JPH10257318A - Method for recording gradation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrences of mismatching, such as a white stripe or a black stripe in the boundary part of a gradation level, even when an image density is continuously changed at the tie of using an extension dither method by establishing a specified conditional expression. SOLUTION: A exposure pattern is set, based on a rule indicated by a expression in the exposure pattern provided with adjacent Z addresses AZ. Here, AX1 and AX2 are arbitrary values within the X-addresses AX (AX=0 to m-1) of the exposure pattern, AY1 and AY2 are arbitrary values within the Y- addresses AY (AY=0 to n-1) of the exposure pattern, AZ is the value of the Z-address corresponding to density information, F (AX1-AX2, AY1-AY2 and AZ) is a function for counting the number of black elements, included in an area within a range from (AX1 and AY1) to (AX2 and AY2) for the exposure pattern which is designated by an arbitrary AZ, and α is a prescribed coefficient.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gradation recording method for halftone reproduction by area gradation, and is used, for example, in an electrophotographic copying machine or a printer.

[0002]

2. Description of the Related Art Conventionally, in order to record an image having a halftone in a printer or a digital copying machine, various recording methods using various area gradations such as a density pattern method and a dither method have been proposed.

In the density pattern method, one pixel of an original image is associated with a threshold value of each dot (element) of an m × n matrix, and is represented by a plurality of different area patterns.

In the dither method, one pixel of the original image is mx
A dot concentration type (Fattening type) in which the threshold values of each dot of the n matrix are made to correspond one-to-one, and the dots are sequentially thickened with the center as a nucleus.
And a dot dispersion type (Bayer type) in which dots are dispersed as much as possible. When a dot dispersion type threshold matrix is used, high γ (gamma) recording characteristics result due to the influence of the light intensity distribution of the laser light. On the other hand, when a dot concentration type threshold matrix is used, the resolution decreases when the matrix size is increased in order to increase the number of gradations, and when the matrix size is reduced in order to improve the resolution. The number decreases.

As a recording method for achieving both gradation and resolution, the density patterns PSH1 to PSH4 shown in FIG.
As shown in (1), a method (IH) in which a modified dot concentration type threshold matrix is combined with pulse width modulation
Has been proposed (Journal of the Institute of Electrophotography, Vol. 25, No. 1, 1986, p. 34). The number of gradations r that can be expressed by the IH method is represented by r = m when all whites are included by the dot division number R by pulse width modulation and the matrix size m × m.
× m × R × 0.5 + 1, and a higher image quality output can be obtained as compared with the dither method before this.

However, even when the IH method is used,
As in the case of the dot concentration type, it is unavoidable to increase the gamma of the output characteristics due to the influence of the light intensity distribution of the laser light. Therefore, in order to obtain a linearized output, it is necessary to extract only the necessary steps so that the density changes at regular intervals. Therefore, in that case, the actual number of gradations is lower than the number of gradations r calculated by the above equation, and the image quality is correspondingly reduced.

[0007] As described above, it is difficult to say that both the gradation and the resolution are sufficiently achieved in the related art. This is based on the premise that the number of light emitting dots in the matrix area, that is, the total exposure time (total exposure time) must be increased in order to increase the output density. For example, in the case of the above-described IH method, it is necessary to increase the matrix size m or increase the number of dot divisions R in order to increase the number of gradations r. And the increase in the number of dot divisions R makes it difficult to cope with high speed.

As one method for solving this problem, it is conceivable to modulate the intensity of the laser beam with three or more values (multi-value). By performing intensity modulation of three or more values, it is possible to increase the gradation without lowering the resolution, but it is difficult to secure the stability of the multilevel laser, which leads to an increase in cost. I will.

In order to solve the above-mentioned problem, the image is scanned in the main scanning direction and the sub-scanning direction, and the gradation is controlled by controlling the amount of energy applied to each pixel of the image. In the gradation recording method, a plurality of regions having the same total irradiation amount of the energy in a region of a fixed size are different from each other by differently irradiating positions of the energy. A method of recording at different gradations has been proposed (Japanese Patent Laid-Open No. Hei 7-115538).

For example, when a rotating photosensitive drum is exposed in a main scanning direction by a light source such as a laser beam or an LED array, a portion exposed with a certain energy or more is visualized by toner. At that time, even if the total exposure time by the light source is the same within a region of a fixed size, if the exposure position by the light source is different, the added state of the energy from the light source is different, so that the image is visualized. Thus, recording is performed at different gray scales.

In this specification, a method of recording at different gradations by making the exposure positions different from each other is referred to as an extended dither method. In the extended dither method, since the number of gradations can be increased by changing the exposure position without increasing the total exposure amount, there is no problem such as a decrease in resolution.

Therefore, the density is set in accordance with the gradation level, and a dot pattern that can obtain the density is freely combined as a gradation pattern (exposure pattern). The gradation can be expressed by setting.

[0013]

However, in the extended dither method, if the dot arrangements in the tone patterns of adjacent tone levels are too far apart, for example, the tone continuously changes. When a gradation image as an image is printed (printed), a white streak (white line) or a black streak (black line) may appear.

That is, when printing a gradation image, there is no problem if the image density changes in a unit of an integral multiple of the set matrix size, but if not, it is shown in FIG. As described above, there is a possibility that an abnormal density change occurs at the boundary portion where the image density has changed (the portion where the Z address is switched in FIG. 9), and this may appear as a white streak or a black streak. .

In the case of the extended dither method, there is a part where the gradation level is changed by changing the dot arrangement. Therefore, the dot arrangement in the matrix existing at the boundary of the change in the image density is changed before and after the boundary. There may be another dot arrangement obtained by synthesizing the gradation pattern, in which case, the total exposure time will be too large or too small as compared with the preceding and following gradation patterns, so the white streaks or They appear as black streaks.

The present invention has been made in view of the above-described problem, and when the extended dither method is used, even if the image density changes continuously, white stripes are formed at the boundary between adjacent gradation levels. Another object of the present invention is to provide a gradation recording method in which inconsistencies such as black streaks do not appear.

[0017]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention compares exposure patterns (tone patterns) of adjacent gradation levels so as to limit the amount of different exposure positions. The exposure pattern is set in accordance with such rules.

That is, the method according to the first aspect of the present invention comprises:
This is a gradation recording method in which an image is scanned in the main scanning direction and the sub-scanning direction, and gradation is obtained by controlling the amount of energy irradiated corresponding to each pixel of the image. In a matrix composed of n pixels (m and n are 0 or positive integers), for a plurality of matrices having the same total energy irradiation amount, the positions where the energy is irradiated are different from each other, so that In a gradation recording method for recording at different gradations, an arbitrary value in an X address in the matrix is set to AX
1 and AX2, any value in the Y address in the matrix is AY1 and AY2, and the value of the address corresponding to the exposure pattern density information indicating the exposure position in the matrix is AZ, and is specified by any AZ The function for counting the number of black elements included in the range of (AX1, AY1) to (AX2, AY2) for the exposure pattern to be performed is represented by F (AX1 to AX2, AY1 to F1).
AY2, AZ) and α is a coefficient, at least in the low density region, the following conditional expression: | F (AX1 to AX2, AY1 to AY2, AZ) -F
(AX1 to AX2, AY1 to AY2, AZ-1) | ≦ |
F (0-m-1,0-n-1, AZ) -F (0-m-
1,0 to n-1, AZ-1) + (m × n × α) |.

In the method according to the second aspect of the present invention, substantially 0.03 is used as the coefficient.

[0020]

FIG. 1 is a schematic diagram of a printer 1 according to the present invention. The printer 1 includes a scanning optical system 1
1, a photosensitive drum 12, a charging device 13, a developing device 14, a transfer device 15, and the like. The photosensitive drum 12
It rotates at a constant speed in the direction of the arrow shown in FIG.
Is charged uniformly to a predetermined potential. Scanning optical system 1
The light beam (laser beam) output from 1 forms an image on the photosensitive drum 12, and an electrostatic latent image is formed on the surface of the photosensitive drum 12. The electrostatic latent image is developed by the developing device 14 and visualized. When the visualized toner image reaches the transfer position, it is transferred to the recording paper PP by the transfer device 15 and fixed by a fixing device (not shown).

FIG. 2 is a perspective view schematically showing the scanning optical system 11. The semiconductor laser 21 is intensity-modulated by image information read from an image memory or the like of a control unit (not shown) and outputs a light beam. The light beam is collimated by the collimating lens 22 and undergoes light deflection by the rotating polygon mirror 23. The polarized light beam forms an image on the photosensitive drum 12 by an imaging lens 24 called an fθ lens, and performs beam scanning. When scanning this beam,
Light at the start end of each scanning line of the light beam is reflected by the mirror 25 and guided to the detector 26. The detection signal from the detector 26 is used as a synchronization signal for scanning in the H direction (horizontal direction).

FIG. 3 is a block diagram of the modulation circuit 30 of the semiconductor laser 21, and FIG. 4 is a diagram showing timing of address designation. In FIG. 3, the modulation circuit 30 includes a pattern RO
M31, Z address counter 32, X address counter 33, Y address counter 34, and driver 35.

The pattern ROM 31 stores a large number of exposure patterns PS composed of an m × n matrix.
With respect to the exposure pattern specified by the Z address counter 32, the value of each element (cell) is sequentially specified by the X address counter 33 and the Y address counter 34, and is read out as 1-bit data DE. The exposure pattern PS will be described later in detail.

The Z address counter 32 is provided for each image data DG, that is, for each pixel, based on image data DG read from an image memory (not shown).
An address AZ is generated and output. The image data DG is
For example, it is a digital signal having density information of about 6 bits or 8 bits for one pixel.

The X address counter 33 counts the pixel clock signal SC and outputs the count value to the X address A.
Output as X. The pixel clock signal SC is a clock signal having a frequency that is k times (k is the number of divisions) the image clock signal. That is, k pixel clock signals SC are input while one image data DG is input. The X address counter 33 repeats counting within a predetermined value range according to the number of elements EM arranged in a horizontal direction (row direction, horizontal direction, or main scanning direction) of the exposure pattern PS described later. For example, the exposure pattern PS
Is 12 (horizontal) × 3 (vertical), the counting is repeated in the range of 0 to 11 as shown in FIG.

The Y address counter 34 counts the horizontal synchronizing signal SH and outputs the count value as a Y address AY. The Y address counter 34 repeats counting within a predetermined value range according to the number of elements EM in the vertical direction (column direction, vertical direction, or sub-scanning direction) of the exposure pattern PS described later. For example, if the size of the exposure pattern PS is 12 (horizontal) × 3 (vertical), counting is repeated in the range of 0 to 2, as shown in FIG.

The driver 35 is configured to operate the semiconductor laser 21 based on the data DE read from the pattern ROM 31.
On / off control. Next, the exposure patterns stored in the pattern ROM 31 will be described.

The exposure pattern has m dots D in both the vertical and horizontal directions.
Each dot DT of the square matrix composed of T is divided into k in the horizontal direction, and each of the divided is set as an element EM (mx
k) consists of a matrix of size m. Each element EM
Takes a value of “0” or “1”. In the case of "0", the semiconductor laser 21 is off, and in the case of "1", the semiconductor laser 21 is on. Therefore, the portion on the photosensitive drum 12 corresponding to the ON state of the semiconductor laser 21 is exposed and its potential decreases, and toner adheres to that portion.
That portion of the recording paper PP becomes black. Conversely, the portion of the recording paper PP corresponding to the off state of the semiconductor laser 21 becomes white. Therefore, the element EM having a value of “0” is replaced with a white element (E
MW) "and the element EM having a value of" 1 "are replaced with a black element (EM
B) ".

It should be noted here that:
This means that the area of an image recorded in black on the recording paper PP by one black element EMB differs depending on the position of the black element EMB. More specifically, the black element E
The element adjacent to MB is a white element EMW or a black element EM
B, and also depending on whether the adjacent position is left or right or up and down. The reason will be described later.

FIG. 5 is a view showing an example of the exposure pattern PSA.
FIG. 6 is a diagram showing a state of irradiation energy corresponding to the exposure pattern PSA shown in FIG. 5, FIG. 7 is a diagram showing a recording state on the recording paper PP corresponding to the exposure pattern PSA shown in FIG.
FIG. 8 is a diagram showing a distribution state of irradiation energy due to the lateral movement of the light beam. 6 and 7 are obtained by simulation, and FIG.
This is a recording state when binarization is performed using g as a threshold.

The exposure patterns PSA1 to PSA4 shown in FIG.
Each dot DT of the square matrix of 3 × 3 dots is divided into four in the horizontal direction to form a 12 × 3 matrix, and “0” is white and “1” is black for each element EM.

The semiconductor laser 21 is turned on corresponding to one black element EMB, and the semiconductor laser 21 irradiates the photosensitive drum 12 with a light beam. Accordingly, irradiation of the photosensitive drum 12 with a light beam for a unit time, that is, exposure is performed by one black element EMB. Black element EM
The total number of B becomes the total exposure time.

Here, each dot DT of the square matrix corresponds to one pixel of the image data DG, and by dividing each dot DT into four, the semiconductor laser 2
1 in which pulse width modulation is performed. For example, the light beam has a half-value width of about 60 μm in a stationary state with respect to one dot DT in both the main scanning direction and the sub-scanning direction.
In the shape of a circle. The energy of the light beam is high at the center thereof, and the energy gradually decreases toward the periphery. The light beam is transmitted to the photosensitive drum 12.
After scanning one line in the main scanning direction in the main scanning direction, the next line is similarly scanned. Therefore, for each element EM of the exposure pattern PSA, the element EM on the same row is continuously scanned, but the column EM is scanned. Are scanned discretely, i.e., discontinuously, at spatial intervals from one another for elements EM that differ from one another.

As a result, when the black elements EMB are adjacent to each other in the horizontal direction, the irradiation energies of the light beams are added to each other and increase, and as a result, the maximum irradiation energy increases as shown in FIG. On the other hand, when the black elements EMB are adjacent in the vertical direction, the maximum irradiation energy does not increase because the contribution of the irradiation energy is small. That is,
Even if the number of black elements EMB is the same, in other words, even if the total exposure time is the same, when the black elements EMB are arranged continuously in the horizontal direction, the maximum irradiation energy is large, When arranged vertically, the maximum irradiation energy becomes smaller. Therefore, even if the total exposure time is the same, an electrostatic latent image is formed on the photosensitive drum 12 with different irradiation energy.

Then, when the electrostatic latent images formed with such different irradiation energies are developed, irradiation energies exceeding a certain threshold are received in accordance with the sensitivity of the photosensitive drum 12 and the magnitude of the developing bias. The toner adheres only to the portion where the toner image adheres, and the image is visualized, and a black image is formed.

As a result, even if the number of black elements EMB is the same, the area of the image recorded on the recording paper PP differs depending on the arrangement position. That is, the black element EM
A plurality of exposure patterns PS having the same total number of B
By making the position of the black element EMB different from that of A, recording of mutually different gradations is performed.

In FIG. 5A, the exposure pattern PSA
1 indicates that the total number of elements EM is “4”, and one dot DT at the center is recorded in black. In this case, as shown in FIG. 6A, the irradiation energy spreads almost concentrically, and when the threshold is set to 5.2 erg, as shown in FIG. A circular black image FG1 is recorded at the center.

5B to 5D, the exposure patterns PSA2 to PSA4 each have a total number of elements "EM" of "5".
And prints 5/4 dots. However, they differ in the position (that is, pattern) of the element EM. That is, in the exposure pattern PSA2, five black elements EMB are continuously arranged in the horizontal direction.
In the exposure pattern PSA3, one black element EMB is arranged in a row above the other four black elements EMB. In the exposure pattern PSA4, two black elements EMB are arranged in a row above the other three black elements EMB. Have been.

Therefore, as shown in FIGS. 6B to 6D, the irradiation energy of the exposure pattern PSA2 in which the five black elements EMB are continuous in the horizontal direction becomes the largest, and the exposure pattern PSA4 having a small number of continuous elements. Has the lowest irradiation energy. As a result, as shown in FIGS. 7B to 7D, the image FG2 based on the exposure pattern PSA2 is the largest and the image FG4 based on the exposure pattern PSA4 is the smallest. When this is shown by numerical values, the exposure patterns PSA1 to PSA1
4, the dot area ratios [%] of the images FG1 to FG4 are “3.7”, “7.0”, “5.2”, and “2.1”, respectively.
5 to 7, as is apparent from the comparison between (A) and (D), when the number of black elements EMB of the exposure pattern PS is different from each other, the number of black elements EMB is small depending on the arrangement position. In some cases, the area of the image recorded on the recording paper PP may be larger.

As described above, even if the matrix size m and the dot division number R are the same, the gradation can be changed by changing the position of the element EM. That is, the number of gradations can be increased without lowering the resolution. Therefore, a linearized gradation characteristic can be obtained by a combination of the total number of the elements EM and the position of each element EM. In addition, since intensity modulation of two or more values is not performed, it is possible to implement the method at low cost without causing a problem of laser stability.

Next, a description will be given of an inconsistency such as a white streak or a black streak by the extended dither method. 9 is a diagram showing an example of white streaks and black streaks appearing by the extended dither method, FIG. 10 is a diagram showing examples of irregular patterns PSX1, 2 appearing by the extended dither method, and FIG. 11 is an irregular pattern P appearing by the extended dither method.
It is a figure showing the example of SX3,4.

When the exposure patterns PSA1 to PSA4 shown in FIGS. 5A to 5D are stored in the pattern ROM 31, the Z address AZ of each exposure pattern PS
Are PSA4,
PSA1, PSA3, and PSA2 are set to increase in this order. The non-exposed exposure pattern P
The Z address AZ of S is the minimum, and the Z address AZ of the exposure pattern PS of all exposures is the maximum.

Here, the exposure patterns PSA4, PSA
1, "1" for PSA3 and PSA2 in this order
It is assumed that Z addresses AZ of “2”, “3”, and “4” are assigned and stored in the pattern ROM 31. When a gradation image whose density changes in the sub-scanning direction is printed,
The switching is performed depending on which part of the Y address AY of the exposure pattern PS in use is switched to the next exposure pattern PS as a boundary, that is, according to the position in the exposure pattern PS when the Z address AZ is switched. There is a possibility that an irregular pattern PSX in which the arrangement of the black elements EMB is different from that of the exposure pattern PS before and after the boundary portion is generated.
Next, a description will be given based on a specific example.

FIG. 9A shows an example of the arrangement of black elements EMB when a gradation image in which the density gradually decreases is printed. In FIG. 9A, when the Z address AZ is “2”, the exposure pattern PSA1 has been read. Immediately before the boundary where the Z address AZ switches, the first row of the exposure pattern PSA1 has already been read, and immediately after the boundary, the Z address AZ switches to “1” and the exposure pattern PSA
4 is read from the second row. As a result, an irregular pattern PSX1 composed of an array of black elements EMB different from any of the exposure patterns PSA1 and 4 at the boundary portion [FIG.
(See (E)). Since the irregular pattern PSX1 has a smaller amount of exposure than the exposure patterns PSA1 and PSA4, when printed, the density of the portion becomes lower, and appears as a white line in the horizontal direction of the figure.

FIG. 9B shows an example of the arrangement of black elements EMB when a gradation image in which the density increases stepwise is printed. In FIG. 9B, when the Z address AZ is “1”, the exposure pattern PSA4 has been read. Immediately before the boundary where the Z address AZ switches, the first row of the exposure pattern PSA4 has already been read, and immediately after the boundary, the Z address AZ switches to “2” and the exposure pattern PSA
1 is read from the second row. As a result, an irregular pattern PSX2 composed of an array of black elements EMB different from any of the exposure patterns PSA1 and 4 at the boundary portion [FIG.
(Refer to (F)). Since the irregular pattern PSX2 has a higher exposure amount than the exposure patterns PSA1 and PSA4, when printed, the density of the portion becomes higher and appears as a black streak in the horizontal direction in the figure.

As described above, when the Z address AZ is switched between "1" and "2", the signals shown in FIGS.
The irregular patterns PSX1 and PSX2 shown in FIG. When the Z address AZ switches between “3” and “4”, the irregular patterns PSX3 and 4 shown in FIGS. 11G and 11I may occur, as described above. .

That is, when the black elements EMB are arranged like the irregular patterns PSX1 to PSX4, the threshold value of the irradiation energy is set to 5.2 erg, and the dot area ratios are set to “0.0” and “7.6”, respectively. "8.8" and "3.7".

Therefore, in FIG. 9A, the arrangement pattern of the black elements EMB generated at the boundary of the gray scale is obtained.
0 (E), the dot area ratio of the irregular pattern PSX1] is considerably smaller than the dot area ratio of the exposure pattern PSA1 (difference of 2.1%), and white streaks occur on the image. Further, in FIG. 9B, the dot area ratio of the array pattern of black elements EMB (irregular pattern PSX2 shown in FIG. 10F) generated at the boundary of the gradation is smaller than the dot area ratio of the exposure pattern PSA1. It becomes quite large (difference of 3.9%) and black streaks appear on the image.

On the other hand, the above-mentioned white streak or black streak does not occur at the boundary where the Z address AZ is “3” or “4”. This is because the difference between the dot area ratios of the irregular pattern PSX3 and the exposure pattern PSA2 and the irregular pattern PSX4 and the exposure pattern PSA3 generated at the boundary of the gradation is 1.8% or 1.5%. Compared with the case where the Z address AZ is at the boundary between “1” and “2”, the difference in the dot area ratio is small and does not become apparent.

In the above example, a gradation image in which the density changes stepwise in the sub-scanning direction is taken. However, the irregular pattern PSX similarly occurs in the main scanning direction, and white streaks or black streaks may occur. There is. When the extended dither method is used, it is an unavoidable problem that irregular patterns PSX having different dot area ratios may occur at the boundary between gradations.

However, as shown in FIG. 9, the irregular pattern PSX of the black element EMB generated at the boundary of the gradation
Is too different from the preceding and subsequent exposure patterns PS, the above-described white streak or black streak occurs, and thus the irregular pattern P that will occur at the boundary of gradations is generated.
It is necessary to set the exposure pattern PS while paying attention to SX. Empirically, when the difference in the dot area ratio between the irregular pattern PSX and the exposure pattern PS before and after the irregular pattern PSX is 2% or more in the low density area and 3% or more in the high density area, white streaks or black streaks are formed. Occur.

Therefore, in the present embodiment, in order to prevent the occurrence of white streaks or black streaks, the number of black elements EMB whose positions are changed in order to make the gradation different in adjacent exposure patterns PS is limited. It was.

The reason why the arrangement of the black elements EMB is different in the exposure pattern PS having the adjacent Z address AZ is that it is necessary to increase the density of the black elements EMB to the target density.
There are two cases: a case where the total number of B is increased, and a case where the position of the black element EMB is moved to achieve the target density.
In the present embodiment, attention is paid to the latter case, and the exposure pattern PS is set based on the rule shown in the following conditional expression.

| F (AX1 to AX2, AY1 to AY2,
AZ) -F (AX1-AX2, AY1-AY2, AZ-
1) | ≦ | F (0−m−1,0−n−1, AZ) −F
(0-m-1,0-n-1, AZ-1) + (m × nx)
α) | where AX1 and AX2 are arbitrary values in the X address AX (AX = 0, 1, 2,..., m−1) of the exposure pattern PS, and AY1 and AY2 are the Y addresses AY ( AY = 0, 1, 2,..., N-1)
AZ is the value of the Z address corresponding to the density information, and F (AX1
To AX2, AY1 to AY2, AZ) are functions for counting the number of black elements included in the region from (AX1, AY1) to (AX2, AY2) for an exposure pattern designated by an arbitrary AZ, and α is a coefficient It is. For example, 0.03 is used as the coefficient α.

The left side of the above conditional expression is a portion for evaluating the difference in the arrangement of the black elements EMB of the exposure pattern PS having the adjacent Z addresses AZ, and the value becomes larger as the arrangement of the black elements EMB is different. The first and second terms in the absolute value symbols on the right side represent the number of black elements EMB that increase with an increase in the Z address AZ. The third term is a part for limiting the number of black elements EMB whose positions are moved to achieve the target density.

The exposure pattern PS is set so that the above conditional expression is satisfied at least in a low density area. This is because black streaks are conspicuous in the low concentration region. In the example shown in FIG. 5, in the relationship between the exposure pattern PSA3 and the exposure pattern PSA2, that is, when the Z address AZ is “4”, the above conditional expression is satisfied for all the X addresses AX and the Y addresses AY. I do. However, in the relationship between the exposure patterns PSA4 and PSA1, that is, when the Z address AZ is “2”, AX1 =
0, AY1 = 0, AX2 = 6, AY2 = 0,
The left side of the conditional expression is “2” and the right side is “0.08”, and the conditional expression is not satisfied. Therefore, it is necessary to reset the contents of either the exposure pattern PSA4 or the exposure pattern PSA1.

Therefore, in the present embodiment, the exposure pattern PS having the Z address AZ of “1” is used as the exposure pattern PS in FIG.
An exposure pattern PSA5 shown in FIG. 12J is used instead of the exposure pattern PSA4 shown in FIG. This exposure pattern P
1 between SA5 and the exposure pattern PSA1.
Irregular patterns PSX5 and 6 shown in 3 (K) and (L) are generated.

The dot area ratios of the irregular patterns PSX4, 5, 6 are "1.5", "5.2" and "0.0", respectively, with 5.2erg as a threshold value. Although the change in the dot area ratio of the exposure pattern PS is large and the gradation jumps slightly, the difference in the dot area ratio between the boundary portion and the pattern portion is less than 2%. No noise appears.

FIG. 14 is a diagram showing an example of another exposure pattern PSB stored in the pattern ROM 31. In FIG. 14, an exposure pattern PSB is obtained by dividing each dot DT of a square matrix of 4 × 4 dots into four parts in the horizontal direction to obtain a 16 ×
4, and each element EM is “0”
Are shown in white and “1” is shown in black.

These exposure patterns PSB1, PSB2
The dot area ratio of -1, PSB2-2, PSB2-3, and PSB3 is "30.2", "31.0", "31.3", and "30.9", respectively, with 5.2 erg as a threshold value.
“32.2”.

Accordingly, the Z address A is stored in the pattern ROM 31 in the order of the exposure patterns PSB1, PSB2 and PSB3.
Z is changed one by one. For example, the Z address AZ is changed to “3”.
0, 31 and 32 are stored. At this time, as the exposure pattern PSB2, the exposure pattern PSB2-1,
A case where one of PSB2-2 and PSB2-3 is used will be described.

First, when the exposure pattern PSB2-1 is used, the above-mentioned conditional expression satisfies the condition that the Z address AZ is "32".
, For example, AX1 = 0, AY1 = 0, AX2 =
8, AY2 = 0 is not satisfied. Further, when a gradation image whose density changes stepwise in the sub-scanning direction is printed, the irregular pattern PSX which is considered to be generated at the boundary portion when the Z address AZ is “32” and “31”
Has a maximum dot area ratio of “40.8” and a minimum of “21.
8 ", and white or black streaks occur.

Next, when the exposure pattern PSB2-2 is used, the above-mentioned conditional expression satisfies all X addresses AX, Y
Address AY and Z
The dot area ratio of the irregular pattern PSX considered to occur at the boundary when the addresses AZ are “32” and “31” is “34.2” at the maximum and “30.7” at the minimum, and the difference is Each becomes "2.0" and "0.6", and no white streaks or black streaks occur.

In the case where the exposure pattern PSB2-3 is used, the above-mentioned conditional expression also satisfies all the X addresses AX, Y
Address AY and Z
The arrangement of the black elements EMB considered to occur at the boundary when the address AZ is “32” and “31” is the same as the exposure pattern PSB2-3 or the exposure pattern PSB3, and the irregular pattern PSX does not occur. Therefore, no white streaks or black streaks occur.

As described above, without using the exposure pattern PSB2-1 which does not satisfy the conditional expression as the exposure pattern PSB2 between the exposure pattern PSB1 and the exposure pattern PSB3, the exposure pattern PSB which satisfies the conditional expression is used.
By employing 2-2 or the exposure pattern PSB2-3, it is possible to prevent occurrence of mismatch such as white streaks or black streaks.

In the above embodiment, the scanning optical system 11 using the semiconductor laser 21 has been described as an example.
An optical system using an LED array, liquid crystal, PLZT, or the like may be used. The present invention can also be applied to the case where an image is recorded by a method other than the electrophotographic method, or the image is visualized and recorded (displayed).

[0067]

According to the first and second aspects of the present invention,
Provided is a gradation recording method in which inconsistency such as a white streak or a black streak does not appear at a boundary portion between adjacent gradation levels even when an image density continuously changes when an extended dither method is used. Can be.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a printer device according to the present invention.

FIG. 2 is a perspective view schematically showing a scanning optical system.

FIG. 3 is a block diagram of a modulation circuit of the semiconductor laser.

FIG. 4 is a diagram showing timing of address designation.

FIG. 5 is a diagram illustrating an example of an exposure pattern PSA.

FIG. 6 is a diagram showing a state of irradiation energy corresponding to the exposure pattern shown in FIG.

FIG. 7 is a diagram showing a recording state on recording paper corresponding to the exposure pattern shown in FIG. 5;

FIG. 8 is a diagram showing a distribution state of irradiation energy due to a lateral movement of a light beam.

FIG. 9 is a diagram showing an example of white streaks and black streaks appearing by the extended dither method.

FIG. 10 is a diagram showing an example of an irregular pattern appearing by the extended dither method.

FIG. 11 is a diagram showing an example of an irregular pattern appearing by the extended dither method.

FIG. 12 is a view showing an exposure pattern used in place of the exposure pattern shown in FIG. 5D in the present embodiment.

FIG. 13 is a diagram showing an example of an irregular pattern appearing by the extended dither method.

FIG. 14 is a diagram showing an example of another exposure pattern stored in a pattern ROM.

FIG. 15 is a diagram showing an example of a density pattern used for the IH method.

[Explanation of symbols]

 1 Printer device PS, PSA, PSB Exposure pattern PSX Irregular pattern EMB Black element

Claims (2)

[Claims] 1. A gradation recording method in which an image is scanned in a main scanning direction and a sub-scanning direction, and gradation is obtained by controlling the amount of energy irradiated corresponding to each pixel of the image. In a matrix composed of m × n pixels (m and n are 0 or positive integers), a plurality of matrices having the same total irradiation amount of the energy are mutually irradiated with the energy irradiation positions. In a gradation recording method for recording gradations different from each other by making them different, an X address (0, 1, 2,...,
m-1) are defined as AX1 and AX2, and the Y address (0, 1, 2,..., n-
The arbitrary values in 1) are AY1 and AY2, the address value corresponding to the density information of the exposure pattern indicating the exposure position in the matrix is AZ, and the exposure pattern specified by the arbitrary AZ is (AX1, A function for counting the number of black elements included in the range of (AY1) to (AX2, AY2) is represented by F (AX1 to AX2, AY1).
ＡAY2, AZ) and α as a coefficient, at least in the low concentration region, the following conditional expression: | F (AX1 to AX2, AY1 to AY2, AZ) -F
(AX1 to AX2, AY1 to AY2, AZ-1) | ≦ |
F (0-m-1,0-n-1, AZ) -F (0-m-
1,0-n-1, AZ-1) + (m × n × α) | 2. The gradation recording method according to claim 1, wherein substantially 0.03 is used as said coefficient.