JP3263945B2 - Linear density conversion method for binary image - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a linear density conversion method for a binary image.

[Conventional technology]

Conventionally, the linear density conversion method for this type of binary image includes SPC
System, logic speech, 9-division method, high-speed projection method, and the like.
(For example, a high-speed pixel density conversion method based on a projection method, Journal of the Institute of Image Electronics Engineers of Japan, Vol. 11, No. 2) Both methods use a logical operation of the density of one original pixel closest to the converted pixel or the density of the four nearest original pixels. , A method of determining the density of the converted pixel.

[Problems to be solved by the invention]

In the above-described conventional linear density conversion method for a binary image, the density of the converted pixel is determined by a logical operation of the density of the nearest one pixel or four original pixels. There is a disadvantage in that interference occurs between the conversion operation and the cycle, if any. Further, since there is no density difference correction function between the original image and the converted image, there is a disadvantage that the density of the original image and the converted image is changed.

[Means for solving the problem]

The linear image linear density conversion method of the present invention is used to determine the density of a converted pixel when obtaining a linear density-converted converted image from an original image composed of a plurality of pixels arranged two-dimensionally. An original image cutout section for extracting an original image block composed of a plurality of pixels set corresponding to the converted pixels from the area of the original image, an original image block density total section for obtaining the total density value of the original image block, and a peripheral area in the image block. A density correction unit that adds a density correction value to the total density value from the total density unit according to a density error between the pixel and the converted pixel; and a threshold value indicated by a dither pattern indicating the total density value corrected by the density correction unit. A conversion pixel density determination unit that binarizes the converted pixel to obtain the density of the converted pixel, an original image / converted image density error calculation unit that obtains a density error between the original image block and the converted pixel, Generating a density correction value by weight distribution to pixels;
An original image conversion image density error distribution unit for performing feedback in the density error correction unit in addition to the total density value.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

 FIG. 1 is a block diagram of one embodiment of the present invention.

The linear density conversion rate can take an arbitrary value, but here, a case of 1/2 times will be described. In order to make it 1/2 times, four pixels of the original image may be converted into one pixel of the converted image. The original image cutout unit 1 cuts out four pixels to be converted from the original image 101. Hereinafter, the pixel set of the original image to be converted is referred to as an original image block 102.

The original image block density summation unit 2 calculates the total value 103 of the density values of the four pixels of the original image block 102. In the present invention, the original image block 102 is divided into four pixels (l row, m column) at the upper left of the original image 101.
Then, since there is one black pixel and three own pixels, the original image block density total value 103 is 1. However, the density values of the black pixel and the white pixel are set to 1,0, respectively.

The density correction unit 3 adds an original image / converted image density error correction value 104, which will be described later, to the original image block density total value 103 and feeds it back to output a density total correction unit 105. In the present invention, if the original image / converted image density error correction value 104 is 0 or 4, the density sum correction value 105 of the original image block 102 is 1.4.

The conversion pixel density determination unit 11 determines the density of the conversion pixel 107 corresponding to the original image block 102. The correction value 105 is binarized by the comparator 5 based on the threshold value 106 calculated by the dither pattern threshold value correction unit 4. In this example, l row and m column indicate the position of i row and j column on the dither pattern 110, and the threshold value at that position is corrected by the threshold value correction unit 4 and the threshold value 106 is sent to the comparator 5. In this case, if the threshold 106 is set to 2.0, the converted pixel 107 becomes 0, that is, white.

In the original image / converted image density error calculating section 6, the original image block 10
The difference between the density sum correction value 105 and the density of the converted pixel 107, that is, the density error 109 is calculated. The weight multiplying unit 7 performs weight correction of the density of one pixel of the converted image and the original pixel in order to calculate the density error 109. In this case, one of the conversion pixels 107
Since the weight of the pixel density is equivalent to four pixels of the original image, the converted pixel 107 is multiplied by a weight of four. The converted pixel 107 is 0, and the density correction value of the converted pixel 107 for the original block 102 is 1.4, so the original / converted image density error 109 is 1.4.

The density error distributing unit 8 sequentially corrects the density difference between the converted image and the original image by feeding back the value obtained by weighting the density error 109 to the pixel blocks around the principle block 102. The density error 109 is weighted by the density error distribution ratio multiplying unit 10, and the peripheral pixel blocks to be distributed are determined by the density error distribution peripheral block determining unit 9. Here, 1.4 of the original / converted image density error 109 generated between the original image block 102 of l rows and m columns is weighted by 1/2.
Is multiplied by (l + 1) rows, m columns and l columns, (m +
1) The difference between the original image and the converted image is corrected by feeding back by adding to the peripheral blocks in the column. 0.4 of the original image / converted image density error correction value 104 is (l-1).
The density error correction value 104 generated between the conversion pixel 107 in the row and m columns and the original image block 102 and the conversion pixel 1 in the l row and (m-1) columns
Density error correction value 1 generated between 07 and original block 102
This is the sum of 04.

〔The invention's effect〕

As described above, according to the present invention, when determining the density of the converted pixel, the density error of the converted image is made closer to the density of the original image as compared with the conventional method by feeding back the density error between the original image block of the original image and the pixel of the converted image. There is an effect that can be. Further, even when the original image is not a halftone image, a halftone image having a very small density difference from the original image can be obtained after the conversion. Further, since the converted image is a halftone image, there is an effect that the compression efficiency is improved.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention. Reference numeral 1 denotes an original image clipping section, 2 ... an original image block density summation section, 3 ... a density correction section, 4 ... a dither pattern threshold value correction section, 5 ... a comparator, 6 ... an original image / converted image density error calculation section, 7 weight multiplying unit, 8 density error distribution unit, 9
... Density error distribution peripheral block determination unit, 10 density error distribution ratio multiplication unit, 11 converted pixel density determination unit, 101 original image, 102 original image block, 103 total density value of block, 104 ... Original / converted image density error correction value, 105... Original image block total density correction value, 106... Threshold value, 107... Converted pixel, 108.
... Original image / conversion image density error, 110 ... Dither pattern.

Claims (1)

(57) [Claims] When obtaining a linear density-converted converted image from an original image composed of a plurality of pixels arranged two-dimensionally, an original image area is converted to a converted pixel in order to determine the density of the converted pixel. An original image clipping unit that cuts out an original block composed of a plurality of pixels set correspondingly; an original block density total unit that calculates a total density value of the original block; and a density error between peripheral pixels and converted pixels in the image block. A density correction unit for adding a density correction value to the density total value from the density total unit in response to the density correction value, and comparing the density total value corrected by the density correction unit with a threshold value indicated by a dither pattern to binarize. A conversion pixel density determination unit that determines the density of the conversion pixel; an original image / conversion image density error calculation unit that obtains a density error between the original image block and the conversion pixel; Wherein to generate the density correction values, linear density conversion method of a binary image, characterized by having said density sum plus the density error corrector original transform image density error distribution unit to perform feedback in.