JPH0494266A - Half tone picture processing unit - Google Patents

Abstract

PURPOSE:To improve the reproducibility of gradation and of a detailed picture by discriminating the of gradient of a density change in a processed part of a half tone picture, selecting one of plural error spread processing means according to the result and outputting an output binarizing signal externally as a picture signal of the pseudo half tone picture. CONSTITUTION:An output of an inter-block dot number difference detection section 18 is small in a region where a density change is low and an output of a comparator 19 is '0'. Then an output binarizing signal of an error spread processing section 12B with a larger error reference picture element block size is selected by a selector 20 in the region in this way. On the other hand, an output of the inter-block dot number difference detection section 18 is small in a region where a density change is high and an output of a comparator 19 is '1'. Since a smaller error reference picture element size is preferred in this region, an output binarizing signal of an error spread processing section 12A is selected by the selector 20. Thus, a pseudo half tone picture with excellent gradation number and reproducibility of gradation is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a halftone image processing apparatus for converting a halftone image into a halftone representation (pseudo halftone image) using binary pixels.

2. Description of the Related Art One method of converting a halftone image into a halftone image using binary pixels, that is, a pseudo halftone image, is a method using so-called error diffusion processing.

Conventionally, a device that performs this error diffusion processing has a basic configuration as shown in FIG. The image signal of the halftone image is input terminal 1.
After being added to a correction signal, which will be described later, in an adder 2, it is input to a converter 4, where it is binarized using a threshold generated by a threshold generator 3, and is output to an output terminal 8. Connoctor 40 input image signal and its binarized signal (
A subtracter 5 obtains a difference signal from the pseudo halftone image signal),
This is stored in the error storage unit 6 as the error of the current pixel of interest. When the next pixel becomes the pixel of interest, the errors of surrounding pixels within the frame of B are read out from the error storage unit 6, with the pixel of interest being A, and the errors are weighted according to the position of each pixel. The addition is performed by the adder 7, and the result is input to the adder 2 as a correction signal and added to the pixel signal of the current pixel of interest.

According to the so-called error diffusion processing described above, the number of gradations and resolution reproducibility can be increased compared to the systematic dither method.

Note that an image signal processing apparatus disclosed in Japanese Patent Laid-Open No. 1-238373, for example, is known as an apparatus for obtaining a pseudo halftone image by similar error diffusion processing.

Problems to be Solved by the Invention However, when emphasizing resolution reproducibility, the pseudo halftone image signal obtained by the error diffusion process as described above has a short run length due to dot dispersion (for example, if the run length is 1 or 2). ) Due to the occurrence of many lumps of dots, MH
There is a problem in that the compression ratio is poor when encoding is performed using a run-length encoding method such as (Modify Huffman) or MR (Modified Read). This problem is very serious when applied to facsimile.

As a countermeasure, it is possible to increase the number of pixels (size of error reference pixel block) that refer to errors in error diffusion processing. References Robert Ulisiny; 6 Digital Halftoning, MI Tea Press 198
7 (Robert Ulichney: “Dig
ital Halftoning”, MI T-Pr
ess.

(1987), when the number of error reference pixels is increased, black or white dots are concentrated and the overall run length becomes longer, which certainly improves the compression ratio, but on the other hand, the image There is another problem in that the concentration of dots in the details of the image causes deterioration in resolution.

The present invention has been made in view of the above-mentioned problems, and it is possible to obtain a pseudo-halftone image that has good gradation reproducibility and reproducibility of image details, and does not deteriorate the compression rate of run-length encoding. An object of the present invention is to provide a halftone image processing device.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention performs error diffusion processing on the image signal of a halftone image using a plurality of error diffusion processing means having different error reference pixel block sizes. Based on the output binarized signal of the error diffusion processing means with the minimum value, the determination means determines the degree of speed of the density change in the portion of the halftone image being processed, and the selection means selects one of the plurality of error diffusion processing means according to the result. The configuration is such that one is selected and the output binary signal is outputted to the outside as an image signal of a pseudo halftone image.

Effect of the Invention With the above-described configuration, the present invention uses error diffusion processing with a large error reference pixel block size in image areas where density changes are gradual and resolution reproducibility is not a problem, so that dots that lead to deterioration of the compression rate of run-length encoding are eliminated. On the other hand, in image areas with large density changes (image details) where high resolution reproducibility is required, error diffusion with a small error reference pixel block size is used with emphasis on resolution. Through processing, a pseudo halftone image signal can be generated.

Embodiment FIG. 1 shows a schematic configuration of a halftone image processing apparatus according to an embodiment of the present invention, in which an image signal of a halftone image is inputted from an input terminal 10, and a pseudo signal generated by error diffusion processing is input. The image signal of the halftone image is output from the output terminal 11.

12A and 12B are both error diffusion processing units, and, for example, an error diffusion processing circuit 13 similar to the part surrounded by the broken line in FIG. 3 and an error memo 4 corresponding to the error storage unit 6 in FIG. 3, respectively. It consists of. One error diffusion processing section 12A has a small error reference pixel block size, and the other error diffusion processing section 12B has a large error reference pixel block size.

FIG. 2 is an explanatory diagram of the error reference pixel block, and (a)
(b) shows an error reference pixel block of the error diffusion processing section 12A, and (b) shows an error reference pixel block of the error diffusion processing section 12B. The square cells in FIG. 2 correspond to individual pixels, the cells marked with ◯ are pixels of interest, and the cells without marks are error reference pixels.

Reference numeral 15 denotes a determination unit that determines the degree of rapidity of density change in the image portion being processed in order to determine which output from the error diffusion processing units 12A and 12B is valid.

In this determination section 15, 16 is an error diffusion processing section 12A.
an in-block dot number counter that counts the number of black dots in a block of a predetermined size by inputting the output image signal of the
17 is a peripheral block dot number memory for temporarily storing the number of black dots in the surrounding blocks of the block containing the pixel of interest, and 18 is a memory for storing the difference in the number of black dots between the block containing the pixel of interest and four adjacent blocks. The inter-block dot number difference detecting section 19 is a comparator that compares the inter-block dot number difference detected by the detecting section 18 with a predetermined threshold value. The output of the comparator 19 is a determination result signal, which becomes a control signal for the selector 20.

The selector 20 selects the output binary signal of the error diffusion processing section 12A or 12B in accordance with the determination result signal, and outputs it to the output terminal 1 as an image signal of a pseudo halftone image.

Regarding the halftone image processing device configured as above,
The operation will be explained below.

The image signals of the halftone image are simultaneously input to the error diffusion processing units 12A and 12B, and the error diffusion processing circuit 13 performs error diffusion processing on the same pixel of interest using each error reference pixel block size, and binarizes the pixel of interest. The signal is output and the error is stored in the error memo January 4.

The intra-block dot number counter 16 of the determining section 15 receives the output binary signal from the error diffusion processing section 12A and counts the number of black dots within the block of interest. The inter-flock dot number difference detection unit 18 calculates the number of black dots in the block of interest input from the intra-block dot number counter 16 and the number of black dots in four blocks adjacent to the block of interest read out from the peripheral block dot number memo January 7. Find the maximum value of the difference between
This is output as the difference in the number of dots between blocks.

In a region where density changes are gradual, there is almost no difference in the number of black dots between adjacent blocks, so the output value of the block-to-block dot number difference detection section 18 is small, and the output of the comparator 19 is "
0". In such areas (flat image areas), even if the error reference pixel block size is increased, the resolution reproducibility will not be a problem, and the larger the error reference pixel block size, the better the compression ratio will be. The selector 20 selects the output binarized signal of the error diffusion processing unit 12B when the error reference pixel block size is the same since the occurrence of short run lengths that cause deterioration is reduced.

On the other hand, in areas with large density changes, that is, in the details of the image, there is a large difference in the number of black dots between adjacent blocks.
The output value of the inter-block dot number difference detection unit 18 becomes large, and the output of the comparator 19 becomes "1". In such a region, it is preferable that the error reference pixel block size is small in order not to deteriorate the resolution reproducibility, so the output binary signal of the error diffusion processing section 12A is
0 is selected.

In addition, by providing three or more error diffusion processing units with different error reference pixel block sizes and comparing the difference in the number of dots between blocks with two or more threshold values, the speed and speed of the density change can be determined in three or more stages. , the output of the error diffusion processing section may be selected based on the determination result.

The configuration of the error diffusion processing section is not limited to the basic configuration shown in FIG. 3, but may be, for example, the configuration shown in Japanese Patent Application Laid-Open No. 1-238373.

Effects of the Invention As is clear from the above explanation, the present invention can perform run-length encoding using error diffusion processing with a large error reference pixel block size in image areas where density changes are gradual and resolution reproducibility is not a problem. It outputs a pseudo-halftone image signal that suppresses the dispersion of dots that causes a deterioration of the compression ratio.On the other hand, in image areas with large density changes (image details) that require high resolution reproducibility, error reference is performed with emphasis on resolution. Since the configuration is capable of outputting a pseudo-halftone image signal through error diffusion processing with a small pixel block size, the overall number of gradations and resolution reproducibility are good, and MH, M
This method has the effect of being able to obtain a pseudo-halftone image signal with a good compression rate by run-length encoding such as the R method.

[Brief explanation of drawings]

FIG. 1 is a schematic block diagram of a halftone image processing device according to an embodiment of the present invention, FIG. 2 is an explanatory diagram of error reference pixel blocks of each error diffusion processing section in FIG. 1, and FIG. 3 is a diagram of a conventional halftone image processing device. 1 is a block diagram showing the basic configuration of a halftone image processing device using error diffusion processing. FIG. 2...Adder, 4...Comparator, 5...Subtractor, 7. Weighting is adder, 10...Input terminal, 11...
- Output terminal, 12A...Error diffusion processing unit with a small error reference pixel block size, 12B...Error diffusion processing unit with a large error reference pixel block size, 15...Determination unit,
16... Intra-block dot number counter, 17... Peripheral block dot number memory, 18... Inter-bronch dot number difference detection unit, 19... Comparator, 20... Selector. Name of agent: Patent attorney Shigetaka Awano and 1 other person

Claims (1)

[Claims] It performs error diffusion processing on the image signal of a halftone image input from the outside and outputs a binarized signal, and there are multiple error diffusion processes each with different sizes of pixel blocks that refer to errors during error diffusion processing. and determining means for determining the degree of slowness or rapidity of density change in the portion of the halftone image being processed based on the binarized signal output from the error diffusion processing means whose pixel block size for reference is the smallest. and a selection means for selecting one of the plurality of error diffusion processing means according to the result of the determination, and outputting the binarized signal outputted therefrom to the outside as an image signal of a pseudo halftone image. Features of halftone image processing device.