JP3184055B2 - Display device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a computer terminal,
The present invention relates to a display device used for various image information processing devices such as a monitor such as a word processor and a viewfinder such as a video camera.

[0002]

2. Description of the Related Art For example, in a display device using a plasma display, a liquid crystal element, or an electrochromic element, it is desired to perform multi-tone display in order to display a high-quality image.

Therefore, conventionally, when one pixel is divided into a plurality of sub-pixels capable of binary display for multi-tone display, the area of 2 ⁰ : 2 ¹ : 2 ² : 2 ³ :. It is known to split into ratios. With this division, if the number of sub-pixels constituting one pixel is ^N , 2 ^N
Level gradation display is possible. Such a gradation display method is disclosed in Japanese Patent Application Laid-Open Nos. 1-267519 and 55-4.
No. 6,783, EP 0219479, Shokai 61-4
No. 2591, US Pat. No. 5,124,695 and the like.

On the other hand, US Pat. No. 4,712,877 discloses a method in which a unit pixel is not spatially divided into sub-pixels, but gradation is displayed by partially inverting the inside of a pixel using an applied electric field distribution in the unit pixel. , USP 4,763,9
94, USP 4,824,218, and the like.

The former method, that is, the method of performing gradation display using sub-pixels, is less expensive than the latter method because of the simple structure of the driving circuit.

FIG. 1 is a schematic diagram showing a unit pixel including a sub-pixel. As shown in the drawing, one pixel is divided into three pixels of R, G, and B colors by a vertical boundary line, and each color pixel (unit pixel) is obtained by:
It is divided into four areas with an area ratio of 2: 4: 8. Therefore, in this case, each color has 16 levels and 409 per pixel.
Six colors can be displayed. In this manner, the division pattern divided into R, G, and B at the vertical boundary line and further divided according to the required number of gradations at the horizontal boundary line is hereinafter referred to as division pattern A. That.

[0007]

However, in such a division method, a problem in image quality may occur. At the time of an experiment of displaying an image with a continuously changing gradation level such as a photograph or computer graphics using a display element having such a pixel configuration, image processing by the dither method was performed in order to draw a more natural image. . As a result, according to an experiment performed by the present inventors, it has been found that a characteristic pattern (line-shaped defect) appears where a specific gradation level changes. Such a pattern may degrade image quality.

An object of the present invention is to suppress the occurrence of "line-shaped defects" in a display device with a simple configuration in a display device in view of such a conventional technical problem. It is in.

[0009]

In order to achieve this object, a display device according to the present invention is arranged such that each unit pixel corresponds to the number of tones required to express a plurality of halftones, and the horizontal direction of the pixel corresponds to the number of gray levels. In a display device having a plurality of pixels divided into three or more sub-pixels having mutually different areas only at the boundary line, the area of the sub-pixel having the largest area is equal to the area of the other sub-pixels. It is characterized by being less than or less than the sum.

[0010]

According to this configuration, the change in the center of gravity of the light spot of the pixel and the change in the pixel pattern when the gradation level changes are suppressed. A certain "line defect" can be avoided with a simple configuration, and smooth continuous tone expression can be achieved.

A gradation bar whose brightness continuously changes from darkest (black) to brightest (white) shown in FIG. 2 will be taken as an example. Image processing for converting halftone image data into binary data by the dither method was applied to the gradation bar data. When this is displayed on the display element having the pixel configuration shown in FIG. 1, a characteristic pattern can be confirmed near the center CP. According to the experiments performed by the present inventors, when the portion CP is magnified and observed, as shown in FIG. 3, the portion CP may have a mixture of 7-level and 8-level pixels (gray level 7.5). found. This is clearly visible because
The position of the center of gravity of the light spot (the center of gravity of the sub-pixel in the bright state) when each pixel shifts to the next gradation level changes significantly, and the light spot (the sub-pixel in the bright state) Is that there is a clear difference in the shape of the periodic pattern formed by. In the case of the division as shown in FIG. 1, the center of gravity of the light spot is shifted by more than a half pitch of the pixel. As a result, a large pattern difference between adjacent pixels is visually recognized.

A similar experiment was conducted on a display device of pixels having a division pattern (hereinafter, referred to as division pattern B) with vertical and horizontal boundaries as shown in FIG. A similar pattern was seen in a place where levels 7 and 8 were mixed (that is, a gradation level of 7.5).

In these pixel division methods, a clear line-like defect is visually recognized at a position that is not included in a desired image to be originally displayed, so that the image quality is inferior. In order to prevent such a shift (shift) of the center of gravity of the light spot, the above-mentioned Japanese Utility Model Application Publication No.
In Japanese Patent No. 42591 and US Pat. No. 5,124,695, one sub-pixel is further divided concentrically around a certain point. However, this method provides a low-cost, high-performance display because the electrode structure is complicated and the aperture ratio (the area of the effective pixel area) is reduced due to the increased number of divisions. Is difficult. On the other hand, according to the present invention, the shift of the light spot can be controlled without using such a method.

In the present invention, the sub-pixels have a division ratio of 1: 2: 3, 2: 3: 4, 3: 4: 5, 4:
5: 6,... 1: 2: 3: 6, 1: 2: 4: 7, 1:
2: 3: 5, 1: 2: 3: 4, 1: 2: 4: 5, 1:
2: 4: 6 ... 2: 3: 4: 5, 2: 3: 4: 6, ...
... and so on.

More preferably, instead of the division such as 2: 3: 4: 5, the minimum sub-pixel and 2 such as 1: 2: 3: 4 are used.
It is better that the ratio to the second smallest sub-pixel is 1: 2.

Further, it is more desirable to divide the image into four parts such as 1: 2: 3: 5 rather than three parts such as 1: 2: 3. This is because, for example, when displaying the fifth halftone in area ratio, only the lighting of one sub-pixel having the maximum area (5/11) or the second and third areas (3/11, 2 /
This is because any one of the display modes of the simultaneous lighting of the two sub-pixels in 11) can be selected. Thus, it is desirable to design the division ratio such that at least one halftone can be selected from at least two (sub-pixel) display modes.

In the case of performing color display, each pixel may be divided into color pixels for color display, and each color pixel may be divided into three or more sub-pixels as described above. The color pixels include red (R), green (G), blue (B), white (W) or yellow (Y), cyan (Cy), magenta (M), and the like.

In the present invention, a dither method or a density pattern is used so that image data having a number of gradations equal to or greater than the number of gradations (number of gradation levels) obtained by division into sub-pixels can be handled. Performs image processing represented by the law.

[0019]

FIG. 5 is an enlarged view of a display section when a display device according to an embodiment of the present invention performs display by image processing by the dither method. In FIG. 5, an area where gradation levels 4 and 5 are mixed at the center of the gradation bar is shown in an enlarged manner. It can be seen that the shift of the image light spot from the adjacent pixel and the difference in the image pattern are smaller than in the case of the display method shown in FIG. This indicates that the "line defect" has been eliminated.

In other words, in the example shown in FIG. 5, a part P4 has a gradation level 4 and a part P5 has a gradation level for performing image processing for halftone display using a total of two unit pixels of 2 × 2. Level 5 is shown, and gradation level 4.
5 is shown. There is no shift of the light spot as shown in FIG. 3 at the time of transition of these three gradation levels, and it can be seen in FIG. 5 that the shift is lightened.

As described above, according to the display device of the present invention, by selecting a light or dark sub-pixel from a plurality of unit pixels, the number of gradation levels is larger than the number of gradation levels that can be displayed by only one unit pixel. Can be processed and displayed.

FIG. 6 is a block diagram of a display device according to an embodiment of the present invention having the above-described image data processing circuit. Reference numeral 101 denotes a display panel using liquid crystal or the like, which has a unit pixel including a plurality of sub-pixels capable of binary display as described above. 102 is a scanning line driving circuit for applying a signal to a scanning line of the display panel, 103 is an information line driving circuit for applying a signal to an information line of the display panel, and the display panel is formed by these circuits 102 and 103. Multiplexing drive is performed. 104 denotes circuits 102 and 103
Is a drive voltage generating circuit for applying reference voltages V ₁ , V ₂ , V ₃ , V ₄ , and V _C.

These circuits are controlled by the control circuit 105 according to control signals. The control circuit 105 has an image processing circuit 108 in addition to the logic control unit 107,
Here, image processing such as dithering is performed. That is, according to the gradation image data provided from the data generation unit 109,
It is determined how to turn on the sub-pixels of the display panel. 110 is a power switch, and 111 is a power supply.

Next, the image processing circuit 108 in this case will be described with reference to an example in which image processing is performed by an error diffusion method as a dither method. FIG. 7 shows an image processing circuit 10.
8 shows an example of the structure of FIG. In the figure, 15a to 15d are flip-flops (hereinafter simply referred to as FFs) for latching data, 16a to 16d are adders, and 17 is a line memory for delaying one line. 18 is a comparator, 19 is A
An ND gate 20 is an error distribution control circuit.

First, the corrected data (original image data corresponding to the target pixel position (i, j)) input via the data line 1000 has an error to be distributed to the pixel position (i, j). The value (stored in the flip-flop 15d) and the adder 16d are added, and the value is added to the comparator 18d.
Is output to the error distribution control circuit 20. The comparator 18 compares the data on the data line 355 with the threshold data (signal line 300) to output a binarized signal. The comparator 18 outputs “1” to the signal line 500 if the data on the data line 355 is larger than the threshold data, and outputs “0” to the signal line 500 if the data is smaller than the threshold data.

In the error distribution control circuit 20, the signal 355 before the binarization processing and the binary signal 500 are multiplied by 255 (that is, 255).
The difference (error) from the value “0” or “255”) is calculated, and the output amount of the error amounts 351 to 354 distributed to the surrounding pixel groups is controlled. The error amount signals 351 to 354 are (i-1, j + 1), (i, j) when the target pixel position is (i, j).
(j + 1), (i + 1, j + 1) and (i + 1, j) are added to the error amounts already distributed to the adders 16a to 16d.

Reference numeral 119 denotes a pattern generation unit such as a video RAM, which has at least memory cells corresponding one-to-one with sub-pixels of the display panel. Each memory cell stores binary data of “1” or “0” based on image data subjected to binarization processing by the error diffusion method. Therefore, if the display state of each sub-pixel is selected based on the binary data stored in the pattern generation unit, halftone display processed by the error diffusion method can be performed on the display panel.

Next, a description will be given of an example in which a number of display panels in which a chiral smectic liquid crystal is arranged between a pair of substrates are manufactured, and a display is performed using these. In each panel,
By appropriately designing the shape of the intersection between the scanning electrode and the information electrode and the shape of the color filter, a unit pixel composed of sub-pixels having an area ratio described later was formed.

(Examples 1 to 6 and Comparative Examples 1 and 2) Based on the pattern A division method as shown in FIG. 1, the division ratio is changed as shown in the column of "each color division ratio" in Table 1. The image quality was evaluated. As a display element, a liquid crystal display device was prepared and used. The pixel pitch was 200 μm. For image processing, the apparent number of gradations was increased by using a dither method. As the image, a photographic image including a woman's face and the gradation bar described above were used. The image quality was evaluated using 10 subjects, and the line defect and the total image quality were evaluated in three stages of "good", "normal" and "bad". The evaluation was performed by setting "○" for two or less persons and "△" for the others. "△" was judged to be at a level that does not cause any problem in image quality of the product. Table 1 shows the evaluation results.

[0030]

[Table 1] From this evaluation result, it can be seen that good results are obtained when the area of the sub-bit (sub-pixel) having the maximum area is equal to or less than the sum of the areas of the other sub-bits. It can also be seen that particularly good results are obtained when the area of the sub-bit having the largest area is smaller than the sum of the areas of the other sub-bits.

Comparative Examples 3 and 4 Examples 1 to 4 were performed except that the image quality was evaluated by changing the division ratio based on the pattern B division method as shown in FIG.
6 and Comparative Examples 1-2. In Comparative Example 4, the area ratio of the sub-pixels is 1: 2 by setting the division ratio in the vertical direction to 1: 3 and the division ratio in the horizontal direction to 1: 2 in FIG.
2: 3: 6 was obtained. Table 2 shows the results.

[0032]

[Table 2] (Examples 7 and 8) Next, using the pattern A division method, the display devices of Examples 7 and 8 having the respective color division ratios shown in Table 3 were manufactured, and an experiment was performed.

[0033]

[Table 3] Which one of Embodiments 7 and 8 has better image quality?
The evaluation was performed by choosing one. As a result, it was found that Example 8 was superior to Example 7. This is because the seventh embodiment lacks halftone continuity (for example, 1/14,
13/14 cannot be displayed).

[0034]

As described above, according to the present invention, a change in the center of gravity of a light spot of a pixel and a change in a pixel pattern when a gradation level changes can be suppressed. It is possible to avoid “line-shaped defects” with a simple configuration and perform smooth continuous tone expression.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating an example of a pixel divided into a plurality of sub-pixels for performing multi-tone display.

FIG. 2 is a schematic diagram showing a gradation bar in which the brightness continuously changes from darkest (black) to lightest (white).

FIG. 3 is an enlarged view of a region where gradation levels 7 and 8 are mixed at the center of a gradation bar displayed by a conventional display device.

FIG. 4 is a schematic diagram showing another example of a pixel divided into a plurality of sub-pixels for multi-tone display.

FIG. 5 is an enlarged view of a region where gradation levels 4 and 5 are mixed at the center of a gradation bar displayed by the display device according to one embodiment of the present invention.

FIG. 6 is a block diagram illustrating an example of a display device according to the present invention.

FIG. 7 is a block diagram illustrating an example of an image processing circuit used in the present invention.

[Explanation of symbols]

15a to 15d: flip-flop, 16a to 16d:
Adder, 17: line memory, 18: comparator, 19: A
ND gate, 20: error distribution control circuit, 101: display panel, 102: scanning line drive circuit, 103: information line drive circuit, 104: drive voltage generation circuit, 105: control circuit, 1
07: logic control unit, 108: image processing circuit, 10
9: data generator, 110: power switch, 111: power supply, 119: pattern generator.

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takeshi Makita 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Kazumi Suga 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon (56) References JP-A-4-230787 (JP, A) JP-A-3-174514 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/133 575 G02F 1/1343 G09G 3/20

Claims (7)

(57) [Claims] 1. A method in which each unit pixel expresses a halftone
Corresponding to the required number of gradations,
In a display device having a plurality of pixels divided into three or more sub-pixels having mutually different areas, the area of the sub-pixel having the largest area is equal to or less than the sum of the areas of the other sub-pixels. A display device having an image processing circuit for expressing, by using a plurality of unit pixels, a grayscale level larger than the number of grayscale levels expressed by one unit pixel. 2. The method according to claim 1, wherein each pixel is divided into color pixels for color display, and each color pixel is divided into three or more sub-pixels as described above. Display device. 3. The area of the sub-pixel having the largest area is:
2. The display device according to claim 1, wherein the area is smaller than the sum of the areas of the other sub-pixels. 4. The pixel according to claim 3, wherein each pixel is divided into color pixels for performing color display, and each color pixel is divided into three or more sub-pixels as described above. Display device. 5. The display device according to claim 1, wherein the image processing circuit converts the image data into binary data corresponding to a display state of each sub-pixel by a dither method. 6. The display device according to claim 1, wherein the image processing circuit converts the image data into binary data corresponding to a display state of each sub-pixel by an error diffusion method. 7. The display device according to claim 1, wherein the image processing circuit converts the image data into binary data corresponding to a display state of each sub-pixel by a density pattern method.