JP2001177722A - Image forming method, image processor and storage medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processing method and an image processor by which gradation and stability are excellent in low and medium gray scale parts and void is not caused in a high gray scale part. SOLUTION: In the image forming method where a multi-level image is quantized by an error propagation method and each pixel of a quantized image is expressed in dots, employing a dither threshold of a pattern where dot distribution type areas with higher thresholds are arranged around dot concentration type areas (half-screen-processed parts) shown in e.g. Figure 2 periodically changes quantization thresholds to produce dots concentratingly in the low medium gray scale parts of the multi-gradation image and to produce dots distributingly to surrounding of the concentrated dots in the high gray scale part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to various apparatuses which handle images such as copiers, printers, scanners and faxes, and more particularly to an image forming method and an image processing apparatus utilizing an error diffusion method.

[0002]

2. Description of the Related Art For example, in a laser printer, a digital copying machine, and other various image processing apparatuses, a dither method or an error diffusion method is generally used to simulate the gradation of an image.

[0003] In general, the dither method has an advantage of being excellent in graininess and being able to smoothly express a halftone image, but also has a disadvantage. For example, in the dither method (area gradation method represented by), the resolution deteriorates in order to obtain gradation. Further, in the dither method for generating a periodic image, moire tends to occur in a printed image such as a halftone dot.

On the other hand, the error diffusion method can obtain a resolution faithful to an original image and is suitable for reproducing a character image. However, in a halftone image such as a photograph, isolated dots are dispersed or irregularly connected and arranged, so that granularity is poor and a unique texture may be generated. Particularly, in an electrophotographic printer, an image is unstable because an image is formed by isolated dots, and deterioration in granularity and banding due to uneven density are likely to occur.

With respect to the error diffusion method, in order to improve texture due to irregular connection of dots, a dither threshold is used as a quantization threshold, and a method of improving texture by disturbing dot connection is described below. Such improved techniques have been proposed. (1) For the purpose of eliminating the occurrence of pseudo contours and unique stripe patterns, a dither threshold is used, and the larger the edge amount, the greater the amount of error diffusion (JP-A-3-34772). (2) A fixed threshold value is used for an edge portion of an image, a variation threshold value is used for a non-edge portion, and a variation threshold level is used for the purpose of preventing white spots in non-edge low-density portions and preventing the occurrence of character notches. Is reduced as the concentration becomes lower (Japanese Patent No. 2755)
No. 307). (3) In order to prevent the occurrence of moiré and false contours when using a multi-valued printer having three or more values, an edge portion of the image
A dither signal having a size corresponding to the edge amount is added to the image data, and a fixed value is added to the image data in the non-edge portion, and the added image data is subjected to multi-level quantization using a fixed threshold (Japanese Patent No. 2801195). issue). (4) In order to smoothly reproduce an image portion in which the density changes smoothly and to reproduce fine lines and characters with enhanced sharpness, the non-edge portion has a large number of quantization levels (for example, 16
The quantization result is selected by the quantization means (value), and the quantization result by the quantization means having a small number of quantization levels (for example, six values) is selected at the edge portion (Japanese Patent No. 2851662).
issue).

[0006]

The present invention relates to an improvement in an image forming method and an image processing apparatus using an error diffusion method.
Image formation with good gradation and stability at low / medium density levels, image formation with few white spots at high density levels, character parts of images, line drawing parts, halftone dot image parts with relatively low frequency It is an object of the present invention to provide a novel image forming method and an image processing apparatus which enable high-resolution image formation and smooth reproduction of a joint portion between regions having different image characteristics.

[0007]

The main feature of the image forming method according to the present invention is that a multi-tone image has a low image quality.
2. An image having good gradation and stability at a medium density level and hardly causing white spots at a high density level is formed.
As described above, in an image forming method in which a multi-tone image is quantized by an error diffusion method and pixels of the quantized image are represented by dots, a low-medium The quantization threshold is periodically set so that the dots are concentrated in the density portion, and the dots are concentrated in the high density portion of the multi-tone image, and the dots are dispersed around the dot. Is to fluctuate. Other purposes and features are as follows. 3. The feature of the invention according to claim 2, wherein by switching the processing from the error diffusion tone to the dither tone according to the image feature, it is possible to form an optimal image for a multi-tone image having various image features. In the image forming method according to the first aspect of the present invention, the variation range of the quantization threshold is changed according to the characteristics of the multi-tone image. In order to obtain a high resolution by performing error diffusion based processing in a character portion, a line drawing portion, or the like of a multi-tone image, the feature of the invention according to claim 3 is as follows.
The purpose is to reduce the fluctuation range of the quantization threshold value in a region where the edge degree of the multi-tone image is large. In order to smoothly reproduce a boundary portion between image regions having different image characteristics, a feature of the invention according to claim 4 is that, in the image forming method according to claim 2, the average value of the quantization threshold is set to a substantially constant value. Is to keep. The feature of the invention according to claim 5 is that the dots are generated in a concentrated manner in the low / medium density part of the multi-tone image, and the dots are generated by being dispersed around the concentrated dots in the high density part. In the image forming method according to the first aspect of the present invention, using a dither threshold matrix composed of a dot concentration type area and a dot dispersion type area around the dot concentration type area having a larger threshold value,
That is to vary the quantization threshold periodically. In order to reduce the continuation of white pixels in the high-density portion, the feature of the invention according to claim 6 is that, in the image forming method according to claim 1, a dot having a larger threshold value in a concentrated area and its periphery is provided. The quantization threshold value is periodically changed by using a dither threshold value matrix that is arranged and shifted by a half phase in the main scanning direction or the sub-scanning direction in a main scanning direction or a sub-scanning direction. In order to enhance the concentration of dots in the low / medium density part and to improve the gradation and stability of the image, the feature of the invention according to claim 7 is the feature of the invention according to claim 5 or 6. In other words, the difference between the minimum threshold value in the dot dispersion type area of the dither threshold value matrix and the maximum threshold value in the dot concentration type area is made larger than the step width of the threshold value in the dot concentration type area. -To vary the quantization threshold with an appropriate vibration width according to the image characteristics of the multi-tone image, the feature of the invention according to claim 8 is:
7. The image forming method according to claim 5, wherein a value obtained by adding a constant value to a value obtained by multiplying a threshold value of the dither threshold value matrix by a coefficient corresponding to a feature of the multi-tone image is used as a quantization threshold value. is there. -To vary the quantization threshold with an appropriate vibration width according to the image characteristics of the multi-tone image, the feature of the invention according to claim 9 is
In the image forming method according to the fifth or sixth aspect, a dither threshold matrix for periodically changing a quantization threshold is switched according to image characteristics of a multi-tone image. The feature of the invention described in claim 10 is that the feature of the invention described in claim 10 is the image formation according to the invention described in claim 8 or 9 in order to enhance the resolution by using error diffusion-based processing in a character portion or a line drawing portion of a multi-tone image. In the method, the fluctuation range of the quantization threshold is reduced in a region having a large edge degree in the multi-tone image. In order to obtain a high resolution by error diffusion processing of a fixed threshold value in a character portion or a line drawing portion of a multi-tone image, the feature of the present invention according to claim 11 is the following. This is to fix the quantization threshold to a constant value in a region where the degree of edge of the toned image is maximum.

The main feature of the image forming method according to the present invention is that the generation of small dots is suppressed in low / medium density parts to enhance the stability of the image, and the number of dots is increased in high density parts. In order to prevent white spots, the multi-tone image is quantized by n values (n ≧ 3) by an error diffusion method, and the pixels of the quantized image are increased as the quantization level is higher. In an image forming method for expressing by dots, m-value (m <n) is quantized in a low / medium density part of a multi-tone image. Other purposes and features are as follows. In order to perform such quantization, a feature of the invention described in claim 13 is that in the image forming method according to claim 12, the image forming method according to claim 12 periodically performs quantization on an image space for quantization of a multi-tone image. By using (n-1) quantizing thresholds that fluctuate, at least two quantizing thresholds corresponding to low and medium density levels of a multi-tone image have the same value at a specific position within the fluctuation cycle. is there. The method according to claim 1, wherein dots are concentrated in low and medium density areas to enhance gradation and stability, and high density areas are dispersed around the concentrated dots to generate dots to prevent white spots.
According to a fourth aspect of the present invention, in the image forming method according to the thirteenth aspect, the specific position is a central portion in a fluctuation cycle of the quantization threshold. 16. The feature of the invention according to claim 15, wherein by switching the processing from an error diffusion tone to a dither tone according to the image characteristics, it is possible to form an optimal image for a multi-tone image having various image characteristics. According to a thirteenth aspect of the present invention, in the image forming method according to the thirteenth aspect, the variation range of the quantization threshold is changed according to the image characteristics of the multi-tone image. The invention according to claim 16 is the image forming method according to claim 15, in which the resolution is improved by processing the error diffusion tone in a character portion or a line drawing portion of the multi-tone image. Is to decrease the fluctuation width of the quantization threshold value as the edge degree of is larger. In order to smoothly reproduce a boundary portion between image areas having different image characteristics, the feature of the invention according to claim 17 is as follows.
In the image forming method according to the invention described above, an average value of the quantization threshold is kept substantially constant. In order to suppress the generation of small dots in the low and medium density parts and increase the number of dots in the high density parts, the feature of the invention according to claim 18 is that in the image forming method according to claim 13,
Using a plurality of dither threshold matrices composed of a binarized region and an n-valued region, periodically changing (n-1) quantization thresholds are generated, and an m-valued region of at least two dither threshold matrices is generated. The thresholds at the same position in are equal. In order to reduce the continuity of white pixels in a high density portion and to prevent white spots, the feature of the invention according to claim 19 is the image forming method according to claim 13, wherein the m-valued area and the n-valued area are used. And a plurality of threshold matrices composed of (n−) are shifted and shifted by a half phase in the main scanning direction or the sub-scanning direction.
1) generating (n-1) periodically varying quantization thresholds using dither threshold matrices, and generating at least 2
The threshold at the same position in the m-valued area of the two dither threshold matrices is equal. The feature of the invention according to claim 20 is that in order to prevent white spots in a high-density part and to prevent small dots from being generated in a low-medium-density part to enhance the stability of an image, In the image forming method of the invention described above, the threshold value at the same position in the m-valued area of all dither threshold value matrices is equalized. The feature of the invention according to claim 21 is that the dots are concentrated in low / medium density parts to enhance gradation and stability, and white spots are prevented in high density parts. In the image forming method of the present invention, n in the dither threshold matrix
This is to arrange the value conversion area around the m-value conversion area. In order to suppress the occurrence of small dots in low / medium density parts and to enhance the stability of the image, the feature of the invention according to claim 22 is that the image forming method according to claim 21 is characterized in that In other words, the maximum threshold value in the binarized region is set to be smaller than the minimum threshold value in the n-valued region. The feature of the invention according to claim 23 is to enhance the gradation and stability by increasing the degree of concentration of dots in the low / medium density portion and suppressing the occurrence of small dots. In the image forming method of the present invention, the difference between the minimum threshold value in the n-valued area and the maximum threshold value in the m-valued area of the dither threshold matrix is made larger than the step width of the threshold value in the m-valued area. is there. The feature of the invention according to claim 24 is to increase the probability of occurrence of small dots in the high-density portion, to prevent white spots, and to enhance the gradation property, by the invention according to any one of claims 18 to 23. In the image forming method of (1), all the thresholds in the n-valued area of the dither threshold matrix for generating the highest quantization threshold are set to constant values larger than the thresholds in the n-valued areas of the other dither threshold matrices. That is. In order to optimize the quantization threshold according to the image characteristics of the multi-tone image, the invention according to claim 25 is based on claims 18 to 24
In the image forming method according to any one of the above aspects, the dither threshold matrix used for generating each quantization threshold is switched according to the image characteristics of the multi-tone image. The feature of the invention described in claim 26 is that the multi-level image forming method according to claim 25 is characterized in that the multi-gradation image has a high resolution by performing processing based on error diffusion in a character portion or a line drawing portion. The purpose is to reduce the fluctuation range of the quantization threshold value in an area where the edge degree of the toned image is large. The feature of the invention according to claim 27 is to generate only large dots from low / medium density part to high density part and obtain a stable image without white spots in the character part and line drawing part of the multi-tone image. In the image forming method according to the twenty-sixth aspect, the same dither threshold matrix composed entirely of m-valued areas is used for generating all quantization thresholds in an area where the degree of edge of the multi-tone image is large. .・ In order to obtain an image having excellent resolution and stability in a character portion or a line drawing portion of a multi-tone image by performing a binary error diffusion process of a fixed threshold from a low / medium density portion to a high density portion. A feature of the invention described in Item 28 is that in the image forming method according to Item 26, all quantization thresholds are fixed to the same constant value in a region where the degree of edge of a multi-tone image is maximum.

The image processing apparatus according to the present invention as described in claims 29 to 42 enables image formation by the image forming method according to claims 1 to 11, and the main feature of the image processing apparatus is as described in claim 29. As described above, the first means for adding an error to the multi-gradation image data, the second means for quantizing the image data after the addition of the error by the first means, the quantized data by the second means, A third means for obtaining an error to be added to the multi-tone image data from the image data after the error addition by the first means and providing the error to the first means; and a quantization threshold for the second means in an image space. And a fourth means for periodically varying the above, so that the quantization threshold becomes a dot concentration type within a value range corresponding to the low / medium density level of the multi-tone image data within the variation cycle. Fluctuating and multi-tone image data And to vary as a dot dispersion type with a value range corresponding to the density level. Other features are as follows. A feature of the invention according to claim 30 is that, in the image processing apparatus according to claim 29, the fourth means changes the variation range of the quantization threshold according to the image characteristics of the multi-tone image data. . A feature of the invention described in claim 31 is that in the image processing apparatus according to the invention described in claim 30, the fourth means reduces the fluctuation range of the quantization threshold as the edge degree of the multi-tone image data increases. is there. A feature of the invention described in claim 32 is that, in the image processing apparatus according to the invention described in claim 30, the fourth means keeps the average value of the quantization threshold at a substantially constant value. According to a thirty-third aspect of the present invention, in the image forming apparatus according to the thirty-ninth aspect, the fourth means comprises a dithered area comprising a dot concentration type area and a dot dispersion type area surrounding the dot concentration type area having a larger threshold value. This is to vary the quantization threshold using a threshold matrix. According to a thirty-fourth aspect of the present invention, in the image processing apparatus of the thirty-ninth aspect, the fourth means comprises a threshold value comprising a dot concentration type area and a dot distribution type area around the dot concentration type area having a larger threshold value. This is to vary the quantization threshold using a dither threshold matrix which is arranged and shifted by a plurality of matrices in the main scanning direction or the sub-scanning direction by a half phase and is enlarged. -The feature of the invention described in claim 35 is the feature of claim 33 or 34.
In the image processing apparatus of the invention described above, the difference between the minimum threshold value in the dot dispersion type area and the maximum threshold value in the dot concentration type area of the dither threshold value matrix is made larger than the step width of the threshold value in the dot concentration type area. That is. -The feature of the invention described in claim 36 is the feature of claim 33 or 34.
The image processing apparatus of the invention described above further comprises a fifth means for extracting and outputting image characteristics of the multi-tone image data,
The means is to change a variation range of the quantization threshold according to the image feature output from the fifth means. According to a thirty-seventh aspect of the present invention, in the image processing apparatus according to the thirty-sixth aspect, the fourth means sets a coefficient corresponding to the image feature output from the fifth means to a threshold value of the dither threshold value matrix. A value obtained by adding a constant value to the multiplied value is set as a quantization threshold. According to a thirty-eighth aspect of the present invention, in the image processing apparatus according to the thirty-fifth aspect, the fourth means changes the quantization threshold in accordance with the image feature output from the fifth means. Switching the matrix. The feature of the invention described in claim 39 is the feature of claim 37 or 38
In the image processing apparatus according to the aspect of the invention, the fifth means outputs the edge degree of the multi-tone image data as an image feature, and
The means is to reduce the fluctuation range of the quantization threshold as the edge degree output from the fifth means is larger. A feature of the invention according to claim 40 is the image processing apparatus according to claim 39, wherein the fourth means fixes the quantization threshold to a constant value when the edge degree output from the fifth means is maximum. It is to be. The feature of the invention described in claim 41 is that in the image processing apparatus according to the invention described in claim 39 or 40, the fifth means outputs the edge degree subjected to the area expansion processing in order to reproduce a halftone image well. That is. The feature of the invention according to claim 42 is that, in order to satisfactorily reproduce a halftone image having the number of lines used in a general printed matter, the image processing apparatus according to claim 41 requires an expansion width of the area expansion processing. This is to select within 0.5 mm in the image space.

The image processing apparatus according to the present invention enables image formation by the image forming method according to the present invention. The main feature of the image processing apparatus is the image processing apparatus according to the present invention. As described above, the first means for adding an error to multi-gradation image data, the second means for quantizing the image data after adding the error by the first means to n values (n ≧ 3), and the second means A third means for obtaining an error to be added to multi-tone image data from the quantized data by the means and the image data after the error addition by the first means and providing the error to the first means; And a fourth means for generating (n-1) quantizing threshold values that vary in the range and providing the same to the second means, wherein at least two quantizing threshold values have the same value at a specific position within the variation period. That is. A feature of the invention described in claim 44 is that, in the image processing apparatus according to the invention described in claim 43, at least two quantization thresholds corresponding to the low / medium density levels of the multi-tone image data are within the fluctuation period. It is to take the same value in the central part. A feature of the invention described in claim 45 is that, in the image processing apparatus according to the invention described in claim 43, the fourth means changes the variation width of the quantization threshold according to the image characteristics of the multi-tone image data. is there. A feature of the invention described in claim 46 is that in the image processing apparatus according to the invention described in claim 45, the fourth means reduces the fluctuation range of the quantization threshold in an area where the degree of edge of the multi-tone image data is larger. It is. A feature of the invention according to claim 47 is that, in the image processing apparatus according to claim 45, the fourth means keeps the average value of the quantization threshold at a substantially constant value. A feature of the invention according to claim 48 is that in the image processing apparatus according to claim 43, the fourth means comprises (n-1) dither threshold matrices composed of an m-valued area and an n-valued area. Are used to generate (n-1) periodically varying quantization thresholds, and the thresholds at the same position in the m-valued area of the dither threshold matrix for generating at least two quantization thresholds are equal. That is. A feature of the invention according to claim 49 is that, in the image processing apparatus according to claim 43, the fourth means includes a plurality of threshold matrices each including an m-valued region and an n-valued region, Using (n-1) dither threshold matrices arranged and shifted by a half cycle in the sub-scanning direction, (n
-1) generate periodically varying quantizing thresholds, wherein at least two dither threshold matrices have equal thresholds at the same position in the m-ary region. The feature of the invention described in claim 50 is the feature of claim 48 or 49
In the image processing apparatus of the invention described above, the thresholds at the same position in the m-valued area of all the dither threshold matrices are equal. -The feature of the invention described in claim 51 is the feature of claim 48 or 49
In the image processing apparatus of the invention described above, the n-valued area of the dither threshold matrix is arranged around the m-valued area, and the m-valued area is a dot concentration type area. A feature of the invention according to claim 52 is that, in the image processing apparatus according to claim 51, m in the dither threshold matrix
In other words, the maximum threshold value in the binarized region is set to be smaller than the minimum threshold value in the n-valued region. A feature of the invention according to claim 53 is that, in the image processing apparatus according to the invention according to claim 52, n in the dither threshold matrix
The difference between the minimum threshold value in the m-valued region and the maximum threshold value in the m-valued region is to be larger than the step width of the threshold value in the m-valued region. -The invention described in claim 54 is the invention according to claim 51, 52 or 53.
In the image processing apparatus according to the aspect of the present invention, all the threshold values in the n-valued area of the dither threshold value matrix for generating the highest quantization threshold value are set to n values of the dither threshold value matrix for generating another quantization threshold value. Is set to a constant value larger than the threshold value in the activation area. -The features of the invention described in claim 55 are as set forth in claims 48 to 54.
The image processing apparatus according to any one of the above, further comprising a fifth means for extracting and outputting image characteristics of the multi-tone image data, wherein the fourth means converts the image characteristics output from the fifth means to Switching the dither threshold matrix used to generate each quantization threshold accordingly. According to a feature of the invention described in claim 56, in the image processing apparatus according to the invention described in claim 55, the fifth means outputs the edge degree of the multi-tone image data as an image feature, and the fourth means outputs
The fluctuation width of the quantization threshold is reduced as the edge degree output from the fifth means is larger. According to a feature of the invention described in claim 57, in the image processing apparatus according to the invention described in claim 56, when the edge degree outputted from the fifth means is large, the fourth means entirely comprises an m-valued area. The use of the same dither threshold matrix for the generation of all quantization thresholds. The invention according to claim 58 is the image processing apparatus according to claim 56, wherein the fourth means sets all quantization thresholds to the same constant when the edge degree output from the fifth means is maximum. It is fixed to a value. The feature of the invention described in claim 59 is that, in the image processing apparatus according to the invention described in claim 56, 57 or 58, the edge degree obtained by performing the area expansion processing by the fifth means is used in order to reproduce a halftone image favorably. Output. The feature of the invention according to claim 60 is that, in order to satisfactorily reproduce a halftone image having the number of lines used in a general printed matter, the image processing apparatus according to claim 59 uses an image processing apparatus according to claim 59 in which the expansion width of the area expansion processing is set to an image. It is to select within 0.5 mm in space.

The features of the computer readable storage medium according to the invention of claim 61 are described in order to enable the invention of claims 1 to 28 to be easily implemented using a general-purpose or special-purpose computer. A program for causing a computer to realize the functions of each means of the image processing apparatus according to any one of the aspects of the invention is recorded.

In order to realize a printer, a display, a scanner, a facsimile, a copying machine, etc. to which the image forming method according to any one of claims 1 to 28 is applied, the invention according to claims 62, 63 or 64 is based on claims 29 to Any one of 60
An image forming means for forming an image using dots for outputting an image of quantized data to the image processing apparatus according to the invention described in the above paragraph, and an image for inputting multi-tone image data by optically scanning a document It is characterized by further comprising one or both of the reading means.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An image processing apparatus according to an embodiment of the present invention will be described below with reference to the accompanying drawings. The image forming method of the present invention can be carried out by supplying the quantized data by the image processing apparatus of the present invention described below to an appropriate image forming means. In order to avoid repetition of the description, the same reference numerals are used for the same or corresponding parts in a plurality of drawings in the accompanying drawings.

<< Embodiment 1 and Modifications >> An image processing apparatus according to Embodiment 1 of the present invention has a block configuration as shown in the block diagram of FIG. 1, and inputs multi-gradation input image data 101 by an error diffusion method. An error diffusion processing unit 100 that quantizes and outputs quantized data 103;
It comprises a quantization threshold value generator 200 for supplying a quantization threshold value that periodically fluctuates in the image space with respect to 0. In this embodiment and each of the following embodiments, the input image data 101
Is image data of 256 gradations representing a density level of 0 to 255 at 8 bits / pixel.

An error diffusion processing unit 100 outputs an error adding unit 104 for adding an error to the input image data 101 and an image data 102 to which the error has been added by the error adding unit 104 from a quantization threshold value generating unit 200. A quantization unit 105 that quantizes using a given quantization threshold and outputs quantized data 103, and an error detection unit 1 that detects an error (quantization error) between the quantized data 103 and the image data 102
06, an error storage unit 107 for temporarily storing the detected error, and an error to be added to a pixel to be processed next (target pixel) using the error data stored in the error storage unit 107. Error calculator 10 to be applied to error adder 104
8.

In this embodiment, the quantization threshold value generating section 2
00, the quantization threshold value given to the quantization unit 105 is one, and the quantization unit 105
Is quantized to a quantization level 0 or a quantization level 1. Therefore, in this embodiment, the error detection unit 10
6 is the quantized data 103 and the error-added image data 10
When calculating the error from 2, the output value corresponding to the quantization level 0 is treated as 0 (decimal), and the output value corresponding to the quantization level 1 is treated as 255 (decimal).

In this embodiment, the error calculation unit 1
As shown in the block 08, the mark * is defined as the position of the pixel of interest, and the error detected at the pixel at each of the positions a, b, and c on the previous line and the pixel at the immediately preceding position d on the same line as the pixel of interest. , And a value obtained by dividing the sum of the values multiplied by the coefficients 1, 5, 3, and 7 by 16 is output as an error to be added to the target pixel (*). Therefore, as the error storage unit 107,
For example, a two-line memory is used. However, the arrangement and the number of pixels around the target pixel for which error data is referred to for calculating the error to be added to the target pixel can be changed, and the coefficient (weight) by which the error in each pixel is multiplied can be changed. It is.

The error detection unit 106 and the error storage unit 10
7 is provided with calculation means corresponding to the error calculation unit 108,
Using the error detected by the error detection unit 106 and the error data stored in the error storage unit 107, the error to be added to the peripheral unprocessed pixels is sequentially recalculated by the calculation unit, and the error storage unit A configuration may be adopted in which the error data in 107 is updated, the error to be added to the next pixel to be processed is directly read out from the error storage unit 107 and given to the error adding unit 104.

In this embodiment, the quantization threshold value generating section 2
Reference numeral 00 uses a 4 × 4 dither threshold matrix shown in FIG. 2 to generate a quantization threshold value that fluctuates in the main and sub scanning directions at a cycle of four pixels. Such a quantization threshold generation unit 200
For example, a counter that counts the main and sub-scanning timing signals of the input image data 101 generates a read address of the ROM storing the dither threshold matrix shown in FIG. 2 and corresponds to the target pixel position of the dither threshold matrix. The threshold value may be read out from the ROM and output.

In the dither threshold matrix (quantization threshold of 4
As shown in FIG. 2, the size and arrangement of the thresholds (within a fluctuation cycle of × 4 pixels) are such that the shaded 3 × 3 region has a spiral shape in which the threshold value increases stepwise from 40 to 104 with a step width of 8. The area is a dot-concentrated type area, and the surrounding area is a dot-distributed area in which threshold values increasing from 160 to 208 with a step width of 8 are distributed. Although the threshold value in the dot concentration type area is selected to be lower than the threshold value of the dot distribution type area, the minimum threshold value of the dot distribution type area ( 160) and the maximum threshold value (104) of the dot concentration type area are set to be larger than the step width 8 of the threshold value.

Since the quantization threshold value is generated by using such a dither threshold value matrix, the influence of the error propagation is neglected. In the low density level region of the input image data, the dot concentration type is generated within the 4 × 4 pixel period. 2x at the center corresponding to the area
Two pixels are quantized to the quantization level 1, and in the medium density level area, 3 × 3 pixels corresponding to the dot concentration type area are quantized to the quantization level 1. In the high density level area, 3 × 3 pixels corresponding to the dot concentration type area are quantized to the quantization level 1, and the discrete pixels corresponding to the surrounding dot dispersion type area are also quantized to the quantization level 1. It becomes to be.

Therefore, if the quantized data 103 of the image processing apparatus of the present embodiment is given to a binary image forming apparatus in which only the pixels of the quantization level 1 are dot-formed, the low-density portion of the image is as shown in FIG. In the middle density portion of the image, dots are generated as shown in FIG. 4, and in the high density portion of the image, dots are generated as shown in FIG. 3 and 4
As shown in Fig. 5, dots are concentrated in the low and medium density areas, so they are not easily affected by dot gain, have good gradation, and have less graining and stability with less banding and uneven density. A good image is formed. In addition, as shown in FIG. 5, 3 ×
Since dots are dispersed around the dots of No. 3 and white pixels are hardly continuous, occurrence of white spots can be prevented. Actually, the dots are spread due to the influence of the dot gain. Therefore, particularly in the case of using an electrophotographic printer having a large dot enlargement ratio, the white spots in the high-density portion become less noticeable. FIGS. 3 to 5 show the tendency of dot generation. Strictly speaking, dots are not always formed as shown in FIGS. 3 to 5 due to the influence of error propagation.

By the way, when a quantization threshold is generated by using a dot concentration type dither threshold matrix as shown in FIG. 28, an image with good gradation can be obtained. As shown, white pixels are continuously generated, and are easily recognized as white spots.

According to a modification of this embodiment, the quantization threshold value generating section 200 uses an 8 × 8 dither threshold matrix as shown in FIG. Generate an activation threshold. The dither threshold matrix of FIG. 6 includes four 4 × 4 dither threshold matrices shown in FIG.
In this configuration, two pixels (half a phase) are shifted in the sub-scanning direction. In the 4 × 4 dither threshold matrix of FIG. 7, the shaded 2 × 3 region is a dot concentration type in which threshold values increasing in steps of 8 from 64 to 104 are arranged in a spiral, and the peripheral region is 112 to 184. Step width up to 8
This is a dot dispersion type in which threshold values that increase with are dispersed. Note that a similar dither threshold matrix shifted by a half phase in the main scanning direction may be used.

However, the quantization threshold generator 200 does not necessarily need to have such an 8 × 8 dither threshold matrix. For example, there is provided a 4 × 4 dither threshold matrix shown in FIG. 7, and control is performed such that the reading position in the sub-scanning direction of the dither threshold matrix is shifted by two pixels (half-phase) in a four-pixel cycle in the main scanning direction. Thus, a quantization threshold substantially similar to the case where the dither threshold matrix of FIG. 6 is used may be generated.

When an 8 × 8 dither threshold matrix in which four dither threshold matrices shown in FIG. 7 are arranged without being shifted in the sub-scanning direction is used for generating a quantization threshold, in a high density portion, as shown in FIG. In this case, white pixels are generated in a 2 × 2 area, and white spots occur. On the other hand, in this modification, since the quantization threshold is generated using the dither threshold matrix of FIG. 6, the continuous white pixels are reduced to two pixels in the high density portion of the image as shown in FIG. Is unlikely to occur.

Also in this modification, the dots corresponding to the dot concentration type area of the dither threshold matrix are concentrated and generated in the middle and low density parts, so that the gradation and the stability in the low and medium density parts are obtained. Is good.

<< Embodiment 2 and its Modifications >> An image processing apparatus according to Embodiment 2 of the present invention has a block configuration as shown in the block diagram of FIG. The difference in the block configuration from the first embodiment is that an image feature extraction unit 300 that extracts an image feature in a region near a target pixel from the input image data 101 is added, and an output signal 301 of the image feature extraction unit 300 is used.
0, and, if necessary, through a signal delay unit 150 such as a line memory to adjust the timing between the image feature extraction unit 300 and the error diffusion processing unit 100. This is to be input to the diffusion processing unit 100.

In this embodiment, the image feature extraction unit 300
As shown in FIG. 11, an edge detection unit 302 for detecting the degree of edge and an area extension processing unit 303 for detecting the periodicity of the density level change (identification of a halftone dot area in a specific frequency range) It is a block configuration consisting of: The edge detecting unit 302 detects the edge amounts in a total of four directions of ± 45 ° with respect to the main scanning direction, the sub-scanning direction, and the main scanning direction, using, for example, four types of differential filters as shown in FIG. Then, as the maximum value (absolute value), 2-bit edge degree data indicating the edge degree from level 0 (non-edge) to level 3 (maximum edge degree) is output. The area extension processing unit 303 performs area extension on the image space with respect to the edge degree data provided by the edge detection unit 302. Specifically, for example, an area of 7 × 7 pixels around the pixel of interest (3 pixels before and after in the main scanning direction, 3 pixels before and after in the sub-scanning direction)
With reference to the edge degree data in the pixel range), the maximum value thereof is selected as the edge degree data of the target pixel, and is output as the output signal 301.

In this embodiment, the image feature extraction unit 30
The variation width of the quantization threshold is controlled according to the edge degree level indicated by the output signal 301 of 0. In order to control the variation width of the quantization threshold, the quantization threshold generator 200 has a block configuration including a dither threshold generator 201, a multiplier 202, and an adder 203, for example, as shown in FIG.

The dither threshold generation unit 201 is, for example, as shown in FIG.
4 is used to output a threshold value corresponding to the pixel position of interest using a 4 × 4 dither threshold matrix shown in FIG. In this dither threshold matrix, the shaded 3 × 3 area is
Threshold values that sequentially increase from −7 to +1 at a step width of 1 are of a dot concentration type arranged in a spiral shape, and the surrounding area is a dot where the threshold values increasing at a step width of 1 from 2 to 8 are distributed. It is distributed. The use of a dither threshold matrix in which dot dispersed areas are arranged around such a dot concentrated area, as described in connection with the first embodiment, concentrates dots in low and medium density portions of an image. This is to enhance gradation and stability, and to generate dots by dispersing the dots around the dots concentrated in the high density portion, thereby making it difficult for white spots to occur. Such a dither threshold generation unit 201 is easily provided by, for example, a ROM storing the dither threshold matrix of FIG. 14 and a counter that counts main and sub-scanning timing signals of image data and generates a read address of the ROM. realizable.

The multiplication unit 202 outputs a value obtained by multiplying a coefficient corresponding to the edge degree indicated by the output signal 301 of the image feature extraction unit 300 by a threshold output from the dither threshold generation unit 201. In this embodiment, level 0
The coefficient at the time of (non-edge) is 8, the coefficient at the level 1 is 4, the coefficient at the level 2 is 2, and the coefficient at the level 3 (maximum edge degree) is 0. Therefore, the multiplication unit 202
The output value of varies in the maximum width from -56 to +64 when the edge degree is at level 0 (non-edge), the fluctuation width decreases at the edge degree level 1, and further decreases at the edge degree level 2. However, at the edge degree level 3, the fluctuation width becomes zero.

The adder 203 adds +128 corresponding to the central level of the image data to the output value of the multiplier 202. The output value of the addition unit 203 is provided to the quantization unit 105 as a quantization threshold. Therefore, the edge degree level 0
, The quantization threshold fluctuates with a maximum fluctuation range from +72 to +192 around +128, and at the edge degree level level 3, the quantization threshold is fixed to +128. As described above, the higher the edge degree level, the smaller the fluctuation width of the quantization threshold is controlled.

Therefore, in an image region having a high edge degree such as a character portion or a line drawing portion of an image, the quantization threshold is fixed at +128 or can be varied with a small variation width. And a fixed threshold of 2
The value error diffusion process or the error diffusion-based process is performed, and an image with good resolution can be formed. On the other hand, in an image region having a low edge degree such as a flat portion of the image, the quantization threshold can be varied with a large variation width around +128 according to the dither threshold matrix of FIG. Are performed, and dots are concentrated and generated in low and medium density portions, and an image with good gradation and stability can be formed, and high density portions are concentrated in the same manner as in the first embodiment. Since dots are dispersed around the generated dots, an image in which white spots are unlikely to occur can be formed. Also, the quantization threshold fluctuates periodically, and the fluctuation width also changes according to the edge degree level. However, since the time average of the quantization threshold is kept at a substantially constant value (+128), the quantization threshold is switched. In other words, it is possible to prevent a dot generation delay at a boundary between image areas having different image characteristics, and therefore, it is possible to smoothly express the boundary between image areas.

Now, if the input image data 101 is
When read at a resolution of 00 dpi, the expansion width of 7 pixels in the area expansion processing is about 0.3 mm on the document, which corresponds to a dot period of about 86 Lpi. Therefore,
The halftone dot image portion having a line number of 86 Lpi or more is evaluated as an edge, and the error diffusion processing portion 100 performs error diffusion processing or error diffusion with good resolution using a fixed quantization threshold or a quantization threshold having a small variation width. Since the base tone processing is performed, halftone dots can be faithfully reproduced with high resolution, and the occurrence of moire can be prevented. In the halftone dot image portion with a lower screen ruling, the inside of each halftone dot is evaluated as a non-edge and, like the flat portion of the image, the dither-based processing is performed using a quantization threshold of a large vibration width, The stability is good, and white spots are less likely to occur in high density areas. It should be noted that such a halftone dot image having a low screen ruling does not have resolution information of the image inside each halftone dot, so that there is no inconvenience in the dither tone processing. As described above, according to the present embodiment, a high-quality image including characters, line drawings, photographs, halftone dots, and the like can be reproduced.

The sharpness of the image is affected by the image change point. Generally, it is sufficient in terms of image quality if halftone dots having a relatively low screen ruling up to about 50 Lpi can be faithfully reproduced. Therefore, strictly speaking, the MT of the scanner used for image reading is
Although it is necessary to consider the influence of the F characteristic, the characteristics of the edge detection filter, and the period difference caused by the density change of the halftone image, the expansion width of the area expansion processing must be within 0.5 mm in the image space (12 pixels at 600 dpi). In general, halftone images can be reproduced with sufficient image quality. Further, image data read by scanning a document with a scanner is generally passed through a smoothing filter in order to smoothly express halftones, and is usually smoothed from about 150 Lpi.
The periodic amplitude of the halftone dot having a higher screen ruling than about 175 Lpi to 200 Lpi remains. Therefore, the halftone dot image portion having such a high screen ruling is determined to be a non-edge portion by the image feature extracting portion 300, and even if dither-based processing is performed similarly to the flat portion of the image, there is no fear that moire will occur. Absent.

In this embodiment, the halftone dot image area in the specific frequency range is identified by the area expansion processing of the edge degree data. Is separately provided, and the halftone image area detected by the means is fixed or has a small fluctuation width regardless of the degree of edge detected by the edge detection unit 302. It may be generated at 200 to perform error diffusion-based processing.

According to the modified example a of the present embodiment, the dither threshold value generating section 201 uses a dither threshold value matrix as shown in FIG. This dither threshold matrix is a dot dispersion in which threshold values increasing by one from 3 to 9 are distributed around a shaded dot concentration type area in which threshold values increasing by one from -8 to 0 are spirally arranged. And a mold area. The difference between the minimum threshold value (3) in the dot concentration type region and the maximum threshold value (0) in the dot concentration type region is selected as 3 which is larger than the step width (1) of the threshold value in the dot concentration type region. ing. When such a dither threshold matrix is used, the quantization threshold is 64 in the image flat part.
However, since the difference between the maximum quantization threshold (128) of the dot concentration type area and the minimum quantization threshold (152) of the dot dispersion type area is as large as 24, the low / medium density is low. As a result, the probability of occurrence of dots in the dot dispersion type region is reduced, the degree of concentration of dots is increased, and the gradation and stability of low / medium density portions are improved.

Although not shown, according to another modified example b of the present embodiment, the quantization threshold value generating section 200 multiplies each threshold value of the dither threshold value matrix shown in FIG.
It has four types of 4 × 4 dither threshold matrices for edge degree levels 0, 1, 2, and 4 obtained by adding quadruple, double, and zero to 128, and outputs the output signal 301 of the image feature extraction unit 300 from among them. The threshold value of the dither threshold value matrix selected according to the edge degree level indicated by is output as a quantization threshold value. According to such a configuration, since the multiplication unit 202 and the addition unit 203 in the configuration shown in FIG. 13 are not required, hardware can be easily implemented, and multiplication that requires processing time is not required even when software is used. There are advantages. It is apparent that the same processing as that of the present embodiment or the above-described modification a can be performed by the modification b. Also, the average value of the quantization threshold is a substantially constant value (+128)
Is kept.

Although not shown, according to another modified example c of the present embodiment, in this embodiment or each of the above embodiments a and b, the image feature extraction unit 300 sends the area expansion processing unit 30
3 is omitted, and the edge degree data output from the edge detection unit 302 is output as the output signal 301 as it is. According to this modification, it is also possible to form a high-resolution image in an area where the density changes sharply, such as characters and line drawings of the image, and has good gradation and stability in the flat part of the image without any white spots. An image can be formed.

This embodiment and its modifications a, b, c
In this case, an enlarged dither threshold matrix in which a plurality of dither threshold matrices as shown in FIG. 14 or FIG. 15 are shifted by half a phase in the sub-scanning method or the main scanning direction is used for generating a quantization threshold. May be.

<< Embodiment 3 and its Modifications >> The image processing apparatus according to Embodiment 3 of the present invention has a block configuration as shown in the block diagram of FIG. 1 as in Embodiment 1 described above. However, this is different from the first embodiment.

One of the differences is that the quantization threshold generator 200
And two quantization thresholds Thr1 and Thr2 (0 <Thr1
≦ Thr2 <255). Another difference is that the quantizing unit 105 quantizes the error-added image data 102 in the case of Thr1 <Thr2, but quantizes the error-added image data 102 in the case of Thr1 = Thr2. It is to make. That is, if the level of the image data 102 after error addition is L, Thr
<Thr2, quantization level 0 when L <Thr1, quantization level 1 when Thr1 ≦ L <Thr2,
When L ≧ Thr2, the quantization level 2 is output. On the other hand, when Thr1 = Thr2, L <Thr
When 1 (= Thr2), the quantization level 0 is set, and L ≧ Thr
When 2 (= Thr1), the quantization level 2 is output. Another difference is that when the error detector 106 calculates an error between the quantized data 103 and the error-added image data 102, the output value corresponding to the quantization level 0 is set to 0 (decimal),
The output value corresponding to the quantization level 1 is, for example, 128 (10
Hex), the output value corresponding to quantization level 2 is 255 (10
Hex).

The quantization threshold generation section 200 has a quantization threshold T
hr1 and Thr2 are generated using different dither threshold matrices. An example of such two dither threshold matrices is shown in FIG. The dither threshold matrices shown in (a) and (c) are for the generation of Thr1, and (b)
And the dither threshold matrix shown in (c) is for the occurrence of Thr2. A pair of the dither threshold matrices (a) and (b) or a pair of the dither threshold matrices (c) and (d) may be used.

Both dither threshold matrices are composed of shaded 3 × 3 binarized areas and surrounding ternary areas. In the binarized regions of both dither threshold matrices, the threshold at the corresponding position has the same value. That is, the pixels corresponding to the binarized area are binarized. In addition, the binarization area is 8 from 42 to 106.
The threshold value, which increases step by step, is a spirally arranged dot concentration type area, which is for concentration of dots in low and medium density portions of an image.

In the ternary area around the binarized area, Thr
The thresholds for 1 and Thr2 are different, and the threshold for Thr2 is selected to be larger than the threshold for Thr1. Therefore, for a pixel corresponding to the ternary region, T
hr1 <Thr2, so that ternary quantization is performed. The ternarization area of the dither threshold matrix for Thr1 is 156 to 2 in the dither threshold matrix of FIG.
The dot concentration type is a dot concentration type in which threshold values increasing in increments of 8 up to 04 are sequentially arranged. In the dither threshold value matrix of FIG. The ternarization area of the dither threshold matrix for Thr2 is also
In the dither threshold matrix of (b), the dot concentration type in which threshold values increasing by 8 from 192 to 240 are arranged in order is used. In the dither threshold matrix of (d), the same threshold value is dispersed and arranged in the dot dispersion type. It has become. Further, in order to increase the degree of concentration of dots, the difference between the minimum threshold value in the ternary area and the maximum threshold value in the binarized area is selected to be larger than the step width of the threshold value in the binarized area. .

According to the image processing apparatus of the present embodiment, small dots are not generated in the low / medium density portion of the image because of the binary quantization, and the binarized region is a dot concentration type region. Therefore, large dots are generated in a concentrated manner, so that an image with good gradation and stability can be formed. Further, in the high-density part of the image, the ternary quantization is performed, so that the probability of occurrence of small dots increases, and white spots are less likely to occur.

In order to clarify such advantages, the quantized data 103 of the image processing apparatus according to the present embodiment is supplied to, for example, an electrophotographic printer or the like which forms an image using small dots and large dots. FIG. 17 shows a state of dot generation in the case where is formed. As described above, Thr1,
The threshold value in the binarized area of the dither threshold matrix for generating Thr2 is lower than the threshold value in the ternary area, and the binarized area is a dot concentration type area. In the middle density portion, as shown in FIGS. 17A and 17B, large dots corresponding to the quantization level 2 are intensively generated in a spiral, and a stable image with good gradation is obtained. It is formed. Dither threshold matrix 3
Since the binarized area is arranged around the binarized area, in the high density part, as shown in FIG. 17C, small dots corresponding to the quantization level 1 are arranged around the concentrated large dots. And white spots are less likely to occur.

According to the modified example a of this embodiment, the quantization threshold value generating section 200 uses the dither threshold value matrix pairs shown in FIGS. 18 to 20 in place of the dither threshold value matrix pairs shown in FIG. Can be These dither threshold matrices differ from the dither threshold matrix of FIG. 16 only in the positional relationship between the binarized area (dot concentrated area) and the ternary area.

According to another modification b of this embodiment,
In order to generate the quantization thresholds Thr1 and Thr2, FIG.
The dither threshold matrix pairs shown in FIGS. For these dither threshold matrix pairs, Th
All thresholds in the ternary area of the dither threshold matrix for r2 are selected to be relatively low and constant (216). When such a dither threshold matrix is used, the tendency of large dots to occur after all small dots have occurred in the ternarization area is further increased, so that the probability of white pixels is further reduced, and more reliable white spots are generated. Prevention is possible.

The number of pixels in the binarized area and the ternary area of the dither threshold matrix can be uniquely determined based on the lowest density level at which white pixels are not desired to be generated as follows. For example, it is assumed that white pixels are not desired to be generated at a level of 200 or more of the multi-tone image data after the gamma conversion. 4x
X is the number of pixels in the binarized area in the dither threshold matrix of 4.
Then, the number of pixels in the ternary region is (16−X).
As described above, the output value of the small dot (quantization level 1) is 128, and the output value of the large dot (quantization level 2) is 255.
Then, (255 × X / 16) + (128 × (16−X) / 1
6) When the value of X that satisfies = 200 is obtained, in this case, X = 9.1. Therefore, the binarized region has 9 pixels, and the ternary region has 7 pixels.
If it is a pixel, no white pixel occurs at a level of 200 or more of the image data.

According to another modification c of this embodiment,
16, 18 to 20, or 22 to 24
A plurality of 4 × 4 dither threshold matrices as shown in
Enlarged dither threshold matrices shifted in the main scanning direction or the sub-scanning direction are used for generating the quantization thresholds Thr1 and Thr2. Examples of such a dither threshold matrix are shown in FIGS. Thr shown in FIG.
The dither threshold matrix for 1 is 4 × 4 in FIG.
This corresponds to an arrangement in which four dither threshold matrices are shifted by a half phase (two pixels) in the sub-scanning direction. The dither threshold matrix for Thr2 shown in FIG.
This corresponds to an arrangement in which four 4 × 4 dither threshold matrices in (b) are shifted by a half phase in the sub-scanning direction. Such a dither threshold matrix can suppress continuation of the ternary region, and is thus effective in preventing white spots.
Note that the quantization threshold generation unit 200 does not necessarily need to have such an 8 × 8 dither threshold matrix, but has, for example, a 4 × 4 dither threshold matrix shown in FIGS. By performing control such that the readout position in the sub-scanning direction of the dither threshold matrix is shifted by two pixels at a cycle of four pixels in the direction, quantization substantially similar to the case of using the dither threshold matrices of FIGS. 25 and 26 is performed. A threshold may be generated.

In this embodiment and its modified examples a, b, and c,
The high-density portion is quantized by three values, and the low- and medium-density portions are quantized by binary. However, the number of quantizations is not limited thereto. For example, three area thresholds Thr1... Using three dither threshold matrices in which a quaternary area having a larger threshold is arranged around the binarized area. By generating Thr2 and Thr3, and setting the thresholds at the same position in the binarization area of the dither threshold matrix for the quantization thresholds Thr1, Thr2 and Thr3 to the same value, the binary in the low / medium density part of the image is obtained. It is also possible to perform quantization and quaternary quantization in a high density part. Also, for example, four quantization thresholds Thr1, Thr2, and Thr4 are provided using four dither threshold matrices in which a ternary region having a larger threshold value is arranged around the ternary region.
Thr3 and Thr4 are generated, the thresholds at the same position in the ternarization area of the dither threshold matrix for Thr1 and Thr2 are set to the same value, and the same positions in the ternarization area of the dither threshold matrix for Thr3 and Thr4 are set. It is also possible to perform ternary quantization in the low / medium density part and quinary quantization in the high density part by setting the threshold values to equal values.

More generally, (n-1) quantization thresholds are generated using (n-1) dither threshold matrices composed of an m-value area and an n-value area (n> m). In the high density part of the image, the occurrence probability of small dots is increased by n-value quantization to prevent white spots. On the other hand, in the low / medium density parts, the occurrence of small dots is reduced by m-value quantization. Image stability can be improved. This is the same in the following fourth embodiment and its modification.

<< Embodiment 4 and its Modifications >> An image processing apparatus according to Embodiment 4 of the present invention is similar to that of Embodiment 2 shown in FIG.
It is a block configuration shown in FIG. The image feature extraction unit 300
This is the same configuration as that of the second embodiment shown in FIG. The quantization threshold generation unit 200 generates two quantization thresholds Thr1 and Thr2 in the same manner as in the third embodiment, but outputs a dither threshold matrix used for generating the quantization thresholds to the output of the image feature extraction unit 300. Embodiment 3 is different from Embodiment 3 in that the switching is performed according to the edge degree level indicated by the signal 301.

The quantization unit 105 of the error diffusion processing unit 100
Are the quantization threshold values Thr1 and Th, as in the third embodiment.
When r2 is different, the error-added image data 102 is ternary-quantized into levels 0, 1, and 2, and the quantization threshold Thr1,
If thr2 is equal, the error-added image data 102
Is quantized to levels 0 and 2.

FIG. 27 shows an example of a dither threshold matrix used for each edge degree level in the quantization threshold generator 200. At the edge degree level 0, a pair of dither threshold matrices having a large threshold variation width shown at the top of FIG. 27 is used. This dither threshold matrix pair is the same as the dither threshold matrix pair of FIG. 21 used in the modification of the third embodiment.

Thr1, Th at edge degree level 1
To generate r2, a pair of dither threshold matrices shown in the second row of FIG. 27 is used. These dither threshold matrices also have a structure in which a ternary area is arranged around a 3 × 3 binarized area, but the threshold of each area is different from the dither threshold matrix for the edge degree level 0. In other words, the minimum value of the threshold value in the binarization area is selected to be 86, which is larger than the case of the edge degree level 0, and the step width is also set to 4, which is smaller than the case of the edge degree level 0. Also,
In the ternarization area of the Thr1 dither threshold matrix, the minimum value of the threshold is smaller than that in the case of the edge level 0.
8, and the step width is also set to 4, which is smaller than the case of the edge degree level 0. Further, the threshold value of the ternarization region of the dither threshold matrix for Thr2 is selected to be a constant value (172) smaller than the case of the edge degree level 0.

Thr1, Th at edge degree level 2
To generate r2, a pair of dither threshold matrices shown in the third row of FIG. 27 is used. These dither threshold matrices are entirely binarized regions, and the minimum value of the threshold is selected to be a larger value (114) than in the case of the edge degree level 1, and the step width is also smaller than that in the case of the edge degree level 1 ( Selected in 2).

Thr1, Th at edge degree level 3
To generate r2, a pair of dither threshold matrices shown at the bottom of FIG. 27 is used. These dither threshold matrices are also binarized regions as a whole, but all thresholds are selected to be the same value (128).

Since such a pair of dither threshold matrices is switched and used according to the edge degree level, appropriate processing can be performed according to the characteristics of the image. That is,
In a character portion or a line drawing portion having a high edge degree level, normal binary error diffusion processing or binary error diffusion-based processing is performed using a fixed threshold value or a quantization threshold value having a small vibration width. Can be reproduced. The same process is applied to a halftone dot region having a relatively low screen ruling that is equal to or greater than a certain line ruling, which is evaluated as an edge by the image feature extracting unit 300, so that the halftone dot can be faithfully reproduced. On the other hand, a halftone dot area with a lower number of lines,
A halftone dot region having a high ruling, for which periodicity is removed by the smoothing filter, is subjected to dither-based processing in the same manner as a flat portion of the image, so that a stable image can be formed with less occurrence of white spots.

The positional relationship between the binarized area and the ternary area of the dither threshold matrix shown in FIG. 27 may be changed. As described in each of the above embodiments, FIG.
A dither threshold matrix in which a plurality of dither threshold matrices as shown in (1) are shifted in the main scanning direction or the sub-scanning direction may be used.

Further, the area extension processing section 303 may be omitted from the image feature extraction section 300 and the edge degree data of the edge detection section 302 may be modified to be output as the output signal 301 as it is. Even in this case, as in the case of the present embodiment, it is possible to clearly reproduce an image region having a sharp change in density such as a character portion and a line drawing portion of an image, and to reproduce a flat portion such as a photograph or an image with high quality. Can be.

<< Implementation of the Present Invention by Software >> The image processing apparatus according to each of the above-described embodiments and its modifications can be realized by software using a general-purpose or dedicated computer. In this case, a program for realizing the function of each unit of the image processing apparatus on a computer, for example, a floppy disk on which the program is recorded,
The image of the present invention is read from various storage media such as an optical disk, a magneto-optical disk, and a semiconductor storage element, or received from an external computer or the like via a network, loaded into a main memory of the computer, and executed by a CPU. The processing device can be realized on a computer. As a storage area such as a line memory necessary for storing various data, for example, a main memory is used. Various computer-readable storage media on which such a program is recorded are also included in the present invention.

<< Other Embodiments >> The image processing apparatus according to each of the above-described embodiments and its modifications includes devices related to image formation such as a printer and a display, devices related to image reading such as a scanner and a facsimile, It can be incorporated into equipment such as a digital copier related to both image reading and image formation. As an example of such an embodiment, an example of a digital copying machine to which the present invention is applied will be described below.

FIG. 30 is a schematic sectional view for explaining the structure of an example of a digital copying machine according to the present invention. The digital copying machine includes an image reading unit 400 that optically scans an original, a laser printer 411 as an electrophotographic image forming unit, and a circuit unit (not shown) (see FIGS. 31 and 32). Be composed.

The image reading section 400 includes a flat platen 4
The document placed on the document 03 is illuminated by an illumination lamp 502, and the reflected light image is formed on an image sensor 507 such as a CCD via mirrors 503 to 505 and a lens 506.
By sub-scanning the document by moving 505, the image information of the document is read and converted into an electrical image signal.
An analog image signal output from the image sensor 507 is input to a circuit unit (not shown) and processed. The image data output from the circuit unit 550 is
11 and an image is formed.

In the laser printer 411, the writing optical unit 508 converts image data input from a circuit unit (not shown) into an optical signal, and exposes an image carrier made of a photoconductor, for example, a photoconductor drum 509. By
An electrostatic latent image corresponding to the document image is formed. The writing optical unit 508 drives, for example, a semiconductor laser in accordance with the image data by the light emission drive control unit to emit laser light whose intensity is modulated, and deflects and scans this laser light by the rotary polygon mirror 510 to perform f / θ lens. And irradiates the photosensitive drum 509 via the reflection mirror 511. Photoconductor drum 50
Reference numeral 9 is rotated by a driving unit, rotates clockwise as indicated by an arrow, is uniformly charged by a charger 512, and is then exposed by a writing optical unit 508 to form an electrostatic latent image. The electrostatic latent image on the photosensitive drum 509 is developed by the developing device 513 to become a toner image. Further, the sheet is fed to the registration roller 520 from one of the plurality of sheet feeding units 514 to 518 and the manual sheet feeding unit 519. The registration roller 520 is connected to the photosensitive drum 50.
9 is sent out in time with the toner image on the sheet 9. A transfer bias is applied to the transfer belt 521 from a transfer power supply, and the transfer belt 521 transfers the toner image on the photosensitive drum 509 to a sheet and conveys the sheet. The sheet on which the toner image has been transferred is conveyed to the fixing unit 522 by the transfer belt 521, where the toner image is fixed, and then discharged to the discharge tray 523. Further, the photosensitive drum 509 is cleaned by the cleaning device 524 after the transfer of the toner image, and is further discharged by the discharger 525 to prepare for the next image forming operation.

FIG. 31 is a simplified block diagram showing an example of a circuit section inside the digital copying machine. The input of this circuit unit is the image sensor 5 of the image reading unit 400.
07 is an analog image signal read at, for example, 600 dpi. The level of this analog image signal is adjusted by an AGC circuit 551, and then converted into 8-bit digital image data per pixel by an A / D conversion circuit 552.
3 corrects variations in sensitivity and illuminance for each pixel of the image sensor 507. Next, the image data is sent to the filter processing circuit 554 and subjected to, for example, MTF correction.
Next, a smoothing filter process for smoothly expressing the halftone image is performed. The image data that has been subjected to such processing is sent to a gamma correction circuit 555, where it is subjected to gamma correction for conversion to a writing density. The image data after the γ correction is input to the halftone processing unit 556. As the halftone processing unit 556, the image processing device according to each of the above-described embodiments or its modification is used. Output image data of the image processing apparatus is sent to a light emission drive control unit of a semiconductor laser in the writing optical unit 508. Halftone processing unit 55
6, the above-described quantization processing is performed, so that an image read from the document can be reproduced with high image quality.

As the halftone processing section 556, the second embodiment
When the image processing apparatus according to the fourth or modified example is used, the image feature extracting unit 300 can be provided in the halftone processing unit 556, but can also be provided in a stage preceding the halftone processing unit 556. For example, the image feature extraction unit 300 may be provided at a position as shown in FIG.

In practice, in a digital copying machine, it is often possible to perform a scaling process, a background removal process, a flare removal process, and other image editing processes on image data. I do. Further, the present invention can be similarly applied to a digital copying machine having an image reading unit for moving a document table and a digital copying machine having image forming means other than a laser printer.

As described above, the present invention is applicable to various devices such as a scanner and a facsimile. In this case, although not shown, a reading unit such as an image reading unit 400 of the digital copying machine for inputting image data to the image processing apparatus according to each of the above-described embodiments or a modification thereof, and an image of the quantized data are used. The image forming apparatus has a configuration in which one or both of image forming units such as a laser printer 411 of the digital copying machine for forming the image are added.

[0073]

As is clear from the above description, according to the image forming method of the present invention as described in any one of claims 1 to 28, it is possible to form an image with good gradation and stability at low / medium density portions. Image formation with few white spots in the density part, character part of image, line drawing part,
This makes it possible to form a high-resolution image of a halftone dot image portion having a relatively low screen ruling, and to form a smooth image of a joint portion between regions having different image characteristics. According to the image forming method of the first aspect of the present invention, it is possible to form an image having good gradation and stability in low and medium density areas and less occurrence of white spots in high density areas. According to the image forming method of the second aspect of the present invention, by switching from error diffusion-based processing to dither-based processing in accordance with image characteristics, it is optimal for a multi-tone image having various image characteristics. Image formation is possible. According to the image forming method of the third aspect of the present invention, it is possible to form a high-resolution image by performing error diffusion-based processing in a character portion or a line drawing portion of a multi-tone image. According to the image forming method of the present invention, it is possible to smoothly reproduce a boundary portion between image regions having different image characteristics. According to the image forming method of the present invention, the dots are concentrated in the low and medium density portions of the multi-tone image and the dots are dispersed in the high density portions, so that the gradation and stability are good. Thus, an image without white spots can be formed. According to the image forming method of the present invention, the continuity of white pixels in the high density portion can be reduced and white spots can be prevented. According to the image forming method of the present invention, the concentration of dots in the low / medium density portion can be enhanced, and the gradation and stability of the image can be improved. According to the image forming method of the invention according to claims 8 and 9,
By varying the quantization threshold with an appropriate vibration width according to the image characteristics of the multi-tone image, it is possible to adapt the processing from the dither to the error diffusion to the image characteristics. According to the image forming method according to the tenth and eleventh aspects of the present invention, high resolution can be obtained by performing error diffusion-based processing in a character portion or a line drawing portion of a multi-tone image. According to the image forming method of the present invention,
It is possible to suppress the occurrence of small dots in the medium density portion and enhance the stability of the image, and increase the number of dots in the high density portion to prevent white spots. According to the image forming method of the present invention, the gradation and stability are improved by concentrating the dots in the low and medium density portions, and the dots are dispersed around the concentrated dots in the high density portions. By generating dots, white spots can be prevented. According to the image forming method of the present invention, by switching the processing from the error diffusion tone to the dither tone according to the image characteristics, the optimum image formation for a multi-tone image having various image characteristics is achieved. Becomes possible. According to the image forming method of the present invention, it is possible to form a high-resolution image in a character portion, a line drawing portion, and the like by processing based on error diffusion. According to the image forming method of the present invention, it is possible to form an image in which boundary portions of image regions having different image characteristics are smoothly combined. According to the image forming method of the invention, a low
It is possible to suppress the occurrence of small dots in the medium-density portion and enhance the stability of the image, and to increase the number of dots in the high-density portion to prevent white spots. According to the image forming method of the nineteenth aspect, the continuity of white pixels in the high density portion can be reduced and white spots can be prevented. According to the image forming method of the twentieth aspect, it is possible to prevent white spots in high-density areas and to prevent small dots from being generated in low- and medium-density areas, thereby improving image stability. . According to the image forming method of the invention described in claim 21,
Concentrate dots in the medium density area to enhance gradation and stability,
White spots can be prevented in high density areas. According to the image forming method of the present invention, a low
It is possible to suppress the occurrence of small dots in the medium density portion and increase the stability of the image. According to the image forming method of the invention described in claim 23,
By increasing the degree of concentration of dots in the medium density portion and suppressing the occurrence of small dots, gradation and stability can be improved. According to the image forming method of the twenty-fourth aspect, by increasing the probability of occurrence of small dots in the high-density portion, it is possible to prevent white spots and enhance the gradation. According to the image forming method of the twenty-fifth aspect, by changing the quantization threshold with an appropriate vibration width in accordance with the image characteristics of the multi-tone image, the processing from the dither key tone to the error key tone is applied to the image characteristic. Can be adapted. According to the image forming method of the twenty-sixth aspect, it is possible to obtain a high resolution by performing error diffusion-based processing in a character portion or a line drawing portion of a multi-tone image. According to the image forming method of the invention described in claim 27,
By generating only large dots from medium density to high density, it is possible to form a stable image without white spots in a character portion or a line drawing portion of a multi-tone image. According to the image forming method of the invention described in claim 28,
By performing a binary error diffusion process with a fixed threshold value from a medium density to a high density, an image having excellent resolution and stability can be obtained in a character portion or a line drawing portion.

As is apparent from the above description, claim 2
According to the image processing apparatus of the present invention, the quantized data is supplied to an appropriate image forming apparatus, whereby the image forming method of the present invention is implemented, and the low / medium density Image formation without white spots in high-density areas, character parts of images, line drawing parts,
It is possible to form a high-resolution image of a halftone dot image portion having a relatively low screen ruling, and to smoothly reproduce a joint portion between regions having different image characteristics. Further, according to the image processing apparatus of the present invention, it is possible to form a favorable image even in a halftone portion of a multi-tone image.

According to the storage medium of the present invention, the image processing apparatus of the present invention can be implemented by:
The present invention can be easily realized by using a general-purpose computer or a special-purpose computer. Therefore, the image forming method according to the present invention can be easily implemented.

According to the image processing apparatus of the present invention, a printer, a display, a scanner, a facsimile, a copying machine, etc. to which the image forming method of the present invention is applied are realized. be able to.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an example of a block configuration of an image processing apparatus according to the present invention.

FIG. 2 is a diagram showing a dither threshold matrix used in the first embodiment of the present invention.

FIG. 3 is a diagram illustrating a state of dot generation in a low density portion according to the first embodiment of the present invention.

FIG. 4 is a diagram illustrating a state of dot generation in a medium density portion according to the first embodiment of the present invention.

FIG. 5 is a diagram illustrating a state of dot generation in a high density portion according to the first embodiment of the present invention.

FIG. 6 is a diagram illustrating a shift-type enlarged dither threshold matrix used in a modification of the first embodiment of the present invention.

7 is a diagram showing a basic dither threshold matrix of the shift type enlarged dither threshold matrix shown in FIG. 6;

8 is a diagram showing how dots are generated in a high density portion when the shift type enlarged dither threshold matrix shown in FIG. 6 is used.

FIG. 9 is a diagram illustrating how dots are generated in a high density portion when a non-shift type enlarged dither threshold matrix is used.

FIG. 10 is a block diagram showing another example of the block configuration of the image processing apparatus according to the present invention.

FIG. 11 is a block diagram illustrating an example of an image feature extraction unit.

FIG. 12 is a diagram illustrating an example of a differential filter for edge detection.

FIG. 13 is a block diagram illustrating an example of a quantization threshold generation unit.

FIG. 14 is a diagram showing a dither threshold matrix used in Embodiment 2 of the present invention.

FIG. 15 is a diagram showing a dither threshold matrix used in a modification of the second embodiment of the present invention.

FIG. 16 is a diagram showing a dither threshold matrix used in Embodiment 3 of the present invention.

FIG. 17 is a diagram illustrating a state of dot generation in a low density portion, a medium density portion, and a high density portion in Embodiment 3 of the present invention.

FIG. 18 is a diagram showing a dither threshold matrix used in a modification of the third embodiment of the present invention.

FIG. 19 is a diagram illustrating a dither threshold matrix used in a modification of the third embodiment of the present invention.

FIG. 20 is a diagram showing a dither threshold matrix used in a modification of the third embodiment of the present invention.

FIG. 21 is a diagram showing a dither threshold matrix used in a modification of the third embodiment of the present invention.

FIG. 22 is a diagram showing a dither threshold matrix used in a modification of the third embodiment of the present invention.

FIG. 23 is a diagram illustrating a dither threshold matrix used in a modification of the third embodiment of the present invention.

FIG. 24 is a diagram showing a dither threshold matrix used in a modification of the third embodiment of the present invention.

FIG. 25 is a diagram illustrating a shift-type enlarged dither threshold matrix used for generation of Thr1 in a modification of the third embodiment of the present invention.

FIG. 26 is a diagram illustrating a shift-type enlarged dither threshold matrix used for generating Thr2 in a modification of the third embodiment of the present invention.

FIG. 27 is a diagram showing a pair of dither threshold matrices for generating Thr1 and Thr2 of the variation width corresponding to the edge degree level in the fourth embodiment of the present invention.

FIG. 28 is a diagram illustrating a dot concentration type dither threshold matrix as a whole;

FIG. 29 is a diagram showing how dots are generated when the dither threshold matrix of FIG. 28 is used.

FIG. 30 is a schematic sectional view showing a schematic configuration of an example of a digital copying machine according to the present invention.

FIG. 31 is a block diagram showing an example of a block configuration of a circuit section of the digital copying machine shown in FIG. 30;

FIG. 32 is a block diagram showing another example of the block configuration of the circuit section of the digital copying machine shown in FIG. 30;

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 error diffusion processing unit 101 input image data 102 error-added image data 103 quantized data 104 error addition unit 105 quantization unit 106 error detection unit 107 error storage unit 108 error calculation unit 150 signal delay unit 200 quantization threshold generation unit 201 Dither threshold generation unit 202 Multiplication unit 203 Addition unit 300 Image feature extraction unit 302 Edge detection unit 303 Area expansion processing unit 400 Image reading unit 403 Platen table 411 Laser printer 502 Illumination lamps 503, 504, 505 Mirror 506 Lens 507 Image sensor 508 Writing Optical unit 509 Photoreceptor drum 510 Rotating polygon mirror 511 f / θ lens and reflection mirror 512 Charger 513 Developing device 514 to 518 Feed unit 519 Manual feed unit 520 Registrow et al. 521 Transfer bell 522 fixing unit 523 discharge tray 524 cleaning device 525 discharger 551 AGC circuit 552 A / D converter circuit 553 the shading correction circuit 554 filter circuit 555 gamma correction circuit 556 halftone processing section

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Etsuro Morimoto 1-3-6 Nakamagome, Ota-ku, Tokyo F-term in Ricoh Co., Ltd. 5C077 LL19 MP01 NN15 PP52 PP53 RR08 RR13 RR16 5C078 AA04 BA44 CA22 DB07

Claims (64)

[Claims] 1. An image forming method for quantizing a multi-tone image by an error diffusion method and expressing pixels of the quantized image by dots, wherein a low-to-medium image of the multi-tone image is specified within a specific period in an image space. The quantization threshold is periodically set so that the dots are concentrated in the density portion, and the dots are concentrated in the high density portion of the multi-tone image, and the dots are dispersed around the dot. An image forming method characterized by varying. 2. The image forming method according to claim 1, wherein the variation range of the quantization threshold is changed according to the characteristics of the multi-tone image. 3. The image forming method according to claim 2, wherein the fluctuation range of the quantization threshold is reduced in an area where the degree of edge of the multi-tone image is large. 4. The image forming method according to claim 2, wherein the average value of the quantization threshold is kept substantially constant. 5. The quantization threshold value is periodically changed by using a dither threshold value matrix including a dot concentration type region and a dot distribution type region having a larger threshold value around the dot concentration type region. The image forming method as described in the above. 6. A dither threshold obtained by arranging and enlarging a plurality of threshold matrices each composed of a concentrated area and a dot dispersed area having a larger threshold value in the vicinity thereof, shifted by a half phase in the main scanning direction or the sub scanning direction. 2. The image forming method according to claim 1, wherein the quantization threshold is periodically changed using a matrix. 7. The method according to claim 1, wherein a difference between a minimum threshold value in the dot dispersion type region and a maximum threshold value in the dot concentration type region of the dither threshold value matrix is larger than a step width of the threshold value in the dot concentration type region. The image forming method according to claim 5. 8. A quantization threshold value, wherein a value obtained by multiplying a threshold value of a dither threshold value matrix by a coefficient corresponding to an image feature of a multi-tone image and adding a constant value is used as a quantization threshold value. The image forming method as described in the above. 9. The image forming method according to claim 5, wherein a dither threshold matrix for periodically varying the quantization threshold is switched according to the image characteristics of the multi-tone image. 10. The image forming method according to claim 8, wherein the fluctuation range of the quantization threshold value is reduced in an area of the multi-tone image having a higher edge degree. 11. The image forming method according to claim 10, wherein the quantization threshold is fixed to a constant value in an area where the degree of edge of the multi-tone image is maximum. 12. An image forming method in which a multi-tone image is quantized by n values (n ≧ 3) by an error diffusion method, and pixels of the quantized image are represented by larger dots as the quantization level is higher. An image forming method, wherein m values (m <n) are quantized in a low / medium density portion of an image. 13. A method for quantizing a multi-tone image, using (n-1) quantization thresholds that periodically fluctuate in an image space, wherein at least two quantization thresholds within the fluctuation period are used. 13. The image forming method according to claim 12, wherein the same value is taken at a specific position. 14. The image forming method according to claim 13, wherein the specific position is a central portion in a fluctuation cycle of a quantization threshold. 15. The method according to claim 13, wherein the variation range of the quantization threshold is changed according to the image characteristics of the multi-tone image.
The image forming method as described in the above. 16. The image forming method according to claim 15, wherein the variation width of the quantization threshold is reduced in an area where the degree of edge of the multi-tone image is large. 17. The image forming method according to claim 15, wherein an average value of the quantization threshold is maintained at a substantially constant value. 18. Using the (n-1) dither threshold matrices consisting of an m-valued area and an n-valued area, generating (n-1) quantization thresholds that vary periodically, 14. The image forming method according to claim 13, wherein the thresholds at the same position in the m-valued area of at least two dither threshold matrices are equal. 19. (n-1) dither threshold matrices in which a plurality of threshold matrices each including an m-valued area and an n-valued area are arranged and shifted by a half phase in the main scanning direction or the sub-scanning direction. Generating (n-1) quantizing thresholds that vary periodically, wherein thresholds at the same position in the m-valued area of at least two dither threshold matrices are equal. 14. The image forming method according to item 13. 20. The image forming method according to claim 18, wherein threshold values at the same position in the m-valued areas of all dither threshold value matrices are equal. 21. The n-valued area of the dither threshold matrix is arranged around the m-valued area, and the m-valued area is a dot concentration type area.
0. The image forming method according to item 1. 22. The image forming method according to claim 21, wherein a maximum threshold value in the m-valued area of the dither threshold value matrix is smaller than a minimum threshold value in the n-valued area. 23. The difference between the minimum threshold value in the n-valued area and the maximum threshold value in the m-valued area of the dither threshold matrix is m
23. The image forming method according to claim 22, wherein the step width is larger than a step width of a threshold value within the binarized area. 24. Setting all thresholds in the n-valued area of the dither threshold matrix for generating the highest quantization threshold to constant values larger than the thresholds in the n-valued areas of the other dither threshold matrices. 18. The method according to claim 18, wherein:
4. The image forming method according to any one of 3. 25. The image forming method according to claim 18, wherein a dither threshold matrix used for generating each quantization threshold is switched according to image characteristics of a multi-tone image. . 26. The image forming method according to claim 25, wherein the fluctuation range of the quantization threshold value is reduced as the edge degree of the multi-tone image increases. 27. The image according to claim 26, wherein the same dither threshold matrix composed entirely of m-valued regions is used for generating all quantization thresholds in a region where the degree of edge of the multi-tone image is large. Forming method. 28. The image forming method according to claim 26, wherein all quantization thresholds are fixed to the same constant value in a region where the degree of edge of the multi-tone image is maximum. 29. A first means for adding an error to multi-tone image data, a second means for quantizing the image data to which the error has been added by the first means, and a quantized data by the second means. A third means for obtaining an error to be added to the multi-gradation image data from the image data after the error addition by the first means and providing the error to the first means; and a quantization threshold for the second means, And fourth means for periodically varying in space, wherein the quantization threshold value is within
Fluctuates to be dot-concentrated in the value range corresponding to the low and medium density levels of multi-tone image data, and to be dot-dispersed in the value range corresponding to the high density level of multi-tone image data An image processing apparatus comprising: 30. An image processing apparatus according to claim 29, wherein said fourth means changes a variation range of a quantization threshold according to an image feature of multi-tone image data. 31. The image processing apparatus according to claim 30, wherein said fourth means reduces the variation width of the quantization threshold value in an area where the degree of edge of the multi-tone image data is large. 32. An image processing apparatus according to claim 30, wherein said fourth means keeps the average value of the quantization threshold at a substantially constant value. 33. The apparatus according to claim 33, wherein the fourth means varies the quantization threshold value using a dither threshold value matrix including a dot concentration type area and a dot dispersion type area having a larger threshold value around the dot concentration type area. Item 30. The image forming apparatus according to Item 29. 34. The fourth means shifts a plurality of threshold matrices each composed of a dot concentration type area and a dot dispersion type area having a larger threshold value around the dot concentration type area by a half phase in the main scanning direction or the sub scanning direction. 30. The image processing apparatus according to claim 29, wherein the quantization threshold value is varied using a dither threshold matrix that has been arranged and enlarged in accordance with the present invention. 35. A difference between a minimum threshold value in the dot dispersion type region and a maximum threshold value in the dot concentration type region of the dither threshold value matrix is larger than a step width of the threshold value in the dot concentration type region. The image processing apparatus according to claim 33 or 34. 36. The image processing apparatus further comprises: fifth means for extracting and outputting an image feature of the multi-tone image data, wherein the fourth means changes a quantization threshold in accordance with the image feature output from the fifth means. 34. The width is changed.
5. The image processing device according to 4. 37. A quantization threshold value, wherein a value obtained by adding a constant value to a value obtained by multiplying a threshold value of a dither threshold value matrix by a coefficient corresponding to an image feature outputted from the fifth means is used as the quantization threshold value. The image processing apparatus according to claim 36, wherein: 38. The image processing apparatus according to claim 36, wherein said fourth means switches a dither threshold matrix for changing a quantization threshold according to the image feature output from said fifth means. . 39. The fifth means outputs an edge degree of the multi-tone image data as an image feature, and the fourth means outputs a change in the quantization threshold as the edge degree output from the fifth means increases. 4. The method according to claim 3, wherein
39. The image processing apparatus according to 7 or 38. 40. The image processing apparatus according to claim 39, wherein said fourth means fixes the quantization threshold to a constant value when the edge degree output from said fifth means is maximum. 41. An image processing apparatus according to claim 39, wherein said fifth means outputs an edge degree subjected to an area expansion process. 42. The image processing apparatus according to claim 41, wherein an expansion width of the area expansion processing is selected within 0.5 mm in an image space. 43. A first means for adding an error to multi-tone image data, a second means for quantizing the image data to which the error has been added by the first means by n values (n ≧ 3), A third means for obtaining an error to be added to multi-tone image data from the quantized data by the two means and the image data after the error addition by the first means and providing the error to the first means; And (4) means for generating (n-1) quantizing thresholds which fluctuate periodically and applying the same to the second means, wherein at least two quantizing thresholds have the same value at a specific position within the fluctuation cycle. An image processing apparatus characterized by taking. 44. The at least two quantization thresholds,
44. The image processing apparatus according to claim 43, wherein the same value is taken at a central portion within the fluctuation period. 45. An image processing apparatus according to claim 43, wherein said fourth means changes a variation range of a quantization threshold according to an image feature of multi-tone image data. 46. The image processing apparatus according to claim 45, wherein said fourth means reduces the variation width of the quantization threshold in an area where the degree of edge of the multi-tone image data is large. 47. An image processing apparatus according to claim 45, wherein said fourth means keeps the average value of the quantization threshold at a substantially constant value. 48. The fourth means uses (n-1) dither threshold matrices each composed of an m-valued area and an n-valued area, and (n-1) periodically varying quantizations. 44. The image processing apparatus according to claim 43, wherein a threshold is generated, and thresholds at the same position in the m-valued area of at least two dither threshold matrices are equal. 49. The fourth means, wherein a plurality of threshold matrices each composed of an m-valued area and an n-valued area are arranged and shifted by a half cycle in the main scanning direction or the sub-scanning direction.
Generate (n-1) periodically varying quantization thresholds using -1) dither threshold matrices, at least 2
44. The image processing apparatus according to claim 43, wherein the thresholds at the same position in the m-valued area of the two dither threshold matrices are equal. 50. The threshold value at the same position in the m-valued area of all dither threshold value matrices is equal to each other.
Or the image processing device according to 49. 51. The image processing apparatus according to claim 48, wherein the n-valued area of the dither threshold matrix is arranged around the m-valued area, and the m-valued area is a dot concentration type area. 52. The image processing apparatus according to claim 51, wherein the maximum threshold value in the m-valued area of the dither threshold value matrix is smaller than the minimum threshold value in the n-valued area. 53. The difference between the minimum threshold value in the n-valued area and the maximum threshold value in the m-valued area of the dither threshold matrix is m
53. The image processing apparatus according to claim 52, wherein the step width is larger than a step width of a threshold value in the binarization area. 54. All thresholds in the n-ary region of the dither threshold matrix for generating the highest quantization threshold
The image processing apparatus according to claim 51, 52 or 53, wherein a constant value larger than a threshold value in an n-value area of a dither threshold value matrix for generating another quantization threshold value is set. 55. A fifth means for extracting and outputting an image feature of the multi-tone image data, wherein the fourth means sets each quantization threshold according to the image feature output from the fifth means. The image processing apparatus according to any one of claims 48 to 54, wherein a dither threshold matrix used for generation is switched. 56. The fifth means outputs the edge degree of the multi-tone image data as an image feature, and the fourth means outputs a change in the quantization threshold as the edge degree output from the fifth means increases. 6. The method according to claim 5, wherein
6. The image processing apparatus according to 5. 57. The fourth means, wherein when the degree of edge output from the fifth means is large, the same dither threshold matrix composed entirely of m-valued regions is used for generating all quantization thresholds. 57. The image processing device according to claim 56, wherein: 58. The apparatus according to claim 56, wherein the fourth means fixes all quantization thresholds to the same constant value when the degree of edge output from the fifth means is maximum.
An image processing apparatus as described in the above. 59. The apparatus according to claim 56, wherein said fifth means outputs an edge degree subjected to an area expansion process.
59. The image processing apparatus according to 57 or 58. 60. The image processing apparatus according to claim 59, wherein an expansion width of the area expansion processing is selected within 0.5 mm in an image space. 61. A computer-readable storage medium storing a program for causing a computer to realize the functions of each means of the image processing apparatus according to claim 29. 62. An apparatus according to claim 29, further comprising an image forming means for forming an image using dots for outputting an image of the quantized data.
The image processing device according to the item. 63. The image processing apparatus according to claim 29, further comprising image reading means for inputting multi-tone image data by optically scanning a document. 64. An image reading means for inputting multi-tone image data by optically scanning a document, and an image forming means for forming an image using dots for outputting an image of quantized data. The image processing apparatus according to any one of claims 29 to 60, comprising: