JPH0762867B2 - Halftone image screen number conversion device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] In order to convert the screen ruling of a halftone dot image represented by black and white pixels, the halftone dot image is divided into small areas, and the number of black pixels in the small area is set to the small area. After converting the representative value according to the overlapping state of the small area and the small area where the screen frequency is converted and the ratio of the area of the small area before and after the conversion of the screen frequency, a target halftone image is created. Therefore, it is possible to perform conversion without disturbance in gradation and screen ruling.

TECHNICAL FIELD The present invention relates to an apparatus for converting pseudo gradation data of a halftone dot image.

A method called a halftone image is used as a method of outputting a grayscale image such as a photograph by an output device capable of only binary recording, for example, a printing device.

For the halftone dot image, for example, a predetermined one in a 10 × 10 area is blacked according to the gradation of one small area of the input grayscale image using the halftone dot pattern table as shown in FIG. Pseudo gradation is obtained as a pixel. For example, if the gradation is “20”, the portion of the numerical value of 20 or less in the table is set as the black pixel.

Pseudo-gradation (halftone conversion) of a grayscale image is performed at the time of image input according to the linear density of the output device. However, when outputting to an output device with a different linear density, pseudo-gradation roughness and trimming are performed. There is a problem that occurs, and this solution is desired.

[Prior Art] Conventionally, in order to output halftone dot image data to output devices having different line densities, halftone dot image data is combined with simple thinning, logical thinning, or simple enlargement processing. I am trying to create the desired halftone image data.

The simple thinning-out is simply thinning out one line every several lines, and the logical thinning-out is a thinning-out after logical ORing in order to prevent loss of important pixels.

A combination of thinning and simple enlargement was used to expand, for example, 2/3.

[Problems to be Solved by the Invention] Since conventional processing for image data conversion is performed by a combination of simple thinning, logical thinning, and simple enlargement, gradation disorder and screen frequency disorder Occurs, and the coordinate value exceeds a predetermined range to cause trimming.

The present invention is intended to provide a novel halftone image screen frequency conversion device which solves the above-mentioned conventional problems.

[Means for Solving Problems] FIG. 1 is a block diagram showing the principle of a halftone image screen frequency conversion device according to the present invention.

In the figure, 1 is a representative value calculating means, which divides a screen frequency conversion target image into square areas having one side of a halftone dot period, and calculates the number of black pixels in each square area as a representative value of each area. To do.

Reference numeral 2 is a representative value converting means, which enlarges (when increasing the number of screen lines) or reduces (the number of screen lines) the representative value group calculated by the representative value calculating means 1 according to the overlapping state of square areas before and after conversion. Then, the representative value is converted according to the area ratio of the small regions before and after the conversion of the screen ruling to create a representative value group after the conversion of the screen ruling.

For example, 70% of the area of one square area after conversion is included in the square area A before conversion, 30% is included in the square area B, the representative value of the area A is 10, and the area B is Has a typical value of 12
If it is, the representative value of the square area is 10 × 0.7 + 1
If the area ratio of the square area before and after conversion is 100: 64, it will be 10.6 × (64/100).
= 6.8 = 7 is converted.

This conversion is to match the ratio of the number of expressed gradations before and after the conversion.

Reference numeral 3 denotes a halftone image generation means, which converts the screen frequency from the representative value group after the conversion of the screen frequency created by the representative value conversion means 2 using a halftone dot pattern table for the screen frequency after conversion of the screen frequency. Create a halftone dot image later.

[Operation] With the configuration of the present invention, the halftone image is replaced with a representative value, data conversion is performed based on the overlapping state of the small regions before and after conversion and the area ratio before and after conversion, and then the halftone image is generated again. Therefore, it is possible to obtain a converted image in which gradation and screen ruling are not disturbed.

[Examples] The present invention will be described more specifically with reference to Examples shown in Figs. 2 to 8.

FIG. 2 is a circuit configuration block diagram of an embodiment of the present invention.

In FIG. 2, reference numeral 11 denotes a black pixel counter, which counts the number of black pixels from the input halftone image scanning data and writes the count value in the memory 14.

The pixel counter 12 and the line counter 13 determine a small area in which one side has a halftone dot period, and control the count range of the black pixel counter 11. The number of black pixels in each small area is stored in the memory 14 as a representative value.

FIG. 3 is a diagram showing an example of a halftone dot image to be converted according to the present embodiment, which is a halftone dot image with a screen ruling called 45 degrees / 64 lines (64 lines per inch in the 45 degree direction). is there.
As shown in the figure, the halftone dot period is 10 pixels.

FIG. 4 shows a representative value group, that is, a representative image, obtained for each small area in the memory 14.

Returning to FIG. 2 again, reference numeral 21 denotes an enlargement / reduction calculation table, which is a table for performing a calculation for converting the representative value and expanding according to the overlap of the small areas generated by the screen number conversion. 22c and 22d are multipliers that multiply each overlapping area and its representative value, 23 is an adder that adds the multiplication results of these multipliers, and the expanded representative value group calculated by this is stored in the memory 24. Remembered.

Reference numeral 25 is a representative value conversion table, a table for converting the representative value stored in the memory 24 according to the area ratio, and 26 is a multiplier for performing the calculation.

The obtained representative image after conversion is stored in the memory 27.

FIG. 5 is a diagram showing the overlap of the small areas of the 45 ° / 64-line halftone dot image shown in FIG. 3 and the 45 ° / 80-line halftone dot image (halftone dot period 8 pixels) to be converted. .

FIG. 6 is a diagram showing a representative image conversion from 45/64 lines to 45/80 lines.

It is calculated and expanded according to the overlap of small areas, and then the representative value is converted by the area ratio, ie 64/100.

Returning to FIG. 2 again, 31 is a halftone dot pattern generator, and white and black pixels are arranged from the converted representative image stored in the memory 27 using a halftone dot pattern table of 45 ° / 80 lines. Then, a halftone image of 45 degrees / 80 lines is created. The created halftone dot pattern is stored in the memory 32.

FIG. 7 shows a 45 ° / 80-line representative image converted by this embodiment.

FIG. 8 shows a 45 ° / 80 line halftone dot image converted by this embodiment.

As can be seen from FIG. 8, it is possible to obtain a good-quality halftone dot image in which gradation and screen ruling are not disturbed.

[Advantages of the Invention] As described above, according to the present invention, it is possible to perform halftone dot image screen frequency conversion without disturbance in gradation and screen frequency, and its practical effect is extremely large.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the principle of the present invention, FIG. 2 is a circuit configuration block diagram of an embodiment of the present invention, and FIG. 3 is a diagram showing a halftone image before conversion according to the present invention. 4 shows a representative image before conversion obtained by the present invention, FIG. 5 shows overlapping of small areas before and after conversion according to the present invention, and FIG. 6 shows a representative image conversion procedure according to the present invention. FIG. 7 is a diagram showing a representative image after conversion according to the present invention, FIG. 8 is a diagram showing a halftone image after conversion according to the present invention, and FIG. 9 is a diagram illustrating a halftone dot pattern table. In the drawing, 1 is a representative value calculating means, 2 is a representative value converting means, 3 is a halftone image reproducing means, 11 is a black pixel counter, 12 is a pixel counter, 13 is a line counter, 14, 24, 27, 32 are memories. , 21 is an enlarging / reducing calculation table, 22a to 22d are multipliers, 23 is an adder, 25 is a representative value conversion table, 26 is a multiplier, and 31 is a halftone dot pattern generator.

Claims (1)

[Claims] 1. Halftone image data generated for outputting a grayscale image as a pseudo-tone image using a halftone dot pattern table is converted into halftone image data in which the screen frequency of halftone dots is changed. A point image screen frequency conversion device, in which a screen frequency conversion target image is divided into square areas with one side having a halftone dot period, and the number of black pixels in each square area is calculated as a representative value of the area. The means (1) and the image divided into the square areas are overlapped with the image divided into the square areas having the halftone dot period of the screen frequency to be converted, and each one of the square areas after the screen frequency conversion. Then, the area ratio of each square area before conversion that overlaps with this is obtained, and after the values obtained by multiplying the ratio by the representative value of the overlapped square area are added together, the area ratio of the square area before and after the conversion of the screen rulings is calculated. The value obtained by multiplying A representative value conversion means (2) for converting each representative value after conversion of the screen frequency and a halftone dot pattern table for the converted screen frequency from the representative value group converted by the representative value conversion means (2). A halftone dot image screen frequency conversion device, comprising: a halftone dot image generating means (3) for generating a halftone dot image after screen frequency conversion.