JP7543098B2 - Display device - Google Patents

Description

Article 30, Paragraph 2 of the Patent Act applies - Website address (1) https://corporate. jp. sharp/news/200511-a. html Publication date: May 11, 2020 (2) https://jp. sharp/support/aquos/doc/4tc65_55cq1_mn. pdf? productId=4T-C65CQ1 (3) https://jp. sharp/support/aquos/doc/4tc65_55cq1_dmn. pdf? productId=4T-C65CQ1 Posting date May 23, 2020

The present invention relates to a display device.

Currently, the trend in high-quality video formats is HDR (High Dynamic Range), which assumes a maximum brightness of 1,000 to 10,000 candelas. On the other hand, the conventional SDR (Standard Dynamic Range) format assumes a maximum brightness of 100 candelas. Compared to the SDR format, the HDR format allocates more gradation values to the low gradation range and expands the brightness that can be expressed in the high gradation range. The HDR format makes it possible to express gradations in light and dark areas that cannot be expressed with the SDR format, and can reproduce realistic images with a high dynamic range.

There is also a known technique in which an image processing device generates a gamma curve based on the luminance histogram of an input image and performs luminance conversion processing using the gamma curve (see, for example, Patent Document 1). The purpose of this is to improve the contrast according to the pattern of the input image.

JP 2009-017200 A

Due to the above-mentioned characteristics, the distribution tendency of the brightness histogram is different between HDR and SDR signals. Therefore, when the video format of the input image is switched, the viewer may feel a strange sensation. This strange sensation cannot be eliminated even by the brightness conversion process based on the brightness histogram described in Patent Document 1.

One aspect of the present invention aims to reduce the discomfort felt by viewers when switching the video format of an input image.

In order to solve the above problem, a display device according to one aspect of the present invention includes a gamma curve determination unit that determines a gamma curve that represents an output luminance, which is the luminance of an output image, relative to an input luminance, which is the luminance of an input image, and a luminance conversion unit that outputs the output image by converting the input luminance of the input image to the output luminance based on the gamma curve, and the gamma curve determination unit determines the gamma curve when the video format of the input image is a high dynamic range format so that the gain of the output luminance relative to the input luminance is greater than when the video format of the input image is a standard dynamic range format.

According to one aspect of the present invention, it is possible to reduce the difference in brightness when switching the video format of an input image, thereby reducing the sense of discomfort felt by the viewer.

2 is a block diagram showing a flow of signal processing in the television receiver according to the first embodiment of the present invention; FIG. 1 is a front view showing an external appearance of a television receiver according to a first embodiment of the present invention. 4 is a graph showing an example of a gamma curve according to the first embodiment of the present invention. 4 is a graph showing characteristics of a video signal according to the first embodiment of the present invention. 4 is a graph showing an example of a gamma curve for each video signal according to the first embodiment of the present invention.

[Embodiment 1]
Hereinafter, the first embodiment of the present invention will be described in detail.

Figure 1 is a block diagram showing the flow of signal processing in a television receiver (display device) 100 according to this embodiment. Figure 2 is a front view showing the exterior of the television receiver 100 according to this embodiment.

(Configuration of the Television Receiver 100)
As shown in FIG. 1, a television receiver 100 includes an input switching circuit 1, a signal level analysis circuit 2, a color space conversion circuit 3, an NR (Noise Reduction) circuit 4, a scaler circuit 5, an active contrast circuit (gamma curve determination section, luminance conversion section) 6, a sharpness circuit 7, a color space conversion circuit 8, a color LUT (Look Up Table) circuit 9, a gamma/WB (White Balance) circuit 10, and a panel display section 11.

(Functional overview of each circuit)
The input switching circuit 1 is connected to external input terminals of HDMIs (High-Definition Multimedia Interface, registered trademark) 1 to 4 provided on the television receiver 100, and to a tuner built into the television receiver 100. The input switching circuit 1 switches to an input signal selected by a user operation from among signals inputted through each external input terminal or received through the tuner.

The signal level analysis circuit 2 acquires the minimum and maximum values of the input signal, a histogram of the input luminance of the video signal, and the video system (format) of the video signal for each frame.

Examples of video formats include the HDR and SDR formats mentioned above. HDR formats include the Hybrid Log Gamma (HLG) format and the Perceptual Quantization (PQ) format.

The luminance characteristics of the HLG method are such that it displays darker from low to mid-tones, allowing it to display a wider range of gradation characteristics than the SDR method. Also, the luminance characteristics of the PQ method are such that it displays darker from low to mid-tones than the HLG method. Since the majority of images are composed of low to mid-tones, in many cases the HLG method appears darker to the user than the SDR method, and the PQ method appears darker than the HLG method.

Examples of PQ methods include HDR10 and HDR10+.

The color space conversion circuit 3 converts the input signal into a color difference signal based on the information acquired for each frame. The NR circuit 4 removes noise contained in the color difference signal. The scaler circuit 5 adjusts the size of the color difference signal according to the resolution of the panel so that it is easy to see.

The active contrast circuit 6 determines a gamma curve that represents the output luminance, which is the luminance of the output image, relative to the input luminance, which is the luminance of the input image. In detail, the active contrast circuit 6 determines the gamma curve by referring to the minimum and maximum values of the input signal, the histogram of the input luminance of the video signal, and the format of the video signal, which are acquired by the signal level analysis circuit 2. Then, the active contrast circuit 6 outputs the output image by converting the input luminance of the input image to output luminance based on the gamma curve that it has determined.

The sharpness circuit 7 performs sharpness processing to detect and enhance the contours and boundaries of each image in the video. The color space conversion circuit 8 converts the color difference signal into an RGB signal. The color LUT circuit 9 performs mapping so that the RGB signal is optimally matched to the color gamut of the panel.

The gamma/WB circuit 10 performs gamma adjustment processing and WB adjustment processing. As part of its gamma adjustment processing, the gamma/WB circuit 10 adjusts the output luminance in accordance with the gamma characteristics of the panel. As part of its WB adjustment processing, the gamma/WB circuit 10 adjusts the color of each image in the video so that the colors appear natural in a variety of scenes. The panel display unit 11 displays the image.

(Example of gamma curve determined by active contrast circuit 6)
Fig. 3 is a graph showing an example of a gamma curve determined by the active contrast circuit 6 according to this embodiment. In other words, Fig. 3 is a diagram showing an outline of the processing of the active contrast circuit 6 according to this embodiment. A gamma curve is a curve that expresses, in an XY coordinate system, the output luminance, which is the luminance of an output image, relative to the input luminance, which is the luminance of an input image. The value on the X-axis of the gamma curve is the input luminance (INPUT), and the value on the Y-axis of the gamma curve is the output luminance (OUTPUT). In the figure, the dashed line indicates a reference gamma curve in which the input luminance and the output luminance do not change, and the solid line indicates a gamma curve determined by the active contrast circuit 6.

In one aspect, the active contrast circuit 6 may determine the gamma curve by setting one or more reference points corresponding to the X coordinate on the gamma curve and adjusting the Y coordinate of each reference point. Below, a case will be described in which at least a first reference point P1, a second reference point P2, a third reference point P3, a fourth reference point P4, and a fifth reference point P5 are set on the gamma curve, but this embodiment is not limited to this, and the number of reference points to be set is not particularly limited.

The first reference point P1 is a point in the low luminance region. The second reference point P2 is a point in the high luminance region. The third reference point P3 is a point in the medium luminance region between the first reference point P1 and the second reference point P2. The fourth reference point P4 is a point corresponding to the minimum luminance Ymin of the input image or an approximate minimum luminance that is approximate to the minimum luminance Ymin. The fifth reference point P5 is a point corresponding to the maximum luminance Ymax of the input image or an approximate maximum luminance that is approximate to the maximum luminance Ymax. It is preferable that the active contrast circuit 6 sets at least the first and second reference points, and it is more preferable that the active contrast circuit 6 sets at least the first, second and third reference points.

(Setting the reference point)
In one embodiment, the active contrast circuit 6 may set each reference point according to a histogram of the input luminance. First, the active contrast circuit 6 creates a histogram of the input luminance based on the input luminance. For example, when the input luminance has 256 gradations, the active contrast circuit 6 divides the input luminance into 32 classes (BIN) and expresses the number of pixels in each class as a frequency. The classes are not limited to this example and are appropriately set according to the number of gradations of the input luminance, etc. For example, a class may be set for each gradation.

Then, the active contrast circuit 6 calculates the input luminance corresponding to each of the key points that determine the gamma curve based on the created histogram. The key points include at least a first reference point P1 in the low luminance region of the input luminance, a second reference point P2 in the high luminance region of the input luminance, and a third reference point P3 between the first reference point P1 and the second reference point P2.

Specifically, the active contrast circuit 6 adds up the ratio of each BIN to the sum of the frequencies of all BINs in the histogram, starting from the lowest BIN, and calculates the input luminance of the first reference point P1 using a predetermined formula that includes the frequency of the BIN that exceeds the low luminance ratio that defines the low luminance region. The active contrast circuit 6 also adds up the ratio of each BIN to the sum of the frequencies of all BINs in the histogram, starting from the highest BIN, and calculates the input luminance of the second reference point P2 using a predetermined formula that includes the frequency of the BIN that exceeds the high luminance ratio that defines the high luminance region. The active contrast circuit 6 also calculates the input luminance of the third reference point P3 based on the input luminance of each of the calculated first and second reference points P1 and P2.

Next, the active contrast circuit 6 calculates the output luminance of the first to fifth reference points P1 to P5 corresponding to the input luminance of the first to fifth reference points P1 to P5, and determines the gamma curve so that it passes through each reference point.

(First Calculation)
In one embodiment, the active contrast circuit 6 may calculate a provisional output luminance for the first reference point P1, as shown in Fig. 3, so as to make the dark parts darker while maintaining the gradation characteristics of the dark parts. The active contrast circuit 6 may also calculate a provisional output luminance for the second reference point P2, so as to make the bright parts brighter while maintaining the gradation characteristics of the bright parts. The active contrast circuit 6 may also calculate a provisional output luminance for the third reference point P3, so as to make the intermediate gradation parts brighter. These provisional output luminances are determined regardless of the video format of the input image. The first calculation may be omitted.

(Second Calculation)
Next, the active contrast circuit 6 determines the final output luminance of each reference point by adjusting the provisional output luminance of each reference point calculated in the first calculation in accordance with the video format of the input image.

First, the characteristics of each of the underlying video formats will be described in detail. Figure 4 is a graph showing the characteristics of the video signal according to this embodiment. The value on the X axis of each graph is the gradation value, and the value on the Y axis of each graph is the intended brightness.

The video signals input to the television receiver 100 come in a variety of formats, including SDR, HLG, and PQ signals. As shown in FIG. 4, each signal has different characteristics. For example, the HLG curve and PQ curve corresponding to the HLG and PQ signals contained in the HDR signal have characteristics in which the intended brightness is suppressed low in the gradation values between 0.0 and 0.6 (i.e., dark areas) compared to the SDR curve corresponding to the SDR signal.

In other words, HDR signals are a display technology with a wider range of brightness (dynamic range) than SDR signals. The PQ and HLG curves are deeper than the SDR curves. This allows HDR signals to produce more natural and realistic images without sacrificing any of the gradations in the areas that are blacked out (dark areas of the photo appear completely black due to insufficient exposure) or white areas (areas that appear completely white due to overexposure due to too much light hitting them) in SDR signals.

Taking these characteristics into consideration, in this embodiment, the gamma curve is adjusted as follows. Figure 5 is a graph showing an example of a gamma curve for each video signal according to this embodiment.

When the video format of the input image is the HLG format or the PQ format (HDR format), the active contrast circuit 6 determines the gamma curve so that the gain of the output luminance relative to the input luminance is greater than when the video format of the input image is the SDR format.

As shown in FIG. 4, the HLG and PQ curves are deeper than the SDR curve in the dark region where INPUT is from 0.0 to 0.6. Therefore, as shown in FIG. 5, when the video format of the input image is the HLG and PQ formats (HDR formats), the active contrast circuit 6 determines a gamma curve so that the gain is greater, particularly in the low luminance region of the input luminance, than when the video format of the input image is the SDR format.

Next, as shown in FIG. 4, the PQ curve is a deeper curve than the HLG curve. Therefore, as shown in FIG. 5, when the video format of the input image is the PQ format, the active contrast circuit 6 determines a gamma curve so that the gain is particularly greater in the low luminance region of the input luminance than when the video format of the input image is the HLG format.

In practice, the active contrast circuit 6 changes the gain corresponding to at least one of a first reference point in the low luminance region of the input luminance on the gamma curve, a second reference point in the high luminance region of the input luminance on the gamma curve, and a third reference point between the first and second reference points on the gamma curve, depending on the video format of the input image.

For example, at the first reference point P1 and the third reference point P3 in FIG. 3, the magnitude of brightness adjustment for each video signal may be SDR signal < HLG signal < PQ signal.

In other words, when the video format of the input image is the HLG format and the PQ format (HDR format), the active contrast circuit 6 may determine a gamma curve so that the gains corresponding to the first reference point P1 and the third reference point P3 are larger than when the video format of the input image is the SDR format.

Then, at the second reference point P2 in FIG. 3, the magnitude of the luminance adjustment for each video signal may be SDR signal < HLG signal and PQ signal < HLG signal.

In other words, when the video format of the input image is the HLG format, the active contrast circuit 6 may determine a gamma curve so that the gain corresponding to the second reference point P2 is larger than when the video format of the input image is the SDR format or the PQ format.

(Effects of the First Embodiment)
As described above, by changing the active contrast parameters in accordance with the characteristics of each video signal, it becomes possible to perform appropriate processing on each video signal and supply video of appropriate image quality.

For example, when the video format of the input image is the HLG format, the gain of the gamma curve for a specified gradation is made higher than the gain for the SDR format, and when the video format of the input image is the PQ format, the gain of the gamma curve for a specified gradation is made higher than the gain for the HLG format, thereby reducing the difference in brightness when switching the input image format and mitigating the discomfort felt by the viewer.

(Variation 1)
The method of setting the reference points shown in Fig. 3 does not necessarily have to be based on the histogram of input luminance, but may instead be based on a reference point corresponding to a predetermined input luminance.

(Variation 2)
In the above, the gamma curve is determined by adjusting the Y coordinate of the reference point, but the gamma curve may be adjusted by other methods as well. As described above, it is sufficient to be able to adjust the gamma curve for the low luminance region, the medium luminance region, and the high luminance region.

(Variation 3)
There is a problem that the images of BS4K broadcasts from private broadcasting stations are displayed darkly. As a countermeasure, the brightness of the high gradation range is increased because private BS4K broadcasts are often transmitted in the HLG format.

In other words, when the video format of the input image is the HLG format, the active contrast circuit 6 determines the gamma curve so that the gain in the high-luminance region of the input luminance is greater than when the video format of the input image is either the SDR format or the PQ format.

And with regard to HLG content, there is a problem that the image appears dark, particularly when private BS2K broadcasting images are converted to BS4K broadcasting images (the color reproducibility standard is converted from BT.709 to BT.2020, and the video format is converted from SDR to HLG) (i.e., the video format of the input image is the converted HLG format), and as a countermeasure, the brightness of low-brightness and high-brightness areas is increased. However, with regard to low-brightness areas, as mentioned above, when the video format of the input image is the HLG format, the brightness is increased more than when it is the SDR format.

In other words, when the video format of the input image is the HLG format, the active contrast circuit 6 determines whether the input image has been converted from an image in the standard dynamic range format, and when it determines that the input image has been converted from an image in the standard dynamic range format, it determines a gamma curve so that the gain in the high luminance region of the input luminance is greater than when the video format of the input image is either the standard dynamic range format or the PQ format.

The active contrast circuit 6 determines whether the video format of the input image is the converted HLG format based on the luminance information from the signal level analysis circuit 2. Specifically, for example, the active contrast circuit 6 determines whether the video format of the input image is the converted HLG format based on whether the maximum luminance value in the luminance information is equal to or less than a threshold value. For example, the active contrast circuit 6 determines that the video format of the input image is the converted HLG format when the maximum luminance value in the luminance information is equal to or less than a threshold value. For example, the active contrast circuit 6 determines that the video format of the input image is not the converted HLG format when the maximum luminance value in the luminance information is not equal to or less than a threshold value.

〔summary〕
A display device according to aspect 1 of the present invention comprises a gamma curve determination unit that determines a gamma curve representing an output luminance, which is the luminance of an output image, relative to an input luminance, which is the luminance of an input image, and a luminance conversion unit that outputs the output image by converting the input luminance of the input image to the output luminance based on the gamma curve, and when the video format of the input image is a high dynamic range format, the gamma curve determination unit determines the gamma curve so that the gain of the output luminance relative to the input luminance is greater than when the video format of the input image is a standard dynamic range format.

According to the above configuration, when the video format of the input image is switched from the standard dynamic range format to the high dynamic range format, the gamma curve is determined so that the gain is increased, thereby reducing the difference in brightness before and after the switch and reducing the sense of discomfort felt by the viewer.

In the display device according to aspect 2 of the present invention, in the above aspect 1, the gamma curve determination unit may determine the gamma curve such that, when the video format of the input image is a high dynamic range format, the gain in the low luminance region of the input luminance is greater than when the video format of the input image is a standard dynamic range format.

According to the above configuration, the high dynamic range method has different characteristics in the low brightness region of the input luminance compared to the standard dynamic range method, so by determining the gamma curve so that the gain in the low brightness region is large, the difference in brightness before and after switching can be reduced, and the sense of discomfort felt by the viewer can be reduced.

In the display device according to aspect 3 of the present invention, in the above aspect 1 or 2, the gamma curve determination unit may determine the gamma curve so that, when the video format of the input image is a PQ (Perceptual Quantization) format, the gain in the low luminance region of the input luminance is greater than when the video format of the input image is a HLG (Hybrid Log Gamma) format.

According to the above configuration, the PQ method has different characteristics in the low brightness region of the input brightness compared to the HLG method, so by determining the gamma curve so that the gain in the low brightness region is large, the difference in brightness before and after switching from the standard dynamic range method to the PQ method can be reduced, and the sense of discomfort felt by the viewer can be reduced.

In the display device according to aspect 4 of the present invention, in the above aspects 1 to 3, the gamma curve determination unit may determine the gamma curve when the video format of the input image is the HLG format, so that the gain in the high luminance region of the input luminance is larger than when the video format of the input image is either the standard dynamic range format or the PQ format.

The above configuration solves the problem of private BS4K broadcasts appearing dark, since private BS4K broadcasts are often transmitted using the HLG format.

In the display device according to aspect 5 of the present invention, in the above aspects 1 to 3, the gamma curve determination unit may determine whether or not the input image has been converted from an image in a standard dynamic range format when the video format of the input image is the HLG format, and when it is determined that the input image has been converted from an image in a standard dynamic range format, determine the gamma curve so that the gain in the high luminance region of the input luminance is greater than when the video format of the input image is either the standard dynamic range format or the PQ format.

The above configuration solves the problem of images appearing dark when the video format is converted from SDR to HLG.

In the display device according to aspect 6 of the present invention, in the above aspects 1 to 5, the gamma curve determination unit may change the gain corresponding to at least one of a first reference point in a low luminance region of the input luminance on the gamma curve, a second reference point in a high luminance region of the input luminance on the gamma curve, and a third reference point between the first and second reference points on the gamma curve, in accordance with the video format of the input image.

According to the above configuration, the gain corresponding to at least one of the first reference point, the second reference point, and the third reference point is changed depending on the video format of the input image, thereby making it possible to adjust the brightness according to the characteristics of the video format.

In the display device according to aspect 7 of the present invention, in the above aspect 6, the gamma curve determination unit may determine the gamma curve such that, when the video format of the input image is a high dynamic range format, the gains corresponding to the first and third reference points are larger than when the video format of the input image is a standard dynamic range format.

According to the above configuration, the high dynamic range method has different characteristics at the first and third reference points compared to the standard dynamic range method, so by determining the gamma curve so that the gain at the first and third reference points is large, the difference in brightness before and after switching the video method of the input image from the standard dynamic range method to the high dynamic range method can be reduced, reducing the sense of discomfort felt by the viewer.

In the display device according to aspect 8 of the present invention, in the above aspects 1 to 7, the gamma curve determination unit may determine the gamma curve by referring to a histogram of the input luminance.

With the above configuration, the gamma curve is determined by referring to the histogram of input luminance, allowing for precise luminance adjustment.

The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. The technical scope of the present invention also includes embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Furthermore, new technical features can be formed by combining the technical means disclosed in the respective embodiments.

6 Active contrast circuit (gamma curve determination section, luminance conversion section)
100 Television receiver (display device)
P1 1st reference point P2 2nd reference point P3 3rd reference point

Claims (7)

a gamma curve determination unit that determines a gamma curve that represents an output luminance, which is the luminance of an output image, relative to an input luminance, which is the luminance of an input image;
a luminance conversion unit that converts the input luminance of the input image into the output luminance based on the gamma curve, thereby outputting the output image,
the gamma curve determination unit determines the gamma curve such that, when the video format of the input image is a high dynamic range format, a gain of the output luminance relative to the input luminance is greater than when the video format of the input image is a standard dynamic range format;
The display device is characterized in that, when the video format of the input image is a PQ (Perceptual Quantization) format, the gamma curve determination unit determines the gamma curve so that the gain in the low luminance region of the input luminance is larger than when the video format of the input image is an HLG (Hybrid Log Gamma) format. a gamma curve determination unit that determines a gamma curve that represents an output luminance, which is the luminance of an output image, relative to an input luminance, which is the luminance of an input image;
a luminance conversion unit that converts the input luminance of the input image into the output luminance based on the gamma curve, thereby outputting the output image,
the gamma curve determination unit determines the gamma curve such that, when the video format of the input image is a high dynamic range format, a gain of the output luminance relative to the input luminance is greater than when the video format of the input image is a standard dynamic range format;
The display device is characterized in that, when the video format of the input image is the HLG format, the gamma curve determination unit determines the gamma curve so that the gain in the high luminance region of the input luminance is larger than when the video format of the input image is either the standard dynamic range format or the PQ format. a gamma curve determination unit that determines a gamma curve that represents an output luminance, which is the luminance of an output image, relative to an input luminance, which is the luminance of an input image;
a luminance conversion unit that converts the input luminance of the input image into the output luminance based on the gamma curve, thereby outputting the output image,
the gamma curve determination unit determines the gamma curve such that, when the video format of the input image is a high dynamic range format, a gain of the output luminance relative to the input luminance is greater than when the video format of the input image is a standard dynamic range format;
The display device is characterized in that, when the video format of the input image is the HLG format, the gamma curve determination unit determines whether the input image has been converted from an image in the standard dynamic range format, and, when it is determined that the input image has been converted from an image in the standard dynamic range format, determines the gamma curve so that the gain in the high luminance region of the input luminance is larger than when the video format of the input image is either the standard dynamic range format or the PQ format. The display device according to any one of claims 1 to 3, characterized in that, when the video format of the input image is a high dynamic range format, the gamma curve determination unit determines the gamma curve so that the gain in the low luminance region of the input luminance is larger than when the video format of the input image is a standard dynamic range format. The display device according to any one of claims 1 to 4, characterized in that the gamma curve determination unit changes the gain corresponding to at least one of a first reference point in a low luminance region of the input luminance on the gamma curve, a second reference point in a high luminance region of the input luminance on the gamma curve, and a third reference point between the first and second reference points on the gamma curve, in accordance with a video format of the input image. The display device according to claim 5, characterized in that the gamma curve determination unit determines the gamma curve so that, when the video format of the input image is a high dynamic range format, the gains corresponding to the first reference point and the third reference point are larger than when the video format of the input image is a standard dynamic range format. 7. The display device according to claim 1 , wherein the gamma curve determination section determines the gamma curve by referring to a histogram of the input luminance.

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