JPH01216843A - Gradation display of image output apparatus - Google Patents

Abstract

PURPOSE:To reduce the number of gradations which are difficult to realize stably and improve image quality by eliminating a gap with the coloring of micropixels existing in the gap at the following gradation level, when a gap only for a single colored micropixel which is difficult to reproduce stably is formed between each halftone dots of each gradation level. CONSTITUTION:An image is subdivided into a minute area of pixels, the pixel is further subdivided into a more minute area of micropixels, and halftone dots P17-P20 are formed by micropixels colored in the pixel. Then, a gradation is displayed by the proportion of colored micropixels to the total micropixels. The halftone pixels P17-P20 are always formed by a group of connected micropixels, and the colored micropixels are decided for each gradation. In order to decide the colored micropixels in each gradation, if a gap for a single micropixel generates between adjoining halftone pixels (P17, P17 or P19, P19) when a certain gradation level is displayed, a micropixels existing in the gap is colored and adjoining halftone dots (P18, P18 or P20, P20) are connected when the following gradation level is displayed. Consequently, the number of gradation levels which are too unstable to be reproduced decreases and the reproduction of an image is stable, when all the gradation levels are reproduced.

Description

Detailed Description of the Invention A0 Object of the Invention (1) Industrial Application Field The present invention relates to a gradation display method in an image output device, and in particular, to a method for dividing an image to be reproduced into pixels of a minute area, and further dividing the pixels into The present invention relates to a gradation display method in which the pixel is divided into micropixels having a microscopic area, and the gradation is displayed based on the ratio of colored micropixels forming a halftone dot within the pixel to the total micropixels.

(2) Conventional technology Conventionally, when creating an image with gradation in an image output device such as a printing press, printer, or digital copying machine,
A method of displaying gradations in a pseudo manner has been adopted.

In the pseudo gradation display method, the gradation is determined by dividing the image into minute unit pixels, and dividing the image into minute unit pixels (
For example, depending on the size of the area occupied by the dots or lines, the shading is displayed in a manner similar to continuous tone.

A method is often adopted in which regularly arranged large and small halftone dots are used as minute elements within the unit pixel.

A density pattern method (ie, area gradation method) is known as a method using the halftone dots. This density pattern method is applied to the display side (image output device side) corresponding to one pixel of the original image.
Divide one pixel into a plurality of micropixels, select a predetermined number of micropixels corresponding to the gradation of the pixel from among the micropixels, and color the selected micropixels a predetermined color (for example, "black"). This is a method of displaying in color. In this method, a halftone dot is formed from a predetermined number of colored fine pixels corresponding to the gradation.

In the density pattern method, gradation display can be performed in stages corresponding to the number of fine pixels forming one pixel on the display side.

For example, as shown in FIG. 6, if the number of micropixels S forming one pixel is 4X4=16, and each micropixel S displays two pixels, the total number of pixels is (4X4). +1
It can be reproduced with =17 gradations. In other words, the 0th gradation is when all of the micropixels S are white, the 1st gradation is when only one of the 16 micropixels S is colored, and the 1st gradation is when only one of the 16 micropixels S is colored. The second gradation is when only the color is colored.
..., the 16th gradation is when all 16 of the 16 micropixels S are colored, and the pixels are set to 1 in total.
It can be displayed with 7 gradations.

Generally, if the number of micropixels 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, depending on how the fine pixels to be colored are selected, the shape of the halftone dot differs even if the number of colored fine pixels is the same. The quality of the displayed image varies depending on the shape of the halftone dots.

Therefore, how to set the shape of halftone dots is an important problem, and various proposals have been made in the past. Currently, there are roughly two methods for setting the shape of halftone dots.

The first method is to use a font-type screen generator, in which the shape of the halftone dot formed by colored micropixels is set appropriately corresponding to each gradation level, and at each gradation level, the shape of the halftone dot is set appropriately. This is a method of coloring minute pixels so as to form halftone dots with a certain shape. In this first method, the halftone dot shape can be set independently for each gradation level, so it is easy to generate the optimal halftone dot shape. However, since it requires memory to store the halftone dot shape corresponding to each gradation level, it is necessary to store data equal to the number of micropixels that form one pixel multiplied by the number of gradation levels. Memory capacity is required.

The second method is to use a dark value type screen generator, in which all of the plurality of micropixels forming one pixel are ranked in order of coloring, and at the first gradation level, the first priority micropixel is colored. At the second gradation level, the first and second order micropixels are colored, and at the final gradation level, all the micropixels from the first to the final order are colored.
This is the method. This second method can generate halftone dot patterns corresponding to all gradation levels by simply retaining the number of threshold data corresponding to the number of micropixels forming one pixel. Although it has the advantage of requiring a small memory capacity, it has the problem that it is difficult to set the optimal halftone dot shape because it is not possible to set the halftone dot shape independently for each gradation level. Contains.

In either of the first and second methods, the shape of the halftone dots differs depending on which micropixel is colored in each gradation, resulting in a difference in the quality of the displayed image.

Therefore, various proposals have been made regarding which micropixel to be colored at each gradation among the micropixels constituting one pixel.

For example, how to determine the colored fine pixels belonging to the second method is described in the "Image Processing Handbook" (published by the Image Processing Handbook Editorial Committee, Shokodo Co., Ltd., June 8, 1986, 75 - pages 76) are known. There, there is a spiral shape shown in Figure 7-A,
Bayer shape or seventh shape shown in Figure 7-B
- It describes how to define colored fine pixels such as the halftone dot shape shown in Figure C. In addition, in this FIG. 7, one pixel is composed of a plurality of micropixels 3l-3I&, and the subscripts 1 to 16 of each micropixel 31'''SI& indicate the order in which the micropixels are colored. .

By the way, the size of the micropixels that make up the halftone dots is usually set close to the resolution limit of the image output device, so the number of colored micropixels that form the halftone dots may be one, or the number of colored micropixels that form the halftone dots may be one. If there is a small protrusion of one fine pixel in one cluster of colored fine pixels, it is not easy to reproduce accurately. Therefore, the Bayer (Bayer) shown in FIG.
er) shape or halftone dot shape shown in Figure 7-C, where a large number of minute colored micropixels are arranged separately, the problem is that the image reproducibility is unstable and the image quality is likely to deteriorate. There is. Such a difficulty is that when a halftone dot is formed from a cluster of colored micropixels like the spiral shape shown in FIG. 7-A, the number of colored micropixels increases and the cluster of colored micropixels becomes larger. It will be eased as time goes on.

(3) Problems to be Solved by the Invention As described above, it is sometimes convenient to form halftone dots by clusters of colored micropixels, but the area ratio of colored micropixels forming halftone dots to all micropixels is 50. There is a problem in that image reproduction tends to become unstable at gradation levels where the halftone dots increase to nearly 50% and adjacent halftone dots begin to come into contact with each other.

According to research conducted by the present applicants, if there is a gap of two micropixels between adjacent halftone dots, the independence of the halftone dots is maintained, and a relatively stable reproduced image can be obtained. However, when the width of the gap between halftone dots becomes one micropixel, the image reproducibility suddenly becomes unstable and the image quality deteriorates.

In the gradation display method described above, in which one halftone dot is formed by coloring the number of micropixels according to the gradation from among a large number of micropixels, adjacent halftone dots are colored somewhere in the gradation level. It is inevitable that a gap of one minute pixel will occur between the points.

The present invention has been made in view of the above-mentioned circumstances, and includes a method for determining colored micropixels that form halftone dots (a method for selecting colored micropixels).
An object of the present invention is to improve image quality by reducing the rate at which a gap of one minute pixel is created between adjacent halftone dots when displaying each gradation level.

B6 Structure of the Invention (1) Means for Solving the Problems In order to solve the problems described above, the halftone display method in the image output device of the present invention divides an image into pixels with a minute area, and divides the pixels into even smaller pixels. The halftone dots are divided into micropixels each having an area of 1, and the halftone dots are formed by the colored micropixels within the pixel, and the gradation is displayed according to the ratio of the colored micropixels to the total micropixels, and the halftone dots are always connected. In a gradation display method in an image output device that is formed from one cluster of micropixels and in which the colored micropixels are determined corresponding to each gradation, the method of determining the colored micropixels in each gradation is based on a certain gradation level. If a gap of one micropixel occurs between adjacent halftone dots when displaying the next gradation level, the micropixel existing in the gap is colored to make the adjacent halftone dots continuous. It is characterized by

(2) Effect The gradation display method in the image output device of the present invention having the above-described configuration is such that when a gap of one minute pixel occurs between adjacent halftone dots somewhere in the gradation level to be reproduced, That is, if a part of unstable reproducibility occurs, the fine pixels in the gap are colored at the next gradation level to make adjacent halftone dots continuous. Therefore, when all gradation levels are reproduced, the number of gradation levels with unstable reproducibility is reduced, so that image reproduction becomes more stable.

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

Figure 2 shows a monochrome digital copying machine F to which the present invention is applied.
FIG. The digital copying machine F is composed of a machine main body F and a cover F2 hingedly connected to the top surface of the machine main body F.

The machine main body F is provided with a platen (original holder) l made of transparent glass on its upper surface. An exposure optical system 2 is disposed below the platen 1. This exposure optical system 2 includes a movable lamp unit 3
The lamp unit 3 is constructed by integrating a lamp 4 for illuminating the document and a first mirror 5. Further, the exposure optical system 2 includes a movable mirror unit 6 that moves at half the moving speed of the lamp unit 3. This moving mirror unit 6 is composed of a second mirror 7 and a third mirror 8. Further, the exposure optical system 2 includes a lens 9, a fourth mirror 10
etc. Then, when the lamp unit 3 moves in the front-back direction parallel to the original, and the movable mirror unit 6 moves by a distance of 1/2 at a speed of 1/2 of the moving speed of the lamp unit 3, the original Since the distance between the lens 9 and the lens 9 is kept constant, the reflected light from the original illuminated by the lamp 4 passes through the exposure optical system 2 and is converged on the image reading section 11. The image reading section 11 converts the amount of reflected light at each pixel of the document into an electrical signal. This electric signal is converted into density data and sent to the image processing unit 12.The image processing unit 12 converts the density data into an area ratio of halftone dots, and scans each scan so that it can be output as a rask image by a laser scanner 13, which will be described later. Convert as binary serial data for each line. An image is written on the photoreceptor 15 on the drum by turning on or off the laser beam 14 emitted from the laser scanner 13 in accordance with the serial data.

A charging charger 16, a developing unit 17, a transfer charger 18, a cleaner unit 19, etc. are arranged around the photoreceptor 15 along the rotational direction of the photoreceptor 15. Further, the machine main body F+ includes a transfer paper storage tray 20, and transfer paper in the transfer paper storage tray 20 is transferred to the photoreceptor 15 and the transfer charger 18.
A paper feeding mechanism 21 is disposed between the
A conveyance mechanism 22 is also provided that passes between the photoreceptor 15 and the transfer charger 18 and conveys the transfer-completed paper on which the transfer has been completed, peeling it off from the photoreceptor 15. Further, the machine main body F1 is provided with a fixing unit 23 for fixing the transfer-completed paper conveyed by the conveyance mechanism 22, and a paper discharge tray 24 for receiving and receiving the transfer-completed paper discharged from the fixing unit 23. has been done.

Next, the configuration of the image processing section 12 will be explained in detail.

The image processing unit 12 has one pixel of 6 pixels as shown in FIG. 4-A.
The 17th to 20th pixels shown in FIG.
It is configured to display gradation halftone dots Pl'l to P20. Note that the pixels in FIG. 1 are fine pixels S similar to the pixels in FIG. 4-A, although the symbols S, ~S3& are not written
336. And fine pixel S1
is the fine pixel that is colored with the lowest darkness value, and the fine pixel S0
is the fine pixel that is colored with the highest threshold value.

FIG. 4-A is a diagram showing the relationship between the micropixels 3l-3S& and the values X and Y of the X and Y addresses that specify each of the micropixels.

Moreover, FIG. 4-B is a diagram showing the relationship between threshold value data A stored in the dark value table 123, which will be described later, and the values X and Y of the X and Y addresses. As shown in FIG. 1, the data A stored in the darkness value table 123 is such that the halftone dot in each gradation is formed from one set of colored fine pixels and one colored fine pixel. It is determined that no more than two protrusions are formed from a pixel. Furthermore, the data A has a certain gradation level (the 17th gradation level shown in FIG. 1).
(See gradation halftone dot P17 or 19th gradation halftone dot P1)
If a gap of one micropixel occurs between adjacent halftone dots when displaying , the next gradation level (the 18th gradation halftone dot pH+ shown in Figure 1 or the 20th gradation halftone When displaying halftone dots (see halftone dot P20), it is determined that minute pixels existing in the gap are colored.

The image processing section 12 shown in FIG. 3 is composed of a so-called dark value matrix type screen generator. This image processing section 12 includes the image reading section 11 (see FIG. 2).
Image data B is input from. This image data B is
This is a signal obtained by analog-to-digital conversion of an analog output signal obtained by a sensor such as a COD in the image reading section 11, and is stored in the line buffer 121 of the image processing section 12. This image data B displays 36 gradations, so it is roooo00J~rl.

This is 6-bit data of 0010J. The line buffer 121 is composed of a shift register, etc.
Image data B for 47 minutes is stored. Since the pixels are formed by a 6×6 matrix with fine pixels 81 to Sff& as elements, the image data indicating the gradation of each pixel is read out six times while outputting one line of pixels. .

The reading and writing of this line buffer 121 is controlled by a controller 122. controller 12
2 consists of a counter etc., and the laser beam 14 (
An X clock signal synchronized with the timing at which the laser beam (see Figure 2) scans each fine pixel, an X reset signal generated every time the laser beam scans each pixel, and an Y clock signal generated every time the pixel rotates,
It operates by a Y reset signal generated every time it rotates by 6 micropixels. Then, in accordance with these signals, the X and Y address signals each take on a value of r000J to "101", and it is specified which fine pixel in the 6×6 matrix belongs to. The threshold table 123 is made up of a storage element such as a ROM, and threshold data A (see Figure 4-B) for coloring the fine pixels specified by the X and Y address signals is used for the fine pixels S, ~SSa. It is stored in correspondence with the X and Y addresses. The comparator 124 compares the threshold value data A successively read out from the threshold value table 123 and the image data B read out from the line buffer 121, and when the image data B is equal to or greater than the threshold value (that is, when A≦B) ” and output A
>B, outputs “0”. The output signal of the comparator 124 is input to the laser scanner 13, and the laser scanner 13 outputs the laser beam 14 when the input signal is "1".
Turn off, r□.

It is configured to turn on when

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

When the image data B is roooooooJ, the fourth-B
As shown in the figure, (darkness value data A)≦(image data B)
The threshold data A is not stored, and all fine pixels 5l
The threshold data A of −336 is (threshold data A)>(image data B). Therefore, the output signal of the comparator 124 is obtained by scanning all fine pixels S+=S3i (see Figure 4-A). Therefore, all the fine pixels S+
"" Since the laser beam 14 is irradiated to Ssa, the shape of the output halftone dot is zero, that is, all the fine pixels s1 to s3h
will be uncolored.

Next, when image data B is roooolJ(7), the fourth
- As shown in figure B, the threshold data A that satisfies (threshold data A)≦(image data B) is the value r0 of the X and Y addresses.
10, r010'', and all other fine pixels S z '=S
The threshold data A stored in sh is A>B.

Therefore, the output signal of the comparator 124 is "1" only when the fine pixel S1 is scanned, and is "0" otherwise.
Therefore, since the laser beam 14 is irradiated to all the fine pixels S8 to S3&, except for the fine pixel S1, only the fine pixel SI is colored, and the other fine pixels S8 to ssi are outputted as uncolored halftone dots. be done.

Next, when the image data B is "roooolo", the fourth -
As shown in figure B, (threshold data A)≦(image data B
) is the value ro1 of the X and Y addresses.
0J, r010J, and the threshold data A that is stored only in the fine pixels S+ and St specified by "rolo" and "roll", and is stored in all the other fine pixels 32 to 336, A>B. Therefore, the output signal of the comparator 124 is "1" only when the fine pixels S1 and S8 are scanned, and is "o" otherwise. Therefore, all the fine pixels S except the fine pixels SI and S□, Laser light 1 to ~S3-
4 is irradiated, so the fine pixel S.

When only S and St are colored, other fine pixels S, ~
In S3&, uncolored halftone dots are output.

Then, in the same way, image data B is "010001"
, roloolo", rolool" and "010
100'', the shape of the output halftone dot is the halftone dot P1F*PII+P19+P20 shown in FIG.

Therefore, by storing the data A shown in FIG. 4-B in the darkness value table 123, a display as illustrated in FIG. 1 corresponding to the value of the image data B, that is, a certain gradation level is displayed. If a gap of one minute pixel is created between adjacent halftone dots when displaying the next gradation level, display is performed such that the minute pixels existing in the gap are colored.

Although the embodiments of the gradation display method in the image output device according to the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and may deviate from the scope of the present invention as set forth in the claims. It is possible to make various design changes without having to do so.

For example, when displaying the 17 types of halftone dots P, ~P0 shown in FIG. 1, instead of using the darkness value matrix type screen generator shown in FIG. 3, the font type screen generator shown in FIG. It is possible to use In this case, the image processing section 12 may use the font memory 125 shown in FIG. 5 instead of the darkness value table 123 and the comparator 124 shown in FIG. is the first
17 types of halftone dots P・~P3 corresponding to each gradation shown in the figure
& is stored, and at this time, 1 is stored for colored micropixels and 0 is stored for non-colored micropixels. For example, in the 0th gradation, 0 is stored for all micropixels sl to S2&, 1st
In the gradation, 1 is stored in the micropixel s1, and 0 is stored in the other micropixels S8 to S3&, and in the second gradation, the micropixel S
1 is stored in I and S2, and other fine pixels S,
The controller 122 stores O in ~Si&, and stores 1 in all the fine pixels S and ~S3& at the 36th gradation.The controller 122 stores such data in the font memory 125. When inputting the X and Y addresses and the image data B output from the line buffer 121 as addresses, the image data B
, ~P3& are specified, and X.

Since the fine pixel S1''336 is specified by the Y address signal, it is possible to output halftone dots of the gradation corresponding to the image data B.

In addition, in the embodiment, a case is shown in which pixels are formed in a matrix of 6 x 6 with a screen angle of 0" and a screen angle of 0" is formed, and 37 gradations are displayed, but other matrix sizes and other screen It is also possible to display angles and other numbers of gradations.In that case, it is sufficient to use a configuration with the number of bits corresponding to those numbers.Furthermore, by setting the number of bits to 8 bits, each configuration can be easily obtained. Using a general-purpose one,
It is also possible to use the required number of lower or upper bits. Furthermore, the fine pixels forming the pixels may be a rectangular matrix instead of a square matrix, or may have other shapes.

In the above embodiment, the case of monochrome display was shown, but it is also possible to apply to each color of color display.Also, although the example of application to a digital copying machine was shown, it can also be applied to a laser printer. It is possible. Furthermore, the present invention can be applied to other image output devices as long as they are capable of displaying gradations using halftone dots and the reproducibility becomes unstable when the gap between halftone dots is small. is possible.

C0 Effects of the Invention According to the gradation display method in the image output device of the present invention described above, a gap of one colored fine pixel, which is difficult to stably reproduce, is formed somewhere between each halftone dot at each gradation level. However, at the next gradation level, the fine pixels in the gap are colored and the gap disappears, so the number of gradations that are difficult to reproduce stably is reduced. Therefore, image quality can be improved.

[Brief explanation of the drawing]

1-A-D are explanatory diagrams of halftone dot shapes in an embodiment of the gradation display method according to the present invention, FIG. 2 is an overall explanatory diagram of a digital copying machine to which the present invention is applied, and FIG. 3 is an image thereof. A diagram showing one embodiment of the configuration of the processing section, FIG. 4-A is a diagram showing the arrangement relationship between X and Y addresses and fine pixels, and FIG. 4-B is a diagram showing the relationship between the dark value data stored in the threshold table and the X , a diagram showing the relationship with the Y address, FIG. 5 is a diagram showing another embodiment of the configuration of the image processing section, FIG. 6 and FIG. 7-A-0 are explanatory diagrams of the prior art,
It is.

Claims (1)

[Scope of Claims] An image is divided into pixels with a minute area, and the pixels are further divided into minute pixels (S_1 to S_3_6) with a minute area, and a halftone dot (P
The halftone dots (P_1_7 to P_2_0) are always formed from a group of connected micropixels, and the halftone dots (P_1_7 to P_2_0) are always formed from a group of connected micropixels, and the halftone dots (P_1_7 to P_2_0) are always connected to each other, and the coloring micropixels are In the gradation display method for an image output device in which the colored fine pixels in each gradation are determined in correspondence with each gradation, the method of determining colored fine pixels in each gradation is as follows: when displaying a certain gradation level, adjacent halftone dots (P_1_7, P_
1_7 or P_1_9, P_1_9), if a gap of one fine pixel occurs between the pixels, the fine pixel existing in the gap is colored and the adjacent halftone dot (
P_1_8, P_1_8 or P_2_0, P_2_0
) is a continuous gradation display method.