JP2010117578A - Display device, liquid crystal display device, and display method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent quality of an image for display from being deteriorated even when the luminance of backlight light is flexibly and finely controlled. <P>SOLUTION: The display device includes: a changing means changing the luminance of backlight light; an image converting means obtaining a converted image by converting the grayscale of each pixel constituting the image in accordance with the luminance of the backlight light after changed; a &gamma; curve converting means calculating a converted &gamma; curve by converting a regular &gamma; curve in accordance with the luminance of the backlight light after changed; an error calculating means calculating, as an error, the difference between the luminance of the pixel before the conversion displayed based on the &gamma; curve and the luminance of the pixel displayed based on the converted &gamma; curve in each pixel constituting the converted image; an error diffusing means diffusing the error of each pixel constituting the converted image into other pixels in the converted image; and a display means displaying the converted image after the error is diffused, by using the converted &gamma; curve. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a display device, a liquid crystal display device, and a display method that flexibly and finely control the luminance of backlight light.

  Conventionally, in a liquid crystal display device, an active backlight control technique has been adopted for the purpose of suppressing power consumption during operation. When an image is displayed using this technology, the brightness of the backlight light is reduced more than usual at the time of image display, and the γ curve (gradation-luminance curve) preset in the apparatus is reduced. It changes according to the brightness | luminance of light, and displays an image using the gamma curve after the said change. Since the brightness of the backlight light is reduced, the power consumption can be reduced accordingly. On the other hand, since the γ curve is changed to one corresponding to a decrease in the luminance of the backlight light and then used for image display, an image is displayed with a quality close to that when the luminance of the backlight light is not decreased.

An example of a conventional active backlight control technique is disclosed in Patent Document 1.
JP 2007-264104 A (publication date: October 11, 2007) JP 2008-107719 A (publication date: May 8, 2008) JP 2002-287709 A (publication date: October 4, 2002)

  However, the conventional technique has a problem that gradation disturbance may occur when an image is displayed with the brightness of the backlight light lowered. Specifically, there is a problem that smooth luminance change cannot be obtained and gradation or human skin cannot be expressed well.

  The reason why this problem occurs will be described with reference to FIG. FIG. 8 is a diagram for explaining the problems of the prior art.

  In a liquid crystal display device that employs active backlight control, the gradation of each pixel of video data is converted according to the intensity of the backlight. As shown in FIG. 8A, when there is no adjustment of the luminance of the backlight light, the gradation of each pixel is converted using a straight line 80. Therefore, there is no change between the gradation before conversion (input gradation) and the gradation after conversion (output gradation).

  On the other hand, when the luminance of the backlight light is lowered, the gradation of each pixel of the video data is converted using, for example, a curve 82 shown in FIG. When the curve 82 is used, linear gain is performed in the range where the input gradation is 0 to 200, and non-linear gain is performed in the range from 201 to 255.

  FIG. 8B is an enlarged view of 84 in FIG. As shown in FIG. 8B, when performing gradation conversion according to the luminance of the backlight light, a dotted curve 82 is theoretically used. However, this curve 82 is theoretical only and cannot be used in practice. This is because the value of the output gradation corresponding to the input gradation on the curve 82 is a value that cannot actually be handled in the liquid crystal display device. More specifically, the gradation values that can be taken in the video data are discrete. For example, an integer value such as 253, 254, and 255 can be taken, but a decimal point such as 253.5 or less. Cannot take a value that contains. Here, when the curve 82 in FIG. 8B is seen, the value of the output gradation obtained by converting the input gradation is a value between 254 and 255 when the input gradation is 253. On the other hand, when the input gradation is 254, the output gradation has values of 254 and 255. Since these values are values that cannot be taken by the converted video data, they are converted to integer values that are closest to these theoretical values.

  Therefore, when the input gradation is 253, it is converted into an output gradation of 254, while when the input gradation is 254, it is converted into an output gradation of 255. As a result, as shown in FIG. 8B, the pixels represented by the gradations of 254 and 255 in the video data before conversion are both represented by the gradation of 255 in the video data after conversion. As a result, when the corrected video data is displayed, all pixels that should be originally expressed with different luminances are expressed with the same gradation. Therefore, there arises a problem that the change in luminance is impaired.

  FIG. 8C is an enlarged view of 86 in FIG. In this figure, the output gradation value obtained by converting the input gradation is a value between 102 and 103 when the input gradation is 101, while the input gradation is 99. In this case, the output gradation is a value between 99 and 100. Since these values are values that cannot be taken by the converted video data, they are converted to integer values that are closest to these theoretical values.

  Therefore, when the input gradation is 101, it is converted into 102 output gradations, while when the input gradation is 99, it is converted into 99 output gradations. When the input gradation is 100, it is converted into an output gradation of 101. As a result, as shown in FIG. 8B, the gradation that changes continuously with 99 and 100 in the video data before conversion changes by 99 and 101 in the video data after conversion. As a result, when the corrected video data is displayed, a portion that should originally be expressed by a continuous luminance change is expressed with one luminance skipped. Accordingly, a problem arises in that the change in luminance is impaired as in FIG.

  As described above, in the related art, the quality of an image to be displayed after changing the luminance of the backlight light is not necessarily the same as that of the original image (that is, the image when displaying without changing the luminance of the backlight light). It may not be close to quality. Therefore, the luminance of the backlight light can only be changed at a rate that does not cause the above-described problem. That is, there is a problem that the brightness of the backlight light cannot be controlled flexibly and finely.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a display device, a liquid crystal display device, and a display method capable of controlling the brightness of backlight light flexibly and finely.

In order to solve the above problems, a display device according to the present invention provides
A display device that displays images using backlight light,
Changing means for changing the brightness of the backlight light;
Image conversion means for obtaining a converted image by converting the gradation of each pixel constituting the image in accordance with the luminance of the backlight light after the change;
Γ curve conversion means for calculating a converted γ curve by converting a prescribed γ curve according to the luminance of the backlight light after the change,
For each pixel constituting the converted image, the difference between the luminance when the pixel before the change is displayed on the pixel based on the γ curve and the luminance when the pixel is displayed based on the converted γ curve is an error. Error calculating means for calculating as
In the converted image, error diffusion means for diffusing the error to other pixels for each pixel constituting the converted image;
And a display means for displaying the converted image after the error is diffused using the converted γ curve.

  The display device is, for example, a liquid crystal display device that displays an image using backlight light. This apparatus has a function of changing the brightness of backlight light (so-called active backlight control function). That is, the power consumption is optimized by increasing or decreasing the luminance of the backlight light in accordance with the image to be displayed.

  This apparatus obtains a converted image by converting the gradation of each pixel constituting the image to be displayed according to the luminance of the backlight light after the change. This converted image is an actual display target. Further, when displaying a converted image, a converted γ curve is calculated by converting a prescribed γ curve in accordance with the luminance of the changed backlight light, and this converted γ curve is used.

  When the converted γ curve is used as it is, the quality of the displayed converted image may be lower than that of the original image. Therefore, this apparatus prevents this problem. Specifically, for each pixel constituting the converted image, the luminance when the pixel before conversion is displayed on the pixel based on a prescribed γ curve, and the luminance when the pixel is displayed using the converted γ curve, Is calculated as an error. Then, in the converted image, an error is diffused to other pixels for each pixel constituting the converted image. In this way, the converted image after the error is diffused is displayed using the converted γ curve.

  By displaying the converted image as described above, the quality of the displayed converted image can be maintained at the same level as that of the original image. Further, no matter what value the intensity of the backlight light is adjusted, the quality of the display image is not degraded. Therefore, this apparatus can control the brightness | luminance of backlight light flexibly and finely.

In order to solve the above problems, a display method according to the present invention provides
A display method for a display device that displays an image using backlight,
A change step of changing the brightness of the backlight light;
An image conversion step for obtaining a converted image by converting the gradation of each pixel constituting the image according to the luminance of the backlight light after the change,
A γ curve conversion step for calculating a converted γ curve by converting a specified γ curve according to the luminance of the backlight light after the change;
For each pixel constituting the converted image, the luminance when the gradation before conversion into the gradation of the pixel is displayed based on the γ curve and the gradation of the pixel are displayed using the converted γ curve. An error calculating step of calculating a difference from the brightness of
In the converted image, an error diffusion step for diffusing the error to other pixels for each pixel constituting the converted image;
And a display step of displaying the converted image after the error is diffused using the converted γ curve.

  According to said structure, there exists an effect similar to the display apparatus which concerns on this invention.

  In the display device according to the present invention, it is preferable that the error diffusion means diffuses the error spatially or temporally.

  According to said structure, an error can be diffused efficiently.

  The display device according to the present invention is preferably a liquid crystal display device.

  According to said structure, the liquid crystal display device which controls the brightness | luminance of backlight light flexibly and finely can be provided.

  The display device according to the present invention diffuses a luminance error in an image after gradation conversion before displaying an image in accordance with the luminance of the backlight light, so that the luminance of the backlight light can be controlled flexibly and finely.

  An embodiment according to the present invention will be described below with reference to FIGS.

  FIG. 1 is a block diagram showing a configuration of a liquid crystal display device 1 (display device) according to the present embodiment. As shown in this figure, the liquid crystal display device 1 includes an image determination circuit 2, an image data conversion circuit 3 (conversion means), an error diffusion circuit 4 (γ curve conversion means, error diffusion means), and a liquid crystal driver 5 (display means). A liquid crystal panel 6, a backlight dimming circuit 7, and a backlight unit 8.

  Image data is input to the liquid crystal display device 1 from the outside. The image determination circuit 2 analyzes the input image data, detects the maximum gradation value included in the image data, and outputs it to the image data conversion circuit 3 and the backlight dimming circuit 7. Note that the image determination circuit 2 also outputs the input image data to the image data conversion circuit.

  The backlight dimming circuit 7 outputs a backlight control signal corresponding to the input maximum gradation value to the backlight unit 8. An example of the waveform of the backlight control signal is shown in FIG. 2A and 2B are diagrams illustrating an example of the waveform of the backlight control signal, where FIG. 2A illustrates the waveform of the backlight control signal when the maximum gradation value is small, and FIG. 2B illustrates the maximum gradation value. The waveform of the backlight control signal when it is large is shown.

  As shown in this figure, the backlight dimming circuit 7 outputs a PWM (Plus Width Modulation) signal having a pulse width (for example, a pulse width proportional to the maximum gradation value) representing the detected maximum gradation value. That is, as shown in FIG. 2A, when the maximum gradation value is small, a backlight control signal in which the High period of the pulse wave is shortened is output. On the other hand, when the maximum gradation value is large, the pulse wave A backlight control signal with a longer High period is output to the backlight unit 8.

  The backlight unit 8 irradiates the liquid crystal panel 6 with light having an intensity (luminance) defined by the High period of the pulse width of the input backlight control signal. The luminance of the backlight light at this time is equal to the luminance displayed by the maximum gradation value in the image data. That is, by reducing the luminance of the backlight light to the maximum luminance necessary for displaying this image, the power required for driving the backlight is reduced, thereby reducing the power consumption of the liquid crystal display device 1.

  The image data conversion circuit 3 of the liquid crystal display device 1 converts the image data so as to match the luminance of the backlight light determined according to the maximum gradation value in the image data. In the present embodiment, conversion is performed to increase the value of each gradation in the image data by the amount that the luminance of the backlight light is lower than the original luminance. More specifically, as shown in FIGS. 3A and 3B, the gradation (input gradation) of each pixel constituting the image represented by the image data is output by using linear interpolation and spline interpolation. Convert to key. 3A and 3B are diagrams for explaining gradation interpolation of image data. FIG. 3A shows the relationship between the input gradation and the output gradation, and FIG. 3B shows the conversion principle of the spline interpolation region.

  In the example of FIG. 3A, the image data conversion circuit 3 converts any gradation from 0 to 180 into an output gradation by linear interpolation. On the other hand, any one of 180 to 220 is converted into an output gradation by spline interpolation. Further, all gradations exceeding 220 are uniformly converted to 255 (that is, a saturation value). Since the gradation conversion shown in FIG. 3 is a known technique, further detailed description is omitted. The image data conversion circuit 3 outputs the gradation-converted image data to the error diffusion circuit 4.

  The error diffusion circuit 4 performs a process of pre-diffusing an error generated when the pixel is displayed on the liquid crystal panel 6 in each pixel constituting the image represented by the image data for the image data after gradation conversion. To do. Before explaining the details of the error diffusion process, the reason why an error occurs in luminance when an image is displayed using image data after gradation conversion will be explained.

  FIG. 4 is a diagram for explaining the principle when a prescribed γ curve is converted to obtain a converted γ curve. The liquid crystal display device 1 is provided with a prescribed γ curve. When the brightness of the backlight light is not adjusted, that is, when the backlight light having the maximum brightness that can be originally output is output from the backlight unit 8, the liquid crystal display device 1 constructs an image by using the prescribed γ curve. The gradation of each pixel to be converted is converted into luminance, and then displayed on the liquid crystal panel 6. In this embodiment, this prescribed γ curve is a curve of γ = 2.2 (γ curve 40 in FIG. 4).

  In the liquid crystal display device 1, when the luminance of the backlight light is reduced and the gradation of the image data is converted, the specified γ curve is converted according to the luminance of the changed backlight light, and the converted γ curve is obtained. Get. In the example of FIG. 4, the γ curve 40 is converted to obtain the γ curve 42. The γ curve is converted to 40 according to the luminance of the backlight light after the change. Specifically, when the luminance of the backlight light is reduced to 1 / X, the luminance value corresponding to each gradation of the γ curve 40 is multiplied by X. If the converted luminance exceeds the maximum luminance (250 in FIG. 4) that can be displayed by the liquid crystal panel 6 in the calculation, the maximum luminance is set.

  The γ curve 42 is only theoretical and cannot be used in the liquid crystal display device 1 in practice. This is because the theoretical luminance after conversion is a value that cannot be selected by the liquid crystal panel 6. The luminance that can be displayed on the liquid crystal panel 6 is limited to the luminance obtained from each gradation in the γ curve 40 shown in FIG. Here, since the gradation takes discrete values such as 1, 2,..., The displayable luminance also takes a discrete value. For example, assuming that 90 luminances can be displayed with 10 gradations and 110 luminances with 11 gradations, the liquid crystal panel 6 cannot display 91 to 109 luminances in principle. This is because the gradation corresponding to these luminances is a value that cannot be taken in the image data.

  Therefore, it is necessary to adjust the theoretical conversion γ curve to a conversion γ curve that can be actually used in the liquid crystal display device 1. Specifically, a converted γ curve to be actually used is obtained by correcting the theoretical luminance after conversion into luminance that can be actually displayed. FIG. 5 is a diagram for explaining the principle when the theoretical conversion γ curve is corrected to an actually usable conversion γ curve.

  As shown in FIG. 5, first, an intermediate value between each possible luminance (indicated by a black diamond) in the original (ie, prescribed) γ curve 40 is obtained. The obtained intermediate value is applied to the theoretical γ curve 42. In FIG. 5, this is schematically shown as a line parallel to the horizontal axis with respect to the γ curve 40 to the γ curve 42. Then, in the γ curve 42, if there is a theoretical luminance after conversion between a certain intermediate value and the next intermediate value, this theoretical luminance is converted into an element existing between these two intermediate values. (Ie, on the γ curve 40). Thus, by converting the theoretical luminance into luminance that can be actually displayed, a γ curve 44 is obtained as a curve connecting the luminances after the conversion. Since each luminance corresponding to each gradation on the γ curve 44 has the same value as any luminance corresponding to each gradation on the original γ curve 40, the liquid crystal display device 1 actually uses it for image display. it can. The γ curve 44 in FIG. 4 corresponds to the γ curve 44 in FIG.

  When an image is displayed using the γ curve 44, the luminance of the backlight light is reduced to 1 / X. Therefore, the luminance when actually displayed on the liquid crystal panel 6 is obtained from the γ curve 44. The value is 1 / X of the luminance. That is, the relationship between the luminance and gradation of the image displayed on the liquid crystal panel 6 is a γ curve 46 shown in FIG. On the other hand, when the brightness of the backlight is not changed, that is, when an image is displayed using the specified γ curve 40, the relationship between the brightness of the displayed image and the gradation matches the γ curve 40.

  In the γ curve 46, the luminance is saturated when the gradation becomes a certain value or more. However, since the image to be displayed originally does not include the gradation exceeding the certain value, there is no problem in image display. At the gradation below the certain level, the relationship between the gradation and the luminance shows a similar tendency between the original image and the image after the change. However, the luminance displayed by each gradation is not the same value before and after the change. This is because, in the changed γ curve 44, the theoretical luminance after conversion is corrected to the luminance that can be actually displayed.

  If the changed image is displayed using the γ curve 44 as it is, each luminance value corresponding to each gradation is the luminance corresponding to the same gradation value when the original image is displayed using the γ curve 42. It will be different from the value of. This difference becomes a luminance error in each pixel constituting the image. Due to the existence of this error, the amount of change in gradation and the amount of change in luminance are inconsistent, and when displaying gradation or human skin, the original image was smooth and changed. In a later image, the original luminance is lost, resulting in a problem of rough display.

  Therefore, the liquid crystal display device 1 performs a process of diffusing the error. Specifically, first, for each pixel constituting the image after conversion, the luminance displayed on the screen when the luminance before change is displayed on the pixel using the original γ curve 40 is expressed as γ curve 40. Ask from. This is obtained as the luminance corresponding to the gradation of the original image in the γ curve 40. Next, for each pixel constituting the converted image, the luminance displayed on the screen when the pixel is displayed using the converted γ curve 44 is obtained from the γ curve 46. The luminance obtained when the image is displayed on the actual screen instead of the γ curve 44 is the value obtained by reducing each luminance of the γ curve 44 to 1 / X as described above, that is, on the γ curve 46. This is because the brightness value of. Thus, for each pixel, the difference between the two luminances obtained from the γ curves 40 and 46 is calculated as a luminance error in each pixel. When the error is calculated from all the pixels, the calculated luminance is diffused to other pixels for each pixel using a known error diffusion technique for the converted image data.

  FIG. 6 is a diagram for explaining luminance error diffusion of pixels. In the example of FIG. 6A, the error diffusion circuit 4 spatially diffuses the error of the pixel A to four adjacent pixels. Specifically, 5/16 of the error is diffused to the right adjacent and right lower pixels, and 3/16 of the error of the lower right and lower left pixels is diffused. If the calculated error is a positive value, the error is subtracted from the pixel A, and the same value is added to the diffusion destination pixel. On the other hand, when the error is a negative value, the error is added to the pixel A, and the same value is subtracted from the other pixels. The diffusion distribution described here is only an example. By diffusing an error as described above, a pixel generated in a certain pixel can be absorbed by a plurality of adjacent pixels, so that unevenness in luminance can be reduced.

  As shown in FIG. 6B, when an error is diffused from one pixel to another pixel, the error that originally occurred and the error diffused from the other pixel are added to the other pixel. To do. When the error diffusion circuit 4 diffuses the error of the other pixel, both of these two types of errors are processed as diffusion targets. For example, when an error is diffused from the pixel A to the pixel B, the error diffused from the pixel A to the pixel B is further diffused to the pixel C.

  As described above, the error diffusion circuit 4 diffuses each error constituting the converted image to each pixel. As a result, the converted image has a mottled pattern of dark luminance and bright luminance, and as a result, the desired luminance is displayed. Since the error diffusion method described above is already a known technique, further detailed description is omitted. The error diffusion circuit 4 can execute any known error diffusion method. For example, the error may be temporally diffused by using the frame rate control. Specifically, the error of a certain pixel in a certain frame may be diffused to the pixel at the same position in another frame from the next onward.

  The error diffusion circuit 4 outputs the converted image data after diffusing the error to the liquid crystal driver 5. The liquid crystal driver 5 displays an image (converted image) represented by the input converted image data on the liquid crystal panel 6. FIG. 7 shows an example of a display image (gradation). 7A and 7B are diagrams showing displayed gradations. FIG. 7A shows a gradation display based on the original γ curve, and FIG. 7B shows a gradation display when a conventional active backlight control is operated. (C) shows a gradation display when error diffusion is performed by the liquid crystal display device 1 of the present invention.

  As shown in FIG. 7A, when an image is displayed based on the original γ curve, the gradation is expressed smoothly. On the other hand, as shown in FIG. 7B, when the image is displayed by controlling the conventional active backlight control, since the luminance does not change even if the gradation changes, the gradation is not expressed smoothly. There are places. However, as shown in FIG. 7C, when the image is displayed using the error diffusion technique of the present invention, the gradation is expressed smoothly as in the case where the image data is displayed based on the original γ curve. The

  As described above, the liquid crystal display device 1 diffuses an error generated in each pixel constituting the image to other pixels, so that the quality of the converted image can be improved regardless of the intensity of the backlight light. It can be kept at the same level as the original image. Therefore, the brightness of the backlight light can be controlled flexibly and finely.

  The present invention is not limited to the above-described embodiments. Those skilled in the art can make various modifications to the present invention within the scope of the claims. That is, a new embodiment can be obtained by combining appropriately changed technical means within the scope of the claims.

  The present invention can be widely used as an apparatus (for example, a liquid crystal display apparatus) that displays an image using backlight light.

1 is a block diagram illustrating a configuration of a liquid crystal display device (display device) according to an embodiment. It is a figure which shows an example of the waveform of a backlight control signal, (a) shows the waveform of a backlight control signal when a maximum gradation value is small, (b) is a backlight when a maximum gradation value is large. The waveform of a write control signal is shown. It is a figure explaining the gradation interpolation of image data, (a) shows the relationship between an input gradation and an output gradation, (b) represents the conversion principle of a spline complementation area | region. It is a figure explaining the principle at the time of converting a regular gamma curve and obtaining a conversion gamma curve. It is a figure explaining the principle at the time of correcting a theoretical conversion gamma curve into a conversion gamma curve which can actually be used. It is a figure explaining the brightness | luminance error diffusion of a pixel. It is a figure which shows the displayed gradation, (a) shows the gradation display based on an original gamma curve, (b) shows the gradation display at the time of operating the conventional active backlight control, (c ) Shows a gradation display when error diffusion is performed by the liquid crystal display device 1 of the present invention. It is a figure explaining the problem of a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Liquid crystal display device 2 Image determination circuit 3 Image data conversion circuit 4 Error diffusion circuit (γ curve conversion means, error calculation means, error diffusion means)
5 Liquid crystal driver (display means)
6 LCD panel 7 Backlight dimming circuit 8 Backlight unit

Claims (4)

A display device that displays images using backlight light,
Changing means for changing the brightness of the backlight light;
Image conversion means for obtaining a converted image by converting the gradation of each pixel constituting the image in accordance with the luminance of the backlight light after the change;
Γ curve conversion means for calculating a converted γ curve by converting a prescribed γ curve according to the luminance of the backlight light after the change,
For each pixel constituting the converted image, the difference between the luminance when the pixel before the change is displayed on the pixel based on the γ curve and the luminance when the pixel is displayed based on the converted γ curve is an error. Error calculating means for calculating as
In the converted image, error diffusion means for diffusing the error to other pixels for each pixel constituting the converted image;
A display device comprising: display means for displaying the converted image after the error is diffused using the converted γ curve.   The display device according to claim 1, wherein the error diffusion means diffuses the error spatially or temporally.   The display device according to claim 1, wherein the display device is a liquid crystal display device. A display method for a display device that displays an image using backlight,
A change step of changing the brightness of the backlight light;
An image conversion step for obtaining a converted image by converting the gradation of each pixel constituting the image according to the luminance of the backlight light after the change,
A γ curve conversion step for calculating a converted γ curve by converting a specified γ curve according to the luminance of the backlight light after the change;
For each pixel constituting the converted image, the luminance when the gradation before conversion into the gradation of the pixel is displayed based on the γ curve and the gradation of the pixel are displayed using the converted γ curve. An error calculating step of calculating a difference from the brightness of
In the converted image, an error diffusion step for diffusing the error to other pixels for each pixel constituting the converted image;
And a display step of displaying the converted image after the error is diffused using the converted γ curve.