JP2003008897A - Gradation processor and laser printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the discontinuity of gradation characteristics due to abrupt increase in the density of dots, constituting a dot dither, accompanying a carriage return by an image output device, such as a color laser printer which concurrently uses a dot dither gradation process and laser pulse width modulation (PWM). SOLUTION: A gradation processor 9 is composed of a main address arithmetic part 68, a subordinate address arithmetic part 67, a default register 6, a select signal generation part 69, and a laser pulse width modulation (PWM) table 22. The select signal generation part 69 further assigns processes to the subordinate address arithmetic part 67 and default register 61 depending upon a threshold nc , even after an input value ni exceeds an intermediate PWM level processed by the main address arithmetic part 68. The laser irradiation time of dots, exceeding the intermediate level represented by PWM, is controlled in two stages and combined with a threshold pattern 84 to shorten the pulse time of dots, before a carriage return at the same time with the generation of the carriage return dots, thereby preventing density discontinuity with accompanies the carriage return of the dots from being generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image output device such as a laser printer, a digital copier, or a laser facsimile which prints an image as a set of dots (pixels), and more particularly to a laser pulse width modulation (PWM) and a plurality of devices. The present invention relates to a gradation processing device for expressing continuous gradation by combining the above-mentioned pixels.

[0002]

2. Description of the Related Art Conventionally, in a raster device such as a laser printer having one pixel and two gradations (binary), the intermediate gradation is artificially supplemented by an intermediate gradation expressing means known as a dither method or an error diffusion method. Techniques have been widely used.

Among them, the laser printer is disclosed in
As disclosed in Japanese Patent Application Laid-Open No. 00-4359, a dot concentrated dither method that expresses an intermediate density by changing the area ratio of halftone dots formed by combining a plurality of dots, and one dot by laser pulse width modulation. A technique for realizing high density and high gradation by using together with a laser pulse width modulation (PWM) process for further subdivision has often been used.

[0004]

[Problems to be Solved by the Invention] Halftone dot formation by the dot concentrated dither method and laser pulse width modulation (PWM)
Laser pulse width modulation when used with dot subdivision
With respect to the pulse width growth direction (main scanning direction) by (PWM), the gradation property is controlled by the pulse time that controls the laser exposure time. Is easy to realize.

However, when adjacent dots are struck in the direction (sub-scanning direction) orthogonal to the pulse width growth direction by laser pulse width modulation (PWM), that is, when the dots are line-fed, the dot line feed leads Since the area of the bottom of the dot and the area of the newly hit dot interfere with each other, the density rises discontinuously.

The influence on the discontinuity of the increase in density due to the line feed of dots forming halftone dots is relatively small in the case of low density and large halftone dots or laser pulse width modulation (PWM) processing with coarse division width. Inconspicuous.

On the other hand, small halftone dots forming a high-density image and laser pulse width modulation (PW) with small division steps are used.
In M) processing, the influence is relatively large and cannot be ignored.

For the same reason, with respect to the increase of the gradation value, the reproduction density of the low density gradation reproduced by the isolated dot until the first dot line break by dither occurs becomes too thin. .

Further, these problems are caused by the fact that the exposure charging potential on the photoconductor below the development threshold exceeds the development threshold due to interference, so that so-called machine difference for each printer engine and environmental change of temperature / humidity. It is likely to be a factor that causes the concentration change due to the influence of.

It is possible to consider one dot, which is the basis of the resolution of the printer engine, as a minimum unit subdivided by laser pulse width modulation (PWM), but in order to avoid confusing the definition, the present invention is used. In the description, the minimum pixel unit as input pixel data given to the gradation processing process of the printer is treated as one dot.

An object of the present invention is to provide a gradation processing apparatus and a laser printer which are provided with means for reducing discontinuity in density increase accompanying dot line feed and realizing smooth gradation processing of high density and high gradation. Is to provide.

[0012]

In order to achieve the above object, the present invention provides a laser pulse width modulation (PWM) circuit for controlling gradation in multiple stages by laser pulse width modulation (PWM) and an input pixel. A gradation processing method for a laser printer engine comprising a threshold value array for sequentially comparing a gradation value with a threshold value and converting the gradation value into an output pixel gradation value. Is an array that forms a halftone dot composed of a plurality of output pixels whose area increases with an increase in the gradation value of the input pixel. The halftone dot growth direction changes with the increase of the value from the laser pulse scanning direction (main scanning direction) to the orthogonal direction (sub scanning direction) (line feed). The laser pulse width in the main scanning direction is shortened as the We propose a gradation processing method of the laser printer engine that switches in this way.

The present invention also provides a laser pulse width modulation (P
A laser pulse width modulation (PWM) circuit for controlling gradation in multiple stages by WM), and a threshold value array table for sequentially comparing the gradation value of an input pixel with a threshold value and converting the gradation value to an output pixel. In the gradation processing device for the laser printer engine, the area of the threshold value array table increases with an increase in the gradation value of the input pixel for the gradation value of the input pixel which is two-dimensionally uniform. It is an array that forms a halftone dot composed of a plurality of output pixels, and in the laser pulse width modulation, the growth direction of the halftone dot with the increase of the input gradation value is orthogonal to the scanning direction (main scanning direction) of the laser pulse. The direction of the laser printer engine that switches the laser pulse width in the main scanning direction shortly as the halftone dots grow in the sub-scanning direction at some input gradation values that change (line feed) in the direction (sub-scanning direction). Propose a tone processing device.

A laser printer, a digital copying machine, and a laser facsimile equipped with the gradation processing device can be provided with a user interface for adjusting the switching amount of the laser pulse width.

Further, a rewritable register for holding the switching amount of the laser pulse width, a sensor for detecting the density inside the laser printer, and a gradation processing device for adjusting the register value based on the output value of the sensor are provided. Is also possible.

The present invention further includes a printing command means for outputting a gradation test pattern which continuously changes from the minimum density to the maximum density and a selection means capable of selecting several printing conditions, and a laser printer engine. Adjustment software provided for the laser printer main body or a computer connected to the laser printer for calibration, paying attention to the density discontinuity point on the test pattern, and selecting the printing state in which the density discontinuity point becomes the smallest. , Adjustment software of laser printer engine for adjusting gradation characteristics is proposed.

That is, according to the present invention, in a gradation processing device such as a laser printer, for some values of input gradation values at which dot line breaks due to dither occur, simultaneously with the occurrence of line break dots, and immediately before line breaks. Means for switching the laser pulse width in the main scanning direction corresponding to the dot to a short one is provided.

A user interface is provided so that the user can select and instruct an appropriate value corresponding to the switching amount while checking the output of the test chart.
Alternatively, the printer is equipped with a sensor for scanning the test pattern and automatically selecting the optimum value of the switching amount of the pulse width.

In the present invention having such a structure, a rapid increase in density accompanying a line break of a dot is offset by a decrease in density due to the reduction of the dot width before the preceding line break. Continuity is achieved.

On the other hand, regarding the variation in the appropriate value of the switching amount of the pulse width due to the machine difference for each printer engine or the influence of the environmental change of temperature / humidity, the user corrects it while checking the output of the test chart or the sensor automatically Can be corrected dynamically.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Next, referring to FIGS.
An embodiment of a gradation processing device and a laser printer according to the present invention will be described.

FIG. 1 shows a resolution of 23.6 dots / mm, ie, a resolution of 600 dp, to which the gradation processing apparatus of the present invention is applied.
It is a figure which shows the flow of the image processing in the color laser printer of i. The image data 1 to be printed is stored in the input buffer 2 as RGB data for one page.

The printer engine 13 uses the YMCK (Yello
w, Magenta, Cyan, Black)
The processes after the input buffer 2 in FIG. 1 are repeated four times for four YMCK planes.

The RGB data sent from the input buffer 2 is subjected to color correction by the color correction means 3 and four-color separation means 5.
After conversion into yellow data and gradation correction by the γ correction means 7, laser pulse width modulation by the gradation processing device 9
The (PWM) signal 12 is output to the printer engine.

The four-color separation means 5 initializes the inside so as to calculate Yellow from the RGB dot-sequential data. In response to this, the γ correction means 7 uses the γ correction value table 8 to determine Yell.
Load the correction value corresponding to ow into the internal lookup table.

The dither circuit 10 loads a threshold value array corresponding to Yellow, size data of the array and a column offset value described later from the table 11 and initializes the inside.

This threshold value array is the threshold value array 2 of FIG.
7, an array of values from 0 to 255, with γ
The correction unit 7 adjusts the density gradation characteristics depending on the characteristics of the gradation processing 9 and the printer engine 13 substantially linearly by the correction values loaded in the internal reference table.

When the processing for one page of Yellow is completed, 4
The color separation unit 5, the γ correction unit 7, and the dither circuit 10 each reload the necessary parameters for Magenta, and after initialization, the same procedure is followed by Magenta 1
A laser pulse width modulation (PWM) signal 12 for a page is sent to the printer engine 13.

Similarly, Cyan and Black are processed.

Each of these color planes corresponds to the printer engine 13
It is switched in synchronization with the vertical synchronizing signal of.

However, in the following discussion, which is limited to the explanation of the function of the gradation processing device, the effect of the γ correction means 7 is excluded in the sense that the correspondence between the logical gradation value and the gradation processing function is not confused. deal with.

FIG. 2 is a diagram showing a system configuration of the gradation processing device 9. The main function of the gradation processing device 9 is the input gradation value n.
Laser pulse width modulation (PWM) corresponding to i and threshold value nc
It is to determine a subscript (address) that refers to each element of the table 22. To keep the explanation simple, here
The description will be made using only positive logic in which true is 1 and false is 0 as the truth value of the logic judgment.

The present embodiment is an extension of the gradation processing apparatus which uses both the dispersed halftone dots and the laser pulse width modulation (PWM) by the sub-matrix method as disclosed in FIG. 3 of Japanese Patent Laid-Open No. 2000-4359. Is realized as.

Particularly in the following description, the product of the dispersion number of the halftone dots and the laser pulse width modulation (PWM) division number is Δh, and
It is assumed that the register 50 of FIG. 2 has a value of Δh, the register 52 has a value of 2Δh, and the register 54 has a value of Δh, and the threshold value array ni for determining the threshold value input ni has a configuration as shown in FIG. 5 described later. .

As is clear from the configuration of FIG. 5, Δh is
It is the minimum interval between thresholds in the threshold array. Also, Δh
Is 16 (= 2 ⁴ ) or 32 (= 2 ⁵ ), and Δh = 16
In the case of, the value of the register 71 is set to 1, and in the case of Δh = 32, the value of the register 71 is set to 0.

The subtraction circuit 14 is a difference signal Δn = ni between the input gradation value ni of the gradation processing device 9 and the threshold value nc from the threshold value array.
Sequential conversion to -nc.

Processing and selection circuit 65 corresponding to the difference signal Δn
The relationship with the signal selection according to is as follows depending on the magnitude relationship between the sign of Δn and Δh.

(1) When Δn <0 First, in the main address operation unit 68, all bits of the output from the NAND circuit 17 are 0 due to the underflow signal generated in the subtraction circuit 14. Next, adder circuit 60
The output value from the address offset calculation unit 66 is added to this by the adder circuit 58 to become the output value from the main address calculation unit 68.

At the same time, in the selection signal generation unit 69, the outputs of the comparison circuits 53 and 55 both become 0, and the value of the selection signal 64, that is, the output signal of the main address calculation unit 68 corresponding to (00) in binary representation. Are selected by the selection circuit 65.

On the other hand, in the address offset calculation unit 66, if nc> Δh, the output of the comparison circuit 51 becomes 0, so the AND circuit 57 outputs the value 0, and nc ≦ Δ
If h, the output of the comparison circuit 51 becomes 1, so that AN
A preset address offset value is output to the offset register 56 by the D circuit 57.

(2) When 0 ≦ Δn <Δh The selection signal 64 is represented by a binary expression (0
0), and the selection circuit 65 causes the main address operation unit 6
8 outputs are selected.

In the main address operation unit 68, the NAND circuit 17 outputs the difference value Δn as it is. The difference value Δn is divided by 4 to round up the fraction, and the upper 2 bits of Δn are rounded down to the lower 2 bits. Is added to the intermediate 4 bits by the adder circuit 60 via the OR circuit 59.

As in the case of (1), the output from the address offset computing section 66 is added to the output of the adder circuit 60 by the adder circuit 58.

Thus, in addition to the case of (1), the threshold value n
The address that refers to the laser pulse width modulation (PWM) table 22 is switched by the offset value set in the offset register 56 depending on whether c is nc ≦ Δh or not.

(3) In the case of Δh ≦ Δn <2Δh In this case, the output of the comparison circuit 53 of the selection signal generation unit becomes 0, the output of the comparison circuit 55 becomes 1, and the selection signal 64 is expressed in binary (01). Becomes From this, the output of the sub-address calculator 67 is selected as the address signal to the laser pulse width modulation (PWM) table 22 by the selection circuit 65.

In the sub-address calculation unit 67, when the value of the register 71 is 0, according to a preset value of the register 71, the upper 3 bits of nc are simply referred to as a subscript to refer to the address table 62 to determine the output. .

When the value of the register 71 is 1, the 3-bit value obtained by ORing the high-order 1 bit by the OR circuit 72 with respect to each intermediate 3 bits obtained by cutting off the low-order 4 bits and the high-order 1 bit of nc is added. The address table 62 is referenced to determine the output.

(4) In the case of Δn ≧ 2Δh In this case, the output of both the comparison circuit 53 and the comparison circuit 55 of the selection signal generation unit 69 is 1, and the selection signal 64 is binary expression (11).

Then, the selection circuit 65 selects the value a ₀ preset in the default register 61 as an address signal to the laser pulse width modulation (PWM) table 22.

FIG. 3 is a flow chart showing an outline of the gradation processing procedure in the gradation processing device 9 of FIG. The gradation process is performed in page units for each color plane.

First, in the initialization step 100 prior to page processing, the registers 50, 52 related to the threshold interval Δh,
54, 71, offset register 56, laser pulse width modulation (PWM) table 22 storing laser pulse width modulation (PWM) pulse patterns (indicated as P in FIG. 3)
The address table 62 (denoted by T in FIG. 3) that indirectly refers to the laser pulse width modulation (PWM) table is initialized.

Normally, the initial values required for these initializations are held in a non-volatile memory that holds system parameters outside the gradation processing device 9, and when the gradation processing device 9 is initialized, each register and Loaded into the table. In FIG. 2, this data flow is omitted.

In the next step 101, successive input pixels n
Load i and threshold nc. However, since the input pixel array is much larger than the threshold array, the threshold array is periodically repeated in both the row direction and the column direction, and compared with each other as an array of the same size as the input pixel array. Will be done.

At the next step 102, the difference value between ni and nc is set to .DELTA.n (= ni-nc). Further, the setting value of the offset register 56 is set to θ ₀ , and the highlight offset θ is set to ₀ when nc <Δh, and θ ₀ when nc ≧ Δh.

In step 103, laser pulse width modulation
The index a of the (PWM) table 22 (P) is θ when (i) Δn <0, and (Δn / when 0 ≦ Δn <Δh).
Round up value less than or equal to 4) + θ, and (iii) Δh ≦ Δn <
In the case of 2Δh, the reference value of the address table 62 corresponding to the rounded-down value of Δc / Δh is a decimal value, and (iv) Δn ≧ 2
In the case of Δh, the fixed value a ₀ is set.

In step 104, the laser pulse width modulation (PWM) pulse pattern P [a] is obtained by referring to the laser pulse width modulation (PWM) table with the index a obtained in step 103.

In step 105, the pulse pattern P
[a] is generated and output as an actual laser pulse width modulation (PWM) waveform.

The above steps 101 to 105 are repeated until all the pixels for one page are completed.

However, in the case of (iii) above, Δh = 16 (= 2
⁴ ), in the example of FIG. 2, since the subscript of the address reference table is 3 bits, the most significant bit of Δc / Δh is clipped by the OR circuit 72. However, this is of course essential. It is not a problem and it is easy to configure the address reference table with 4 bits if there is a margin in mounting.

Further, when this processing is realized by a circuit, as in the example of FIG. 2, 16 (= dispersion (4) × PWM (4 stages)) in which the magnitude of Δh is adjusted to the bit boundary from the beginning. , 32
Although it is effective in terms of mounting cost and processing efficiency to limit the value to (= dispersion (4) × PWM (8 stages)) or the like,
If the algorithm of FIG. 3 is implemented by software, Δh is 24 (= dispersion (4) × PWM (6 stages)) or 28 (= dispersion (4) × PWM in the same algorithm.
It is easy to set a value such as (7 steps)).

FIG. 4 shows the difference Δ with respect to the gradation processing circuit 9 of FIG.
h = 32, the number of halftone dot dispersions is 4, the number of laser pulse width modulation (PWM) steps for one dot is 8, and the offset register 56
The correspondence table of the laser pulse width modulation (PWM) table reference address with respect to the set of the input pixel value ni and the threshold value nc when the address offset value set in is set to θ and the default address value set to the default register 61 is set to a ₀ FIG.

Each row of FIG. 4 has an input gradation value n of 0 to 255.
i, each column has a threshold value nc = 32k + i, (k = 0, 1, ..., 7, i
= 0, 1, 2, 3), and the values listed in the table correspond to (ni, nc) at the position in the table.
The (PWM) table reference address value is shown. Also,
In FIG. 4, the corresponding value in the address table 62 is represented by T [i].

FIG. 5 is a diagram showing an example of the configuration of the input threshold value array to the gradation processing circuit 9 of FIG. Similar to FIG. 4, Δ
It is assumed that h = 32, the number of dispersed dots is 4, and the number of laser pulse width modulation (PWM) steps for 1 dot is 8.

First, let K be a basic threshold value pattern 25 that is a unit of halftone dots, and let K be Δh × K, Δh × K + 1, Δh ×.
An extended threshold pattern 26 is formed by four threshold patterns generated by K + 2 and Δh × K + 3.

As a result, as the input gradation increases, halftone dots that are dispersed and grow while circulating among the four threshold patterns forming the extended threshold pattern 26 are formed.

Here, when the number of gradations of the basic threshold pattern 25 is simply referred to as the number of basic gradations, (total number of gradations) = (number of basic gradations) × (number of dispersed dots) × (number of PWM steps) ) = (Basic gradation number) × Δh.

Therefore, when Δh = 32,
Since the basic gradation number can only be 8 (= 256 / Δh) gradations, in the basic threshold value pattern 25 of FIG. 5, some threshold values are overlapped and the basic gradation number is 8 gradations.

Next, when the extended threshold pattern 26 is filled in a rectangular area which periodically closes both rows and columns, the threshold array 2
7 is obtained. The dither circuit 10 cyclically and repeatedly uses this threshold value array 27 to generate a threshold value nc.

The threshold value array 27 has a structure in which the upper two rows are shifted to the left at the position (the sixth column) shown by the position of the arrow A in the figure and are stacked 10 stages downward.

Therefore, instead of the threshold value array 27, the simplified threshold value array 28 consisting of the upper two rows is used as the printer engine 1.
If the column address of the starting position is repeatedly shifted by 6 columns for every 2 rows of the input image in synchronization with the horizontal synchronizing signal of 3, the mounting memory can be further saved.

In the threshold array 27 composed of the basic threshold pattern 25 of FIG. 5, the dots formed form an angle of 18.4 ° with respect to the main scanning direction. This angle is called the screen angle.

FIG. 6 is a diagram showing an example of a basic threshold pattern 25 forming a screen angle different from the example of FIG.
In a color printer, when a threshold array having different screen angles is assigned to each of CMYK colors, more stable reproduced colors can be obtained against color misregistration.

FIG. 7 is a diagram showing an example of setting the table of the gradation processing device 9, and shows an example of the address table 62 and the laser pulse width modulation (PWM) table 22. In the laser pulse width modulation (PWM) table 22, the right 16-digit binary number is converted from left to right by the laser pulse width modulation (PWM) generation circuit 23 into a pulse train. 16 pulses correspond to 1 dot of the printer engine.

Further, the laser pulse width modulation (PWM) of FIG.
The setting value of the table 22 is assumed to be θ = 9 as the address offset value θ (setting value of the offset register 56 in FIG. 2).

Therefore, laser pulse width modulation (PWM)
Input address 09H to 1 in hexadecimal representation of table 22
The table area 80 up to 1H particularly corresponds to the highlight portion (low density portion) where nc <Δh, and corresponds to θ to (8 + θ) in the table of FIG. A long pulse is assigned to the highlight part to suppress an extreme decrease in density.

In FIG. 7, a part of the correspondence between the output of the address table 62 and the input of the laser pulse width modulation (PWM) table 22 and the correspondence of the default value a ₀ stored in the default register 61 are indicated by arrows. Indicated by.

The value of the address table 62 is T in FIG.
Corresponds to [0] ... T [6]. In this embodiment, the input value 7 of the address table 62 is not used.

FIG. 8 is a diagram conceptually showing the effect of gradation processing by the setting of FIG. The threshold pattern 84 of FIG.
It shows a part of the threshold array 27 that constitutes a halftone dot.

With respect to the threshold pattern 84, when the input gradation value ni is ni = 32, the address value referring to the laser pulse width modulation (PWM) table 22 is only "cell with threshold 0" from FIG. Becomes 8 + θ = 11H, and the output pulse pattern of FIG. 7 is (11111111 1111 11
11).

As a result, the dot at the cell position of 0 of the threshold pattern 84 is printed, but in reality, due to the influence of the intensity distribution of the laser light, the peripheral region 8 of the cell of the threshold value 0 is printed.
It is printed as a density distribution dot having a foot in 1a.

On the other hand, the input gradation value ni is ni = 33.
When the number of cells of the threshold 0 is T [0],
The cell with the threshold 32 is 01.

At this time, according to the address table 62 of FIG. 7, laser pulse width modulation (PWM) corresponding to T [0] is performed.
Since the address value referring to the table 22 is 12H, as a result, the pulse pattern corresponding to ni = 33 is (11111111 1111 10
00), the cell with the threshold value 32 is (1100 0000 0000
0000).

In this case, as shown in FIG. 7, in the interference area 83 between the peripheral area 81b of the cell having the threshold value of 0 and the peripheral area 82 of the cell having the threshold value of 32, the density is higher than that in the case of each cell alone. Will increase.

The increase in density due to the interference region 83 leads to a gradation jump between the input gradations ni = 32 and ni = 33 as it is, but the pulse length at which the cell having the threshold 0 is turned ON is about 3/16. Since it is shorter (black arrow in FIG. 8), the influence of the increase in density due to the interference region 33 is offset.

FIG. 9 is a diagram schematically showing the growth pattern of the printing dots with respect to the increase of the input gradation value ni, corresponding to the threshold pattern 84 of FIG. 8 and the table setting of FIG. The gray display portion represents a logical printing area, and T [i] for i = 0, ..., 6 is simply denoted as Ti in the figure. Further, in the figure, a black arrow indicates the growth direction of the print region by laser pulse width modulation (PWM).

As shown in FIG. 9, in the printing portion corresponding to the threshold pattern 84, the input gradation values ni = 32 to ni = 3.
Position of line feed of dot to 3 and ni = 128 to ni =
Dot 8 at the position where the line feed of dot to 129 occurs
5,88 is corrected shortly before the line feed, and at other growth dot switches 86,87,89,90,91, this correction amount is corrected so as to be absorbed little by little, and the entire gradation characteristic is adjusted. There is.

However, in practice, in the growth dot switching 91 of FIG. 9, the reproduction density is almost saturated in most cases, and the truncation process corresponding to T [6] is virtually unnecessary.

FIG. 10 is a diagram showing an example of a test chart 90 for detecting gradation jump. The test chart 90 shows CMY corresponding to input gradation values ni = 0 to 255.
K gradation bar 91c, 91m, 91y, 9
1k and a scale 94 indicating the value of the corresponding input tone value ni.

FIG. 10 shows Cyan's gradation bar 91.
An example in which gray scale gaps 92 and 93 occur is shown in c. In the case of the basic threshold pattern 25 shown in FIGS. 5 and 6, discontinuous gradation skipping occurs at the input gradation value ni = 32 or ni = 128 (in the case of FIG. 6B, ni).
= 32 only).

The occurrence of such gradation gaps 92 and 93 is due to the fact that the address values T [1] and T [3] assigned to the pre-line feed dots 85 and 88 shown in FIG. 9 are not appropriate.

Originally, the rapid increase in the density due to the line feed of dots is caused by the interference of the skirts of the dots, and therefore it may be different due to the influence of the machine difference such as the development characteristic change due to the environmental change and the variation of the laser intensity spot diameter.

In the present invention, this problem is addressed by correcting the address value of the laser pulse width modulation (PWM) table indicated by the address table 62, as shown in FIG.

For example, if the grayscale gap 93 in FIG. 10 is a density reversal, it means that the pulse pattern assigned to T [3] is too short.

Therefore, the address table 62 of FIG.
The corresponding address T [3] of the input 3 of
Corrected to H etc., and related T [4], T [5], T [6] have T [4] = 13H, T [5] = 13H, T [6] = 08H
An intermediate pattern of 10H and 08H may be allocated as shown in FIG.

On the contrary, if the gradation gap 93 in FIG. 10 has a sharp increase in density, it means that the pulse pattern assigned to T [3] is still too long. In this case, the address table 62 shown in FIG. 7 may be displayed as, for example, T [3] = 0fH, T
It is conceivable to reallocate as [4] = 12H, T [5] = 10H, T [6] = 13H.

Although such an allocation change is complicated, it is not necessary so frequently. Therefore, several kinds of combinations are prepared in advance as the maintenance software on the host computer, and the address table is prepared. You can download it to 62.

FIG. 11 shows a laser pulse width modulation (PWM) table maintenance software user interface 120.
It is a figure which shows an example. In this maintenance software, as a set of address table values corresponding to the gradation gap 92 of FIG. 10, a table of standard value combinations, a set of table combinations of three types of long pulses, and a set of three types of short pulses are used. A total of seven types of table combinations are prepared for each of CMYK, and slides 121c, 121m, 121 are prepared.
The selection is made according to y and 121k.

A combination of address table values corresponding to the gradation gap 93 of FIG.
23c, 123m, 123y, and 123k are selected.

The test print button 123 temporarily downloads the combination of the address table values selected by these slides to the address table 62 and prints the test chart 90.

The download button 124 writes the selected address table value in the nonvolatile memory holding the initialization data of the gradation processing device and holds the setting.

FIG. 12 is a diagram showing an example of the configuration of a color laser printer incorporating a controller board 130 having the gradation processing device 9 mounted therein. In FIG. 12, the controller board 130 is mounted on the bottom surface of the printer in parallel with the mechanical section, and is therefore indicated by a broken line.

The gradation processing device of the present invention is the photosensitive belt 1
The input image signal is developed in real time in synchronization with the horizontal synchronizing signal and the vertical synchronizing signal for controlling the operations of the laser 31 and the laser optical device 132, and an electrostatic latent image is formed on the photosensitive belt 131.

When automatically optimizing the address table, the brightness sensor 133 paired with the illumination mechanism is installed close to the intermediate transfer member 134.

The control system of the printer controller is
When the calibration command to the printer is executed,
While changing the combination of the address table values, the test chart printing is repeated with the paper conveyance stopped, and the signal change via the bandpass filter and the differential filter with respect to the change in the reflection brightness of the test chart on the intermediate transfer body is minimized. A combination of address table values is automatically selected and downloaded to the nonvolatile memory holding the initialization data of the gradation processing device.

[0105]

According to the present invention, laser pulse width modulation
In a laser printer equipped with a gradation processing device that uses both (PWM) and halftone dot dither processing, at the same time as the occurrence of line feed dots for some of the input gradation values that cause line breaks in dots due to dither Since a means for switching the laser pulse width in the main scanning direction corresponding to the dot just before the line feed to short is provided, the rapid increase in the density accompanying the line feed of the dot is caused by the decrease in the density due to the reduction of the dot width before the line feed preceding the line. They are offset, and the continuity of gradation change is maintained well.

On the other hand, a user interface is provided so that the user can select and instruct an appropriate value corresponding to the switching amount while checking the output of the test chart, or the test pattern is scanned to change the switching amount of the pulse width. Since the printer is equipped with a sensor for automatically selecting the optimum value, the user can check the variation in the appropriate value of the pulse width switching amount due to the effects of machine differences between printer engines and environmental changes in temperature and humidity. It is possible to correct while checking the output of the test chart, or it is possible to correct automatically by the sensor.

[Brief description of drawings]

FIG. 1 is a resolution 23. to which a gradation processing device of the present invention is applied.
FIG. 6 is a diagram showing a flow of image processing in a color laser printer having 6 dots / mm, that is, a resolution of 600 dpi.

FIG. 2 is a diagram showing a system configuration of a gradation processing device 9 of the present invention.

3 is a flowchart showing an outline of a gradation processing procedure in the gradation processing device 9 of FIG.

FIG. 4 is a diagram showing a correspondence table of laser pulse width modulation (PWM) table reference addresses for a set of an input pixel value ni and a threshold value nc.

FIG. 5 is a diagram showing an example of a method of forming a threshold array from a threshold pattern.

FIG. 6 is a diagram showing an example of a basic threshold pattern 25 forming a screen angle different from the example of FIG.

7 is a diagram showing a setting example of a table of the gradation processing device 9. FIG.

8 is a diagram conceptually showing the effect of gradation processing by the setting of FIG.

9 is a diagram schematically showing a growth pattern of printing dots with respect to an increase in input tone value ni corresponding to the threshold pattern 84 of FIG. 8 and the table setting of FIG.

FIG. 10 is a diagram showing an example of a test chart 90 for assisting a user in optimizing table values to detect gradation skip.

FIG. 11 is a diagram showing an example of a user interface 120 of laser pulse width modulation (PWM) table maintenance software.

FIG. 12 is a diagram showing an example of the configuration of a color laser printer incorporating a controller board 130 in which the gradation processing device 9 is mounted.

[Explanation of symbols]

1 image data 2 input buffer 3 color correction means 5 4 color separation means 7 γ correction means 8 γ correction data 9 gradation processing device 10 dither circuit 11 Threshold array table 12 Laser pulse width modulation (PWM) signal 13 Printer engine 14 Subtraction circuit 16 OR circuit (for Δh overflow) 17 NAND circuit 18 Switching means 19 registers (for network Δ) 22 Laser pulse width modulation (PWM) table 23 Laser pulse width modulation (PWM) generation circuit 27 threshold array 50 registers 51 Comparison circuit 52 registers 53 Comparison circuit 54 registers 55 Comparison circuit 56 offset register 57 AND circuit 58 Adder circuit 59 AND circuit 60 adder circuit 61 Default register 62 address table 64 selection signal 65 selection circuit 66 Address offset calculator 67 Sub address calculation unit 68 Main address calculator 69 selection signal generator 70 selection circuit 71 register 72 OR circuit 80 Highlight area 81a, 81b, 82 areas 83 Interference area 84 Threshold pattern 90 test chart 91c, 91m, 91y, 91k gradation bar 94 scale 120 user interface 121c, 121m, 121y, 121k slides 123 Test print button 124 Download button 130 Controller board 131 photoconductor belt 132 Laser Optical Device 133 Brightness sensor

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H04N 1/23 103 B41J 3/00 M 5C077 (72) Inventor Takashi Onoda 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture F-term in the Digital Media Systems Division, Hitachi, Ltd. (reference) 2C362 AA32 AA36 CA03 CA09 CA14 CA18 CA30 CB73 5B057 AA11 CA01 CA08 CA12 CA16 CB01 CB07 CB12 CB16 CE13 CE18 CH07 CH09 5C051 AA02 CA07 DB02 DB07 DB30 DE FA01 5C062 AA05 AB05 AB22 AC04 AC61 AE04 5C074 AA05 BB03 BB26 DD05 DD07 EE12 FF05 FF15 HH02 5C077 LL19 MP08 NN04 NN08 NN17 PP33 PQ08 PQ12 PQ20 PQ22 PQ23 RR10 RR11 TT03

Claims (9)

[Claims] 1. A laser pulse width modulation (PWM) circuit for controlling gradation in multiple steps by laser pulse width modulation (PWM), and a gradation value of an input pixel is sequentially compared with a threshold value to obtain a gradation value of an output pixel. In the gradation processing method for a laser printer engine including a threshold value array for conversion, the threshold value array has a gradation value of an input pixel with respect to a gradation value of an input pixel that is planarly uniform. Is an array that forms a halftone dot composed of a plurality of output pixels whose area increases with an increase in the pulse width, and the laser pulse width modulation is such that the growth direction of the halftone dot with the increase of the input gradation value is a laser pulse. In some values of the input gradation values that change (line feed) from the scanning direction (main scanning direction) to the direction orthogonal to (sub scanning direction), the main scanning direction is changed as the halftone dots grow in the sub scanning direction. Laser characterized by short switching of laser pulse width Printer engine gradation processing method. 2. A laser pulse width modulation (PWM) circuit for controlling gradation in multiple stages by laser pulse width modulation (PWM), and a gradation value of an input pixel is sequentially compared with a threshold value to obtain a gradation value of an output pixel. In a gradation processing device for a laser printer engine, which comprises a threshold value array table for conversion, the threshold value array table is such that the threshold value of the input pixel is It is an array that forms a halftone dot composed of a plurality of output pixels whose area increases with an increase in the tone value, wherein the laser pulse width modulation is such that the growth direction of the halftone dot with an increase in the input gradation value is At some values of the input gradation values that change (line feed) from the laser pulse scanning direction (main scanning direction) to the orthogonal direction (sub scanning direction), main scanning is performed as the halftone dots grow in the sub scanning direction. Direction to change the laser pulse width to short Characteristic laser printer engine gradation processing device. 3. A laser printer including the gradation processing device according to claim 2, further comprising a user interface for adjusting the switching amount of the laser pulse width. 4. A laser printer including the gradation processing device according to claim 2, wherein a rewritable register that holds the switching amount of the laser pulse width, and a sensor that detects the density inside the laser printer. And a gradation processing device that adjusts the register value based on the output value of the sensor. 5. A digital copying machine equipped with the gradation processing device according to claim 2, further comprising a user interface for adjusting the switching amount of the laser pulse width. 6. A digital copying machine equipped with the gradation processing device according to claim 2, wherein a rewritable register that holds the switching amount of the laser pulse width, and the density is detected inside the laser printer. A digital copying machine comprising: a sensor; and the gradation processing device that adjusts the register value based on an output value of the sensor. 7. A laser facsimile equipped with the gradation processing apparatus according to claim 2, further comprising a user interface for adjusting the switching amount of the laser pulse width. 8. A laser facsimile equipped with the gradation processing device according to claim 2, wherein a rewritable register that holds the switching amount of the laser pulse width, and a sensor that detects the density inside the laser printer. And a gradation processing device for adjusting the register value based on the output value of the sensor. 9. A laser printer engine for calibrating, comprising: a print command means for outputting a gradation test pattern which continuously changes from a minimum density to a maximum density; and a selection means capable of selecting several print conditions. Adjustment software provided in a laser printer main body or a computer connected to the laser printer, paying attention to a density discontinuity point on the test pattern, selecting a printing state in which the density discontinuity point is the smallest, Adjustment software for a laser printer engine, which adjusts the adjustment characteristics.