JPH04284781A - Image forming device - Google Patents

Abstract

PURPOSE:To form always stable density even in an unstable area for image formation based upon electrophotographic method of less than prescribed intermediate density value. CONSTITUTION:An error correcting circuit 206 executes the error correcting processing of remarked picture element data based upon a binary error stored in an error memory 20. The corrected picture element data are sent to a low density part detecting part 201 to detect whether the data part is a low density part or not. An average density computing circuit 203 calculates an average density value from the binary data of near-by picture elements stored in a binary memory 204 and sends the calculated density to an error computing circuit 208 and a binarizing processing circuit 202. The circuit 202 binarizes the corrected picture element data by using the average density calculated by the circuit 203 and outputs binary data. When the remarked picture element is a low density part, a selector 205 outputs the binary signal, and when the picture element is not the low density part, outputs of the corrected image data.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to an image forming apparatus that forms a multivalued image on a photoreceptor by pulse width modulating a laser drive signal for each pixel based on an input multivalued image signal. The present invention relates to an image forming apparatus that inputs data and forms a halftone visible image.

0002

2. Description of the Related Art Laser beam printers employing an electrophotographic method have been attracting attention in recent years as high-speed, low-noise printers, and are mainly used for binary recording of characters, line drawings, figures, and the like. Since such binary recording of characters, figures, etc. does not require halftone processing, the structure of the printer and the circuit for image processing can be simplified.

[0003] Incidentally, in recent years, a method for reproducing halftones using such a binary printer has been developed. As conventional halftone reproduction methods, the dither method, density pattern method, etc. are well known. However, as is well known,
Printers that employ the dither method or the density pattern method have a problem in that high resolution cannot be obtained. Therefore, in recent years, printers have been developed that are capable of forming halftones while employing a binary recording method by driving a laser by pulse width modulating (PWM) an image signal. According to this PWM method, it is possible to form images with high resolution and high gradation, and this technology has become indispensable especially for color image formation.

0004

However, even the PWM type laser beam printer has the following problems. That is, one is related to the instability of image forming density inherent in the electrophotographic technique itself, and the other is related to pulse width modulation of the laser beam.

Generally, characteristics such as exposure sensitivity and residual potential of a photosensitive drum change due to changes in the surrounding environment and the passage of time. Furthermore, the developing density of the developing material (toner, etc.) in the developing device changes greatly due to changes in the amount of charge, etc. of the developing material (toner, etc.). These problems are density instability problems inherent in electrophotographic technology itself. In particular, it has a great influence on the formation of minute concentrations by PWM laser beam printers.

Furthermore, in a PWM type laser beam printer, if the maximum light emission time of a semiconductor laser element per pixel is T (sec), ideally the PWM signal should be 0.
When changing the pulse width between ~T (sec), it is desirable that the laser beam of the semiconductor laser element also emit light during the period of the pulse width. However, in reality, as shown in FIG. 12, there is a difference between the PWM signal shown by f in FIG.
A response delay in emitting/stopping the laser beam as shown by waveform g occurs. For example, it is good if the pulse width of the PWM signal is large to a certain extent, such as pulse width T and t2, but when the pulse width is small, such as pulse width t3, the semiconductor laser element cannot be completely turned on. Further, when the pulse width t4 is reached, the semiconductor laser element is practically not operating.

The beam effect indicated by h in FIG. 12 shows the emission state of the laser beam two-dimensionally. Since the first pixel is black, the laser element is on during the entire period of one pixel. However, the pulse width of the PWM signal is, for example, t3 = 10n
If the time is extremely short like s, not only is there a problem as to whether the beam is actually generated, but even if the beam is emitted, it may be in the extremely unstable region for electrophotographic image formation. Because of this, it is not possible to form a stable concentration.

As described above, in gradation expression using the PWM method, there is a limit to the minimum pulse width that can form density, and if this limit is set to t3 = 10 ns, the pulse width below this (highlight area) will be limited. All gradations will be white.

[0009]

[Means for Solving the Problems] The present invention has been made for the purpose of solving the above-mentioned problems, and has the following configuration as a means for solving the above-mentioned problems. That is, in an image forming apparatus that forms a multivalued image on a photoreceptor by pulse width modulating a laser drive signal for each pixel based on an inputted multivalued image signal, the pixel density of the inputted multivalued image signal is a determining means for determining whether or not the value is equal to or less than a predetermined intermediate density value "M";
and a binarization means for converting the image density value of the input multivalued image signal into a density value of either "M" or "0" when it is detected that the image density value is less than or equal to "M", The binarization means performs binarization processing using a predetermined intermediate density value unique to each color when the input multi-value image signal is composed of a plurality of color signals.

0010

[Operation] With the above configuration, even when forming a density in an unstable area in image formation by electrophotography below a predetermined intermediate density value, it is possible to determine whether the density is formed at the predetermined intermediate density value [M] or whether density formation is not performed [0]. Since it is binarized into two values, it is possible to always form a stable concentration.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the accompanying drawings. [Description of Schematic Structure (FIG. 1)] FIG. 1 is a block diagram showing a schematic structure when the present invention is applied to a laser beam printer.

In the figure, 200 is a low-density binarization processing circuit (details will be described later) that binarizes pixel data of a predetermined intermediate density or lower, and 300 is an image forming unit (having a photosensitive drum 301). ), 400 is P for pulse width modulating the image signal
WM circuit, 500 is a laser driver circuit, and 501 is a semiconductor laser element. In this configuration, for example, an 8-bit input image signal A having normal 256 gradations is converted by the low-density binarization processing circuit 200 into an 8-bit output image signal B suitable for a PWM method laser beam printer. . Further, this image signal B is input to a PWM circuit 400, a laser driver circuit 500, a semiconductor laser element 50
1, a high gradation image is formed by electrophotography in the image forming section 300.

[Description of Structure of Image Forming Section (FIGS. 2 to 4)] First, the structure of the electrophotographic printer mechanism of this embodiment will be explained with reference to FIG. The printer mechanism section is indicated by arrow A,
Chargers 302 are sequentially arranged in the rotational direction of a photosensitive drum as an image carrier that is rotationally driven in the direction B, and chargers 302 for four colors (Y, M,
Developing device 303 and transfer drum 3 for each color (C, BK)
04, a cleaning device 305 and an optical system 306 disposed above the photosensitive drum 301 (at the top of the drawing).

The optical system 306 includes a semiconductor laser section (not shown), a polygon mirror 307 that rotates at high and low speeds,
It is composed of an f-θ lens 308, a light shielding plate (not shown), and the like. The semiconductor laser section outputs a laser beam by being oscillated by a laser drive signal that is PWM-modulated in response to time-series digital pixel signals that are calculated and output by an image reading device, electronic calculator, etc. (not shown). are doing.

This laser beam is transmitted through a polygon mirror 307.
is irradiated. Since the polygon mirror 307 is rotating at a high and constant speed, the laser beam irradiated on the side surface of the polygon mirror 307 passes through the drum surface between the charger 302 and the developer 303 in the direction of the generatrix of the drum 301 (in the drawing). scanning (exposure) in the vertical direction). Next, with reference to FIGS. 3 and 4, in response to time-series digital pixel signals calculated and output by an image reading device, electronic calculator, etc. (not shown),
Details of a pulse width modulation (PWM) circuit and a laser driver circuit that generate a PWM modulation signal for causing a semiconductor laser section to emit light will be described.

In FIG. 3, 401 is a TTL latch circuit that latches an 8-bit digital image signal, and 402 is a TTL latch circuit that latches an 8-bit digital image signal.
A level converter converts the TL logic level to a high-speed ECL logic level, 403 is a D/A converter that converts the ECL logic level to an analog signal, 404 is an ECL comparator that generates a PWM signal, and 405 converts the ECL logic level to the TTL logic level. It is a level converter that converts to .

406 is a clock oscillator that generates the clock signal 2f; 407 is a triangular wave generator that synchronizes with the clock signal 2f to generate a substantially ideal triangular wave signal; and 408 is a pixel clock that divides the clock signal 2f by half. This is a 1/2 frequency divider that generates the signal f, and the 1/2 frequency divider 408 converts the clock signal 2f into a signal having twice the period of the pixel clock signal f. In this embodiment, ECL logic circuits are arranged everywhere in order to operate the circuit at high speed.

FIG. 12 shows the operation of a circuit having such a configuration.
This will be explained with reference to the timing chart. Signal a represents a clock signal 2f, and signal b represents a pixel clock signal f having a period twice that of the clock signal 2f, which are related to the pixel signals as shown. Furthermore, inside the triangular wave generator 407, in order to maintain the duty ratio of the triangular wave signal at 50%, the clock signal 2f is once frequency-divided by 1/2 and then the triangular wave signal c is generated.

Furthermore, this triangular wave signal c has an ECL level (
0 to -1V) and becomes a triangular wave signal d. on the other hand,
The input image signal is a signal with 256 gradations from 00H (white) to FFH (black), and the input level also changes in 256 levels. Note that the symbol "H" indicates hexadecimal representation. The ECL voltage level obtained by D/A converting this input image signal with respect to several image signal values is shown as an image signal e. Note that this image signal is at the ECL voltage level. For example, the first pixel has a black level of FFH, the second pixel has a halftone level of 80H, and the third pixel has a halftone level of 40H.
H and the fourth pixel indicate voltages of 20H at the intermediate tone level.

The comparator 404 generates a PWM signal having pulse widths T, t2, t3, t4, etc. according to the pixel density to be formed by comparing the triangular wave signal d and the image signal e. This PWM signal is then converted to a TTL level of 0V or 5V to become a PWM signal f, which is input to the laser driver circuit 500. Next, the detailed configuration of the laser driver circuit will be explained with reference to FIG.

In FIG. 4, 500 is a constant current type laser driver circuit, and 501 is a semiconductor laser element. This semiconductor laser element 501 is a switching transistor 50
When transistor 2 is on, it emits laser light, and when transistor 502 is off, it stops outputting the laser light. This switching transistor 502 forms a current switch circuit together with the transistor 504 that forms a pair with it, and controls on/off (commutation) of the current to be passed through the semiconductor laser element 501 according to the input PWM signal f. .

The current flowing through the laser element 501 is controlled to be constant by a transistor 505. Furthermore, this constant current value can be set to any current value by changing the base potential of the transistor 505. That is, the input 8-bit laser power value is input to the D/A converter 5.
The laser drive current value corresponding to the laser power value is determined by converting the voltage into an analog voltage in step 03 and inputting the voltage value obtained by comparing this voltage with a reference voltage to the base of the transistor 505.

[Description of low-density portion binarization processing circuit (Fig. 5~
[Fig. 9)] In order to simplify the explanation, in this example, the minimum pulse width of PWM that allows the laser beam printer to form a stable density is set to t3 = 10 ns, for example.
Assuming that, the corresponding predetermined halftone density value “M” is M
=30. Here, "30" is a numerical value in decimal system, and in this example, it is 0 to 255 levels, that is, 25
It is set to "30" out of 6 gradations.

FIG. 5 shows the low density portion binarization circuit 20 of this embodiment.
FIG. 2 is a block configuration diagram showing details of 0. In Figure 5,
When the image data "G" of the pixel of interest is input, the binarization error to be distributed to the pixel of interest stored in the error memory 207 is given, and error correction processing is performed in the error correction circuit 206. .

Next, the image data "G" of the pixel of interest that has been subjected to the error correction process is sent to the low-density portion detection circuit 201.
Based on the predetermined intermediate density value M, the low density part (0 to M-1
level) is detected. Then, when it is detected that the pixel of interest is a low density portion, the average density calculation circuit 203 calculates the average density (S) from the binary data of neighboring pixels stored in the binary memory 204. This average density (S) is sent to the error calculation circuit 208 and the binarization processing circuit 202.

The binarization processing circuit 202 performs a binary judgment on the pixel of interest using the average density (S) calculated by the average density calculation circuit 203 as a threshold, and as a result, the binary output value is "0".
or “M” is output. The selector 205 selects an error correction circuit 206 according to the detection result from the low density portion detection circuit 201.
Either the signal from the output signal or the signal from the binarization processing circuit 202 is selected and output as image data "G'". That is, when the pixel of interest is in a low density area, the signal from the binarization processing circuit 202 is output as image data "G'".

On the other hand, the error calculation circuit 208 calculates the binarization error generated in this binarization process, and stores the error memory 20 in order to distribute the calculation result to adjacent pixels when processing the current pixel.
Store within 7. Note that if it is determined that the image data of the pixel of interest is (M~255) in the low density area, the selector 205 selects the image data (M~255) and outputs it as image data B as is. Become. Note that in this case, since binarization processing is not performed, binarization error correction is not performed either.

The flow of the above processing is shown in the flowchart of FIG. By performing the processing shown in FIG. 6 on the input continuous 256-level gradation data of 0 to 255 levels using the above method, 0 and M to 255 levels, that is, 1 to (M-1) levels, are processed. This will be output as image data that does not use any data.

Details of the above-mentioned binarization processing and details of distributed control of errors generated by the binarization processing will be explained below with reference to FIGS. 7 to 9. FIG. 7D shows image data input to the low-density binarization processing circuit 200, such as image data (including corrected data) from an external host computer or image reader.

FIG. 7(c) shows the input image data “G(
This shows the conversion sequence "L (i, j)" of the image data after only the low-density part is detected based on The conversion is performed based on a predetermined halftone density "M" that takes reproducibility into consideration.Hereinafter, it will be explained in detail using FIG.

FIG. 7(d) is the input image data string “G
(i, j)'', the pixel of interest G is indicated by a thick frame in the figure. Here, the pixel of interest G is compared with the predetermined halftone density value M (M=30), According to the conditions, only low density values below the predetermined halftone density value "M" are detected (1).Furthermore, if a high density value having a value above "M" is input, as will be described later (Fig. 9). ), but (2), the compared predetermined halftone density value "M" is input.

According to FIG. 7(c), when the pixel of interest G is detected to be a low-density portion, the pixel of interest G is binarized using the average density value calculated from neighboring pixels as a threshold. Processing takes place. The average density is calculated using binary data near the pixel of interest (
As the weighted average value obtained from Fig. 7(b)), Fig. 7
Calculation is performed using the weight mask shown in (a).

[0033] The weight mask (a) is used for the neighborhood 1 to the pixel of interest.
Two pixels have a constant gradient, and the sum of the gradients is equal to a predetermined halftone density value M (M=30). The average density S is calculated as follows by superimposing the neighborhood binary data (b) and the weight mask (a). S=6×0+10×0+5×1+9×1=14 Using this average density S=14 as a threshold, the pixel of interest G=20 is subjected to a binary determination. FIG. 8 illustrates this binary determination.

In FIG. 8, the average density S is shown as a variable threshold value that changes between 0←→M based on the information of binary data (b) near the pixel of interest. Now, since the average density S is calculated as S=14 in the above example, it is compared with the density data G=20 of the pixel of interest, and a binary determination is made as follows. G > S → B=1 20>14 → B=1 Based on the above results, binary data “B(i,j)” (b
), the result of the binary judgment (B=1) is input.

Based on the output result of this binary determination, image data of the pixel of interest is output. That is, B=1
→ G'=30 B=0 → G'= 0, and the target pixel G=20 has been binarized to G'=30. This image data is selected by a selector (selected only when it is determined to be a low density portion), and image output data "G'(i,j))" is output.

[Binarization error correction] The binarization error is the difference value between the density G of the pixel of interest and the average density S used as a threshold value. By distributing the density to adjacent pixels and performing density correction, the low density area 2
The density of the input image will be saved on the image after the value conversion process.

In FIG. 8, the binarization error “e” is e=
G-S (e=20-14=6), and the error is corrected by distributing this error to adjacent pixels as shown in FIG. 7(d). The result of this error correction is the corrected input data shown in FIG. 7(h).

25+1/(2e)=28 (Adjacent pixel correction) 30+1/(2e)=33 (Adjacent pixel correction) FIG.
, the density value of the pixel of interest G is a predetermined intermediate density value M(M-3
0) is shown. Figure 9(d)
In the input data shown in , the density value of the pixel of interest G is G≧
If it is determined that the input data is M, the input data "G(i,j)" is selected by the selector (h) and is output as is as the output data "G'(i,j)". That is, when the input data is at a value between M and 255 levels, the value is output as is, and the density value within this range is stored as is. In this case, no binarization process is performed and no binarization error occurs. Therefore, error correction is not performed.

Furthermore, when it is determined that G≧M, the predetermined intermediate density M (M
= 30) is input, so B = 1 is always input as binary data in the subsequent binary judgment process (Fig. 9(b)
)). In other words, the density value greater than or equal to the intermediate density M is always B=1.
A binary determination is made, and the data is input as binary data. These binary data in FIGS. 7(b) and 9(b) are used as neighborhood binary data (b) for calculating the average density value when the next pixel of interest is subjected to binary determination. As a result, a binary memory is required to delay the weight mask pattern by two lines from the weighted mask pattern of FIGS. 7 and 9(b).

In the above embodiment, the minimum pulse width of PWM that can form a visible image was fixed as t3 = 10 ns (=predetermined intermediate density value 30), but the scope of application of the present invention is limited to this. This value can be changed for each color: yellow (Y), magenta (M), cyan (C), and black (Bk). In particular, B
Regarding k, it is possible to set a smaller halftone density value "MBk" for the halftone density values "MY", "MM", and "MC" set for each of the other Y, M, and C colors. is important.

For example, suppose that the minimum pulse width of PWM that can form a visible image is t3 = 10 ns, and the density value is <3.
0>, a more stable image forming area (0,40 to 255
), the gradation reproducibility in highlight areas is more stable and image noise such as roughness can be reduced.

In particular, in a color image forming apparatus that places importance on gradation reproducibility, uniformity, color balance (gray balance), etc. in highlight areas, it is important to is the predetermined halftone density value “MY
”, “MM”, and “MC” are important for stable formation of color images. However, if “M” is set too high to stabilize tone reproduction, characters and There is a possibility that the reproducibility of fine lines will deteriorate.
``The above-mentioned binarization process will be performed even on thin text information that has density data lower than the value, and there is a possibility that the text density data of that pixel will be distributed to surrounding pixels.
This is because the sharpness of the characters may be reduced as a result.

Therefore, in black (Bk), where reproduction of characters, thin lines, etc. is important, it is important to set the minimum density value "M" as low as possible. That is, instead of making all the predetermined halftone density values "M" of each color the same,
In the case of black (Bk), which is used to express characters, thin lines, etc., the minimum density value “M” is [MY, MM, MC >
By setting it to a low value such as MBk], stable gradation reproduction can be achieved in color image areas, and clear line reproduction can be reestablished in black character areas.

[0044] By the above method, the input 0 to 255
By performing the processing shown in Figure 3 on the 256-level continuous gradation data, it can be output as image data that does not use data from 0 and M to 255 levels, that is, from 1 to (M-1) levels. becomes. As a result, even in laser beam printers using electrophotographic processes,
By arbitrarily setting the midtone density value "M" that enables stable visible image formation, it is possible to reproduce the density of highlight areas without using the unstable image forming area unique to the electrophotographic process. This enables stable gradation reproduction in all density ranges.

[0045]

[Other Embodiments] FIGS. 10 and 11 show other embodiments according to the present invention. FIGS. 10 and 11 are diagrams showing examples of weight mask patterns for calculating the average density used as a threshold value during binarization determination. The wider the area of the weighting coefficient distribution for average value calculation, the greater the edge enhancement effect appears, so by expanding the mask pattern from FIG. 10 to FIG. 11, it is possible to strengthen the edge enhancement effect. .

Further, as a binarization means, although the surrounding average density is used as a threshold value in the present invention, the present invention is not limited to this, and a fixed threshold value within a value of "0 to M" (for example, "M
/2'', etc.) is also effective as the present invention. It goes without saying that the present invention is effective regardless of whether or not the binarization error correction processing is performed.

The present invention may be applied to a system made up of a plurality of devices, or to an apparatus made up of one device. It goes without saying that the present invention can also be applied to cases where the present invention is achieved by supplying a program to a system or device.

[0048]

As explained above, according to the present invention, it is possible to form the density of the highlight portion of an output image stably, and characters can also be reproduced clearly.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a schematic configuration of a laser beam printer according to an embodiment of the present invention.

FIG. 2 is a diagram showing the detailed configuration of an electrophotographic printer mechanism adopted in the apparatus of this embodiment.

FIG. 3 is a diagram showing a detailed circuit configuration of a PWM circuit adopted in the device of this embodiment.

FIG. 4 is a diagram showing the detailed configuration of a laser driver circuit adopted in the device of this embodiment.

FIG. 5 is a block diagram showing the detailed configuration of the low-density portion binarization circuit of this embodiment.

FIG. 6 is a flowchart showing image data conversion processing in this embodiment.

FIG. 7 is a diagram illustrating an operation process when validating the binarized data from the low-density portion binarization circuit in the low-density region of the present embodiment.

FIG. 8 is a diagram illustrating binary determination processing in this embodiment.

FIG. 9 is a diagram illustrating an operation process when validating input data in a case other than a low concentration region according to the present embodiment.

[Figure 10] and

FIG. 11 is a diagram showing an example of a weight mask pattern according to another embodiment of the present invention.

FIG. 12 is a timing chart showing the operation of the PWM circuit.

[Explanation of symbols]

200 Low density part binarization circuit, 202 Binarization processing circuit, 203 Average concentration calculation circuit, 204 Binary memory, 205 Selector, 206 Error correction circuit, 207 Error memory, 208 Error calculation circuit, 300 Image forming section, 301 Photosensitive drum, 400 PWM circuit, 500 Laser driver circuit, 501 Semiconductor laser element

Claims (4)

[Claims] 1. An image forming apparatus that forms a multi-value image on a photoreceptor by pulse width modulating a laser drive signal for each pixel based on an input multi-value image signal, the image forming apparatus comprising: The pixel density value of the image signal is a predetermined intermediate density value "M"
a determination means for determining whether or not the image density value of the input multi-valued image signal is equal to or less than a predetermined intermediate density value "M"; and a binarization means for converting the density to either "M" or "0", and the binarization means has a predetermined value unique to each color when the input multi-valued image signal is composed of a plurality of color signals. An image forming apparatus characterized by performing binarization processing using an intermediate density value. [Claim 2] The binarization means uses a density value between "0" and "M" as a fixed threshold when performing the binarization process, and when the input multivalued image has four colors of magenta, yellow, cyan, and black. 2. The image forming apparatus according to claim 1, wherein the binarization threshold for black is set to a lower density value than the binarization thresholds for other colors. [Claim 3] The binarization means performs 2 bits when performing the binarization process.
2. The image forming apparatus according to claim 1, wherein the binarization process is performed using an average density value obtained from the density values of neighboring pixels of a pixel to be converted into a value as a threshold value. 4. The image forming apparatus according to claim 3, wherein the binarization means corrects the density value of the pixel to be binarized later based on a binarization error generated when performing the binarization process. .