JP2743364B2 - Gradation display method in image output device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A. Object of the Invention (1) Field of the Invention The present invention relates to a gradation display method in an image output device, and in particular, divides a reproduced image into pixels having a small area. The pixel is further divided into fine pixels having a smaller area, a halftone dot is formed by the colored fine pixels among the fine pixels, and the halftone dot is formed by the ratio of the colored fine pixels forming the halftone dot to all the fine pixels. The present invention relates to a gradation display method for displaying a tone.

(2) Conventional technology Conventionally, in an image output device such as a printing machine, a printer, or a digital copying machine, a method of pseudo-displaying a gradation when an image having the gradation is created has been adopted.

In the pseudo gradation display method, the gradation is obtained by dividing an image into minute unit pixels and determining the size of the area occupied by minute elements (for example, points or lines) in the unit pixels.
The shade is displayed in a manner similar to continuous tone.

And, a method of using regularly arranged large and small halftone dots as minute elements in the unit pixel has been adopted.

As a method using the halftone dot, a density pattern method (that is, an area gradation method) is known. According to this density pattern method, one pixel on the display side (image output device side) corresponding to one pixel of an original image is divided into a plurality of fine pixels, and a predetermined number of fine pixels corresponding to the gradation of the pixel are selected from the fine pixels. In this method, a pixel is selected, and the selected fine pixel is colored and displayed in a predetermined color (for example, black). In this method, halftone dots are formed from a predetermined number of colored fine pixels corresponding to the gradation.

According to the density pattern method, it is possible to perform gradation display at a stage corresponding to the number of fine pixels forming one pixel on the display side.

For example, as shown in FIG. 5, assuming that the number of the fine pixels S forming one pixel is 4 × 4 = 16 and that each fine pixel S performs a binary display, the total number of the one pixel is ( 4 × 4) +1
= 17 tones. That is, the 0th gradation is when all the subpixels S are all white, the 1st gradation when only one of the 16 subpixels S is colored, and the 2nd of the 16 subpixels S .., The 16th gradation when all 16 of the 16 fine pixels S are colored,
By doing so, the pixels can be displayed with a total of 17 gradations.

In general, if the number of fine pixels forming one pixel is m, the number of gradations that can be expressed is m + 1.

As described above, halftone dots are formed from colored micropixels among a plurality of micropixels forming one pixel, and the gradation is determined by the number of colored micropixels forming the halftone dot. Further, the shape of the halftone dot differs depending on the method of selecting the minute pixels to be colored, even if the number of the minute pixels is the same. Then, the quality of the displayed image varies depending on the shape of the halftone dot.

Therefore, how to set the shape of the halftone dot is an important problem, and various proposals have been made. At present, there are roughly two methods of setting the shape of a halftone dot.

The first method is a method using a font type screen generator, in which the shape of a halftone dot formed by colored fine pixels is appropriately set corresponding to each gradation level, and the setting is performed at each gradation level. This is a method of coloring the fine pixels so as to form a halftone dot of a given shape. In the first method, since the halftone dot shape can be set independently for each gradation level, it is easy to generate an optimum halftone dot shape. However, since a memory for storing a halftone dot shape corresponding to each gradation level is required, data of a number obtained by multiplying the number of fine pixels forming one pixel by the number of gradations must be held. Requires memory capacity.

The second method is to use a threshold-type screen generator, in which all of a plurality of micropixels forming one pixel are colored, and the first gradation micropixel is assigned to the first gradation level. In this method, the first and second order fine pixels are colored in the second gradation level, and all the fine pixels from the first to last order are colored in the final gradation level. . According to the second method, a halftone dot pattern corresponding to all gradation levels can be generated only by holding threshold data of a number corresponding to the number of fine pixels forming one pixel. Although it has the advantage of being able to use a small memory capacity, it is difficult to set the optimum dot shape because the dot shape cannot be set independently for each gradation level. Contains.

In any of the first and second methods, since the halftone dot shape is different depending on which fine pixel is colored in each gradation, the quality of the displayed image is different.

Therefore, various proposals have been conventionally made as to which of the fine pixels constituting the one pixel is to be colored in each gradation, that is, how to determine the shape of the halftone dot.

Then, as a method of determining the colored fine pixels, for example,
"Image Processing Handbook" (edited by the Image Processing Handbook Editing Committee, Shokodo Co., Ltd., published June 8, 1987, 75
To page 76) are known. There,
A method of defining colored micropixels such as a spiral shape shown in FIG. 6-A, a Bayer shape shown in FIG. 6-B, or a halftone dot shape shown in FIG. 6-C is described. Incidentally, in this FIG. 6, one pixel is composed of a plurality of micro-pixels S ₁ to S _16, subscript 1-16 of each micro pixel S ₁ to S ₁₆ is the order in going colored fine pixels Is shown.

By the way, usually, the size of the fine pixels forming the halftone dot is set close to the resolution limit of the image output device, so that the number of the colored fine pixels forming the halftone dot is one or the halftone dot is not formed. If there is a small protruding portion corresponding to one fine pixel in one group of colored fine pixels, it is not easy to reproduce accurately. Therefore, the Bayer shown in FIG.
r) When a large number of minute colored fine pixels are arranged as in the shape or the halftone dot shape shown in FIG. 6-C, the reproducibility of the image is unstable and the image quality is easily deteriorated. There is. Such difficulty is caused by the fact that when halftone dots are formed from a group of colored micropixels as in the spiral shape shown in FIG. 6-A, the number of colored micropixels increases and the cluster of the colored micropixels increases. Somewhat relaxed.

(3) Problems to be Solved by the Invention However, when the _fifth , _seventh , _tenth , and thirteenth gradations are displayed in the spiral shape shown in FIG. 6A, the fine pixels S ₅ , S ₇ , and S ₁₀ are displayed.
And small protruding portion of the area in a halftone dot is formed by S _13. In addition, the protruding portion for one fine pixel also has a problem that the reproducibility of the image is unstable and the image quality is easily deteriorated.

It is considered that such a problem arises for the following reasons. That is, high resolution is required to reproduce minute colored micropixels and minute projections from a group of colored micropixels as an image, and such portions cannot be reproduced by a normal image output device. Stable and
It is difficult to reproduce the image while keeping the colored area at a predetermined value. Therefore, accurate gradation reproduction becomes difficult.

Such problems are not limited to the second method, but the first method.
The same applies to the above method.

Therefore, when the halftone dot has a shape having a protruding portion of one fine pixel, the image quality is easily degraded (a so-called poor graininess is likely to be a rough image), and it is difficult to accurately reproduce the gradation.

However, in the pseudo gradation display method in the image output device, in order to display all gradation levels, a protruding portion formed from one fine pixel colored somewhere in the gradation level occurs. It is unavoidable.

By the way, in various image output devices in which the method of reproducing an image is different, when reproducing a minute projection portion of a halftone dot,
Depending on the type of the image output device, the direction in which the minute protruding portion can be stably reproduced is different. For example, in electrophotography using a photoreceptor in which a toner image is developed on the surface while moving in a process direction perpendicular to the scanning direction, development in the process direction (the direction in which the photoreceptor moves mechanically) is faster than development in the scanning direction. It has the characteristic that it can be performed stably.

For example, a colored fine pixel projecting in the scanning direction from a colored fine pixel forming a halftone dot is difficult for toner to adhere to the tip of the outer peripheral portion and both ends in the process direction, but a colored fine pixel projected in the process direction. The toner is relatively easy to adhere, and the toner image can be stably developed.

Therefore, it is only necessary to form the minute projections of the halftone dots in a direction that can always be reproduced stably. In the second method, that is, the method using the threshold type screen generator, somewhere in the gradation level, There is a situation in which a minute projection must be formed in a direction that is difficult to reproduce stably.

Further, when the halftone dot shape is not symmetrical with respect to the rotation of ± 90 ° or 180 °, an unnatural pattern called a texture tends to appear on the reproduced image.

The graininess and the texture are not completely contradictory characteristics. However, if the halftone dot shape is determined by paying attention to only one characteristic, the other characteristic may be significantly inferior.

For these reasons, it is difficult to obtain a halftone dot shape that has good graininess and does not cause texture in the second method in which an arbitrary halftone dot shape cannot be set according to each gradation.

The present invention has been made in view of the above-described circumstances, and in the first method, that is, a method of coloring fine pixels according to a predetermined pattern according to gradation, forming halftone dots at each gradation level. By devising the method of selecting (selecting) the colored micropixels to be generated, a pattern of halftone dots that can accurately reproduce the gradation of each level is generated, thereby improving the reproducibility of each gradation level and improving the image quality. The task is to improve it.

B. Configuration of the Invention (1) Means for Solving the Problems In order to solve the above problems, a gradation display method in an image output device of the present invention divides an image into pixels having a small area, and further divides the pixels into pixels. The pixel is divided into fine pixels having a small area, a halftone dot is formed by the colored fine pixels in the pixel, and a gray scale is displayed by a ratio of the colored fine pixels to all the fine pixels, and the halftone dots are always connected. An image output device formed from a set of fine pixels and wherein the colored fine pixels are determined corresponding to each gradation, wherein the toner image is developed on the surface while moving in the process direction perpendicular to the main scanning direction. In the gradation display method in the electrophotographic image output device using a photosensitive member, when displaying each gradation level, a colored fine pixel forming one halftone dot is formed from one colored fine pixel. With defined as protruding portions becomes 1 or less to be the one colored protruding portion formed of fine pixels is characterized in that it is arranged in the direction of easy reproducible stable.

(2) Function In the gradation display method of the image output device of the present invention having the above-described configuration, the number of protruding portions that are difficult to accurately reproduce in the fine pixels forming halftone dots is reduced to one or less. In addition, since the protruding portions are arranged in a direction that is easy to reproduce stably, gradation reproduction is stable.

(3) Embodiment An embodiment of a gradation display method in an image output device according to the present invention will be described below with reference to the drawings.

FIG. 2 is an overall explanatory diagram of a monochrome digital copying machine F to which the present invention is applied. Digital copier F is composed of hinged Copper F ₂ Metropolitan a machine body portion F ₁ to the upper surface of the machine body portion F _1.

The machine body portion F ₁ is provided with a platen (document holder) 1 and the transparent glass on its upper surface. An exposure optical system 2 is provided below the platen 1. The exposure optical system 2 has a movable lamp unit 3, and the lamp unit 3 is configured by integrating a lamp 4 for document illumination and a first mirror 5. Further, the exposure optical system 2 has a movable mirror unit 6 that moves at half the speed of the lamp unit 3. This moving mirror unit 6
It comprises a second mirror 7 and a third mirror 8.
The exposure optical system 2 includes a lens 9 and a fourth mirror 10.
And so on. When the lamp unit 3 moves in the front-rear direction parallel to the document and the movable mirror unit 6 moves at a speed half the moving speed of the lamp unit 3 and at a distance of 1/2, Since the distance from the lens 9 is kept constant, the reflected light of the original illuminated by the lamp 4 is reflected by the exposure optical system 2 during that time.
Through the image reading unit 11. The image reading unit 11 converts the amount of reflected light at each pixel of the document into an electric signal. This electric signal is transmitted to the image processing unit 12 as density data. The image processing unit 12 converts the density data into an area ratio of halftone dots, and also converts the density data into binary serial data for each scanning line so that the laser scanner 13 described later can output the raster image. By turning on or off the laser beam 14 emitted from the laser scanner 13 in accordance with the serial data, an image is written on the photoconductor 15 on the drum.

Around the photoconductor 15, a charging charger 16, a developing unit 17, a transfer charger 18, a cleaner unit 19, and the like are arranged along the rotation direction of the photoconductor 15. Further, the machine body portion F ₁ is transfer paper storage tray
20 and a paper feed mechanism 21 for supplying the transfer paper in the transfer paper storage tray 20 between the photoconductor 15 and the transfer charger 18 are provided. A transfer mechanism 22 is also provided for transferring the transfer-completed paper, which has passed through the space between the sheet and the transfer-completed sheet, from the photoreceptor 15 to be transferred. Further, the machine body portion F _1, the transfer mechanism 22
Unit that fixes the transfer end paper conveyed by
23, and a paper discharge tray 24 for receiving the transfer completed paper discharged from the fixing unit 23 are provided.

 Next, the configuration of the image processing unit 12 will be described in detail.

The image processing section 12 is constituted by a so-called font type screen generator. Then, the fine pixel one pixel to form a 4 × 4 matrix as shown in 4-A view S ₁₁ to S
₄₄ and a dot P ₀ as shown in FIG.
PP ₁₆ to display a total of 17 gradations from the 0 th gradation to the 16 th gradation. Note that the pixel of FIG. 1 is code S ₁₁ to S ₄₄ is not described, are formed from the 4-A view of the same fine pixels and pixel S ₁₁ to S _44, the hatched portions colored fine Represents a pixel. And FIG. 4-A is
A fine pixel S ₁₁ ~S _44, X specifying each micro pixel S ₁₁ to S ₄₄ thereof, Y address values X, is a diagram showing the relationship between Y.

FIG. 4-B is a diagram showing a relationship between the halftone data P stored in the font memory 123 described later and the X and Y address values X and Y. Halftone data P stored in the font memory 123, the 0th gradation to be able to view the dot P ₀ to P ₁₆ of the total 17 gradations up to the 16 gray levels, each halftone dot P
₀ Each correspondingly colored fine pixels to P ₁₆ "1", the non-colored pixels which are data stored to "0". And
Dot P ₀ to P _16, which is formed by a colored fine pixels, as shown in the first figure, halftone dots in gradations and one as being formed from the colored fine pixels of one mass It is determined that two or more protruding portions formed from the colored micropixels are not generated. In addition, as shown by halftone dots P ₅ , P ₇ , and P ₁₁ in FIG. 1, one colored fine pixel projecting from a cluster of four or more colored fine pixels is located in the sub-scanning direction, that is, the process direction. (In the Y direction in FIG. 1),
One colored fine pixel projecting from three or more colored fine pixels (see halftone dots P ₇ and P _{11 in} FIG. 1) is a colored fine pixel arranged at the center of the three or more colored fine pixels. It is arranged connected to the pixel.

Image data B is input from the image reading unit 11 to the image processing unit 12 shown in FIG. This image data B
Is a signal obtained by performing an analog-to-digital conversion on an analog output signal obtained by a sensor such as a CCD in the image reading unit 11, and is stored in the line buffer 121 of the image processing unit 12. The image data B is a 5-bit data of "00000" to "10000" because 17-gradation display is performed.

The line buffer 121 is constituted by a shift register or the like, and stores image data B of pixels for one line. Then, 4 × of pixels and the micro-pixels S ₁₁ to S ₄₄ elements
Each pixel is read out four times while a pixel of one line is output because the matrix is formed of four matrices. The line buffer 121 is read out by the controller 122,
Writing is controlled. The controller 122 includes a counter or the like, and an X clock signal synchronized with the timing at which the laser light 14 (see FIG. 2) scans each fine pixel;
An X reset signal generated each time a laser beam scans each pixel, a Y clock signal generated each time the drum-shaped photoconductor 15 (see FIG. 2) rotates by one fine pixel, rotates by four fine pixels. It operates according to a Y reset signal generated every time.
Then, the X and Y address signals take on the values of “00” to “11” in accordance with those signals, and it is specified which fine pixel of the 4 × 4 matrix.

The font memory 123 is composed of a storage element such as a ROM, and stores dot data P corresponding to each gradation level. Then, the image data B signal dot P ₀ is stored in the font memory 123 by to P ₁₆ specifying the gradation level (see 4-B view) are identified. That is, the image data B input to the font memory 123
Is “00000” representing the 0th gradation, the data for displaying the 0th gradation halftone dot P ₀ stored in the font memory 123 is selected, and “00001” representing the 1st gradation is selected. if a ", data for displaying the dot P ₁ of the first gray level stored in the font memory 123 is selected, ... if it is the" 10000 "representing the 16th gradation, font memory 123 data for displaying the dot P ₁₆ of the 16 gradation stored is selected.

Data of the dot P ₀ to P _16, which is stored corresponding to the gradation is specified by the image data B representing the gradation, fine pixel S ₁₁ to S ₄₄ for forming the halftone dots the X, Y Specified by the address signal.

Therefore, when the image data B and the X and Y address signals are inputted as addresses, the font memory 123 stores the data of the fine pixels S _{11 to} S ₄₄ forming the halftone dots P _{0 to} P ₁₆ defined by the image data B. (Data in which “1” is stored in the colored fine pixel and “0” is stored in the uncolored fine pixel shown in FIG. 4-B), and the fine pixels S _{11 to} S ₄₄ specified by the X and Y addresses are read out. Output data "1" or "0".

The output signal of the font memory 123 is input to the laser scanner 13. The laser scanner 13 is configured to turn off the laser beam 14 when the input signal is "1" and to turn on the laser beam 14 when the input signal is "0". I have.

Next, the operation of the embodiment of the present invention having the above-described configuration will be described.
This will be mainly described with reference to FIGS. 1 and 4.

When the image data B is “00000” indicating the 0th gradation, the halftone dot P ₀ displaying the 0th gradation is specified.
As shown in Figure B, all fine pixel data of the halftone dot P ₀ is S ₁₁ to S ₄₄
0 for Therefore, the output signal of the font memory 123 remains “0” while all the fine pixels S _{11 to} S ₄₄ are scanned. Therefore, the laser beam 14 is all fine pixel S ₁₁ to S
Since irradiation with _44, the shape of the halftone dots to be output is a dot P ₀ no colored fine pixel shown in Figure 1.

Then, when the image data B is "00001", halftone dots P ₁ to display the first gray level is specified. At this time, as shown in 4-B Figure, "1" is stored in the data of the halftone P ₁
X, the value of the Y address is "10", "01", X, when the value of the Y address other is the data of the dot P ₁ is stored is "0". In addition, the X and Y addresses "10" and "01"
Is a small pixel S as shown in FIG. 4-A.
₂₃ . Therefore, the font memory 123 stores the fine pixel S ₂₃
Outputs “1” when is scanned, and outputs “0” otherwise. Therefore, the laser beam 14 to the entire fine pixels except fine pixels S ₂₃ is illuminated, the shape of the halftone dots to be output,
The halftone dots P ₁ shown in Figure 1.

When the image data B is "00010", halftone dots P ₂ for displaying the second gradation is specified. At this time, as shown in 4-B view, X "1" in the data of the dot P ₂ is stored, Y address values are "10", "01", and "10",
It is "10" and "0" when the X and Y addresses are other values. Further, the X, the fine pixel corresponding to the Y address is a fine pixel S ₂₃ and S ₃₃ as shown in 4-A Figure. Therefore, the font memory 123, micro-pixels S ₂₃ and
"1" is output when _S33 is scanned, and "0" is output otherwise. Therefore, the laser beam 14 is irradiated to the entire fine pixels except fine pixels S ₂₃ and S _33, the shape of the halftone dots to be output is dot P ₂ shown in Figure 1.

Then, similarly, the image data B is “00011”, “0”
0100 ", ..., when" 10000 ", the shape of the halftone dots to be output halftone shown in FIG. 1 P _3, P _4, ... the P _16.

Therefore, by storing the dot data P shown in 4-B view of the font memory 123, dot P ₀ to P ₁₆ shown in FIG. 1 corresponds to the value of the image data B is output, 17 A gradation display is performed.

At this time, as is clear from FIG. 1, in the ninth gradation, the fine pixels S arranged in the matrix form
₁₁ to S _44, the upper and lower one edge and the left and right of one of the fine pixels disposed along the edge _{S 11, S 12, S 13} , S 14, S 21, S 31, S
₄₁ are all non-colored and the remaining fine pixels are all colored, and the fine pixels S arranged along one of the upper and lower edges and the left and right edges until the ninth gradation is exceeded. ₁₁ ,
S ₁₂ , S ₁₃ , S ₁₄ , S ₂₁ , S ₃₁ , and S ₄₁ are not colored. When such gradation display is performed, the number of gradations at which the projecting portions of adjacent halftone dots contact each other is reduced, so that the granularity of the reproduced image is improved. Similarly, as is apparent from FIG. 1, in the twelfth gradation, the fine pixels S ₁₁ , arranged in the four corners of the fine pixels arranged in the matrix form.
S ₁₄ , S ₄₁ , and S ₄₄ are all uncolored and the remaining fine pixels are all colored. Such gradation display also improves the graininess of the reproduced image.

As described above, the embodiment of the gradation display method in the image output device according to the present invention has been described in detail. However, the present invention is not limited to the above-described embodiment, and may deviate from the present invention described in the claims. It is possible to make various design changes without performing.

For example, in the embodiment, the pixel has 4
Formed by a matrix of × 4 screen angles 0 °,
Although the case where the display of 17 gradations is performed has been described, display of another matrix size, another screen angle, and another number of gradations can be performed. In that case, a configuration of the number of bits corresponding to those numbers may be used. Further, the number of bits may be set to 8 bits, and the required number of lower or upper bits may be used by using a general-purpose one in which each configuration is easily available. Furthermore, it is also possible to arrange the fine pixels forming the pixels in a rectangular matrix instead of a square matrix, or to arrange them in another shape.

In the above-described embodiment, the case of monochrome display has been described, but it is also possible to apply to each color of color display,
Also, an example in which the present invention is applied to a digital copying machine has been described, but the present invention is also applicable to a laser printer. Further, the present invention can be applied to other image output devices as long as the image output device is capable of gradation display by halftone dots and has different reproducibility depending on directions.

C. Effects of the Invention According to the above-described gradation display method in the image output device of the present invention, the number of protruding portions that are difficult to accurately reproduce among the fine pixels forming halftone dots is set to one or less. At the same time, since the protruding part is arranged in a direction that is easy to reproduce stably,
Reproduction of gradation is stabilized. Therefore, among the halftone dots corresponding to each gradation level, the number of halftone dots having unstable reproducibility is reduced, so that the image quality can be improved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a halftone dot shape of a gradation display method according to an embodiment of the present invention, FIG. 2 is an overall explanatory diagram of a digital copying machine to which the present invention is applied, and FIG. FIG. 4-A is a diagram showing an arrangement relationship between X and Y addresses and fine pixels, and FIG. 4-B is a diagram showing halftone dot data and X and Y addresses stored in a font memory. FIG. 5 and FIGS. 6A to 6C are explanatory diagrams of the prior art. P ₀ ~P ₁₆ ...... network point, S ₁₁ ~S ₄₄ ...... fine pixels

 ────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-61-23467 (JP, A) JP-A-58-182374 (JP, A) JP-A-56-117478 (JP, A)

Claims (3)

(57) [Claims] An image is divided into pixels having a small area, and the pixels are further divided into fine pixels (S11 to S44) having a small area. P16), and the gradation is displayed by the ratio of the colored fine pixels to all the fine pixels.
P16) is an image output device which is always formed of a group of connected fine pixels and in which the colored fine pixels are defined corresponding to each gradation, and which moves in the process direction perpendicular to the main scanning direction. In the gradation display method in the electrophotographic image output device using a photoconductor on which a toner image is developed, when each gradation level is displayed, one colored fine pixel forming one halftone dot is provided. The number of protruding portions formed from the colored micropixels is determined to be one or less.
A gradation display method, wherein a protruding portion formed from a plurality of colored slightly different pixels is arranged in a process direction which is a direction in which reproduction is stable and easy. 2. The method according to claim 1, wherein the pixels are formed of a plurality of fine pixels (S11 to S44) arranged in a matrix. At a predetermined gradation level, the pixels of the fine pixels (S11 to S44) are arranged. , Micropixels (S11 to S14, S21, S31, S4) arranged along one of the upper and lower edges and the left and right edges.
1) is completely uncolored and all the remaining fine pixels are colored, and fine pixels arranged along the upper and lower edges and the left and right edges until the predetermined gradation level is exceeded. 2. The gradation display method in an image output device according to claim 1, wherein the pixels (S11 to S14, S21, S31, S41) are not colored. 3. The pixel is formed of a plurality of fine pixels (S11 to S44) arranged in a matrix, and at a predetermined gradation level, the fine pixels (S11 to S44) are arranged in a matrix. 2. The micro-pixels (S11, S14, S41, S44) disposed in the four corners are all uncolored, and the remaining micro-pixels are all colored. 5. A gradation display method in the image output device according to 1.