JP2002055648A - Gradation display method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that conventionally unnaturalness such as the case in which as if there is a luminance difference between pixels, in which there is hardly luminance difference generated in the halftone gradation display method of a display device, in which a sub-field method displaying respectively weighted plural binary images of a moving image, having halftone gradation by overlapping them timewise is used. SOLUTION: This method is a gradation display method, in which when binary images are arranged in ascending numeric order according to the size of weights, the halftone gradation display is performed by giving weights to the respective binary images, so that the absolute value of the difference (the primary difference) between weights respectively given to adjacent binary images is equal to or smaller than 6% of the total number of gradations which can be displayed by the superposition of the plural images.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plasma display panel (PDP).
In a gray scale display device having a binary memory, such as a digital micromirror device (DMD) or a digital micromirror device (DMD), a halftone is displayed by temporally superimposing a plurality of binary image subfields weighted according to luminance. The present invention relates to a halftone display method for a display device using a so-called subfield method.

[0002]

2. Description of the Related Art A conventional so-called subfield method is disclosed in Japanese Unexamined Patent Publication No.
In a display device having a binary memory effect such as
This is used to display halftones. FIG.
a and FIG. 30b show an example of this display method. The display device is
The control data for turning on and off the light is written in advance for all the pixels of the display screen, and then all the pixels are caused to emit light simultaneously based on the control data. With this method, the display device can display a television image of 8 bits and 256 gradations. Hereinafter, this example will be described.

As a conventional example, as shown in FIG.
A case where an image of one field is composed of binary images of eight subfields will be described. Each sub-field has a light-emitting period (a period in which light is emitted when the sub-field is ON) and a period in which light is not emitted, and a hatched portion is a light-emitting period. The period during which no light is emitted is substantially the same for each subfield, but the time length of the light emitting period or the number of light emitting pulses within this light emitting period corresponds to the weight given according to the luminance. Each subfield is assigned a subfield number, and a different weight is assigned to each subfield of the subfield number.

In this subfield method, the gradation of luminance is obtained by changing the length of light emission time or the number of light emission pulses within the time when visual afterimage effect can be used, that is, within one field period (elapsed time). ing. A person perceives the integration of the light emission time of each subfield within the period of one field for each pixel, or the number of light emission pulses, and perceives it as the luminance of the pixel.

In the example shown in FIG. 30, each subfield is represented by 1, 2, 4, 8, 16, 32,
Weights (hereinafter referred to as luminance levels) corresponding to the luminances of 64 and 128 are given. For example, a subfield having a subfield number of 1 (hereinafter, referred to as subfield 1) emits light once to obtain a luminance level “1”, and a subfield of subfield 8 has a luminance level of “128”. Is performed 128 times to obtain.

FIG. 30B is a diagram showing subfields to be illuminated in order to display an expected luminance gradation. The horizontal axis shows the subfields and the weight given to each subfield number, and the vertical axis shows the gradation of luminance to be displayed. In the figure, “ON” indicates a subfield to be illuminated in order to display the luminance gradation on the vertical axis.

More specifically, the display of the gradation 1 causes the subfield 1 to emit light. Similarly, the display of gradation 2 is subfield 2, and the display of gradation 3 is subfields 1 and 2.
The display of gradation 4 is subfield 3, the display of gradation 5 is subfields 1 and 3, the display of gradation 6 is subfields 2 and 3, and the display of gradation 7 is subfields 1 and 2. And 3 are displayed by combining subfield 4 with gray levels 0 to 7 for display of gray scales 8 to 15, and are displayed by combining subfield 5 and gray levels 0 to 15 with display of gray scales 16 to 31. 32-63 display is subfield 6 and gradation 0
32, the display of gradations 64-127 is a combination of subfield 7 and gradations 0-64, and the display of gradations 128-255 is a combination of subfield 8 and gradation 0.
To 128 in combination.

[0008] Each pixel of the PDP displays a halftone by a combination of which subfield emits light. For example, in order to obtain luminance equivalent to “173”, subfield 8 with a weight of “128”, subfield 6 with a weight of “32”, subfield 4 with a weight of “8”, and subfield 4 with a weight of “4” The sub-field 3 and the sub-field 1 of the sub-field 1 whose weight is “1” emit light. As a result PD
P 2 emits light in accordance with the respective weights (or emits light the number of times in accordance with the weights), and the luminance perceived by a human corresponds to a temporal addition value of the light emission.

As described above, in the subfield method, the number of times of light emission in each subfield is integrated to obtain the gradation of the screen.

[0010]

When such a halftone display method is used, in the case of a still image, since the line of sight of a person who views the screen is almost fixed on the screen, the person is given to each subfield. The weights are normally added within the elapsed time of one field, and the luminance of each pixel is perceived, and a desired halftone is obtained without any discomfort, so that there is no problem in image quality. However, in the conventional display method using the subfield method, a document (“Pseudo-contour noise seen in pulse width modulation moving image display”): Technical Report of the Institute of Television Engineers of Japan, Vol.
2, IDY95-21, pp.61-66), there is a problem that a unique pseudo contour noise (so-called moving image pseudo contour) is generated in a moving image and the image quality is deteriorated.

A person who views an image in a moving image perceives and perceives a moving object moving on the image. In the subfield method, the luminance level at a certain position (pixel) of a certain image captured by the eye is a normal image obtained by adding the emission time or the number of emission pulses to the elapsed time (period) of one field in the case of a still image. Corresponding. However, in the case of a moving image, the human eye moves to the position where the image moves before the light emission completely stops at that position, so that the luminance level at that position (pixel) of the image is the locus of the movement. It corresponds to the sum of the light emission time or the number of light emission pulses received during. That is, the addition of the light emission time or the number of light emission pulses is performed not over one pixel but over a plurality of pixels.

Accordingly, the luminance level of each pixel of the moving image cannot be perceived as a normal luminance level, so that the image quality deteriorates. This image quality deterioration is noticeable on a screen in which the luminance level gradually changes between adjacent pixels, for example, a human face or skin, and a pseudo contour pattern such as a contour line appears. This phenomenon will be briefly described with reference to the drawings.

FIG. 31 shows the state of light emission of the adjacent four pixels, pixel A, pixel A, pixel C, and pixel D, with the passage of time (horizontal axis). In this example, pixel a and pixel a are subfields 1, 2, 3, 4, 5, 6, and 7
, And does not emit light in subfield 8. Pixels c and d are subfields 1, 2, 3, 4, 5,
No light is emitted in 6 and 7, and light is emitted in subfield 8.

That is, FIG. 31 shows that the luminance level of the pixel A and the pixel A is "127", the luminance level of the pixels c and d is "128", and "127" and "127" whose luminance levels differ only by one. This is a diagram schematically showing a case where pixels of “128” are adjacent to each other.

When the image is stationary and the line of sight does not move, the viewer of the screen moves the pixel of the luminance level "127" along the arrow marked with the fixed line of sight 127 in FIG. Light emission is detected, the light emission time or the number of light emission pulses is correctly integrated, and the brightness of the luminance level “127” is perceived. Similarly, the viewer of the screen detects the light emission of each subfield in the direction of the arrow marked with the fixed line of sight 128 for the pixel of the luminance level “128”, and perceives the brightness of the luminance level “128”. .

However, when the image on the screen moves, the image formed on the retina follows the line of sight,
As the time elapses, the relationship between the pixel position and the corresponding subfield shifts, and the gradation is disturbed. For example, assume that an image moves by a distance of three pixels during one field period. That is, the elapsed time (period) of one field
Then, it is assumed that an image moves from the position of the pixel a to the position of the pixel d. In this case, the line of sight of the person follows the movement of the image, watching the pixel a at the time when the subfield 1 is emitting light, and predicting that the image moves to the position of the pixel d after one field period, The user moves his / her line of sight according to the moving speed of the image, and moves to the pixel d. This situation is shown in FIG.
It is indicated by a dotted line falling to the right.

The line of sight moves from upper left to lower right in FIG. In this case, the eye sees the pixel A of the luminance level “127”, all the subfields from the subfield 1 to the subfield 7 of the pixel A, the pixel C of the luminance level “128”, and the subfield 8 of the pixel D. Therefore, the brightness level 255 (= (1 + 2 + 4 + 8 + 16 + 32 + 6
4) You will feel the brightness of +128).

Conversely, when the line of sight shifts from the pixel d to the pixel a, that is, when the line of sight moves from the lower left to the upper right in FIG. You may feel zero brightness.

This phenomenon of perceiving a luminance level which is not the intended level when the line of sight of a person who views a moving image follows the movement of the image is caused by mistakenly seeing the light emission of a subfield having a large weight (luminance level). It becomes remarkable when

As described above, in the conventional halftone display method, when the screen is viewed following the movement of the image, the unnaturalness in which there is a luminance difference even between pixels having almost no luminance difference is perceived. There was a problem that there is.

[0021]

In order to solve this problem, according to the present invention, when a plurality of binary images are arranged in ascending order according to the magnitude of the weight, the distance between adjacent binary images is reduced. Are weighted such that the difference between the weights is less than or equal to 6% of the total number of gradations that can be represented by the superposition of the plurality of binary images.

Further, in the present invention according to claim 2, when the plurality of binary images are arranged in ascending order according to the magnitude of the weight, a difference in weight between adjacent binary images (primary difference).
Each binary image is weighted such that the absolute value of the difference (secondary difference) between the image and another primary difference adjacent thereto is 3% or less of the total number of gradations.

According to the third aspect of the present invention, when the binary images are arranged in ascending order according to the magnitude of the weight, the binary images located in the first half of the total number of the binary images are arranged. Are weighted such that the average value of the weighting differences (primary differences) is smaller than the average value of the primary differences between the binary images located in the latter half.

Further, in the present invention according to claim 4, when the binary images are arranged in ascending order according to the magnitude of the weight, of the total number of the binary images, The average of the weighted differences (primary differences)
Each binary image is shifted such that the average value (moving average value) obtained by shifting the range of the primary difference between the binary images included in the average one by one toward the rear of the array of weights monotonically increases. Is weighted.

Further, in the present invention according to claim 5, when the binary images are arranged in ascending order according to the magnitude of the weight, the difference (primary difference) between the weights of adjacent binary images is equal to the weight. Each binary image is weighted so as to increase monotonically from the smaller side to the larger side.

Further, in the present invention according to claim 6, the combination of the binary images expressing an arbitrary halftone level is a combination preferentially selected from binary images having a small weight. .

Further, in the present invention according to claims 7 and 8, the order in which the binary images are temporally superimposed and emitted is such that the weights of the binary images are ascending or descending. It was made.

According to the ninth and tenth aspects of the present invention,
This is a gradation display method in which two binary images are temporally superimposed and displayed in halftone, and specifies the weighting ratio of each binary image.

The invention according to claim 11 and claim 12 is as follows:
This is a gradation display method in which eleven binary images are temporally superimposed and displayed in halftone, and specifies the weighting ratio of each binary image.

The invention according to claims 13 and 14 is:
This is a gradation display method in which ten binary images are temporally superimposed and displayed in halftone, and the weighting ratio of each binary image is specified.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to solve these problems,
The present invention is characterized in that the weighting (luminance level) of each subfield is appropriately set.

According to the present invention, there is provided a gradation display method for displaying a plurality of binary images, each of which is weighted based on luminance, in halftone by temporally superimposing the binary images. When arranged in ascending order, the absolute value of the difference in the weight (primary difference) given to each of the adjacent binary images is 6% or less of the total number of gradations that can be displayed by superimposing the plurality of binary images. The present invention relates to a gray scale display method characterized by giving a weight to each of the binary images so as to perform halftone display, or a difference (secondary difference) between the primary differences adjacent to each other. The present invention relates to a gradation display method characterized in that each binary image is weighted so as to display a halftone so that an absolute value is 3% or less of the total number of gradations.

An embodiment of the present invention will be described with reference to the drawings. (Embodiment 1) FIG. 1 shows a first embodiment of the present invention.
This will be described with reference to FIGS.

FIG. 2 shows an example of weights (luminance levels) when one field is composed of 12 subfields. The first line shows the subfield number, and the second line shows the weight given to each subfield. The subfields are arranged in order of weight for convenience, but they do not necessarily have to be emitted in this order. The third row indicates the value of the primary difference (the difference in weight between adjacent subfields, that is, the difference in weight between adjacent binary images).

According to the subfield number, the weight given to each subfield is 1, 2, 4, 6, 9, 1
4, 29, 34, 36, 39, 40, and 41. With the combination of the binary images having the 12 subfields, the image signal can express 8-bit 256 gradations.

FIG. 1 explicitly shows the order of light emission of the subfields and the light emission state using the weighting of each subfield shown in FIG. This figure shows four adjacent pixels (a horizontal, vertical, and an oblique direction that produce the same phenomena and effects as described below), pixel a, pixel c, and pixel d. The length of each of the rectangles in the horizontal direction represents a period (or the number of times of light emission) during which light is emitted in the subfield. The blank period between the squares is a period associated with each subfield and is a period in which no light is emitted.

In this example, the luminance level of pixel A and pixel A is “40”, and the luminance level of pixel C and pixel D is “4”.
1 ", the pixel A, the pixel A and the pixel C, and the pixel D are adjacent to each other. In this situation, how much difference occurs between the luminance level at which the line of sight following the moving image is captured and the original luminance level will be described.

The reason why the luminance level "40" and the luminance level "41" are selected is that the light-emitting state of the subfield having the heaviest weight among the 12 subfields is switched on or off. This is because the difference between the luminance level captured by the line of sight following the moving image and the luminance level originally desired is the largest. There are several methods for selecting or combining subfields for displaying an arbitrary luminance level. Here, the selection is given priority to the larger subfield.

A feature of the present invention is that when subfields are arranged in ascending order based on subfield weights, the primary difference is 6% of the total number of gradations 256, that is, "15" or less. Weighting each subfield as described above. It should be noted here that, in FIG. 2, the subfields are arranged in ascending order of the weights. However, when actually driving a display device such as a PDP, the subfields are arranged in time. The order of light emission is not limited to the ascending order of the subfield weights. That is, the order of arrangement in FIG. 2 is different from that of FIG. 1 including the actual light emission order, and is arranged in ascending order for the sake of simplicity of explanation.

FIG. 1 shows an example different from the order of the subfields in FIG.
4, 2, 6, 9, 14, 29, 34, 36, 39, 4
The case of emitting light in the order of 0, 41 is shown. The primary difference in the third row in FIG. 2 is a difference in weight between adjacent subfields. For example, the primary difference between subfield 1 and subfield 2 is 1 (= 2-1), and subfield 4 and subfield The primary difference between 5 is 3 (= 9−6). Similarly, the primary differences are 1, 2,.
2, 3, 5, 15, 5, 2, 31, and 1.

In the present embodiment, the maximum value of the primary difference is the primary difference “1” between subfield 6 and subfield 7.
5 ", which satisfies the condition that 6% of the total number of gradations 256, that is," 15 "or less.

Returning to FIG. 1, how the gradation is expressed by the combination of the weighted subfields will be described. When the line of sight of a person watching a display device such as a TV is stationary, the person correctly adds the light emission of each subfield for each pixel along the arrow shown as the fixed line of sight in the figure, and thus the luminance level is “40”. And the luminance of the luminance level "41" is correctly perceived. On the other hand, when the image moves, for example, it is assumed that the image moves a distance of three pixels during one field period. The gaze follows the movement,
It moves from pixel A to pixel D at the elapsed time of one field. The oblique arrow in FIG. 1 is the locus of the line of sight.
Due to the movement of the line of sight, the person adds up the light emission of the subfields that emit light at different timings of the pixels A, A, C, and D along the trajectory, so that the luminance level that can be captured by the eyes should be originally captured A deviation from the luminance level may lead to a misunderstanding that “40” should be “41”, “41” should be “40”, and so on.

However, the difference between the perceived luminance level and the original luminance level is smaller than in the prior art shown in FIGS. 30 and 31 in which halftones are displayed in eight subfields.
FIGS. 3 and 4 show this state. 3 and 4 show the relationship between the input luminance level and the perceived luminance level. The input video signal used here is a ramp signal which changes as a picture signal in a horizontal direction from a luminance level of 0 to 255 by one step. The ramp signal is a signal that moves in the horizontal direction at a speed of 6 pixels / field.

Hereinafter, a predetermined weight according to the luminance is given to each subfield, and the deviation of the perceived luminance level from the original luminance level occurring in that case is calculated using this signal. Further, a deviation of the luminance level perceived from the original luminance level is simply referred to as a “luminance level deviation”. It was confirmed that the information obtained from the calculation result coincided with the result when the image was actually visually evaluated.

FIG. 3 shows the relationship between the input luminance level and the perceived luminance level when the signal is input when the weights shown in FIG. 31 are applied to the eight subfields of the conventional example. Show. If there is no error as described above, the relationship between the input luminance level and the perceived luminance level should be indicated by a straight line. However, due to the mistakes mentioned above,
At some input luminance levels, the perceived luminance level is significantly different than it should be.

FIG. 4 shows the relationship between the input luminance level and the perceived luminance level when the weights shown in FIG. 1 are applied to the twelve subfields of the present embodiment. Comparing FIG. 3 with FIG. 4, it can be seen that the method of the present embodiment shown in FIG. 4 reduces the magnitude of the deviation (peak value) from the original value.

Using a variety of moving images including a lamp signal image (for example, “PDP moving image quality evaluation image list” produced by the PDP Development Council in 1996), the magnitude of the luminance level shift and the image quality, that is, the moving image The appearance of the pseudo contour was compared and verified. As a result, in the conventional example shown in FIG.
There is a close relationship between the peak value of the luminance deviation and the appearance of the pseudo contour of the moving image.
When the luminance peak value appears near 0 or less, it has been found that the pseudo contour of the moving image is visually barely permissible. Therefore, the line A connecting the vertexes of the two peak values is determined as the “permissible line” of the pseudo contour of the moving image and used as an index. FIG. 3 shows the permissible line A.

It is known that a person's ability to discriminate between light and shade (the ratio of the luminance difference dL to the luminance level L, dL / L) is constant in photopic viewing regardless of the absolute value of luminance. Therefore, it is expected that the allowable line A passes through the origin. However, in the display device, when the luminance level is 30 or less, the visual characteristic shifts from photopic vision to the mesopic region, and the ability to discriminate light and darkness is reduced (or exists along with a portion where the luminance level is relatively high). Since it is considered that the ability to discriminate the contrast of a portion having a relatively low brightness level is reduced by simultaneously viewing a portion having a low brightness level, the permissible line A does not pass through the origin. As a result, the permissible line A becomes the straight line shown in FIG. Hereinafter, description will be made based on the permissible line A.

On this assumption, when the subfields are conveniently arranged in ascending order based on the weights of the subfields, the smaller the difference between the weights of two adjacent subfields, that is, the smaller the primary difference, The appearance of the pseudo contour tends to be small, but when the primary difference is less than about 6% of the total number of gradations, the luminance shift is within the allowable line A,
It was found that the appearance of the pseudo contour of the moving image was reduced, and the image quality acceptable as a moving image could be secured. Note that, as shown in FIG. 1, the order of light emission in the subfield is not limited to ascending or descending order with respect to weight.

On the other hand, to express an arbitrary luminance level,
There are several ways to combine any of the 12 weights, and there is redundancy. here,
The combination was performed by intentionally preferentially selecting subfields with a large weight so that the luminance shift at a low luminance level is likely to be large. Even under such conditions, if the primary difference is 6% or less of the total number of gradations, the image quality is acceptable as described above.

In FIGS. 5 and 6, the weight given to each subfield is set as follows. As shown in FIG. 5, the weight (luminance level) given to subfields 1 to 12 is:
1, 2, 4, 8, 9, 10, 11, 21, 3 respectively
8, 49, 50, and 52. As shown in FIG. 5, the primary differences are 1, 2, 4, 1, 1, 1, 10, 17,
11, 1, and 2.

FIG. 6 shows the relationship between the input luminance level and the perceived luminance level when the same lamp input signal used in FIG. 3 is input when this weight is given to each subfield. In FIG. 6, the order of light emission in the subfield is ascending order of weight. In the case of the weighting shown in FIG. 5, the maximum value of the primary difference is "17" and the total number of gradations is 256.
In the case of this weighting, the deviation of the luminance level exceeds the allowable value. The numerical value of 6% is shown in FIG.
It can be seen that this is a significant numerical value even when compared with and 6.

FIGS. 7 and 8 show another example. FIG.
Then, the weights (luminance levels) given to the subfields 1 to 12 are 1, 2, 4, 8, 12, 26,
28, 30, 32, 34, 37, and 41. The primary differences resulting from the weighting are 1, 2, 4, 4, 1
4, 2, 2, 2, 2, 3, and 4.

FIG. 8 shows the relationship between the input luminance level and the perceived luminance level when the lamp input signal used in FIG. 3 is input when this weight is given to each subfield.

In the case of the weighting shown in FIGS. 7 and 8,
The maximum value of the primary difference is “14”, and the total number of gradations is 256.
5%, which is lower than “15” in FIG. 2, the deviation of the luminance level is within the allowable line A even in this case. The appearance of the pseudo contour of the moving image is shown in FIG. 5 and FIG.
The maximum value of the next difference is smaller than that in the case of “17”, and it can be seen that the image quality acceptable as a moving image has been secured.

In the case of the weighting shown in FIG.
The maximum value of the next difference is “12”, which is 4.7 of the total number of gradations 256.
%. Similarly, in the case of the weighting shown in FIGS. 9 and 27, the maximum value of the primary difference is “11”, which is 4.3% of the total number of gradations 256. Each of these examples is "15" in FIG.
In this case, the deviation of the luminance level is within the allowable line A.

FIGS. 5 and 6 show how a pseudo contour of a moving image looks.
The maximum value of the first-order difference is further reduced as compared with the case of “17”, and an improved image quality as a moving image can be secured.

Further, in the case of the weighting shown in FIGS. 10, 25, and 26, the maximum value of the primary difference is "8", which is 3.1% of the total number of gradations 256. Since these values are all well below “15” in FIG. 2, the deviation of the luminance level falls within the allowable line A.

FIGS. 5 and 6 show how a pseudo contour of a moving image looks.
The maximum value of the first-order difference is further reduced as compared with the case of “17”, and excellent image quality as a moving image can be secured.

In the case of the weighting shown in FIGS. 15 and 24, the maximum value of the primary difference is "7" and the total number of gradations is 256.
2.7%. Since these values are all significantly lower than “15” in FIG. 2, the deviation of the luminance level falls within the allowable line A.

FIGS. 5 and 6 show how a pseudo contour of a moving image looks.
The maximum value of the first-order difference is significantly reduced as compared with the case of “17”, and further excellent image quality can be secured as a moving image.

(Embodiment 2) Next, a second embodiment of the present invention will be described with reference to FIG.

In FIG. 9, the weight given to each subfield is 1, 2, 4,
8, 12, 23, 28, 32, 33, 35, 36, and 41, and the primary differences are 1, 2, 4, 4, 11, 5,
4, 1, 2, 1, and 5. This primary difference is 6% of the total number of gradations 256, that is, “15” or less.

The secondary difference is shown on the fourth line in FIG. The secondary difference indicates a difference between adjacent primary differences. For example, the difference “1” between the primary difference “1” between the subfield 1 and the subfield 2 and the primary difference “2” between the subfield 2 and the subfield 3 is the secondary difference. The secondary differences are 1, 2, 0, 7, -6, -1, and-in order from the left of FIG.
3, 1, -1, and 4. The characteristic point in this embodiment is that the absolute value of the secondary difference is 3% of the total number of gradations 256.
That is, the point is weighted so as to be “7” or less.

The aim of this is to weight each subfield so that the primary difference is 6% or less of the total number of gradations, and to further suppress the change in the primary difference relatively small. , The first-order difference tends to increase as the weight increases, the first-order difference between the sub-fields with smaller weights is made as small as possible, and the first-order difference between the sub-fields with larger weights becomes larger. Is to allow that.

For comparison, first, the case of weighting the subfields of FIG. 2 shown in (Embodiment 1) will be described as an example. In FIG. 2, the primary difference suddenly increases from a small value of “5” or less to “15” between subfield 6 and subfield 7, and a small primary difference continues in the latter half. At this time, the secondary difference is “10” between the fifth and sixth primary differences from the left, “−10” between the sixth and seventh primary differences, and The absolute value indicates a value corresponding to 4% of the total number of gradations 256.

On the other hand, with the weighting shown in FIG. 9, the primary difference has increased to "11" between subfield 5 and subfield 6, but the sudden increase to "15" seen in FIG. 2 is avoided. The second half has a relatively large first-order difference as compared with FIG. At this time, since the primary difference between subfields 4 and 5 is “4” and the primary difference between subfields 5 and 6 is “11”,
Between these primary differences, the value increases to the maximum value “7”, but this value remains at 3% of the total number of gradations.

As described above, FIG. 4 shows the difference between the luminance level perceived from the original luminance level caused by the weighted subfield configuration shown in FIG. 2 of the first embodiment and the lamp input signal. (Hereinafter, abbreviated as “brightness level shift” as described above) is shown.

FIG. 29 shows the result of calculating the shift in the luminance level caused by the weighted subfield configuration shown in FIG. 9 of the present embodiment with respect to the lamp input signal.

When FIG. 4 is compared with FIG. 29, the peak value of the luminance shift is shown in FIG. 9 where the secondary difference is set to 3% or less of the total number of gradations.
Is decreasing little by little as a whole, and the improvement ratio is high in a portion where the luminance level is low. In order to more clearly show this state, evaluation is performed using a mean square error as a quantitative index. The mean square error is calculated as: mean square error = [{(perceived luminance level i−input luminance level i) 2} / N] 1/2. Here, N is the number of data to be included in the calculation. When the above mean square error is calculated with respect to the difference between the luminance levels shown in FIGS. 4 and 29, for each range included in the calculation,

[0071]

[Table 1]

Is obtained. The range to be included in the calculation is the whole: luminance level “0” to luminance level “255” Low luminance level part: luminance level “0” to luminance level “127” Luminance level Higher part: luminance level “128” to luminance level “255”.

From this result, it can be seen that the deviation of the luminance level is reduced as a whole, and the improvement ratio is high in the portion where the luminance level is low. The method of reducing the secondary difference is verified by a method of adding the weight of each subfield along the line of sight movement used in (Embodiment 1). In the example used here, from the viewpoint that the effect appears more easily, the maximum value of the absolute value of the secondary difference shown in FIG. 7 is “12” and the secondary difference shown in FIG. The maximum value of the absolute value of is "1"
This is an example of a very small value.

The secondary difference shown on the fourth line in FIG.
1, 2, 0, 10, -12, 0, 0, 0, 1, and 1
It is.

On the other hand, the secondary differences shown in the fourth row of FIG. 10 are 1,1,1,1,1, -1, -1,1,1,1, and 0 from the left, and the maximum value of the secondary differences Is 3% of the total number of gradations 256,
That is, it is "7" or less.

In the two examples of FIGS. 7 and 10, attention is paid to the luminance level at which the light emission of the subfield 6 with the largest weight is switched from off to on, including the subfield 6 where the influence of the secondary difference starts to appear. These are shown in FIG.
FIG. 1B shows an example in which four pixels of pixel a, pixel a, pixel c, and pixel d are adjacent to each other. Here also, an example will be described in which a combination of subfields for expressing an arbitrary luminance is preferentially selected from subfields having a large weight. Therefore, the brightness levels 25 and 26 corresponding to FIG.
And the switching between the luminance levels 15 and 16 corresponding to FIG. Although the horizontal axis in FIG. 11 is the time axis, the order of light emission in the subfields is not limited to the ascending or descending order of the subfield weights.

FIG. 11A corresponds to the weighting of FIG. 10, in which the subfield 2 and the subfield 5 are turned on to express the luminance level "15". At the luminance level "16", only the subfield 6 is turned on. . Also, FIG. 11B corresponds to the weighting of FIG. 7, and the subfield 1, subfield 2, subfield 4, and subfield 5 are turned on to express the luminance level “25”. Only is on. In such a light emission state, when the line of sight is stationary, as indicated by the arrow of the fixed line of sight, the luminance level of each pixel is normally added from subfield 1 to subfield 6, and the original luminance level Capture.

In the case of a moving image, for example, when the line of sight moves by three pixels during the elapsed period from subfield 1 to subfield 6 in one field, the direction moves from the upper left to the lower right along the arrow from pixel a to pixel d. As my eyes move,
The luminance level captured in FIG. 11a is about “20 (= 4
+16) ”, on the contrary, when the line of sight moves from pixel d to pixel a, the luminance level of“ 11 ”is noticeable to the eyes.

On the other hand, in FIG. 11B, in the case of the line of sight along the arrow from pixel A to pixel D, the perceived luminance level is about “51 (= 1 + 4 + 8 + 12 + 26)”, and the movement of the line of sight from the opposite pixel D to pixel A. In, the captured luminance level is about "0", and the deviation from the original luminance level is large.

When comparing FIG. 11A and FIG. 11B, it can be seen that the shift of the luminance level detected according to the movement of the line of sight is clearly smaller in FIG. 11A, and the second order difference is smaller in this verification method. It is effective.

That is, if the secondary difference is suppressed to 3% or less of the total number of gradations, the change of the primary difference can be made relatively small, and the primary difference becomes smaller as the weight of the subfield becomes lower. It can be seen that since it is possible to have a tendency to increase, it is possible to suppress the deviation of the luminance level detected in accordance with the movement of the line of sight at a relatively low luminance.

(Embodiment 3) Next, a third embodiment of the present invention will be described. The magnitude of the deviation of the perceived luminance level from the original luminance level is smaller in a subfield having a higher luminance level than in a subfield having a higher luminance level.
It is desirable to be smaller. This means that, when the weights of the subfields are arranged in ascending order for convenience, the average value of the first-order primary difference (hereinafter referred to as AF) of the total number of subfields and the first-order primary difference of the latter half (Hereinafter, this value is referred to as AS) as a parameter.

For example, when 12 subfields are used, AF is an average value of primary differences obtained from subfield numbers 1 to 6 arranged in ascending order according to luminance.
Is the average value of the primary differences obtained from the subfield numbers 7 to 12.

Although the primary difference is not more than 6% of the total number of gradations, the secondary difference is not more than 3% of the total number of gradations. It is stated that the deviation is small. FIG. 12 shows an example of weighting the subfield. In the example shown in FIG. 12, the maximum value of the primary difference is “14”, which is 6% or less of the total number of gradations, but the maximum value of the absolute value of the secondary difference is “12”, which is 3% of the total number of gradations. Not less than%. The parameters (AF, AS) are (3.6, 6.8), and the latter half is larger than the former half.

FIG. 13 shows the luminance level shift due to the lamp signal input in the example of FIG. 12 (the order in which the subfields emit light is described by the weight of the subfields, and 1, 4, 2, 8, 15, 15). 19, 21, 24, 26, 3
9, 41, 55). FIG. 13 and FIG.
Compared to FIG. 4, which shows the luminance level shift corresponding to the luminance level shift, the peak value of the luminance level shift is almost the same in the portion where the luminance level is 150 or less, but the number of occurrences of the large peak is the former 6
It can be seen that the luminance difference is less likely to occur with the former and the latter 12 with the former. Further, if the idea of using the average value of the primary difference as a parameter is extended not only to the first and second half points of the total number of subfields but also to a moving average in the middle, the average value becomes monotonous in order. It was found that the increase is more effective for the luminance shift.

As an example, regarding the weighting shown in FIG. 10, when the average value of the primary differences is examined from the first half AF to the second half AS as the average value of several five primary differences, In the order (3.0, 3.6, 4.2, 4.8, 5.4,
6.0, 6.8) and monotonically increased. On the other hand, when the weights shown in FIG. 12 are given, the average value of the primary differences is
From the AF to the AS in the latter half, when examining the average value of each of the five primary differences, (3.6, 3.8, 4.
0, 3.6, 4.8, 4.4, 6.8), which is not monotonically increasing.

FIG. 14 shows the result of calculating the luminance shift caused by the weighted subfield shown in FIG. 10 with respect to the lamp input signal. Similarly, FIG. 13 shows a result of calculating a luminance shift caused by the weighted subfield shown in FIG. 12 with respect to the lamp input signal. This FIG.
From the comparison of FIG. 13 and the visual verification, in FIG. 14, the peak of the luminance shift is dispersed without concentrating, and the effect that the pseudo contour becomes difficult to understand is higher in FIG. ,found.

Up to this point, the effect of reducing the luminance deviation has been described using the average value of the primary differences as a parameter. However, if more limited conditions for obtaining the effect are obtained, the individual values of the primary differences themselves increase monotonically. We come to the condition that we do. An example is shown in FIG.

The example shown in FIG. 15 is composed of 12 subfields. The first line is the subfield number, and the second line is the weight given to each subfield. The subfields are conveniently arranged in ascending order of weight. The third row is the value of the primary difference, and the fourth row is the value of the secondary difference.

According to the subfield number, the weight given to each subfield is 1, 2, 4, 7, 11,
16, 21, 26, 32, 38, 45, and 52, and the primary differences are 1, 2, 3, 4, 5, 5, 5, 6, 6,
7, 7, the secondary differences are 1, 1, 1, 1, 0, 0, 1,
0, 1, and 0. In this example, the value of the primary difference monotonically increases from the primary difference between the subfields with smaller weights to the primary difference between the subfields with larger weights.

In this example, the shift of the luminance level was calculated at the input of the ramp signal (the order of emitting the subfields was described by the weights of the subfields, and 1, 4, 2, etc.).
7, 11, 16, 21, 26, 32, 38, 45, 52
It can be seen from FIG. 16 that the peak values of the deviation are dispersed without being concentrated, and that the peak value itself is suppressed as a whole as compared with FIG. This can be confirmed by visual verification.

(Embodiment 4) Next, a fourth embodiment of the present invention will be described. In the examples described in (Embodiment 1) to (Embodiment 3), a large weighting is used to represent an arbitrary luminance level, while there is some redundancy as a combination of subfields having different weights. In the description, the combination is selected preferentially from the subfields with However, for the following reasons, it has been found that it is better to preferentially select and combine subfields having smaller weights in terms of luminance saturation characteristics.

Here, the weighting of FIG. 10 is given as an example. That is, 1, 2, 4, 7, 11, 16, 20, 25, 3
Weights of 1, 38, 46, and 54 are given (subfields are conveniently arranged in ascending order of weight). FIGS. 17 and 18 show two examples in which these subfields are selected and displayed in combination from the luminance level “1” to the luminance level “30”.

FIG. 17 preferentially uses a subfield having a small weight to represent an arbitrary luminance, and FIG. 18 preferentially uses a subfield having a large weight to represent an arbitrary luminance. . This means that the subfields marked with a circle in the figure are used for light emission. For example, when expressing the luminance level “25”, in the selection method shown in FIG. 18 that preferentially uses the subfields having the larger weight, only the subfield 8 is illuminated, whereas the selection method shown in FIG. In the selection method shown in FIG. 17 which is used preferentially from subfields, subfield 1 (luminance level “1”), subfield 2 (luminance level “2”), subfield 3 (luminance level “4”),
Five subfields, subfield 4 (brightness level “7”) and subfield 5 (brightness level “11”), emit light.

When the light emission luminances of these two types are compared,
The latter feels brighter than the former. This is because if the number of light emission of the light emitter is increased or the light emission time is increased within a short time width, saturation of the luminance of the light emitter generally observed occurs. In order to alleviate the luminance saturation, for example, measures such as lowering the absolute luminance, dispersing the light emission temporally within the integration time of the eyes, and the like are effective, but in the case of a device for displaying an image, Since high luminance is desirable, a method of dispersing the number of times of light emission within the integration time of the eyes is preferable. That is, when the luminance is expressed by dispersing it into a plurality of subfields as shown in FIG. 17, the temporal concentration of the light emission is eliminated, and the luminance saturation can be reduced and the luminance can be approximated to the correct luminance.

In addition to the brightness level “25”, the brightness level “1” to the brightness level “30” can be expressed by using a sub-field with a small weight preferentially or not. When the average value of the number of subfields selected and combined per gradation is given priority to a subfield with a small weight, “3.0 sheets / 1 luminance” (= 89/30 luminance) When the subfield is used with priority, "1.9 luminance / 1 luminance"
(= 58/30 luminance), and it can be seen that the luminance is expressed over more subfields when the smaller subfield is used with priority.

That is to say, when priority is given to subfields having smaller weights when expressing an arbitrary luminance, the light emission is dispersed in more subfields, so that the luminance saturation of the luminous body is alleviated. As a result, good gradation can be obtained regardless of a still image and a moving image, and the image quality can be improved.

Further, for both examples, the same ramp waveform as described above was used as an input signal, and the luminance level shift when this signal was moved at a constant speed was calculated.
Comparing the example in which the large subfield is preferentially used in FIG. 14 with the present embodiment in FIG. 19, the use of the small subfield preferentially in the low luminance area
It can be seen that the peak value of the luminance shift can be greatly improved, and it is also effective in terms of the pseudo contour of the moving image. FIG. 1
In the calculations of FIG. 4 and FIG. 19, the order of light emission in the subfields is 1, 3, 2,
4, 5, 6, 7, 8, 9, 10, 11, 12 in that order,
It is not limited to ascending order.

In addition, this effect is not limited to the weighting of FIG. 10, but works similarly for all weighting examples given in (Embodiment 1) to (Embodiment 3). is there.

(Embodiment 5) A fifth embodiment of the present invention will be described. While it is generally said that this method is effective as a method for reducing false contours of moving images, the timing for emitting an arbitrary luminance level and the timing for emitting a luminance level close to the luminance level are as close as possible. There is a condition that it is better. Here, an embodiment of the present invention based on this concept is described with reference to FIG.
Description will be made using an example of weighting assigned to each subfield shown in FIG.

In this example, the luminance levels from "0" to "255" are displayed by preferentially using the sub-fields with the smaller weights. Are driven in such a manner that the weights are arranged in ascending order. In the above-described embodiments, the explanation has been made by arranging the subfield weights in the order of convenience, but here, the light emission order itself is limited to the ascending order, which is different from the above. is there.

In order to quantitatively represent the timing at which an arbitrary luminance level is emitted, a value called “average emission subfield position” is defined.

"Average light emission subfield position" = (1 /
A) × (B / C) where A: number of subfields constituting a field B: sum of subfield numbers that emit light when displaying an arbitrary luminance C: subfield that emits when displaying an arbitrary luminance FIG. 20 shows the average light emission subfield position calculated based on the above equation with respect to the input luminance level. For example, a description will be given taking a luminance level “20” as an example of one point in the drawing. A in the above equation is 12, because it is the number of subfields constituting the subfield. Brightness level "2
Since light is preferentially emitted in a small subfield to represent "0", the subfield marked with a circle at the level of the luminance level "20" in FIG. 17 is selected to emit light. That is, three subfields, subfield 2 (luminance level “2”), subfield 4 (luminance level “7”), and subfield 5 (luminance level “11”) emit light. Therefore, B is 11 (= 2 + 4 +
5) Since C is 3, the “average light emission subfield position” = (1/12) × (11/3) = 0.305. In other words, it is understood that the average position of the subfield that emits light when expressing the luminance level “20” exists at about 30% from the beginning of the period of one field.

In the present embodiment, light emission is performed in time series in the order of subfield numbers, and driving is performed. Therefore, the vertical axis in FIG. 20 indicates a temporal position in a case where the time width of one field is "1". You may think. Referring to FIG. 20, the average light emission subfield position smoothly increases with the luminance level. From this fact, the light emission timing gradually shifts to the rear in a time zone one field period forward with the increase in the luminance level. You can see that it is moving to the time zone.

The same effect as in the ascending order can be obtained also when the light emission order of the subfields is set to the descending order opposite to the ascending order, that is, when the light is driven by emitting the light in order from the subfield side with the larger weight. FIG. 21 shows the average light emission subfield position in this case with respect to the input luminance level.
However, in this case, it can be seen that the average light emission subfield position is biased toward the time zone after one field period as the luminance level increases, and gradually moves to the time zone ahead.

The light emission timing driven according to the present embodiment (ie, light emission timing at which pixels having similar luminance levels emit light in a subfield existing in a similar time zone during one field period) has a similar luminance level. Consider a case where pixels are spatially adjacent to each other. Even if the line of sight following the movement of the image captures the light emission of a plurality of subfields over a plurality of adjacent pixels, as described above, these pixels emit light of the subfields existing in a similar time zone of one field. Light emission timing, the probability of occurrence of luminance shift is low,
Disturbance in gradation is less likely to occur.

The perceived brightness level calculated for the ramp signal input is shown in FIG. 22 for ascending order and FIG. 23 for descending order.

When an arbitrary luminance level is preferentially selected and combined from subfields with a small weight to display, and the light emission order is ascending or descending, the deviation of the luminance level is small as a whole, and the moving image can be viewed from the viewpoint. It can be said that the disturbance of the gradation at the time of chasing, that is, the false contour of the moving image is unlikely to occur. This applies not only to the weighting example of FIG. 10 but also to all the weighting examples described above. In the above description, the number of subfields has been limited to twelve. However, the number of subfields is not necessarily limited to twelve. The thing which can obtain the same effect.

For example, in the case of 11 subfields,
Weighting is 1, 2, 4, 8, 13, 19, 26, 34,
42, 49, 57 (FIG. 25) or 1, 2,
The ratio may be set to 4, 8, 14, 20, 26, 33, 41, 49, 57 (FIG. 26). In the example of ten subfields, the weights are set to 1, 2, 4,.
8, 16, 25, 34, 44, 55, 66 (FIG. 27) or 1, 2, 4, 8, 15, 24, 33, 4
Even in the case of the ratio of 4, 56, 68 (FIG. 28), a similar effect can be obtained in that a shift in the luminance level when the visual line follows the moving image, that is, a pseudo outline of the moving image is unlikely to occur.

[0110]

According to the gradation display method of the present invention, it is possible to remarkably reduce the appearance of a pseudo contour of a moving image and improve the quality of a moving image as compared with the conventional example.

Further, according to the gradation display method of the present invention, especially in a low luminance area, the appearance of the pseudo contour of a moving image can be reduced, and the image quality of a moving image can be improved. Further, the gradation display method of the present invention can further reduce the overall appearance of the pseudo contour of the moving image from low luminance to high luminance.

Further, according to the gradation display method of the present invention, a good gradation can be obtained regardless of a still image or a moving image, and the image quality can be improved. Can be remarkably reduced, and the image quality of a moving image can be improved.

Further, according to the gradation display method of the present invention, the appearance of a pseudo contour of a moving image can be significantly reduced in a low-luminance to high-luminance region, and the image quality of a moving image can be improved.

In the embodiment of the present invention described above, the case where the total number of gradations is 256 has been described.
It goes without saying that the total number of gradations is not limited to 256. Further, the present invention can have various other modifications. Accordingly, all modifications that come within the true spirit and scope of the invention are intended to be covered by the appended claims.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of subfield light emission for explaining improvement of image quality in a moving image according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating weighting based on luminance given to each subfield according to the first embodiment of the present invention.

FIG. 3 is a diagram illustrating a problem of image quality in a moving image in a conventional example, and is a diagram illustrating a relationship between an input luminance level and a perceived luminance level.

FIG. 4 is a diagram illustrating an improvement in image quality of a moving image when weighting based on luminance illustrated in FIG. 2 is applied to a subfield according to the present invention, and illustrates a relationship between an input luminance level and a perceived luminance level; Figure

FIG. 5 is a diagram for comparison in the first embodiment of the present invention, illustrating another weighting based on luminance given to a subfield.

6 is a diagram showing a state of image quality in a moving image when weighting based on luminance shown in FIG. 5 of the present invention is given to a subfield, and is a diagram showing a relationship between an input luminance level and a perceived luminance level.

FIG. 7 is a diagram illustrating weighting based on luminance given to another subfield according to the first embodiment of the present invention.

8 is a diagram showing an improvement in image quality of a moving image when weighting based on luminance shown in FIG. 7 of the present invention is given to a subfield, and shows a relationship between an input luminance level and a perceived luminance level. Figure

FIG. 9 is a diagram illustrating weighting based on luminance given to a subfield according to the second embodiment of the present invention.

FIG. 10 is a diagram illustrating weighting based on luminance given to another subfield according to the second embodiment of the present invention.

FIG. 11 is a schematic diagram of subfield light emission for explaining improvement of image quality in a moving image according to the second embodiment of the present invention.

FIG. 12 is a diagram illustrating weighting based on luminance given to a subfield according to the third embodiment of the present invention.

13 is a diagram showing a relationship between an input luminance level and a perceived luminance level when weighting based on luminance shown in FIG. 12 of the present invention is given to a subfield.

FIG. 14 is a diagram showing a relationship between an input luminance level and a perceived luminance level when weighting based on luminance shown in FIG. 10 is given to a subfield in the third embodiment of the present invention.

FIG. 15 is a diagram illustrating weighting based on luminance given to another subfield according to the third embodiment of the present invention.

16 is a diagram showing a relationship between an input luminance level and a perceived luminance level when weighting based on luminance shown in FIG. 15 of the present invention is applied to a subfield.

FIG. 17 is a diagram illustrating a selected combination of subfields according to the fourth embodiment of the present invention.

FIG. 18 is a diagram illustrating a selected combination of subfields according to the fourth embodiment of the present invention.

FIG. 19 is a diagram illustrating a relationship between an input luminance level and a perceived luminance level corresponding to FIG. 17 of the present invention.

FIG. 20 is a diagram illustrating an average light emitting position of a subfield according to the fifth embodiment of the present invention.

FIG. 21 is a diagram illustrating an average light emitting position of a subfield according to the fifth embodiment of the present invention.

FIG. 22 is a diagram showing a relationship between an input luminance level and a perceived luminance level corresponding to FIG. 20 of the present invention;

FIG. 23 is a diagram showing a relationship between an input luminance level and a perceived luminance level corresponding to FIG. 21 of the present invention;

FIG. 24 is a diagram illustrating weighting based on luminance given to a subfield according to the fifth embodiment of the present invention.

FIG. 25 is a diagram illustrating weighting based on luminance given to a subfield according to the fifth embodiment of the present invention.

FIG. 26 is a diagram illustrating weighting based on luminance given to a subfield according to the fifth embodiment of the present invention.

FIG. 27 is a diagram illustrating weighting based on luminance given to a subfield according to the fifth embodiment of the present invention.

FIG. 28 is a diagram illustrating weighting based on luminance given to a subfield according to the fifth embodiment of the present invention.

FIG. 29 is a diagram showing a relationship between an input luminance level and a perceived luminance level corresponding to FIG. 9 of the present invention;

FIG. 30 is a view for explaining emission weighting and selection combinations of subfields in a conventional example.

FIG. 31 is a schematic diagram of subfield light emission explaining a problem of image quality in a moving image in a conventional example.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) H04N 5/66 G09G 3/28 K (72) Inventor Mitsuhiro Kasahara 1006 Kazuma, Kazuma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Inventor Shizuo Inohara 1006 Kazuma Kadoma, Kazuma, Osaka Prefecture F-term in Matsushita Electric Industrial Co., Ltd.

Claims (4)

[Claims] 1. A gradation display method in which a plurality of weighted binary images are temporally superimposed to display a halftone, wherein a combination of the binary images having different weights to represent an arbitrary luminance is provided. When the binary images are arranged in ascending order according to the magnitude of the weight, the primary difference, which is the difference between the weights of the adjacent binary images, is changed from the side having the smaller weight of the binary image to the side having the larger weight. A gradation display method characterized by monotonically increasing toward the center. 2. The gradation display according to claim 1, wherein the combination of the binary images expressing an arbitrary halftone level is a combination preferentially selected from binary images having a small weight. Method. 3. The gradation display method according to claim 1, wherein the order in which the binary images are temporally superimposed is an order in which the weights of the binary images are in ascending order. 4. The gradation display method according to claim 1, wherein the order in which the binary images are temporally superimposed is an order in which the weights of the binary images are in descending order.