JP7390849B2 - Video signal conversion device and program - Google Patents

Description

The present invention relates to a video signal conversion device and program that performs gradation conversion between HDR (High Dynamic Range) video and SDR (Standard Dynamic Range) video.

Since the start of new 4K8K satellite broadcasting in Japan, broadcasters have been considering efficient integrated production that simultaneously produces HLG (Hybrid Log Gamma) video with different dynamic ranges and SDR video. There is. HLG video is video based on one of a plurality of HDR methods.

HDR video is produced taking into consideration the HDR standard white level defined in the ITU-R report (see, for example, Non-Patent Document 1). The conversion method from HDR video to SDR video in integrated production of HDR video and SDR video is based on the premise that gain processing is performed so that the reference white level of the HDR video becomes white even in the SDR video. There is a conversion method (see, for example, Patent Document 1) that generates an SDR image from a linear signal of scene light acquired in . There is also a conversion method that produces only HDR video and generates SDR video from the linear signal of display light at the final stage of the production system.

In the former conversion method in which conversion is performed using a camera system, the color reproducibility of the finally obtained HDR video and SDR video differs. This is because there is a difference in EOTF (Electro Optical Transfer Function) when converting a video signal (RGB signal) into display light in HDR and SDR.

On the other hand, in the latter conversion method, which performs conversion at the final stage, if the conversion is performed by considering only the standard white level of the HDR image, the brightness of the skin of the person will be darker than in the conventional SDR image, and the HDR There has been a problem in that signals in highlight areas that exceed the reference white level of the video are reproduced as blown out highlights.

Comparing these two conversion methods, since the latter conversion method is more efficient, it was necessary to solve the aforementioned problems in the latter conversion method.

In order to solve the above-mentioned problem, the gain value is determined based on the correspondence between the skin levels of HDR video and SDR video, and while taking into consideration the standard white level of the HDR video signal, the signal in the highlight area is prevented from being reproduced as blown out highlights as much as possible. A conversion method was proposed and formulated as an ITU-R report (see, for example, Non-Patent Document 2 and Non-Patent Document 3). This conversion method is also described in Japanese Patent Application No. 2018-191560 and Japanese Patent Application No. 2019-045293, which were filed by the same applicant of the present patent application and which were unpublished at the time of filing the present patent application.

Here, the tone mapping function used for compression processing when converting HDR video into SDR video is often configured with an S-shaped curve similar to the characteristics of photographic film (for example, see Patent Document 2). This is because, in a video with a limited dynamic range such as an SDR video, reducing the gradation of dark areas increases the difference in brightness from the intermediate tones and increases the sense of detail or contrast.

Japanese Patent Application Publication No. 2016-197854 JP 2018-050293 Publication

Report ITU-R BT.2408-0, “Operational practices in HDR television production” Report ITU-R BT.2446-0, “Methods for conversion of high dynamic range content to standard dynamic range content and vice-versa” K. Nomura, Y. Ikeda, Y. Kusakabe, Y. Nishida, “Method for HDR to SDR Conversion Considering HDR Reference White”, IDW’18, VHF1-4L, p.945-948, (2018)

In a video signal conversion device that converts HDR video to SDR video, the conversion methods of Non-Patent Document 2 and Non-Patent Document 3 described above do not reduce the signal level of the human skin, do not change the white level, and do not change the highlight. This is to suppress overexposure.

However, with this conversion method, the gradation of dark areas cannot be adjusted. For this reason, there is a problem in that, for example, it is not possible to increase the difference in brightness between dark areas and intermediate areas, and it is not possible to emphasize the sense of detail or contrast in dark areas.

In order to adjust the gradation of dark areas, it is assumed that the conversion method of Patent Document 2 mentioned above is used, but the conversion method of Patent Document 2 is applied as is to the conversion method of Non-Patent Document 2 and Non-Patent Document 3. It is difficult to do so.

This is because the conversion method of Patent Document 2 uses a function composed of an S-shaped curve to take into account overexposure of highlights in order to convert an input luminance histogram to an output luminance histogram. The skin signal level and white level are not considered. On the other hand, the conversion methods of Non-Patent Document 2 and Non-Patent Document 3 take into account the signal level of a person's skin, the white level, and the blown-out highlights, and the purpose of the conversion is fundamentally different.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and its purpose is to prevent the signal level of a person's skin from becoming low, the white level to not change, and to suppress blown-out highlights. Furthermore, it is an object of the present invention to provide a video signal conversion device and a program that can correct the gradation of dark areas according to the production intention.

In order to solve the above problem, a video signal conversion device according to claim 1 is a video signal conversion device that converts HDR (High Dynamic Range) video to SDR (Standard Dynamic Range) video, comprising: Converting the linear RGB signal of the HDR video into tristimulus values, converting the tristimulus values into chromaticity coordinates, outputting a luminance signal of the tristimulus values as an HDR luminance signal, and converting the chromaticity coordinates into Compressing the dynamic range of the HDR luminance signal output by the RGB/tristimulus value and chromaticity converter using the output RGB/tristimulus value and chromaticity converter and a predetermined tone mapping function; a compression processing unit that obtains a luminance signal of an SDR video as an SDR luminance signal, the SDR luminance signal obtained by the compression processing unit, and the RGB/tristimulus values and the chromaticity coordinates output by the chromaticity conversion unit, a tristimulus value and chromaticity/RGB conversion unit that converts the tristimulus values into linear RGB signals of the SDR video, and the compression processing unit converts the values of the HDR luminance signal into The tone mapping function has a characteristic that the value of the SDR luminance signal increases as the value of the SDR luminance signal increases, and the tone mapping function has a characteristic of the skin level of the SDR image corresponding to the skin level of the HDR image, and a reference white level of the HDR image. The characteristics of the corresponding white level of the SDR video and the value of the SDR video signal corresponding to the linear RGB signal of the SDR video correspond to the upper limit value of the HDR video signal corresponding to the linear RGB signal of the HDR video, the SDR video signal The tone mapping function has a characteristic of increasing up to an upper limit value of , and further uses the tone mapping function in which the characteristics of the dark area vary according to a gamma value for dark area correction that can adjust the gradation of the dark area in a predetermined area of the SDR luminance signal. , the SDR luminance signal is obtained by compressing the dynamic range of the HDR luminance signal.

The video signal conversion device according to claim 2 is the video signal conversion device according to claim 1, in which the compression processing unit uses an area smaller than a predetermined inflection point in the HDR luminance signal as the tone mapping function. Then, the dynamic range of the HDR luminance signal is compressed by using a power function whose exponent is the gamma value for dark area correction, and by using a logarithmic function in a region above the predetermined inflection point in the HDR luminance signal. It is characterized in that it is obtained as the SDR luminance signal.

Furthermore, the video signal conversion device according to claim 3 is a video signal conversion device that converts an SDR (Standard Dynamic Range) video to an HDR (High Dynamic Range) video, in which linear RGB of the SDR video is An RGB/tristimulus device that converts a signal into tristimulus values, converts the tristimulus values into chromaticity coordinates, outputs a luminance signal of the tristimulus values as an SDR luminance signal, and outputs the chromaticity coordinates. A value and chromaticity converter and a predetermined tone mapping function are used to expand the dynamic range of the SDR luminance signal output by the RGB/tristimulus value and chromaticity converter, and convert the HDR video luminance signal into an extension processing unit that obtains an HDR luminance signal, and converts the HDR luminance signal obtained by the extension processing unit and the RGB/tristimulus values and the chromaticity coordinates output by the chromaticity conversion unit into tristimulus values. , a tristimulus value and chromaticity/RGB conversion unit that converts the tristimulus values into linear RGB signals of the HDR video, and the expansion processing unit converts the HDR luminance signal into The tone mapping function has a characteristic that the value of the luminance signal increases, and the tone mapping function has a characteristic of the skin level of the SDR image corresponding to the skin level of the HDR image, and a characteristic of the skin level of the SDR image corresponding to the reference white level of the HDR image. The white level characteristic and the value of the SDR video signal corresponding to the linear RGB signal of the SDR video increase to an upper limit value of the SDR video signal with respect to the upper limit value of the HDR video signal corresponding to the linear RGB signal of the HDR video. further, the tone mapping function has a characteristic of the dark part in a predetermined area of the SDR brightness signal, and the tone mapping function has different characteristics of the dark part according to a gamma value for dark part correction that can adjust the gradation of the dark part in a predetermined area of the SDR brightness signal. The method is characterized in that the HDR luminance signal is obtained by expanding the dynamic range.

The video signal conversion device according to claim 4 is the video signal conversion device according to claim 3, in which the extension processing unit uses an area smaller than a predetermined inflection point in the SDR luminance signal as the tone mapping function. Then, the dynamic range of the SDR luminance signal is expanded by using a power function whose exponent is the reciprocal of the gamma value for dark area correction, and using an exponential function in the region above the predetermined inflection point in the SDR luminance signal. and obtaining the HDR luminance signal as the HDR luminance signal.

Furthermore, the program according to claim 5 causes a computer to function as the video signal conversion device according to claim 1 or 2.

Furthermore, the program according to claim 6 causes a computer to function as the video signal conversion device according to claim 3 or 4.

As described above, according to the present invention, the signal level of a person's skin does not decrease, the white level does not change, blown-out highlights are suppressed, and the gradation of dark areas can be adjusted according to the production intention. It becomes possible to make corrections.

FIG. 1 is a block diagram showing the configuration of a video signal conversion device according to an embodiment of the present invention. 3 is a flowchart showing processing of the video signal conversion device. FIG. 2 is a block diagram showing the configuration of an RGB/tristimulus value and chromaticity conversion section. FIG. 2 is a block diagram showing the configuration of a tristimulus value and chromaticity/RGB conversion section. FIG. 3 is a diagram illustrating the correspondence between the skin level of an HLG video signal and the skin level of an SDR video signal. FIG. 3 is a diagram illustrating parameters of a tone mapping function used in a compression processing section. FIG. 3 is a diagram illustrating a tone mapping function used in a compression processing section. FIG. 6 is a diagram showing characteristics of display brightness when compressing an HDR brightness signal Y _HDR into an SDR brightness signal Y _SDR . FIG. 9 is an enlarged view showing the characteristics of the dark area in FIG. 8; FIG. 7 is a diagram showing display brightness characteristics when compressing an HDR brightness signal Y _HDR into an SDR brightness signal Y _SDR when the corresponding points of the HDR reference white level Y _HDR,Ref are changed. FIG. 1 is a block diagram showing the configuration of a video signal conversion device that performs inverse conversion according to an embodiment of the present invention. 3 is a flowchart illustrating inverse conversion processing of the video signal conversion device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail using the drawings. When compressing the HDR luminance signal Y _HDR into the SDR luminance signal Y _SDR , the present invention corrects the gradation of dark areas in addition to parameters that take into account the skin level of a person, the reference white level, and the color reproduction of highlights. It is characterized by using a tone mapping function defined by the parameter (gamma value γ _dark for dark part correction). The same applies when expanding the SDR luminance signal Y _SDR to the HDR luminance signal Y _HDR .

As a result, when compressing the HDR luminance signal Y _HDR to the SDR luminance signal Y _SDR , it is possible to perform gradation correction from dark areas to midtones, and dynamic The range can be compressed. Furthermore, when extending the SDR luminance signal Y _SDR to the HDR luminance signal Y _HDR , it becomes possible to perform gradation correction from dark areas to midtones, and expand the dynamic range by using a function that has characteristics opposite to the S-curve. be able to. Note that the opposite characteristics of the S-curve will be described later.

Therefore, the signal level of a person's skin does not decrease, the white level does not change, and blown-out highlights are suppressed, and it is also possible to correct the gradation of dark areas according to the production intention.

[Video signal conversion device]
First, a video signal conversion device that compresses the HDR luminance signal Y _HDR into the SDR luminance signal Y _SDR will be described. FIG. 1 is a block diagram showing the configuration of a video signal conversion device according to an embodiment of the present invention, and FIG. 2 is a flowchart showing processing of the video signal conversion device.

This video signal conversion device 1 is a device that converts an HDR linear RGB signal E _HDR into an SDR linear RGB signal E _SDR . The video signal conversion device 1 includes an RGB/tristimulus value and chromaticity conversion section 10, a compression processing section 11, and a tristimulus value and chromaticity/RGB conversion section 12.

The RGB/tristimulus value and chromaticity conversion unit 10 inputs the HDR linear RGB signal E _HDR (step S201). Then, the RGB/tristimulus value and chromaticity conversion unit 10 converts the HDR linear RGB signal E _HDR into HDR tristimulus values X _HDR , Y _HDR , and Z _HDR (step S202).

The RGB/tristimulus value and chromaticity conversion unit 10 converts the HDR tristimulus values X _HDR , Y _HDR , and Z _HDR into chromaticity coordinates x, y (step S203). Then, the RGB/tristimulus value and chromaticity conversion section 10 outputs the HDR luminance signal Y _HDR of the HDR tristimulus values X _HDR , Y _HDR , and Z _HDR to the compression processing section 11 . Further, the RGB/tristimulus value and chromaticity conversion section 10 outputs the chromaticity coordinates x, y to the tristimulus value and chromaticity/RGB conversion section 12 . Details of the RGB/tristimulus value and chromaticity conversion unit 10 will be described later.

The compression processing unit 11 receives the HDR luminance signal Y _HDR from the RGB/tristimulus value and chromaticity conversion unit 10 . Then, the compression processing unit 11 compresses the dynamic range of the HDR luminance signal Y _HDR using a tone mapping function defined by parameters such as a predetermined dark part correction gamma value γ _dark , and obtains an SDR luminance signal Y _SDR . (Step S204). The compression processing section 11 outputs the SDR luminance signal Y _SDR to the tristimulus value and chromaticity/RGB conversion section 12 .

Here, the parameters such as the predetermined dark part correction gamma value γ _dark may be input as preset data. Furthermore, some of the parameters, such as the predetermined gamma value γ _dark for dark area correction, may be calculated by predetermined arithmetic processing, as will be explained later with reference to FIG. Details of the compression processing unit 11 will be described later.

The tristimulus value and chromaticity/RGB conversion section 12 receives the SDR luminance signal Y _SDR from the compression processing section 11 and also receives the chromaticity coordinates x, y from the RGB/tristimulus value and chromaticity conversion section 10 . Then, the tristimulus value and chromaticity/RGB conversion unit 12 converts the SDR luminance signal Y _SDR and chromaticity coordinates x, y into SDR tristimulus values X _SDR , Y _SDR , and Z _SDR (step S205).

The tristimulus value and chromaticity/RGB conversion unit 12 converts the SDR tristimulus values X _SDR , Y _SDR , Z _SDR into an SDR linear RGB signal E _SDR (step S206), and outputs the SDR linear RGB signal E _SDR ( Step S207). Details of the tristimulus values and chromaticity/RGB conversion section 12 will be described later.

(RGB/tristimulus value and chromaticity conversion unit 10)
Next, the RGB/tristimulus value and chromaticity conversion unit 10 shown in FIG. 1 will be described in detail. As described above, the RGB/tristimulus value and chromaticity conversion unit 10 converts the HDR linear RGB signal E _HDR into HDR tristimulus values X _HDR , Y _HDR , Z _HDR , and converts the HDR luminance signal Y _HDR and chromaticity coordinate x , y.

FIG. 3 is a block diagram showing the configuration of the RGB/tristimulus value and chromaticity conversion section 10. As shown in FIG. The RGB/tristimulus value and chromaticity conversion unit 10 includes an RGB/tristimulus value conversion unit 20 and a tristimulus value/chromaticity conversion unit 21.

The RGB/tristimulus value converter 20 receives the HDR linear RGB signal E _HDR ={R _HDR , G _HDR , B _HDR }. Then, the RGB/tristimulus value conversion unit 20 calculates the HDR tristimulus values X _HDR , Y HDR , Z _HDR from the HDR linear RGB signal E _HDR = {R _HDR , G _{HDR ,} B _HDR } using the following formula. .

Here, the HDR linear RGB signal E _HDR corresponds to both HLG and PQ (Perceptual Quantization) in the HDR method, and the scene light in each method and the scene light are OOTF (OptoF). Optical Transfer Function: Compatible with both processed display light. The SDR linear RGB signal E _SDR , which will be described later, also corresponds to both scene light and display light.

The RGB/tristimulus value conversion section 20 outputs the HDR luminance signal Y _HDR of the HDR tristimulus values X _HDR , Y _HDR , and Z _HDR to the compression processing section 11 . Further, the RGB/tristimulus value converter 20 outputs the HDR tristimulus values X _HDR , Y _HDR , and Z _HDR to the tristimulus value/chromaticity converter 21 .

The tristimulus value/chromaticity conversion section 21 receives the HDR tristimulus values X _HDR , Y _HDR , and Z _HDR from the RGB/tristimulus value conversion section 20 . Then, the tristimulus value/chromaticity conversion unit 21 calculates the chromaticity coordinates x, y from the HDR tristimulus values X _HDR , Y _HDR , and Z _HDR using the following formula. The tristimulus value/chromaticity conversion section 21 outputs the chromaticity coordinates x, y to the tristimulus value and chromaticity/RGB conversion section 12 .

(Compression processing unit 11)
Next, the compression processing section 11 shown in FIG. 1 will be explained in detail. As described above, the compression processing unit 11 compresses the dynamic range of the HDR luminance signal Y HDR converted by the RGB/tristimulus value and chromaticity conversion unit 10 using a predetermined tone mapping function, and converts the dynamic range of the HDR luminance signal Y _HDR into an SDR luminance signal Y. Find _SDR .

This tone mapping function is composed of a combination of a linear function and a logarithmic function. In addition to a function having straight line characteristics, this linear function includes a power function for correcting the sense of detail and contrast of dark gradations. The linear function is used for the purpose of matching the appearance of dark areas and midtones of the HDR image, and the logarithmic function is used for the purpose of preserving the gradation of highlights as much as possible in the SDR image.

The compression processing unit 11 compresses the dynamic range of the HDR luminance signal Y _HDR using the following formula to obtain the SDR luminance signal Y _SDR . The function shown in the upper part of Equation (3) below is a linear function (a power function whose exponent is the preset dark area correction gamma value γ _dark (if γ _dark = 1, a function with linear characteristics)) , and the function shown at the bottom is a logarithmic function.

Here, the gain value k ₁ , the correction values k ₂ , k ₃ , k ₄ , the gamma value γ _dark for dark area correction, the SDR inflection point Y _SDR,ip and the HDR inflection point Y _HDR,ip are expressed by the above formula (3 ) are the parameters of the tone mapping function shown in

The tone mapping function of equation (3) is a formula for calculating the SDR brightness signal Y _SDR from _the HDR brightness signal Y _HDR , and as shown in FIGS. 7 to 10, which will be described later, The SDR luminance signal Y has an S-curve characteristic in which the value of _SDR increases. More specifically, this tone mapping function is such that as the value of the HDR brightness signal Y _HDR increases from 0 to the upper limit value, the amount of change in the value of the SDR brightness signal Y _SDR is initially small, then becomes large, and It has an S-curve characteristic that becomes smaller.

In particular, in the dark region that is the region of the upper equation (Y _HDR <Y _HDR,ip ) in the above equation (3), the HDR luminance signal Y _HDR is changed from the SDR so that the gradation of the HDR luminance signal Y _HDR remains. The luminance signal Y is converted into _SDR .

Specifically, this tone mapping function has characteristics of the skin level of the SDR image corresponding to the skin level of the HDR image, has characteristics of the white level of the SDR image corresponding to the reference white level of the HDR image, The value of the SDR video signal has a characteristic of increasing to the upper limit value of the SDR video signal corresponding to the upper limit value of the HDR video signal. Here, the SDR video signal is a signal corresponding to the SDR linear RGB signal E _SDR , and the HDR video signal is a signal corresponding to the HDR linear RGB signal E _HDR . Furthermore, this tone mapping function has a preset gamma value _γdark for dark area correction that can adjust the gradation of the dark area in the region (Y _HDR < Y _HDR,ip ) of the upper equation of equation (3). This is a function that is set so that the characteristics of the area differ depending on the parameters.

The gain value k ₁ is a parameter set based on the correspondence between the skin levels of the HDR video and the SDR video. The correction value k ₂ is a parameter calculated from the continuity condition of the first-order differential value at the inflection point (Y _HDR,ip , Y _SDR,ip ) of the linear function and the logarithmic function. The correction value k ₃ is a parameter determined from the constraint condition of the HDR reference white level Y _HDR,Ref . The correction value k ₄ is a parameter calculated from the continuity condition of the values at the inflection points (Y _HDR,ip , Y _SDR,ip ) of the linear function and the logarithmic function.

The dark area correction gamma value γ _dark is a parameter for correcting the gradation of the dark area, and takes a real number satisfying γ _dark ≧1. The SDR inflection point Y _SDR,ip is a parameter representing a linear signal value of the SDR luminance signal Y _SDR corresponding to a preset inflection point. The HDR inflection point Y _HDR,ip is a parameter representing a linear signal value of the HDR luminance signal Y _HDR corresponding to the SDR inflection point Y _{SDR ,ip} in the SDR luminance signal Y SDR.

Of these seven parameters _, five parameters (gain value k ₁ , correction values k ₂ , k ₃ , k _{4 and} HDR inflection point Y _HDR,ip ) is an unknown quantity. However, these unknowns can be reduced to one parameter of the correction value _k3 . The five parameters will be explained below in the order of gain value k ₁ , HDR inflection point Y _HDR,ip , correction value k ₂ , correction value k ₄ , and correction value k ₃ .

(gain value k ₁ )
As described above, the gain value k ₁ is a parameter that is set based on the correspondence between the skin levels of the HDR video and the SDR video.

Generally, gain processing is performed to make the HDR luminance signal 200 [cd/m ² ] correspond to the SDR luminance signal between 88 and 100 [cd/m ² ]. Thereby, it is possible to realize conversion from HLG video to SDR video in consideration of the reference white level. As an example of preventing a sudden loss of gradation in HLG highlight areas when considering the reference white level, a gain that makes the reference white level 75% of the HLG video signal correspond to the white level 96% of the SDR video signal is used. processing is used.

FIG. 5 is a diagram illustrating the correspondence between the skin level of the HLG video signal and the skin level of the SDR video signal. In this figure, the results of analyzing the correspondence of skin levels of the same person in HLG video, which is a type of HDR video, and SDR video of the same individually produced content are plotted (circled). ). This HLG video program is content produced based on the standard white level of 75% (0.75 in FIG. 5) of the HLG video signal.

The horizontal axis shows the level of the HLG video signal, and the vertical axis shows the level of the SDR video signal. The solid line shows the characteristics of the prior art, and the broken line shows the characteristics of an example in which skin level is considered. Point α2 will be explained later in FIG. 8, and point β2 will be explained later in FIG. 10.

Note that the HLG video signal is a signal corresponding to the HDR linear RGB signal E _HDR inputted to the RGB/tristimulus value and chromaticity conversion section 10. Further, the SDR video signal is a signal corresponding to the SDR linear RGB signal E _SDR output by the tristimulus value and chromaticity/RGB conversion section 12.

From the analysis results shown in FIG. 5, the average value of the skin level Vh _skin of the HLG video signal is approximately 50% of that of the HLG video signal, while the average value of the skin level Vs _skin of the SDR video signal is approximately 50% of that of the SDR video signal. It can be seen that it is about 67% of the total.

The characteristics of the prior art are the characteristics that apply gain processing to make the HLG luminance signal 200 [cd/m ² ] correspond to the SDR luminance signal 88 [cd/m ² ], and the skin level Vh _skin of the HLG video signal is approximately The skin level of the SDR video signal corresponding to 50% is about 55%. Therefore, the skin level of the SDR video in the prior art is lower than the skin level of the SDR video in the example in which the skin level is considered.

In other words, in order to take the skin level into consideration, it is necessary to perform gain processing to match the skin levels as shown by the broken line in FIG. Therefore, for the gain value k ₁ , one of the following three equations is used depending on the assumed linear signal.

The above equation (4) indicates the gain value k ₁ when the normalized signal of the display light is used as a reference. The above equation (5) indicates the gain value k ₁ when the luminance value [cd/m ² ] of the display light is used as a reference. The above equation (6) indicates the gain value k ₁ when the normalized signal of the scene light is used as a reference. L _W in the above equation (5) is the peak brightness of the display.

Here, from the analysis results shown in FIG. 5, as an example of gain processing that takes the skin level into consideration, the HLG skin level Vh _skin 50% is made to correspond to the SDR skin level Vs _skin 67%. The gain value k ₁ at that time is 7.5439 in the above equation (4), and 0.7544 in the above equation (5) when the display peak luminance of the HLG video signal is 1000 [cd/m ² ], In the above equation (6), it becomes 5.4272. Note that in device implementation, this gain value _k1 may be a parameter arbitrarily set by the user.

(HDR inflection point Y _HDR,ip )
As described above, the HDR inflection point Y _HDR,ip is a parameter representing the linear signal value of the HDR luminance signal Y _HDR corresponding to the SDR inflection point Y _{SDR ,ip in the SDR luminance signal Y SDR} .

The HDR inflection point Y _HDR,ip is calculated using the following formula, and the SDR inflection point Y _{SDR ,ip} corresponding to the knee point when converting the HDR brightness signal Y _HDR to the SDR brightness signal Y SDR and the skin level are It is calculated from the reflected gain value _k1 .

Here, the SDR inflection point Y _SDR,ip is a value obtained by converting the inflection point corresponding to the knee point in the SDR video signal into the SDR luminance signal Y _SDR of the scene light or display light. If this SDR inflection point Y _SDR,ip is set to a high value, signals above the reference white level of the HDR video signal will be compressed from that inflection point to 109%, which is the upper limit of the HDR video signal. , a banding effect with reduced gradation tends to occur in the highlight area. Therefore, it is desirable to set the inflection point at a point slightly higher than the skin level of the SDR video shown in FIG. 5. Therefore, the range of values that the SDR inflection point Y _SDR,ip can take is from 80% or more of the SDR video signal to the signal level corresponding to the point corresponding to the reference white level described later, and can be set arbitrarily by the user. You can.

(Correction value k ₂ )
As described above, the correction value k ₂ is a parameter calculated from the continuity condition of the first-order differential value at the inflection point (Y _HDR,ip , Y _SDR,ip ) of the linear function and the logarithmic function.

Since the first-order differential values at the inflection point (Y _HDR,ip , Y _SDR,ip ) match in the upper linear function and the lower logarithmic function of the equation (3), the following equation is derived.

The above equation (8) is calculated using the unknown correction value k ₃ , the gain value k ₁ set by the user, the SDR inflection point Y _SDR,ip , and the HDR inflection point Y calculated using the above equation (7). By rearranging using _HDR,ip , the following formula is derived.

(Correction value k ₄ )
As described above, the correction value k ₄ is a parameter calculated from the continuity condition of the values at the inflection points (Y _HDR,ip , Y _SDR,ip ) of the linear function and the logarithmic function.

In the tone mapping curve, which is the characteristic of the lower equation of the above equation (3), the gain value k ₁ and the SDR inflection point Y _SDR,ip set by the user, and the HDR inflection calculated using the above equation (7). By rearranging using the point Y _HDR,ip and the correction value k ₂ calculated by the above equation (9), the following equation is derived.

The four parameters shown so far, the gain value k ₁ , the HDR inflection point Y _HDR,ip , and the correction values k ₂ , k ₄ , are based on the gain value k ₁ and the SDR inflection point Y _SDR,ip set by the user in advance. The correction value k ₃ is set and is uniquely determined by calculating the unknown value.

(Correction value k ₃ )
As described above, the correction value k ₃ is a parameter determined from the constraint condition of the HDR reference white level Y _HDR,Ref .

The correction value k ₃ is calculated by determining the correspondence between the HDR reference white level Y _HDR,Ref and the SDR white level Y _SDR,WP . Specifically, the correction value k ₃ is calculated between the inflection points (Y _{HDR,ip ,} Y _{SDR ,ip ) of the HDR inflection point Y HDR,ip and the SDR inflection point Y SDR,ip which are knee points, and the HDR inflection point Y HDR,ip} which is the knee point. The amount of change in the SDR luminance signal Y _SDR between the reference white level Y _HDR,Ref and the reference white level point (Y _HDR,Ref , Y _{SDR,WP )} of the SDR white level Y SDR,WP and the above equation (3) It is calculated by the following equation in which the amount of change in the SDR luminance signal Y _SDR of the tone mapping curve in the lower equation matches.

Note that the HDR reference white level Y _HDR,Ref is the linear signal value of the scene light or display light of 75% of the HLG video signal, as well as the linear signal value of the scene light or display light of 75% to 109% of the HLG video signal. Among these, parameters that can be set arbitrarily by the user may be used.

The correction value k ₃ calculated by the above equation (11) is uniquely obtained. As a result, all five parameters (gain value k ₁ , correction values k ₂ , k ₃ , k ₄ and HDR inflection point Y _HDR,ip ) out of the seven parameters in equation (3) can be set. Can be done.

(tone mapping function parameters)
The tone mapping function is as shown in equation (3) above, and its parameters are gain value k ₁ , correction values k ₂ , k ₃ , k ₄ , gamma value for dark area correction γ _dark , SDR inflection point Y _SDR,ip and HDR inflection point Y _HDR,ip , a total of seven.

These seven parameters may be set in advance by the user in the video signal conversion device 1 shown in FIG. 1, taking into consideration the equations (4) to (11).

Furthermore, among these seven parameters, the gain value k ₁ , the gamma value γ _dark for dark area correction, and the SDR inflection point Y _SDR,ip are set in advance by the user, and the correction values k ₂ , k ₃ , k ₄ and HDR The inflection point Y _HDR,ip may be calculated by the compression processing unit 11 using equations (7), (9), (10), and (11). In this case, the reference white level points (Y _HDR,Ref , Y _SDR ,WP ) of the HDR reference white level Y _HDR,Ref and the SDR white level Y _SDR,WP are also set in advance by the user.

Specifically, the compression processing unit 11 calculates the correction value k ₃ using the above equation (11), calculates the HDR inflection point Y HDR,ip using the above equation (7), and calculates the HDR inflection point Y _HDR,ip using the above equation (9). A correction value k ₂ is calculated using the equation (10), and a correction value k ₄ is calculated using the equation (10). Since seven parameters are thereby set, the tone mapping function of equation (3) above can be configured.

FIG. 6 is a diagram illustrating parameters of the tone mapping function used in the compression processing section 11. In this figure, the compression processing unit 11 uses a preset gain value k ₁ , an SDR inflection point Y _{SDR,ip ,} a gamma value γ _dark for dark area correction, an HDR reference white level Y _HDR,Ref, and an SDR white level Y _{SDR , WP} is input and the HDR inflection point Y _HDR,ip and correction values k ₂ , k ₃ , k ₄ are calculated.

The compression processing unit 11 includes calculation units 24, 25, 26, and 27 for calculating the HDR inflection point Y _HDR,ip and correction values k ₂ , k ₃ , and k ₄ .

The calculation unit 24 inputs the preset gain value k ₁ and the SDR inflection point Y _SDR,ip , and calculates the above equation (7) to obtain the HDR inflection point Y _HDR,ip . Then, the calculation unit 24 outputs the HDR inflection point Y _HDR,ip to the calculation units 25, 26, and 27.

The calculation unit 27 inputs a preset gain value k ₁ , an SDR inflection point Y _SDR,ip , a gamma value γ _dark for dark area correction, an HDR reference white level Y _HDR,Ref, and an SDR white level Y _SDR,WP. At the same time, the HDR inflection point Y _HDR,ip is input from the calculation unit 24. Then, the calculation unit 27 calculates the correction value k ₃ by calculating the equation (11), and outputs the correction value k ₃ to the calculation units 25 and 26.

The calculation unit 25 inputs the preset gain value k ₁ , the SDR inflection point Y _SDR,ip , and the gamma value γ _dark for dark area correction, and also inputs the HDR inflection point Y _HDR,ip from the calculation unit 24. , further inputs the correction value k ₃ from the calculation unit 27. Then, the calculation section 25 calculates the correction value k ₂ by calculating the above equation (9), and outputs the correction value k ₂ to the calculation section 26 .

The calculation unit 26 inputs the preset gain value k ₁ , the SDR inflection point Y _SDR,ip , and the gamma value γ _dark for dark area correction, and also inputs the HDR inflection point Y _HDR,ip from the calculation unit 24. . Further, the calculation unit 26 inputs the correction value k ₂ from the calculation unit 25 and further inputs the correction value k ₃ from the calculation unit 27 . Then, the calculation unit 26 calculates the correction value k ₄ by calculating the above equation (10), and outputs the correction value k ₄ .

As a result, in the compression processing unit 11, the preset gain value k ₁ , the SDR inflection point Y _SDR,ip , the gamma value for dark area correction γ _dark , the HDR reference white level Y _HDR,Ref and the SDR white level Y _{SDR , WP} , the HDR inflection point Y _HDR,ip and the correction values k ₂ , k ₃ , k ₄ are calculated. Then, with a total of seven parameters: gain value k ₁ , correction values k ₂ , k ₃ , k ₄ , gamma value γ _dark for dark area correction, SDR inflection point Y _SDR,ip , and HDR inflection point Y _HDR,ip , The tone mapping function of equation (3) is configured, and compression processing is performed by the compression processing unit 11.

FIG. 7 is a diagram illustrating a tone mapping function used in the compression processing section 11. This tone mapping function has a tone mapping curve made up of seven parameters, and has an S-curve characteristic of increasing values as a whole.

The inflection points (Y _HDR,ip , Y _SDR,ip ) of this tone mapping function are, for example, the SDR inflection point Y _SDR,ip =58.5357 and the HDR inflection point Y _HDR,ip =77.5924. The tone mapping function has the characteristic of the linear function in the upper stage of the equation (3) in the region where the HDR luminance signal Y _HDR <Y _HDR,ip = 77.5924, and the HDR luminance signal Y _HDR ≧Y _HDR,ip = In the region of 77.5924, it has the characteristics of the logarithmic function in the lower half of the equation (3).

More specifically, as the value of the HDR brightness signal Y _HDR increases from 0 to the upper limit value, the value of the SDR brightness signal Y _SDR changes in the region of HDR brightness signal Y _HDR < Y _HDR,ip = 77.5924. The amount is small at first and then becomes large. Further, in the region where the HDR brightness signal Y _HDR ≧Y _HDR,ip =77.5924, the amount of change in the value of the SDR brightness signal Y _SDR is large at first and then becomes small.

By using a tone mapping function with such characteristics, a linear function is used in the dark area where the HDR luminance signal Y _HDR < Y _HDR,ip = 77.5924, and the gradation does not decrease. . Therefore, the sense of detail or contrast can be emphasized in dark areas.

As shown in FIGS. 8 and 9, which will be described later, the characteristics of the linear function in the dark region can be changed depending on the preset gamma value γ _dark for dark region correction. Therefore, by changing the gamma value γ _dark for dark area correction, the gradation expression of dark areas can be adjusted in accordance with the user's production intention.

(Example of parameters)
An example of tone mapping function parameters will be explained. For example, the gamma value for dark area correction γ _dark = 1, the SDR inflection point Y _SDR,ip is set to a value corresponding to 80% of the SDR video signal, and the correspondence of the skin level is set to 50% of the HLG video signal and 67% of the SDR video signal. Assume that, as a constraint on the reference white level, the HDR reference white level Y _HDR,Ref corresponds to 96% of the SDR video signal. In this case, HDR inflection point Y _HDR,ip = 77.5924, gain value k ₁ = 0.7544, correction value k ₂ = 17.1209, correction value k ₃ = 0.7075, correction value k ₄ = 79. It becomes 5822.

Furthermore, when the gamma value for dark area correction γ _dark = 1.1, the HDR inflection point Y _HDR,ip = 77.5924, the gain value k ₁ = 0.7544, the correction value k ₂ = 15.8855, the correction value k ₃ =0.7533, and correction value k ₄ =80.7673.

In this way, when the gamma value γ _dark for dark area correction is adjusted, the HDR inflection point Y _HDR,ip and the gain value k ₁ are not changed, but only the correction values k ₂ , k ₃ , and k ₄ are changed. .

(Example of tone mapping curve)
Next, an example of a tone mapping curve will be explained. FIG. 8 is a diagram showing display brightness characteristics when compressing the HDR brightness signal Y _HDR into the SDR brightness signal Y _SDR . Specifically, when deriving the correction value k ₃ , the characteristics are shown when a constraint condition is given such that the HDR reference white level Y _HDR,Ref =203 becomes the SDR white level Y _SDR,WP =90 (point α1). Point α1, which is the reference white level point (Y _HDR,Ref , Y _SDR,WP ) in FIG. 8, corresponds to, for example, 75% of the HLG video signal and 96% of the SDR video signal (point α2 in FIG. 5). FIG. 9 is an enlarged view showing the characteristics of the dark area in FIG. 8.

FIG. 10 is a diagram showing display brightness characteristics when compressing the HDR brightness signal Y _HDR into the SDR brightness signal Y _SDR when the corresponding points of the HDR reference white level Y _HDR,Ref are changed. Specifically, when deriving the correction value k ₃ , the characteristics are shown when a constraint condition is given such that the HDR reference white level Y _HDR,Ref =1000 becomes the SDR white level Y _SDR,WP =123 (point β1). Point β1, which is the reference white level point (Y _HDR,Ref , Y _SDR,WP ) in FIG. 10, corresponds to, for example, 100% of the HLG video signal and 109% of the SDR video signal (point β2 in FIG. 5). These characteristics show a tone mapping curve when the dark part correction gamma value γ _dark is adjusted within the range of 1≦γ _dark ≦1.5.

As shown in FIGS. 8 to 10, by changing the gamma value γ _dark for dark area correction, the tone mapping curve changes, and the characteristics of the tone mapping function according to the value of the gamma value γ _dark for dark area correction can be obtained. Can be done. In particular, in the dark region shown in FIG. 9, the lower linear function of equation (3) is used as the tone mapping function.

When the conventional S-curve function is used, the change in the SDR luminance signal Y _SDR with respect to the change in the HDR luminance signal Y _HDR is small in the dark area.

On the other hand, when the tone mapping function of the embodiment of the present invention is used, the change in the SDR luminance signal Y _SDR with respect to the change in the HDR luminance signal Y _HDR is larger in the dark area than in the conventional S-shaped curve. In the dark area shown in FIGS. 8 to 10, in contrast to the straight line characteristic when the gamma value for dark area correction γ _dark = 1, the characteristic when 1<γ _dark ≦1.5 is downward (HDR luminance signal Y The curve is sunken in the direction of _{the HDR} axis. Therefore, when converting the HDR luminance signal Y _HDR to the SDR luminance signal Y _SDR , the gradation of the HDR luminance signal Y _HDR remains.

In the tone mapping function of the embodiment of the present invention, the degree of change in the SDR luminance signal Y _SDR with respect to the change in the HDR luminance signal Y _HDR can be set according to the value of the gamma value γ _dark for dark area correction.

Therefore, the sense of detail or contrast can be emphasized in dark areas, and by changing the gamma value γ _dark for dark area correction, the gradation expression of dark areas can be adjusted according to the user's production intention.

(Tristimulus values and chromaticity/RGB conversion unit 12)
Next, the tristimulus value and chromaticity/RGB conversion unit 12 shown in FIG. 1 will be described in detail. As described above, the tristimulus value and chromaticity/RGB conversion unit 12 converts the SDR luminance signal Y _SDR and the RGB/tristimulus value obtained by the compression processing unit 11 and the chromaticity coordinate x, converted by the chromaticity conversion unit 10, y is converted into SDR tristimulus values X _SDR , Y _SDR , and Z _SDR to obtain an SDR linear RGB signal E _SDR .

FIG. 4 is a block diagram showing the configuration of the tristimulus value and chromaticity/RGB conversion section 12. The tristimulus value and chromaticity/RGB conversion unit 12 includes a chromaticity/tristimulus value conversion unit 22 and a tristimulus value/RGB conversion unit 23.

The chromaticity/tristimulus value conversion section 22 receives the SDR luminance signal Y _SDR from the compression processing section 11 and receives the RGB/tristimulus values and chromaticity coordinates x, y from the chromaticity conversion section 10 . Then, the chromaticity/tristimulus value conversion unit 22 converts two of the SDR tristimulus values X _SDR , Y _SDR , and Z _SDR from the SDR luminance signal Y _SDR and the chromaticity coordinates x, y using the following formula. Find the stimulus values X _SDR and Z _SDR .

The chromaticity/tristimulus value converter 22 outputs the SDR tristimulus values X _SDR , Y _SDR , and Z _SDR to the tristimulus value/RGB converter 23 .

The tristimulus value/RGB converter 23 inputs the SDR tristimulus values X _SDR , Y _SDR , and Z _SDR from the chromaticity/tristimulus value converter 22 . Then, the tristimulus value/RGB conversion unit 23 calculates the SDR linear RGB signal E _SDR = {R _SDR , G _SDR , B _SDR } from the SDR tristimulus values X _SDR , Y _SDR , Z _SDR using the following formula. . The tristimulus value/RGB conversion unit 23 outputs an SDR linear RGB signal E _SDR ={R _SDR , G _SDR , B _SDR }.

Here, if there is a signal (R _SDR , G _SDR , B _SDR ) in which the SDR linear RGB signal E _SDR <0, the tristimulus value/RGB conversion unit 23 sets the signal to 0 and outputs it.

As described above, according to the video signal conversion device 1 of the embodiment of the present invention, the RGB/tristimulus value and chromaticity conversion unit 10 converts the HDR linear RGB signal E _HDR into the HDR tristimulus values X _HDR , Y _HDR , The _HDR tristimulus values X _HDR , Y _HDR , and Z _HDR are converted to chromaticity coordinates x, y.

In addition to parameters that take into account the skin level of a person, the standard white level, and the color reproduction of highlights, the compression processing unit 11 uses a tone mapping function defined by a gamma value γ _dark for correcting dark areas to correct the gradation of dark areas. is used to compress the dynamic range of the HDR luminance signal Y _HDR and obtain the SDR luminance signal Y _SDR . This tone mapping function is composed of a combination of a linear function and a logarithmic function.

The tristimulus value and chromaticity/RGB conversion unit 12 converts the SDR luminance signal Y _SDR and chromaticity coordinates x, y into SDR tristimulus values X _SDR , Y _SDR , Z _SDR , and converts the SDR tristimulus values X _SDR , Y _SDR , Z Convert _SDR to SDR linear RGB signal E _SDR .

Thereby, by changing the gamma value γ _dark for dark area correction, the shape of the tone mapping curve can be changed, and gradation correction from dark areas to intermediate tones can be performed. Then, the dynamic range of the HDR luminance signal Y _HDR can be compressed using a commonly used function having S-curve characteristics. Therefore, the signal level of a person's skin does not decrease, the white level does not change, and blown-out highlights are suppressed, and it is also possible to correct the gradation of dark areas according to the production intention.

In addition, in the production of a program that integrates HLG video and SDR video, by producing a program using HLG video and passing it through the video signal conversion device 1, the skin level is as if the program was produced using SDR video, and the reference white level is It is possible to generate an SDR image in which blown-out highlights are also suppressed.

In addition, for example, it is possible to convert a PQ signal that takes into account the standard white level into SDR video, and it is now possible to correct the color reproduction of highlights according to the production intention, and to adjust the gradation of dark areas according to the production intention. It becomes possible to make corrections. The video signal conversion device 1 can be introduced into an integrated production system that is expected to become popular in the future, and furthermore, the video signal conversion device 1 can be installed in an external tuner or receiver.

[Video signal conversion device that performs inverse conversion]
Next, a description will be given of a device that performs inverse conversion to the processing of the video signal conversion device 1 shown in FIG. 1, that is, a video signal conversion device that expands the SDR luminance signal Y _SDR to the HDR luminance signal Y _HDR .

FIG. 11 is a block diagram showing the configuration of a video signal conversion device that performs inverse conversion according to an embodiment of the present invention, and FIG. 12 is a flowchart showing the inverse conversion process of the video signal conversion device.

This video signal conversion device 2 is a device that converts an SDR linear RGB signal E _SDR into an HDR linear RGB signal E _HDR . The video signal conversion device 2 includes an RGB/tristimulus value and chromaticity conversion section 30, an extension processing section 31, and a tristimulus value and chromaticity/RGB conversion section 32.

The RGB/tristimulus value and chromaticity conversion unit 30 receives the SDR linear RGB signal E _SDR (step S1201). Then, the RGB/tristimulus value and chromaticity conversion unit 30 converts the SDR linear RGB signal E _SDR into SDR tristimulus values X _SDR , Y _SDR , and Z _SDR (step S1202).

The RGB/tristimulus value and chromaticity conversion unit 30 converts the SDR tristimulus values X _SDR , Y _SDR , and Z _SDR into chromaticity coordinates x, y (step S1203). Then, the RGB/tristimulus value and chromaticity conversion section 30 outputs the SDR luminance signal Y _SDR of the SDR tristimulus values X _SDR , Y _SDR , and Z _SDR to the expansion processing section 31 . Further, the RGB/tristimulus value and chromaticity conversion section 30 outputs the chromaticity coordinates x, y to the tristimulus value and chromaticity/RGB conversion section 32 . That is, the RGB/tristimulus value and chromaticity converter 30 performs the reverse process of the tristimulus value and chromaticity/RGB converter 12 shown in FIG.

The internal configuration of the RGB/tristimulus value and chromaticity conversion unit 30 is the same as that of the RGB/tristimulus value and _chromaticity conversion unit 10 shown in _FIG. This is the same as replacing the HDR luminance signal Y _HDR with the SDR luminance signal Y _SDR , and replacing the HDR tristimulus values X _HDR , Y _HDR , Z _HDR with the SDR tristimulus values X _SDR , Y _SDR , Z _SDR . , the explanation is omitted.

The expansion processing section 31 receives the SDR luminance signal Y _SDR from the RGB/tristimulus value and chromaticity conversion section 30 . Then, the expansion processing unit 31 expands the dynamic range of the SDR brightness signal Y _SDR using a tone mapping function defined by parameters such as a predetermined gamma value γ _dark for dark area correction, and obtains the HDR brightness signal Y _HDR. (Step S1204). The extension processing section 31 outputs the HDR luminance signal Y _HDR to the tristimulus value and chromaticity/RGB conversion section 32 . That is, the expansion processing section 31 performs the reverse processing of the compression processing section 11 shown in FIG.

Here, similarly to the compression processing unit 11 shown in FIG. 1, parameters such as the predetermined dark part correction gamma value γ _dark may be set in advance by the user. Further, the gain value k ₁ etc. may be set in advance by the user, and the correction value k ₂ etc. may be calculated by calculation.

This tone mapping function is an inverse function of the tone mapping function used by the compression processing unit 11 shown in FIG. 1, and is composed of a combination of a linear function and an exponential function. In addition to a function having straight line characteristics, this linear function includes a power function for correcting the sense of detail and contrast of dark gradations.

The expansion processing unit 31 expands the dynamic range of the SDR brightness signal Y _SDR and obtains the HDR brightness signal Y _HDR using the following equation. In equation (14) below, the function shown in the upper row is a linear function (a power function whose exponent is the reciprocal of the preset dark area correction gamma value γ _dark ), and the function shown in the lower row is an exponential function. .

Here, the gain value k ₁ , the correction values k ₂ , k ₃ , k ₄ , the gamma value γ _dark for dark area correction, the SDR inflection point Y _SDR,ip and the HDR inflection point Y _HDR,ip are expressed by the above formula (14 ) is the parameter of the tone mapping function shown in (3), and is as explained in equation (3) above.

The tone mapping function of formula (14) is a formula for calculating the HDR luminance signal Y _HDR from the SDR luminance signal Y _SDR , and has the characteristics with the horizontal and vertical axes of FIGS. It has a characteristic opposite to an S-shaped curve in which the value of the HDR luminance signal Y _HDR increases as the value of Y _SDR increases. More specifically, this tone mapping function is such that as the value of the SDR brightness signal Y _SDR increases from 0 to the upper limit value, the amount of change in the value of the HDR brightness signal Y _HDR is initially large, then becomes small, and It has the opposite characteristics of an S-shaped curve, such that it becomes larger.

Specifically, this tone mapping function has characteristics of the skin level of the SDR image corresponding to the skin level of the HDR image, has characteristics of the white level of the SDR image corresponding to the reference white level of the HDR image, The value of the SDR video signal has a characteristic of increasing to the upper limit value of the SDR video signal corresponding to the upper limit value of the HDR video signal. Here, the SDR video signal is a signal corresponding to the SDR linear RGB signal E _SDR , and the HDR video signal is a signal corresponding to the HDR linear RGB signal E _HDR . Furthermore, this tone mapping function has a preset gamma value _γdark for dark area correction that can adjust the gradation of the dark area in the region (Y _SDR < Y _SDR,ip ) of the upper equation of equation (14). This is a function that is set so that the characteristics of the area differ depending on the parameters.

The tristimulus value and chromaticity/RGB conversion section 32 receives the HDR luminance signal Y _HDR from the expansion processing section 31 and receives the chromaticity coordinates x, y from the RGB/tristimulus value and chromaticity conversion section 30 . Then, the tristimulus value and chromaticity/RGB conversion unit 32 converts the HDR luminance signal Y _HDR and chromaticity coordinates x, y into HDR tristimulus values X _HDR , Y _HDR , and Z _HDR (step S1205).

The tristimulus value and chromaticity/RGB conversion unit 32 converts the HDR tristimulus values X _HDR , Y _HDR , and Z _HDR into an HDR linear RGB signal E _HDR (step S1206), and outputs the HDR linear RGB signal E _HDR ( Step S1207). That is, the tristimulus value and chromaticity/RGB converter 32 performs the reverse process of the RGB/tristimulus value and chromaticity converter 10 shown in FIG.

The internal configuration of the tristimulus value and chromaticity/RGB conversion unit 32 is _the same as that of the tristimulus value and _chromaticity /RGB conversion unit 12 shown in FIG. This is because it is the same as replacing the SDR tristimulus values X _SDR , Y _SDR , Z _SDR with the HDR tristimulus values X _HDR , Y _HDR , Z _HDR and replacing the SDR linear RGB signal E _SDR with the HDR linear RGB signal E _HDR . , the explanation is omitted.

As described above, according to the video signal conversion device 2 of the embodiment of the present invention, the RGB/tristimulus value and chromaticity conversion unit 30 converts the SDR linear RGB signal E _SDR into the SDR tristimulus values X _SDR , Y _SDR , The _SDR tristimulus values X _SDR , Y _SDR , and Z _SDR are converted into chromaticity coordinates x, y.

The expansion processing unit 31 expands the dynamic range of the SDR brightness signal Y _SDR using a tone mapping function defined by parameters such as a predetermined gamma value γ _dark for dark area correction, and obtains the HDR brightness signal Y _HDR . This tone mapping function is composed of a combination of a linear function and an exponential function.

The tristimulus value and chromaticity/RGB conversion unit 32 converts the HDR luminance signal Y _HDR and chromaticity coordinates x, y into HDR tristimulus values X _HDR , Y _HDR , Z _{HDR , HDR} , Z Convert _HDR to HDR linear RGB signal E _HDR .

Thereby, by changing the gamma value γ _dark for dark area correction, the shape of the tone mapping curve can be changed, and gradation correction from dark areas to intermediate tones can be performed. Then, the dynamic range of the SDR luminance signal Y _SDR can be expanded using a function having characteristics opposite to the commonly used S-curve. Therefore, the signal level of a person's skin does not decrease, the white level does not change, and blown-out highlights are suppressed, and it is also possible to correct the gradation of dark areas according to the production intention.

Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the embodiments described above, and can be modified in various ways without departing from the technical concept thereof.

Note that a normal computer can be used as the hardware configuration of the video signal conversion devices 1 and 2 according to the embodiments of the present invention. The video signal conversion devices 1 and 2 are configured by a computer including a CPU, a volatile storage medium such as a RAM, a nonvolatile storage medium such as a ROM, an interface, and the like.

Each function of the RGB/tristimulus value and chromaticity conversion unit 10, the compression processing unit 11, and the tristimulus value and chromaticity/RGB conversion unit 12 provided in the video signal conversion device 1 is implemented by a program that describes these functions. Each of these is realized by causing the CPU to execute the following.

In addition, the functions of the RGB/tristimulus value and chromaticity conversion unit 30, the expansion processing unit 31, and the tristimulus value and chromaticity/RGB conversion unit 32 provided in the video signal conversion device 2 are also described. Each of these is realized by having the CPU execute the program.

These programs are stored in the storage medium, and are read and executed by the CPU. Additionally, these programs can be stored and distributed in storage media such as magnetic disks (floppy (registered trademark) disks, hard disks, etc.), optical disks (CD-ROMs, DVDs, etc.), semiconductor memories, etc., and can be distributed via networks. You can also send and receive messages.

The video signal conversion device and program of the present invention are useful as a technique for converting dynamic range, for example, in integrated production in which HDR video and SDR video are simultaneously produced. Further, the video signal conversion device and program of the present invention can be installed in a receiver.

1 Video signal conversion device 2 Video signal conversion device 10 RGB/tristimulus value and chromaticity conversion unit 11 Compression processing unit 12 Tristimulus value and chromaticity/RGB conversion unit 20 RGB/tristimulus value conversion unit 21 Tristimulus value/color Degree converter 22 Chromaticity/tristimulus value converter 23 Tristimulus value/RGB converter 24 Calculator 25 Calculator 26 Calculator 27 Calculator 30 RGB/tristimulus value and chromaticity converter 31 Extension processing unit 32 Tristimulus Value and _chromaticity /RGB conversion section E _HDR HDR linear RGB signal E _SDR SDR linear RGB signal _{X HDR} , Y _HDR , Z _{HDR HDR} tristimulus value Y _HDR HDR luminance signal Y _SDR SDR luminance signal k ₁ Gain value k ₂ Correction value k ₃ Correction value k ₄ Correction value γ _dark Gamma value for dark area correction Y _SDR,ip SDR inflection point Y _HDR,ip HDR inflection point Y _HDR,Ref HDR reference white level Y _SDR,WP SDR white level Vh _skin Skin level of HLG video signal Vs _skin Skin level of SDR video signal

Claims (6)

In a video signal conversion device that converts HDR (High Dynamic Range) video to SDR (Standard Dynamic Range) video,
Converting the linear RGB signal of the HDR video into tristimulus values, converting the tristimulus values into chromaticity coordinates, outputting a luminance signal of the tristimulus values as an HDR luminance signal, and converting the chromaticity coordinates into an RGB/tristimulus value and chromaticity converter to output;
A compression processing unit that uses a predetermined tone mapping function to compress the dynamic range of the HDR luminance signal output by the RGB/tristimulus value and chromaticity conversion unit, and obtains the luminance signal of the SDR video as an SDR luminance signal. and,
The SDR luminance signal obtained by the compression processing unit and the RGB/tristimulus values and the chromaticity coordinates output by the chromaticity conversion unit are converted into tristimulus values, and the tristimulus values are converted into tristimulus values of the SDR video. A tristimulus value and chromaticity/RGB conversion unit that converts into a linear RGB signal,
The compression processing section includes:
The tone mapping function has a characteristic that the value of the SDR luminance signal increases with respect to an increase in the value of the HDR luminance signal, the characteristic of the skin level of the SDR image corresponding to the skin level of the HDR image, The characteristics of the white level of the SDR video corresponding to the reference white level of the HDR video and the value of the SDR video signal corresponding to the linear RGB signal of the SDR video are the same as those of the HDR video signal corresponding to the linear RGB signal of the HDR video. The SDR video signal has a characteristic of increasing up to an upper limit value with respect to an upper limit value, and further, the characteristics of the dark area vary depending on a dark area correction gamma value that can adjust the gradation of the dark area in a predetermined area of the SDR luminance signal. A video signal conversion device characterized in that the SDR luminance signal is obtained by compressing the dynamic range of the HDR luminance signal using the tone mapping function. The video signal conversion device according to claim 1,
The compression processing section includes:
As the tone mapping function, a power function whose exponent is the gamma value for dark area correction is used in a region smaller than a predetermined inflection point in the HDR luminance signal, and a power function whose exponent is the gamma value for dark part correction is used in an area smaller than a predetermined inflection point in the HDR luminance signal. A video signal conversion device characterized in that in the region, a logarithmic function is used to compress the dynamic range of the HDR luminance signal and obtain the SDR luminance signal. In a video signal conversion device that converts SDR (Standard Dynamic Range) video to HDR (High Dynamic Range) video,
Converting the linear RGB signal of the SDR video into tristimulus values, converting the tristimulus values into chromaticity coordinates, outputting a luminance signal of the tristimulus values as an SDR luminance signal, and converting the chromaticity coordinates into an RGB/tristimulus value and chromaticity converter to output;
an extension processing unit that uses a predetermined tone mapping function to expand the dynamic range of the SDR luminance signal output by the RGB/tristimulus value and chromaticity conversion unit, and obtains the luminance signal of the HDR video as an HDR luminance signal; and,
The HDR luminance signal obtained by the expansion processing unit and the RGB/tristimulus values and the chromaticity coordinates output by the chromaticity conversion unit are converted into tristimulus values, and the tristimulus values are converted into tristimulus values of the HDR video. A tristimulus value and chromaticity/RGB conversion unit that converts into a linear RGB signal,
The expansion processing unit includes:
The tone mapping function has a characteristic that the value of the HDR luminance signal increases with respect to an increase in the value of the SDR luminance signal, the characteristic of the skin level of the SDR image corresponding to the skin level of the HDR image, The characteristics of the white level of the SDR video corresponding to the reference white level of the HDR video and the value of the SDR video signal corresponding to the linear RGB signal of the SDR video are the same as those of the HDR video signal corresponding to the linear RGB signal of the HDR video. The SDR video signal has a characteristic of increasing up to an upper limit value with respect to an upper limit value, and further, the characteristics of the dark area vary depending on a dark area correction gamma value that can adjust the gradation of the dark area in a predetermined area of the SDR luminance signal. A video signal conversion device characterized in that the HDR luminance signal is obtained by expanding the dynamic range of the SDR luminance signal using the tone mapping function. The video signal conversion device according to claim 3,
The expansion processing unit includes:
As the tone mapping function, in a region smaller than a predetermined inflection point in the SDR luminance signal, a power function whose exponent is the reciprocal of the gamma value for dark area correction is used, and the predetermined inflection point in the SDR luminance signal is A video signal conversion device characterized in that in a region of points or more, an exponential function is used to expand the dynamic range of the SDR luminance signal and obtain the HDR luminance signal. A program for causing a computer to function as the video signal conversion device according to claim 1 or 2. A program for causing a computer to function as the video signal conversion device according to claim 3 or 4.

Images