JP2701310B2 - Halftone image generation method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a halftone image for outputting image information read by a solid-state imaging device or the like as a high-definition, high-gradation image to an electrophotographic laser printer or the like. The present invention relates to a generation method and an apparatus.

[Conventional technology]

Generally, when a halftone image is reproduced by using a dot matrix, the number of gradations and the resolution are in an inverse relationship. That is, when the matrix is enlarged and the number of gradations is increased, the resolution is deteriorated. On the other hand, when the resolution is increased, the number of gradations is reduced. In order to satisfy both the number of gradations and the resolution, it is necessary to miniaturize one pixel forming a matrix. As a method for this, brightness modulation and pulse width modulation in a laser beam printer are known. The luminance modulation controls the amount of light of the laser, and the pulse width modulation controls the lighting time of the laser. Both types of modulation are means for forming fine pixels that are subdivided into multiple values in the laser scanning direction. However, the reproducibility of a fine pixel obtained by dividing one dot subdivided by such means is generally unstable compared to a pixel of one dot not subdivided. For this reason, it is known to reproduce dots by a method of growing dots while concentrating them as much as possible, in particular, a method of growing dots so as to connect them in a straight line like a line screen (Japanese Patent Application Laid-Open No. 61-1986). No. 214662). As a specific means for performing the dot concentration type reproduction, the resolution of the output for recording should be extremely high,
Alternatively, a pseudo line screen has been conventionally performed in consideration of the arrangement of threshold values in a threshold matrix of the dither method. FIG. 7 shows this dot reproduction method.
As shown in FIG. 3A, one pixel is divided into 5 pixels in the main scanning direction of the laser.
A threshold matrix having five fine pixels in one pixel is formed by dividing, and the threshold values are dispersedly arranged in each fine pixel. The numbers in the figure indicate the level of the density threshold, and the lower the numerical value, the lower the density.

[Problems to be solved by the invention]

However, in the conventional method, since the video frequency for ON / OFF control of the laser for recording and the number of rotations of the rotary polygon mirror for scanning the laser are very large, the control is difficult and not practical. In the method (1), since the thresholds are dispersedly arranged, as shown in FIG. 7 (b), dots on the highlight side (that is, pixels with small numerical values) are dispersed and the reproduction becomes poor. In addition to the generation of moire due to the generation of the periodic structure in the sub-scanning direction, a texture in which a part of the screen in the sub-scanning direction is cut off as shown in FIG. I can't. In order to avoid these, it is conceivable to arrange a threshold value connected in the sub-scanning direction. However, in this case, since the number of gradations cannot be increased, image reproduction is significantly deteriorated, which is not preferable.

Therefore, an object of the present invention is to provide a halftone image generating method and apparatus for reproducing a high-quality image with a high number of gradations and a high resolution.

[Means for solving the problem]

In order to achieve the object described above, the present invention provides a halftone image generation method in which one pixel of an input image is an output image with respect to N fine pixels in a main scanning direction. Preparing a threshold pattern memory storing a plurality of thresholds forming a predetermined threshold pattern at the obtained address, preparing an output density memory storing an output density value at an address determined by the output value for the N fine pixels; Adding a correction value corresponding to an error between the input image and the output image to the input image to output a corrected input image; comparing the corrected input image with the plurality of thresholds; Generating said output image comprising said output values for said N fine pixels; said output gray value determined according to said output in said corrected input image and said output gray memory Output the error by comparing the error, calculate a density correction amount of a pixel near a pixel at the current scanning position from the error, and correct the input pixel from the density correction amount to obtain the correction value as the corrected input image. Is calculated, and a halftone image generation method is provided.

According to another aspect of the present invention, there is provided a halftone image generating apparatus which uses one pixel of an input image as an output image with respect to N fine pixels in a main scanning direction. A threshold pattern memory that stores a plurality of threshold values forming a predetermined threshold pattern at an address corresponding to: an output density memory that stores an output density value at an address determined by the output value for the N fine pixels; An adder that adds a correction value corresponding to an error between an image and the output image to the input image to output a corrected input image; and the correction input image output from the adder and read from the threshold pattern memory. A comparator for comparing the plurality of thresholds to generate the output image including the output values for the N fine pixels from one pixel of the corrected input image; An error output circuit that compares the corrected input image output from the output and the output grayscale value read from the output grayscale memory in accordance with the output value output by the comparator, and outputs the error; An arithmetic circuit for calculating a density correction amount of a pixel near a pixel at the current scanning position from the error output from the output circuit, and calculating the correction value to be output to the adder from the density correction amount. A characteristic halftone image generating apparatus is provided.

〔Example〕

FIG. 1 is a block diagram showing the configuration of a system to which the present invention is applied. After image information from an image input device 1 is processed by an image processing device 3, the image information is recorded by an image output device 5. ing. The image input device 1 includes a reading unit such as a solid-state imaging device (CCD), and reads an original image as 8-bit gray values of 0 to 225 at a resolution of, for example, 400 dots / inch (dpi). The read image information is stored in an image memory 2 capable of storing one page in A4 paper size. The image processing device 3 performs MTF correction processing, reduction / enlargement processing, and other image processing on the grayscale values of the image information from the image memory 2, and finally binarizes the data by a halftone image generation processing. The image output device 5 has, for example, a resolution of 2000 dpi in the main scanning direction and a resolution of 400 dpi in the sub-scanning direction.
i has an electrophotographic laser printer. This electrophotographic laser printer receives the image data from the image processing apparatus.
The recording of the halftone image is performed by the digitized signal. The signal control of the entire system as described above is performed by the control device 4.

FIG. 2 is a block diagram of an apparatus for realizing the halftone generation method of the present invention, which is incorporated in the image processing apparatus 3. The main configuration is adder 302, comparator 303, output gray value storage ROM 304,
A subtractor 305 and an arithmetic circuit 307 are fed back from the arithmetic circuit 307 to the adder 302.
The adder 302 has been subjected to various processes by the image processing device 3.
The gradation value data 30 is input, and the correction amount C obtained by the arithmetic circuit 307 is input as described later, and the gradation value data 30 and the correction amount C are added. Comparator 303 has two inputs, one of which has adder 30.
The gray value data 31 obtained by the addition processing by 2 is input. The other input of the comparator 303 has a threshold pattern memory 3
After the threshold data 32 from 01 is delayed by the shift register (SR) 308, it is addressed and input by the clock CLK1. The comparator 303 compares the gray value data 31 with the threshold data 32, and sets “1” (or ON) when the gray value data is equal to or larger than the threshold data, and “0” (or OFF) when the gray value data is smaller. ) Is output and binarized.

FIG. 3 shows examples of the patterns stored in the threshold pattern memory 301. FIG. 4 is a graph showing the densities of these patterns. As shown in FIG. 3, one pixel is divided into five in the main scanning direction of the laser beam to form fine pixels, and a threshold value is arranged for each fine pixel. In this case, the threshold value is one cycle for two pixels, and this cycle is repeated in the main scanning direction. That is, the degree of freedom is 10, and one cycle of the screen is constituted by the ten threshold values. In such a case, the resolution in the main scanning direction is
A 200-line screen is generated for 2000 dpi.

3A and 4A, FIG. 3A shows a saw-shaped pattern in which the threshold value has a stepped shape within one cycle, and FIG.
Is a triangular pattern with the highest central threshold. These two types of patterns generate a line screen that is expressed in gradation by dot growth in the main scanning direction. The selection of the saw pattern and the triangular pattern is considered depending on the characteristics of the printing apparatus. However, the triangular pattern is preferable in terms of the stability of the printing apparatus. In the threshold pattern memory 301, only the threshold data for one cycle is stored, and the comparator 30 is designated by the address of every ten pixels by the clock of CLK1.
Read to 3. The frequency of this clock is CLK2
The frequency is N times (five times in the embodiment) of the clock of FIG. In FIG. 2, reference numeral 309 denotes a shift register.
Delay the binarized data output from 03. In this embodiment, four stages of shift registers 309 are provided. The output gray value storage ROM 304 has an 8-bit output gray value,
Data of a total of 5 pixels (5 bits) of the binarized data up to four pixels before and the original binarized data delayed by the shift register 309 of the stage is read from the comparator 303. The reason why the output gray value is obtained in units of five pixels is as follows.
This is because the ratio N between the resolution (2000 dpi) of the recording device in the main scanning direction and the resolution (400 dpi) of the input device is 5. If the ratio of these resolutions is different, the number of pixels corresponding to the ratio is determined. Is the unit. Therefore, the number of shift registers 309 is also provided according to the ratio.

FIG. 5 shows a lookup table inside the output density storage ROM 304. This figure shows a case where the threshold pattern is a triangular pattern. d1, d2, d3, d4, and d5 are data input from the comparator 303 directly after the binary data binarized by the comparator 303 or after being delayed by the four-stage shift register 309. In this embodiment, five pixels are input as a unit, and the gray value of the output signal is determined by the number of “1” among the five pixels. This is because the output grayscale value is output by dividing an 8-bit input grayscale value (0 to 255) into five. Therefore, for example, when three of the five pixels are “1”, the output grayscale value of 153 is obtained, and all of them are “1”.
In the case of, the output gray value is 255. The subtractor 305 obtains an error generated between the actual output gray value 33 obtained as described above and the input gray value 31 corrected by the adder 31,
Error data 34 is obtained. The error data 34 is divided into data directly input to the shift register 310 and data input to the line buffer 306. And the shift register
The error value e _{4 of} 310 is output to the arithmetic circuit 307. On the other hand, the line buffer 306 is outputted to the arithmetic circuit 307 as an error value e ₃ the error data 34 delayed by one pixel. Further, two shift registers 311 and 312 are connected to the line buffer 306 in series, and one shift register 311 and 312
The delayed error data e ₂ and e ₁ are sequentially output to the arithmetic circuit 307 for each pixel. The arithmetic circuit 307 is provided with a window (Window) 7 shown in FIG.
The four error data e ₁ , e ₂ , e ₃ , and e ₄ are sequentially output into the window 7. In FIG. 6, reference numeral 6 denotes a pixel at the current scanning position, which is located at the coordinates (i, j). The error data e ₁ , e ₂ , e ₃ , and e ₄ are input to each pixel of the window 7 at an input timing. That is, the shift register 31
Error data e ₄ from 0 is assigned to the pixel at coordinates (i−1, j),
The error data e ₃ from the line buffer 306 is (i + 1, j
-1), the error data e ₂ from the shift register 311 in the (i, j-1), the error data e ₁ from the shift register 312 are input to (i-1, j-1 ). The arithmetic circuit 307 calculates such error data e ₁ , e ₂ , e ₃ , e ₄
Is weighted by four weighting factors W ₁ , W ₂ , W ₃ , and W _4, thereby obtaining the correction amount C of the current scanning pixel 6, and feeding it back to the adder 302. The correction amount C is obtained by the following (1).

Here, W ₁ + W ₂ + W ₃ + W ₄ = 1.0, and the weighting factors W ₁ , W ₂ , W ₃ , and W ₄ are set in advance so as to be larger as the pixel is closer to the current scanning pixel 6 and smaller as the pixel is farther away. ing. For example, W ₁ = 0.1, W ₂ = 0.4, W ₃ = 0.1, W ₄ = 0.4
, But is not limited to this. In addition, the size of the window 7 can be appropriately changed. The correction amount C thus obtained is fed back to the adder 302, and the adder 302 corrects the gray value of the current scanning pixel by adding the gray value data 30 of the current scanning pixel and the correction amount C. As a result, even when the degree of freedom of the output pattern is low, the density of the input pixel and the output pixel can be matched and reproduced with a size corresponding to the window size. By repeating the above processing, a high-quality halftone image can be reproduced. This means that even if the degree of freedom is low, the errors generated there are dispersed when the dots on the next pixel and on are turned on, so that errors can be absorbed in a certain minute area, and as a result, the grayscale reproduction between the input pattern and output pattern can be reproduced by the input grayscale. This can be guaranteed within the data quantization number (8 bits). As a result, it is possible to realize halftone generation using a complete line screen using a high-resolution screen pattern that changes only in the main scanning direction.

〔The invention's effect〕

As described above, since the gray value of the pixel at the current scanning position is corrected based on the gray correction amount near the current scanning position from the error of the gray value between the input pixel and the output pixel, the density of the input pixel and the image Can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of a system to which the present invention is applied, FIG. 2 is a block diagram showing an apparatus for implementing the present invention, FIG. 3 is a diagram showing a threshold pattern, and FIG. FIG. 5 is a diagram showing a pattern in a graph, FIG. 5 is a diagram showing a look-up table of an output gradation value storage ROM, FIG. 6 is a diagram showing an error calculation window (Window) for obtaining a correction amount, and FIG. FIG. 7 is a diagram showing a threshold matrix and its output state in dot reproduction of FIG. DESCRIPTION OF SYMBOLS 1 ... Image input device 2 ... Image memory 3 ... Image processing device 4 ... Control device 5 ... Image output device 6 ... Current scanning pixel 7 ... Window 30, 31 … Gray value data 32… Threshold data, 33… Output gray value 34… Error data 301… Threshold pattern memory 302… Adder, 303… Comparator 304… Output gray value storage ROM 305… subtractor 306 ...... line buffer 307 ...... arithmetic circuit 308,309,310,311 ...... shift register C ...... correction amount _{e 1, e 2, e 3} , e 4 ...... error data

Claims (3)

(57) [Claims] 1. A halftone image generating method wherein one pixel of an input image is an output image with respect to N fine pixels in a main scanning direction, wherein a predetermined threshold pattern is assigned to an address corresponding to the N fine pixels. Preparing a threshold pattern memory storing a plurality of threshold values forming the output image, preparing an output density memory storing an output density value at an address determined by the output value for the N fine pixels, the input image and the output image Adding a correction value corresponding to the error of the correction input image to the input image to output a correction input image; comparing the correction input image with the plurality of thresholds; Generating the output image composed of the output value; outputting the error by comparing the corrected input image with the output density value determined according to the output value in the output density memory; Calculating an amount of density correction of a pixel near a pixel at the current scanning position from the error, and calculating the correction value as the corrected input image by correcting the input pixel from the amount of density correction. Halftone image generation method. 2. A halftone image generating apparatus in which one pixel of an input image is an output image of output values for N fine pixels in a main scanning direction, wherein a predetermined threshold pattern is assigned to an address corresponding to the N fine pixels. A threshold pattern memory storing a plurality of threshold values forming an output image, an output density memory storing an output density value at an address determined by the output value for the N fine pixels, and a difference between the input image and the output image. An adder that adds the corrected value to the input image to output a corrected input image, and compares the corrected input image output from the adder with the plurality of thresholds read from the threshold pattern memory. A comparator for generating the output image including the output values for the N fine pixels from one pixel of the corrected input image; and the corrected input image output from the adder. An error output circuit that compares the output grayscale value read from the output grayscale memory in accordance with the output value output by the comparator, and outputs the error; and A halftone image generating apparatus, comprising: an arithmetic circuit that calculates a density correction amount of a pixel near a pixel at a current scanning position and calculates the correction value to be output to the adder from the density correction amount. 3. A latch for serially inputting and outputting in parallel the preceding (N-1) fine pixels out of the N fine pixels sequentially output from the comparator. The output density memory, wherein the N gray scales are composed of the (N-1) fine pixels output in parallel from the latch means and the Nth fine pixels output from the comparator. 3. The halftone image generating apparatus according to claim 2, wherein the pixel is accessed as an address signal.