * ip (Lightness equivalent to 55% HLG) is shown. * ref = 100, and L * ip =63.

[0039] When using equation (1) (thin solid line graph), the hue angle h ab,min and h ab,max The brightness of the inflection point L * f In contrast, when formula (2) is used (thick solid line graph), the hue angle h ab,min and h ab,max In each vicinity of * f is made to change continuously.

[0040] In other words, in the thick solid line graph, the hue angle h ab From 0° to h1, the brightness L * f is L * ref (Lightness equivalent to 75% HLG) and the hue angle h ab is between h1 and h2 (h1≦h ab,min ≦h2), the lightness of the inflection point L * f L * ref(75% HLG equivalent brightness) to L * ip (Lightness equivalent to 55% HLG) ab From h2 to h3, the brightness of the inflection point L * f is L * ip (Lightness equivalent to 55% HLG) ab is between h3 and h4 (h3≦h ab,max ≦h4), the lightness of the inflection point L * f L * ip (55% HLG equivalent brightness) to L * ref (Lightness equivalent to 75% HLG) ab In the area where h4 is larger than h4, the lightness L * f is L * ref (brightness equivalent to 75% HLG) is constant.

[0041] In addition, the brightness of the inflection point L * f As shown in this graph, ab The method of linearly changing the brightness L at the inflection point (for each range) is just an example, and it is not necessary to use such a method. However, as shown in this graph (as in formula (2)), * f The method for determining ρ can simplify the calculation.

[0042] In other words, the lightness L of the inflection point * f Equation (2) was shown above as an example of a function to calculate the brightness of the inflection point L * f It is desirable that the function for determining the value of the inflection point L be as follows: * f The function to calculate the hue angle h ab It is desirable that the function be continuous over the entire domain of h1.ab In the region where h ≤ h2, the brightness L of the inflection point * f is L * ref (or in its vicinity) and decreases (it may be monotonically decreasing) towards L * ip (or in its vicinity). Note that within the region where h1 ≤ h ab ≤ h2, the point where h ab = h ab,min is included. Also, in the region where h2 < h ab < h3, the brightness L of the inflection point * f is L * ip (or in its vicinity) and is constant (or almost constant). Also, in the region where h3 ≤ h ab ≤ h4, the brightness L of the inflection point * f is L * ip (or in its vicinity) and increases (it may be monotonically increasing) towards L * ref (or in its vicinity). Note that within the region where h3 ≤ h ab ≤ h4, the point where h ab = h ab,max is included. And in other regions (that is, h ab < h1 and h4 < h ab ), the brightness L of the inflection point * f is L * ref (or in its vicinity) and is constant (or almost constant).

[0043] Next, the amount for correcting the saturation is determined with the above L * f as the inflection point. When performing correction using Equation (1), σ (where σ > 0) arbitrarily adjusted by the user can also be used as the saturation correction parameter. However, in the case of the above Equation (2), since the inflection point of the brightness changes continuously, it is desirable for the saturation correction parameter to also change according to the change. Therefore, the saturation correction parameter also depends on the brightness L of the inflection point * fFor example, as shown in the following equation (3), the saturation correction parameter σ f Determine.

[0044]

number

[0045] In formula (3), σ1 and σ2 are parameters that are set appropriately. The values ​​of σ1 and σ2 can be adjusted arbitrarily by the user within the range that satisfies σ1≦σ2. Note that the σ calculated using formula (3) f The value of σ f < 1, then σ f =1.

[0046] Figure 3 shows the hue angle h when σ1=1 and σ2=2 as an example. ab and the saturation correction parameter function value σ f In this graph, the horizontal axis represents the hue angle h ab The unit is degrees (°). The domain of definition is 0≦h ab <360. The vertical axis is the saturation correction parameter σ f is.

[0047] The thick solid line in the graph shown in FIG. 3 represents the saturation correction parameter σ f and is calculated using equation (3). The thin solid line graph is for comparison, and saturation when using equation (1) is an example of a positive parameter (σ f = 1.0).

[0048] The saturation correction parameter σ shown in the thick solid line in Figure 3 f is the lightness L shown in the graph in Figure 2. * f In other words, the hue angle h ab From 0° to h1, the saturation correction parameter σ f is constant at 1.0. abis between h1 and h2, and the saturation correction parameter σ f The hue angle h changes linearly (increases) from 1.0 to 2.0. ab From h2 to h3, the saturation correction parameter σ f is constant at 2.0. ab is between h3 and h4, and the saturation correction parameter σ f The hue angle h changes linearly from 2.0 to 1.0. ab In the region where h is greater than h4, the saturation correction parameter σ f is a constant 1.0.

[0049] Figure 4 shows the lightness L * and the saturation correction parameter σ f Graph G0 in the figure shows the saturation correction parameter when using the conventional technology, and its value is the lightness L * The graph G1 shows the saturation correction parameters when σ1=1 and σ2=2 in this embodiment. In the graph G1, the lightness L * =L * ip (=63) when σ f = 2.0, and L * ip <L * <L * ref In σ f varies linearly from 2.0 to 1.0, and L * ≧L * ref (=100) is σ f = 1.0. Graph G2 shows the saturation correction parameters when σ1 = 1 and σ2 = 3 in this embodiment. In graph G2, the lightness L * =L * ip (=63) when σ f = 3.0, and L * ip <L * <L * ref In σ f varies linearly from 3.0 to 1.0, and L* ≧L * ref (=100) is σ f = 1.0. Graph G3 shows the saturation correction parameters when σ1 = 1 and σ2 = 4 in this embodiment. Graph G2 shows the saturation correction parameters when the lightness L * =L * ip (=63) when σ f = 4.0, and L * ip <L * <L * ref In σ f varies linearly from 4.0 to 1.0, and L * ≧L * ref (=100) is σ f = 1.0. In addition, in the case of the combination of values ​​of σ1 and σ2 other than those illustrated in FIG. 4, the formula (3) (where σ f The lower limit of σ is 1.0) f is required.

[0050] The saturation correction processing unit 22 further performs correction processing to maintain the saturation of highly saturated colors that are not perceived as skin colors within the hue angle range of skin colors without changing it. To achieve this, the saturation correction processing unit 22 performs calculations using a decreasing function that makes the skin color correction processing less effective as the saturation becomes higher.

[0051] FIG. 5 is a graph showing an example of the relationship between the input and output of a decreasing function that reduces the effectiveness of skin color correction processing as the saturation increases. In this graph, the horizontal axis represents the saturation C * ab The vertical axis corresponds to the coefficient C * func Corresponds to.

[0052] The effect of this decrease function is as follows: 0≦C where there is skin color above 55% HLG, as shown. * ab In the region ≦50, the decreasing function value C *func is greater than 0.95. That is, C * ab When is less than about 50, skin color correction is effective. And this reduction function is * ab In the region >50, the decreasing function value C * func has the characteristic that C * ab If C exceeds 50, the skin color correction process becomes less effective. * ab In the region of ≧150, the decreasing function value C * func is less than 0.02 and is very close to 0. In other words, C * ab If the value is 150 or more, the skin color correction process is almost ineffective.

[0053] As a decreasing function that satisfies the above requirements, for example, a sigmoid function can be used to realize a function such as the following formula (4).

[0054]

number

[0055] C in Equation (4) * cusp,min is the range of hue angles h1≦h in the CIELAB space corresponding to HDR to which skin color correction processing defined by equation (2) is applied. ab It is the smallest saturation among the saturations that form the color gamut boundary when h1=34° and h4=66°. For example, when h1=34° and h4=66°, C * cusp,min is approximately 253.3, and the hue angle is then 66°.

[0056] The saturation correction processing unit 22 uses the lightness function L * f and the saturation correction parameter function σ f and the decreasing function C * func and a correction factor f to reduce the saturation of skin tones.cor Calculate the correction factor f cor is calculated using, for example, the following equation (5).

[0057]

number

[0058] However, the result calculated by equation (5) is f cor If <0, we force f cor = 0. Then, the saturation correction processing unit 22 sets the correction factor f cor Chroma C * ab Multiply by to get the corrected chroma C * ab,cor That is, we obtain the following equation (6).

[0059] C * ab,cor =C * ab ×f cor (6)

[0060] As shown in equation (5), the correction factor f cor has the effect of reducing the saturation of the pixel. In other words, the saturation correction processing unit 22 reduces the saturation C of the pixel before correction. * ab The higher the correction factor f cor Reduces the degree of desaturation caused by

[0061] As a specific processing method, the saturation correction processing unit 22 calculates the saturation C * ab Depending on the value of the decreasing function C * func That is, the reduction function calculates the saturation C of the pixel before correction. * ab The decreasing function value C * func is the correction factor f cor As already explained with reference to the example of FIG. 5, the decreasing function value C* func is the pixel saturation before correction, C * ab As an example, this decreasing function is as follows: * ab If is less than 50, the decreasing function value C * func is 0.95 or more, and the pixel's uncorrected saturation C * ab When the value of the decreasing function C is 150 or more, * func is 0.02 or less. In other words, in this example, 50≦C * ab In the region of ≦150, the decreasing function value C * func As a result, the effect of saturation correction is limited in pixels with high saturation, and the effect of saturation correction is strong in pixels with low saturation. Note that the saturation correction processing unit 22 uses a predetermined value (σ in Equation (5)) determined based on the lightness and hue angle of the pixel. f and (L * -L * f ) / (L * max -L * f ) and the product of the decreasing function value C * func By multiplying by the correction factor f cor Determine the degree of saturation loss due to

[0062] As described above, the saturation correction processing unit 22 calculates the correction factor f cor is calculated using equation (5). However, the result calculated using equation (5) is f cor If <0, we force f cor = 0. The meanings of the values ​​that appear in the formula are as follows: L * max is the maximum value of lightness. * is the brightness of the pixel (0≦L * ≦L * max ) is L* f is the hue angle h of the pixel ab h1 and h4 are the predetermined inflection point values ​​for the lightness that are determined to change continuously according to the change in hue angle h ab These are parameters (where h1≦h4) that are determined appropriately depending on the region. f is a saturation correction parameter that is determined so as to change continuously in accordance with the change in the brightness of the pixel. * func is the decreasing function value.

[0063] The saturation correction processing unit 22 corrects the chroma C * ab,cor And brightness L * and the hue angle h ab Thus, the corrected LAB signal is calculated.

[0064] The saturation correction processing unit 22 does not correct the saturation of areas other than those that satisfy the above conditions for correction.

[0065] The saturation correction processing unit 22 passes the video signal after the saturation correction processing to the color space reconversion processing unit 23.

[0066] The color space reconversion processing unit 23 converts the signal passed from the saturation correction processing unit 22 into an HDR output video signal and outputs it. In other words, the color space reconversion processing unit 23 reconverts the signal in the CIELAB color space corresponding to HDR after being processed by the saturation correction processing unit 22 into an HDR video and outputs it.

[0067] Specifically, the color space reconversion processing unit 23 converts the received saturation-corrected signal into tristimulus values ​​X from the CIELAB color space corresponding to HDR. HDRout Y HDRout Z HDRout Furthermore, the color space reconversion processing unit 23 converts the display luminance signal Fd HDRout ={R HDRout ,G HDRout ,B HDRoutThis process converts the HDR output video signal E' into the inverse EOTF function that complies with the HDR standard. hout ={R′ HDRout ,G′ HDRout ,B′ HDRout Note that if conversion to a scene luminance signal has been performed in the processing of the color space conversion processing unit 21, the color space reconversion processing unit 23 converts the signal into a video signal using an OETF function that complies with the HDR method.

[0068] 6 is a flowchart showing the processing steps of the image correction processing device 1. As shown in the figure, in step S31, the color space conversion processing unit 21 converts the input HDR input video signal into a signal in the CIELAB color space that corresponds to HDR. Next, in step S32, the saturation correction processing unit 22 corrects the saturation of the CIELAB color space signal converted in step S31 in a region within the image that satisfies a condition. Specifically, the saturation correction processing unit 22 corrects the saturation of a skin color region that satisfies a predetermined condition. Next, in step S33, the color space reconversion processing unit 23 converts the CIELAB color space signal corrected in step S32 into an HDR output video signal for output.

[0069] Through the processing steps described above, the image correction processing device 1 can correct images. That is, even if the facial skin level of a person in an HDR program production is higher than the video signal level assumed in the conventional technology (Patent Document 1) due to production intent, gender, etc., it is possible to reproduce skin tones similar to those in conventional SDR program production. Furthermore, the processing by the image correction processing device 1 makes it possible to perform correction only on skin-tone areas.

[0070] FIG. 7 is a block diagram showing an example of the internal configuration of the image correction processing device 1 of this embodiment. The image correction processing device 1 can be realized using a computer. As shown in the figure, the computer includes a central processing unit 901, a RAM 902, an input / output port 903, input / output devices 904 and 905, and a bus 906. The computer itself can be realized using existing technology. The central processing unit 901 executes instructions contained in a program read from the RAM 902 or the like. In accordance with each instruction, the central processing unit 901 writes data to the RAM 902, reads data from the RAM 902, and performs arithmetic and logical operations. The RAM 902 stores data and programs. Each element included in the RAM 902 has an address and can be accessed using the address. RAM is an abbreviation for "random access memory." The input / output port 903 is a port through which the central processing unit 901 exchanges data with external input / output devices. The input / output devices 904 and 905 are input / output devices. Input / output devices 904 and 905 exchange data with the central processing unit 901 via an input / output port 903. A bus 906 is a common communication path used within the computer. For example, the central processing unit 901 reads and writes data from and to RAM 902 via the bus 906. Also, for example, the central processing unit 901 accesses the input / output port via the bus 906.

[0071] At least some of the functions of the image correction processing device 1 can be implemented by a computer and a program. In this case, the program for implementing these functions may be recorded on a computer-readable recording medium and then loaded and executed by a computer system. Note that the term "computer system" as used herein includes hardware such as an operating system and peripheral devices. Furthermore, the term "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs, CD-ROMs, DVD-ROMs, and USB memory, as well as storage devices such as hard disks built into computer systems. In other words, a "computer-readable recording medium" may be a non-transitory computer-readable recording medium. Furthermore, the term "computer-readable recording medium" may also include media that temporarily and dynamically store programs, such as communication lines used when transmitting programs over networks like the Internet or telephone lines, or media that store programs for a certain period of time, such as volatile memory within the computer systems that serve as the server or client in such cases. The program may also be designed to implement some of the aforementioned functions, or may be capable of implementing the aforementioned functions in combination with a program already stored in the computer system.

[0072] Although the embodiment has been described above, the present invention can also be carried out in the following modified examples.

[0073] In the above embodiment, various parameters (L * min , L * max , h ab,min , h ab,max , h1, h2, h3, h4, etc.) have been described, different values ​​may be used as parameter values. Also, these values ​​may be variable.

[0074] In the above embodiment, the skin color area is the area to be subjected to saturation correction, but other areas may also be subjected to saturation correction. * ,b * The color condition for a region may alternatively be expressed in terms of hue angle, saturation, and brightness.

[0075] In the above embodiment, the image correction processing device 1 is configured to include all of the color space conversion processing unit 21, the saturation correction processing unit 22, and the color space reconversion processing unit 23. As a modified example, the image correction processing device 1 may include only the saturation correction processing unit 22. In this case, it is still possible to perform the same saturation correction as in this embodiment. Note that functions corresponding to the color space conversion processing unit 21 and the color space reconversion processing unit 23 may be provided in a device external to the image correction processing device 1.

[0076] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0077] As described above, the image correction processing device 1 of this embodiment (including the modified examples) makes it possible to correct the saturation of only areas in an image that satisfy specific conditions. More specifically, when the level of an area having human skin color becomes high, the image correction processing device 1 makes it possible to correct only the area having human skin color while maintaining the saturation of areas of other colors. This makes it possible to express skin color included in an input HDR video signal in the same way as skin color in a conventional SDR video signal.

[0078] Furthermore, according to the image correction processing device 1 of this embodiment (including the modified examples), the numerical values ​​of the processing results are made continuous at the boundary parts of the correction processing within the image, thereby achieving the effect of enabling smooth gradation expression in the corrected image.

[0079] According to the image correction processing device 1 of this embodiment (including the modified examples), the effect is obtained that for pixels that have the same hue angle as the area of ​​the color to be corrected (for example, skin color) but do not belong to the color to be corrected (saturation is out of range), the original color is (almost) maintained even after correction.

[0080] Next, the effects of this embodiment will be explained using numerical data. Tables 1, 2, and 3 below show the output saturation values, which are the results of correction processing of pixels, corresponding to the values ​​of pixels included in the input signal to the image correction processing device 1, comparing the output saturation values ​​of the conventional technology with those of this embodiment. In Tables 1, 2, and 3, R, G, and B are pixel values ​​of the R (red), G (green), and B (blue) components, respectively, in the RGB color space of the HDR input signal. Also, L * is the lightness, h ab is the hue angle, C * ab is the saturation (input side). Also, C * ab,out is the saturation after correction processing.

[0081] [Table 1]

[0082] [Table 2]

[0083] [Table 3]

[0084] Tables 1, 2, and 3 show data for hue angles of 36°, 50°, and 66°, respectively. For each of these hue angles, 15 (3 x 5) input signals are assumed, with three lightness levels (65, 90, and 120) and five saturations (40, 80, 120, 160, and 200). In other words, a total of 45 input signals are assumed across the three phase angles.

[0085] "Prior Art 1" is the result (output saturation value) of the technique described in Patent Document 1 (when skin color correction is attempted with a saturation correction parameter of σ=2). "Prior Art 2" is the result (output saturation value) of the technique described in Patent Document 1 when the inflection point of the saturation correction is changed to a lightness equivalent to 55% HLG (similarly, when skin color correction is attempted with a saturation correction parameter of σ=2). "Present Embodiment" indicates the output saturation value when skin color correction, as already explained above, is attempted. Note that the parameter values ​​for the saturation correction process according to this embodiment are h1=34°, h2=38°, h3=62°, h4=66°, σ1=1, and σ2=2.

[0086] As can be seen from the results shown in Tables 1 to 3, Conventional Technique 1 was unable to reduce the saturation of skin tones of signals with a brightness of 65 or 90, whereas this embodiment was able to reduce the saturation of skin tones of signals with these brightness levels. * ab In contrast to the above, when the saturation is high (for example, 160 or 200), the saturation after correction drops, whereas in this embodiment, when the saturation is high, the saturation is largely maintained even after correction. These results demonstrate the effectiveness of this embodiment. [Industrial Applicability]

[0087] The present invention can be used, for example, to process images (HDR images) that include specific colors. More specifically, the image correction processing device of the present invention can be used in a wide range of fields, such as imaging devices, video signal conversion devices, and video monitoring devices. However, the scope of use of the present invention is not limited to the examples exemplified here. [Explanation of symbols]

[0088] 1. Image correction processing device 21 Color space conversion processing unit 22 Saturation correction processing section 23 Color space reconversion processing unit 901 Central Processing Unit 902 RAM 903 Input / Output Ports 904,905 Input / Output Devices 906 Bus

Claims

1. A correction factor f for correcting the saturation of pixels in an image whose color meets a predetermined condition ｃｏｒ is calculated, and the saturation of the pixel is calculated by the correction factor f ｃｏｒ a saturation correction processing unit that performs correction based on Equipped with The saturation correction processing unit calculates the correction factor f, which continuously changes in response to a change in the lightness, based on at least the lightness and the hue angle of the pixel. ｃｏｒ and the correction factor f is continuously changed in response to the change in the hue angle. ｃｏｒ , and the correction factor f determined for the saturation of the pixel is ｃｏｒ The saturation of the pixel is corrected by multiplying The correction factor f ｃｏｒ has the effect of reducing the saturation of the pixel, The saturation correction processing unit calculates the saturation C of the pixel before correction. ＊ ａｂ The higher the correction factor f ｃｏｒ This reduces the degree of saturation loss caused by The saturation correction processing unit calculates the saturation C of the pixel before correction. ＊ ａｂ The correction factor f ｃｏｒ A decrease function value C representing the degree of decrease in saturation due to ＊ ｆｕｎｃ It calculates The decreasing function value C ＊ ｆｕｎｃ is the saturation C of the pixel before correction ＊ ａｂ It decreases monotonically with increasing The saturation C of the pixel before correction ＊ ａｂ When is 50 or less, the decreasing function value C ＊ ｆｕｎｃ is 0.95 or more, The saturation C of the pixel before correction ＊ ａｂ is 150 or more, the decreasing function value C ＊ ｆｕｎｃ is less than or equal to 0.02, The saturation correction processing unit adjusts the decreasing function value C to a predetermined value determined based on the lightness and hue angle of the pixel. ＊ ｆｕｎｃ By multiplying by ｃｏｒ Determine the degree of desaturation caused by Image correction processing device.

2. The saturation correction processing unit calculates the correction factor f ｃｏｒ The formula below: [Equation 1] (However, as a result of the calculation using the above formula, f ｃｏｒ If f<0, ｃｏｒ = 0), (However, L ＊ ｍａｘ is the maximum value of lightness, and L ＊ is the brightness of the pixel (0≦L ＊ ≦L ＊ ｍａｘ ) and L ＊ ｆ is the hue angle h of the pixel ａｂ is a predetermined inflection point value for the lightness that is determined to change continuously according to the change in h １ and h ４ Each of these is the hue angle h ａｂ The parameters are appropriately determined depending on the region (where h １ ≦h ４ ) and σ ｆ is a saturation correction parameter determined to change continuously in accordance with the change in the brightness of the pixel, and C ＊ ｆｕｎｃ is the decreasing function value) The image correction processing device according to claim 1 .

3. A saturation correction processing unit that calculates a correction factor f cor for correcting the saturation of pixels of a color that meets predetermined conditions in an image, and corrects the saturation of the pixels based on the correction factor f cor ; a color space conversion processing unit that converts an image included in an input high dynamic range (HDR) video into a signal in a CIELAB color space corresponding to HDR and passes the converted signal to the saturation correction processing unit; a color space reconversion processing unit that reconverts the signal in the CIELAB color space corresponding to HDR after being processed by the saturation correction processing unit into an HDR image and outputs the HDR image; Equipped with the saturation correction processing unit determines, based on at least the lightness and hue angle of the pixel, the correction factor f cor that continuously changes in response to a change in the lightness and that continuously changes in response to a change in the hue angle, and corrects the saturation of the pixel by multiplying the saturation of the pixel by the determined correction factor f cor ; the saturation correction processing unit corrects the saturation of the pixel based on a signal in a CIELAB color space corresponding to HDR passed from the color space conversion processing unit, The saturation correction processing unit corrects the saturation of only a skin color region exceeding 55% HLG in the CIELAB color space corresponding to HDR passed from the color space conversion processing unit by converting to equal lightness and equal hue. Image correction processing device.

4. The correction factor f cor has the effect of reducing the saturation of the pixel, the saturation correction processing unit reduces the degree of reduction in saturation due to the correction factor f cor as the pre-correction saturation C* ab of the pixel increases; The image correction processing device according to claim 3 .

5. A program for causing a computer to function as an image correction processing device described in any one of claims 1 to 4.

Images