JPH08204964A - Image processor - Google Patents

Abstract

PURPOSE: To provide an image processor which can improve the picture quality of an image at a part where an error diffusion method and a dither method are switched to each other. CONSTITUTION: Plural different threshold value (th) are previously stored in a dither matrix table 15, and these value (th) are selectively and successively read out for every pixel and stored in a threshold value storage register 16. Then the value (th) are applied to a value decider 23. The decider 23 compares the output value INij of an adder 11 with the value (th). Then the decision value x (=1) is applied to an output decider 24 when Inij >th is satisfied, and the value x (=0) is applied to the decider 24 in other cases. The decider 24 outputs the output OUTij (=255) to a printer 9 when the value (x) is equal to 1 and then outputs OUTij (=0) to the printer 9 when the value (x) is equal to 0 respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus for processing image data by an error diffusion method in a digital copying machine, a digital facsimile apparatus or the like.

[0002]

2. Description of the Related Art Conventionally, as this type of image processing apparatus,
For example, as shown in Japanese Unexamined Patent Publication No. 1-115272, image data is quantized by multi-valued dither in the non-edge portion and quantized by an error diffusion method in the edge portion to emphasize the edge in the edge portion, Proposes a method of preventing the generation of pseudo contours and reproducing a halftone image.

That is, this method is a method of selectively utilizing the error diffusion method and the dither method according to the edge amount in order to reduce the bit number (quantization), and the error diffusion method is used for a portion having a large edge amount. The purpose is to prevent the generation of moire by the dither method and the graininess and texture by the error diffusion method by sequentially switching the portion with a small edge amount by the dither method to lower the bit.

[0004]

However, the above-mentioned conventional method has a problem that an image is deteriorated at a portion where the error diffusion method and the dither method are switched.

In view of the above-mentioned conventional problems, it is an object of the present invention to provide an image processing apparatus capable of improving the image quality in a portion where the error diffusion method and the dither method are switched.

[0006]

In order to achieve the above-mentioned object, a first means includes an error diffusion processing means for processing digital image data by an error diffusion method, and data processed by the error diffusion processing means. And a dither processing unit for performing binarization by performing dither processing while changing the threshold value with a dither matrix.

The second means is an error diffusion processing means for processing digital image data by an error diffusion method, and the data processed by the error diffusion processing means is dithered while changing the threshold value by a dither matrix. And a dither processing means for converting into a multi-value level equal to or more than a value.

A third means is to shift the threshold values of the error diffusion processing means for sequentially processing the YMCK digital image data by the error diffusion method and the YMCK data processed by the error diffusion processing means for each YMCK. And a dither processing means for performing binarization by sequentially performing dither processing.

A fourth means is to shift the threshold values of the error diffusion processing means for sequentially processing the YMCK digital image data by the error diffusion method and the YMCK data processed by the error diffusion processing means for each YMCK. And a dither processing unit that sequentially performs dither processing to convert to a multi-valued level of three or more values.

[0010]

In the first means, since the data subjected to the error diffusion processing is binarized by performing the dither processing while changing the threshold value with the dither matrix, the gradation can be improved and the graininess can be reduced. Further, since the error diffusion method is also used, the resolution can be improved, and the image quality at the portion where the error diffusion method and the dither method are switched when the binary printer is used is improved.

In the second means, the error-diffused data is converted into a multi-valued level of three or more values by performing dithering while changing the threshold value in the dither matrix, so that the gradation is improved and the granularity is improved. In addition, since the error diffusion method is also used, the resolution can be improved, and the image quality at the portion where the error diffusion method and the dither method are switched is improved when a multi-valued printer with three or more values is used.

In the third means, since the error-diffused data is binarized by sequentially shifting the threshold value for each YMCK and performing dither processing, the gradation is improved and the graininess can be reduced. Moreover, since the error diffusion method is also used, the resolution can be improved, and the image quality at the portion where the error diffusion method and the dither method are switched is improved when a color printer is used.

In the fourth means, the error-diffused data is binarized by sequentially shifting the threshold value for each YMCK and dithering it, so that the gradation is improved and the graininess can be reduced. Moreover, since the error diffusion method is also used, the resolution can be improved, and the image quality at the portion where the error diffusion method and the dither method are switched is improved when a color printer is used.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. 1 is a block diagram showing a bit reduction circuit which is an embodiment of an image processing apparatus according to the present invention, FIG. 2 is a block diagram showing a monochrome image processing apparatus equipped with the bit reduction circuit of FIG. 1, and FIG. FIG. 4 is a block diagram showing in detail the sum calculator of the surrounding errors, FIG. 4 is an explanatory diagram showing error diffusion matrix coefficients stored in advance in the error matrix table of FIG. 1, and FIG. 5 is an output value calculation of FIG. FIG. 6 is a block diagram showing the error calculator in detail, and FIG. 6 is a block diagram showing the error calculator in FIG. 1 in detail.

First, a monochrome image processing apparatus having the bit reduction circuit 8 of this embodiment will be described with reference to FIG. FIG. 2 shows the flow of data until the original image is read by the scanner 1 and output to the printer 9, and the image signal input through the scanner 1 is converted by the AD conversion circuit 2 into 256-values each pixel having 8 bits. To be converted. The γ characteristic of the scanner 1 is corrected by the scanner γ correction circuit 3 for this image data, and then the background of the original is removed by the background removal circuit 4.

The low-pass filter 5 removes the moiré from the image data read by the scanner 1 and the moiré in the bit reduction circuit 8, and then the MTF filter 6 prevents character breaks and image blurring.
The .gamma. Characteristic of the printer 9 is corrected by the printer .gamma. Correction circuit 7, and the image data is output to the printer 9 after it has been converted into a bit by the bit reduction circuit 8 shown in detail in FIG.

FIG. 1 shows, as an example, a bit reduction circuit 8 which performs binarization. The adder 11 adds the input data INPUT _ij from the printer γ correction circuit 7 and the value ERROR _ij from the summation calculator 12 of the surrounding errors, and the value IN _ij
Is output to the output value calculator 14. In the surrounding error sum calculator 12, as shown in detail in FIG.
1, the error DFF for two lines from the error calculator 17
_{Based on ij} and the error diffusion matrix coefficient α stored in advance in the error matrix table 13, the addition value SUM is obtained by the following equation (1).

SUM = Σα _{k, l} DFF _{i + k, j + l} (1) The error matrix table 13 stores error diffusion matrix coefficients α _{k, l} as shown in FIG. 4, for example. The added value SUM for the pixel is obtained. The added value SUM is divided by the error diffusion matrix coefficient α _{k, l} in the divider 22 to obtain the error ERROR _ij , and the adder 11 calculates IN _ij = INPUT _ij + ERROR _ij (2).

The output value IN _ij of the adder 11 is the output value calculator 14 shown in detail in FIG. 5 and the error calculator 1 shown in FIG. 6 in detail.
7 is applied. In FIG. 5, the output value I of the adder 11
N _ij is compared with the threshold value th by the magnitude judging unit 23, and when IN _ij > th, x = 1, and in other cases, x = 0 (3) is applied to the output determining unit 24. . The output determiner 24 outputs OUT _ij = 255 and x = 0 when x = 1.
In the case of, the error calculator 1 outputs the output OUT _ij of OUT _ij = 0.
7 and the printer 9.

Here, a plurality of different threshold values th are stored in advance in the dither matrix table 15 shown in FIG. 1, and one of them is selectively read out sequentially for each pixel and stored in the threshold value storage register 16 to determine the magnitude. Applied to the device 23.
Therefore, since the threshold th changes, the gradation is improved and the graininess can be reduced when the dither processing is performed. Moreover, since the error diffusion method is also used, the resolution can be improved.

[0021] The error calculator 17 as shown in detail in FIG. 6, the error DFF _ij = OUT _ij -IN _ij output value OUT _ij output value IN _ij and the output value calculator 14 of the adder 11 ... (4) It is calculated and applied to the sum calculator 12 of the surrounding errors shown in FIG.

Next, a second embodiment will be described with reference to FIGS. 7 and 8. FIG. 7 shows, as an example, a low-bit conversion circuit 8 that performs four-value conversion, and FIG. 8 shows the output value calculator 14 ′ shown in FIG. Since other configurations are the same as those of the first embodiment, the same reference numerals are given and detailed description thereof will be omitted.

The dither matrix table 15 shown in FIG.
, Three thresholds th1, th2, th3 (0 <t
h3 <85 <th2 <170 <th1 <255) is 24
Calculated and stored from a dither matrix of × 8 pixels,
It is read out, stored in the threshold value storage register 16, and applied to the magnitude determining units 231 to 233 shown in FIG. 8, respectively.

The magnitude determiner 231 determines the output value I of the adder 11.
The threshold value th1 is compared with N _ij, and when IN _ij > th1, x = 1, and in other cases, x = 0 is output to the output determiner 24 ′. The large / small discriminator 232 compares the value IN _ij with the threshold value th2. When IN _ij > th2, y = 1, and y = 0 in other cases, and the large / small discriminator 233 compares the value IN _ij with the value IN _ij . The threshold value th3 is compared, and the judgment value z of z = 1 if IN _ij > th3, and z = 0 otherwise is output to the output determiner 24 ′.

The output determiner 24 'outputs OU when x = 1.
If T _ij = 255, x = 0 and y = 1, then OUT _ij =
170, OUT when x = 0, y = 0 and z = 1
_{ij when ij} = 85, x = 0, y = 0 and z = 0
Output T _ij = 0.

Next, a third embodiment will be described with reference to FIG. FIG. 9 shows a color image processing apparatus provided with the bit reduction circuit of the above-described embodiment, in which each RGB signal read by the color CCD scanner 1 is converted by the AD conversion circuit 2 into one.
The pixel is converted to an 8-bit 256 value. The RGB image data is corrected by the dot correction circuit 31 for a time lag in reading by the CCD, and then the γ characteristic of the scanner 1 is corrected by the scanner γ correction circuit 3.

The background image of the original is removed by the background removal circuit 4 from the RGB image data, and then the color conversion circuit 32 is used.
Is converted into YMCK data for printing. This YM
The CK data is removed by the low-pass filter 5 from the moiré when read by the scanner 1 and the moiré in the bit reduction circuit 8, and then the MTF filter 6 prevents character breaks and image blurring. The γ characteristic of the printer 9 is corrected by the printer γ correction circuit 7 for this image data, and then the number of bits is lowered by the one-bit lowering circuit 8 as shown in detail in FIGS.
Is output to

In such a color image processing apparatus, the YM
Since the processing after conversion to CK data is frame sequential,
The bit reduction circuit 8 can be dealt with by the circuits shown in FIGS. Here, if the dither matrices are all the same for YMCK data, interference occurs and causes moire. Therefore, set the coefficients of each dither matrix so that halftone dots grow with a sufficient difference in angle. Therefore, moire can be prevented.

[0029]

As described above, according to the first aspect of the present invention, the data subjected to the error diffusion processing is binarized by performing the dither processing while changing the threshold value with the dither matrix.
It can improve the gradation ga and reduce the graininess.
Since the error diffusion method is also used, the resolution can be improved. Therefore, when a binary printer is used, it is possible to improve the image quality at the portion where the error diffusion method and the dither method are switched.

According to the second aspect of the present invention, the error-diffused data is converted into a multivalued level of three or more values by performing dithering while changing the threshold value with a dither matrix.
Gradation is improved, graininess can be reduced, and
Since the error diffusion method is also used, the resolution can be improved. Therefore, it is possible to improve the image quality in a portion where the error diffusion method and the dither method are switched when a multi-value printer with three or more values is used.

According to the third aspect of the present invention, since the error-diffused data is binarized by sequentially shifting the threshold value for each YMCK and performing dither processing, the gradation is improved and graininess is reduced. Moreover, since the error diffusion method is also used, the resolution can be improved. Therefore, when a color printer is used, it is possible to improve the image quality at the portion where the error diffusion method and the dither method are switched.

According to the fourth aspect of the present invention, the error-diffused data is binarized by sequentially shifting the threshold value for each YMCK and performing dither processing. Therefore, gradation is improved and graininess is reduced. Moreover, since the error diffusion method is also used, the resolution can be improved. Therefore, when a color printer is used, it is possible to improve the image quality at the portion where the error diffusion method and the dither method are switched.

[Brief description of drawings]

FIG. 1 is a block diagram showing a bit reduction circuit which is an embodiment of an image processing apparatus according to the present invention.

FIG. 2 is a block diagram showing a monochrome image processing apparatus including the bit reduction circuit of FIG.

FIG. 3 is a block diagram showing in detail the summation calculator for the surrounding errors in FIG.

FIG. 4 is an explanatory diagram showing error diffusion matrix coefficients stored in advance in the error matrix table of FIG.

5 is a block diagram showing in detail the output value calculator of FIG. 1. FIG.

FIG. 6 is a block diagram showing the error calculator of FIG. 1 in detail.

FIG. 7 is a block diagram showing a low bit circuit of a second embodiment.

8 is a block diagram showing the output value calculator of FIG. 7 in detail.

FIG. 9 is a block diagram showing a color image processing apparatus of a third embodiment.

[Explanation of symbols]

 11 adder 12 sum calculator of surrounding errors 13 error matrix table 14, 14 'output value calculator 15 dither matrix table 16 threshold value storage register 17 error calculator

Claims (4)

[Claims] 1. An error diffusion processing means for processing digital image data by an error diffusion method, and dither processing the data processed by the error diffusion processing means while changing a threshold value by a dither matrix.
An image processing apparatus comprising: a dither processing unit for binarizing. 2. An error diffusion processing unit for processing digital image data by an error diffusion method, and dither processing the data processed by the error diffusion processing unit while changing a threshold value by a dither matrix.
An image processing apparatus comprising: a dither processing unit for converting into a multi-valued level equal to or higher than a value. 3. An error diffusion processing means for sequentially processing each digital image data of YMCK by an error diffusion method, and a dither processing for each YMCK data processed by the error diffusion processing means by shifting a threshold value for each YMCK. And a binarizing dither processing unit. 4. An error diffusion processing means for sequentially processing each digital image data of YMCK by an error diffusion method, and a dither processing for each YMCK data processed by the error diffusion processing means by shifting a threshold value for each YMCK. And a dither processing means for converting into a multi-valued level of three or more values,
An image processing apparatus equipped with.