JP2003110852A - Halftone processing method and processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a screening method mixing the merits of systematic dithering having an advantage that dots can be formed in a specified shape and of error diffusion method exhibiting excellent reproducibility of detail, and a halftone processor implementing that method. SOLUTION: In the halftone processing method and processor, a step for storing a threshold for binarizing gray scale image values as dither matrices in a memory and binarizing one objective pixel selected from a gray scale image through sequential scanning using threshold values selected sequentially from the dither matrices, and a step for feeding an error obtained by subtracting the binarized value from the original value of the objective pixel back to not yet processed gray scale image data on the periphery of the objective pixel of the objective image are repeated alternately and the gray scale image is binarized thus obtaining a pseudo gray scale image.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pseudo grayscale (halftone) image processing method and apparatus for a digital image used for printing or the like.

[0002]

2. Description of the Related Art There are two typical algorithms, a dither method and an error diffusion method, as processing methods used when generating a halftone image from a gradation image in printing or the like.

The dither method is one of the algorithms for converting an original gradation image into a black and white binary pseudo gradation image, and makes pseudo contours and the like generated when binarizing them look inconspicuous and beautiful. For this purpose, a random dither that intentionally adds a small amount of random noise to the original image data and then performs binarization processing, and a dither matrix in which the binarization threshold is described in a regular pattern are repeated. It is roughly classified into two methods of ordered dither in which binarization processing is performed by applying it to an original image. The latter is a so-called screening method. FIG. 1 is a block diagram conceptually expressing the processing of such an organized dither method. The matrix values of the above-mentioned dither matrix are stored in the dither mask of FIG. 1, and the threshold value processing is performed according to the sequentially fetched values.

The AM screen is a type of organized dither. An example of the dither matrix is shown in FIG. For printing, an AM screen having excellent printability is often used.

On the other hand, the error diffusion method is also an image processing method for converting an original gradation image into a binary image. This is a method in which the error that occurs during binarization is dispersed to surrounding pixels so that the cumulative color error of the entire image is substantially zero. A good filter for diffusing the binarization (more generally, quantization) error is Floyd & Steinberg shown in FIG.
Two types of filters are well known, a Jarvis, Judice & Ninke type filter shown in FIG. FIG. 2 is a block diagram conceptually expressing the processing of such an error diffusion method. In FIG. 2, threshold processing is always performed with a fixed threshold. For example, in the case of 8-bit grayscale image data for one pixel, the threshold value is a constant value such as 127. Then, as a result of the binarization process, the output value is either 0 or 255, so the value obtained by subtracting 0 or 255 from the original value becomes an error, and through the diffusion filter, the unprocessed pixels around the pixel just processed are unprocessed. Is fed back (added) to the pixels of.

[0006]

In the systematic dither method, halftone dots having a specific shape can be formed by a threshold matrix (called a dither mask, a halftone mask, a dither matrix, etc.) designed in advance. The reproducibility of details is not sufficient. In particular, a dither pattern having excellent printability has a drawback that the reproducibility of details is not good. On the other hand, although the error diffusion method is generally excellent in reproducibility of details, it has a drawback that a unique texture (worm noise, granular noise, etc.) is generated, and the printability is originally poor.

The present invention has been made in consideration of the above problems, and systematic dither having an advantage that halftone dots having a specific shape can be formed, and error diffusion excellent in reproducibility of details. It is an object to provide a screening method that mixes the advantages of both methods.

[0008]

A first invention for solving the problem is a half for outputting a monochrome gradation image as a pseudo gradation image by an output device which can take only a binary output state of black and white. A tone processing method, wherein a plurality of thresholds for binarizing a gradation image value are stored in a storage device as a threshold matrix, and the gradation images are sequentially scanned with thresholds selected in order from the threshold matrix. A binarization step of binarizing one selected target pixel, and an error obtained by subtracting the value of the binarization result from the original value of the target pixel is unprocessed around the target pixel of the target image. The error diffusion step of feeding back to the gradation image data under constant conditions is alternately repeated, and the gradation image is binarized to obtain a pseudo gradation image. It is a method. FIG. 3 is a block diagram conceptually explaining the halftone processing method of the present invention.

A second invention for solving the problem is a halftone processing device for outputting a monochrome gradation image as a pseudo gradation image by an output device which can take only a binary output state of black and white. , A plurality of thresholds for binarizing the gradation image value as a dither matrix, and a coefficient value matrix for error diffusion as an error diffusion filter are stored in a storage unit, respectively, and the dither matrix is sequentially stored. A binarization processing unit that binarizes one target pixel selected by scanning sequentially from the gradation image with a selected threshold, and a difference between an original value of the target pixel and a binarized result value. And a feedback processing unit for diffusing the obtained error to the periphery of the target pixel of the target image through the error diffusion filter, and converting the monochrome gradation image into a pseudo gradation image. Features A halftone processing apparatus.

A third invention for solving the problem is a second invention.
A halftone processing device for a color image, wherein a plurality of sets of processing units of the halftone processing device of the invention according to the invention are provided according to the number of colors so that a pseudo gradation image for each color can be obtained from a color image.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The method of the present invention and a halftone processing apparatus 1 which is an embodiment of the present invention based on the method will be described below with reference to the drawings. The halftone processing device 1 is obtained by converting the block diagram shown in FIG. 3 into a device. FIG. 4 is a block diagram illustrating the configuration of the main part of the halftone processing apparatus 1.

Reference numeral 30 is an original image data storage unit. This is a memory in which original image data is stored.
Reference numeral 31 is a generated halftone image data storage unit. The memory stores halftone image data. Reference numeral 10 is a binarization processing unit. A memory 11 stores a dither matrix. 12 is an error calculation unit. The difference between the current pixel value of the original data and the binarized value of the pixel is calculated and sent to the feedback processing unit 13. Reference numeral 13 is a feedback processing unit. The system value for diffusion is read from the error diffusion filter 14 that stores the coefficient value of the error diffusion filter, and the error calculation unit 1
The error calculated by 2 is diffused to the pixels around the target pixel. Reference numeral 20 is a data signal line, and 21 is an address signal line.

Below, the gradation data of the original image is 2
56 levels (0 to 255), halftone image data is 0
Only binary values of (black) or 255 (white) are taken.

The operation of the binarization processing unit 10 will be further described. The binarization processing unit 10 has a built-in function of generating an appropriate address, and the original image data storage unit 30
Is sequentially scanned according to the address generated by itself, and the data of the target pixel is read. In the dither matrix storage unit 11,
The dither mask is recorded. It is, for example, 16x
In the 16 matrices, values as shown in FIG. 5 are recorded as an example. FIG. 5 is a dither mask for an AM screen.
Now, the position of the target pixel on the original image data is (i,
In the case of j), the stored value at the position (imod16, jmod16) of the dither matrix 11 is taken out and binarized using this as a threshold value. However, imod16 is the remainder obtained by dividing i by 16. The result is recorded in the address (i, j) of the halftone image storage unit 31.

At the same time, the error calculation unit 12 calculates e = {(binarization result)-(original gradation value at the position of the original image (i, j))} × (-1), and the error is calculated. The value e is sent to the feedback processing unit 13.

An error diffusion coefficient matrix is recorded in the error diffusion filter processing section 14. FIG. 6 shows an example of the error diffusion coefficient matrix. The feedback processing unit 13 multiplies the error value e by the coefficient of the error diffusion matrix as (i,
Add to the value of the unprocessed pixel around j). According to the error diffusion coefficient matrix shown in FIG. 6, (i + 16, j) to (i + 31, j)
For 16 pixels of, e × (1/32) is added to the value of the pixel. After the error e is fed back to the original image data, the binarization processing unit 10 moves to the next position (i + 1, j).
The original image data is read with the target pixel as, and the subsequent processing is repeated. In this way, the halftone processing apparatus 1 alternately repeats the binarization step using the dither matrix and the step of feeding back the error generated by the binarization to the original image data, and as a result, a pseudo gradation image is obtained.

The halftone processing apparatus 1 according to the present invention has been described above.
Said. At the same time, the halftone processing method of the present invention is described. FIG. 7 is a halftone dot image obtained by the halftone processing method of the present invention. FIG. 8 is an image for comparison, which is a halftone dot image that has been binarized by the dither mask for the AM screen of FIG. It can be seen that the halftone dot image of FIG. 7 is superior in expression of gradation characteristics and details, probably because the quantization error accumulation is eliminated by the error diffusion method.

The binarization processing unit 10, the dither matrix storage unit 11, the error calculation unit 12, the feedback processing unit 13, and the error diffusion filter storage unit 14 of the halftone processing apparatus 1.
It is also possible to configure a halftone processing device that obtains a pseudo gradation binarized image for each color channel from a color image by providing the above for each color number of original image data.

Although the AM screen matrix of FIG. 5 is used as the dither mask and the error diffusion coefficient matrix of FIG. 6 is used as the error diffusion count matrix in the description of the present specification, the dither mask or the error diffusion used in the method of the present invention is used. The matrix is
It is not limited to those specific values. An appropriate dither mask and an appropriate error diffusion coefficient matrix may be selected and used according to the use and purpose. If it is desired to obtain a halftone image excellent in printability, the dither mask for AM screen shown in FIG. 5 or similar to FIG. 5 may be used as described in the description of the present embodiment.

[0020]

As described in detail above, by the halftone processing method of the present invention or the halftone processing apparatus for realizing the method, the quantization error accumulation obtained by the error diffusion method and the dither method can be obtained. It is possible to obtain the advantages of both generation of halftone dots of an arbitrary shape, and it is possible to obtain a remarkable effect that a good halftone image can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram conceptually explaining a conventional dither method.

FIG. 2 is a block diagram conceptually explaining a conventional error diffusion method.

FIG. 3 is a block diagram conceptually explaining the processing mechanism of the method and apparatus of the present invention.

FIG. 4 is a configuration diagram of a halftone processing device.

FIG. 5: Threshold matrix of AM screen, which is a type of organized dither

FIG. 6 shows an example of an error diffusion filter.

FIG. 7: Halftone image obtained by the method of the present invention

FIG. 8: Halftone image by systematic dither method (AM)

FIG. 9: Example of error diffusion filter (Floyd & Steinberg
Filter by)

FIG. 10: Example of error diffusion filter (Javis, Judice & Nin
filter by ke)

[Explanation of symbols]

10 binarization processing unit 11 Dither matrix storage 12 Error calculator 13 Feedback processing unit 14 Error diffusion filter storage unit 20 data signal lines 21 Address signal line 30 Original image data storage 31 Halftone image data storage section

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Claims (3)

[Claims] 1. A halftone processing method for outputting a monochromatic gradation image as a pseudo gradation image by an output device capable of only a binary output state of black and white, wherein the gradation image value is binarized. A plurality of thresholds at this time are stored in a storage device as a dither matrix, and a threshold value selected in order from the dither matrix is used to binarize one target pixel selected by scanning sequentially from the gradation image. An error diffusion step of feeding back an error obtained by subtracting the value of the binarization result from the original value of the target pixel to unprocessed gradation image data around the target pixel of the target image under a certain condition, The halftone processing method is characterized in that the pseudo gradation image is obtained as a result by binarizing the gradation image by repeating the above two steps alternately. 2. A halftone processing device for outputting a monochromatic gradation image as a pseudo gradation image by an output device which can only output a monochrome binary value, and binarizes the gradation image value. A plurality of thresholds at this time are stored as a dither matrix, and a coefficient value matrix for error diffusion is stored as an error diffusion filter in a storage unit. A binarization processing unit that binarizes one target pixel that is sequentially scanned and selected, an error calculation unit that calculates the difference between the original value of the target pixel and the value of the binarization result, and the calculated error And a feedback processing unit for diffusing the image around the target pixel of the target image through the error diffusion filter, and converting a monochrome gradation image into a pseudo gradation image. 3. A halftone processing apparatus, wherein a plurality of sets of the respective processing units according to claim 2 are provided according to the number of colors to obtain a pseudo gradation image for each color from a color image.