JP2878318B2 - Image area determination method for binary image - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for determining an image area of a binary image, and more particularly, to a binary image for determining whether binary image data belongs to a character area or a pseudo halftone area. In the image area determination method.

[Prior Art] Conventionally, as a method of determining an image area from a binary image, a technique disclosed in JP-A-61-157158 is known. This publication discloses a technique for distinguishing between a halftone image and a character image from the spatial frequency characteristics of binary image data. Specifically, the density of adjacent pixels sandwiching a pixel of interest in a block is When the number of cases different from the target pixel is large, the image is determined to be a halftone image.

[Problems to be Solved by the Invention] However, according to the technology disclosed in the above publication, although a character image and a halftone image can be distinguished, an image subjected to dither processing,
There is a drawback that the image subjected to the error diffusion method cannot be determined, and the determination accuracy of the image area is poor.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems of the related art, and has been made in consideration of an image of a binary image in which an image area can be accurately determined in a binary image in which a character area and a pseudo halftone image are mixed. The purpose of the present invention is to provide an area determination method.

Means for Solving the Problems In order to achieve the above object, the present invention provides a binary image image area determination method for determining an image area of input binary image data, comprising a plurality of pixels. A first discriminating step of counting the number of times that the density is inverted in adjacent pixels in the block, and discriminating to which of the photograph image and the character image the input binary image belongs from the count value; A second determining step of determining whether the input binary image belongs to a dither image or a character image based on the periodicity of the binary image data; And a third determining step of counting the number of isolated pixels in which all the pixels of the input pixel differ from the density of the pixel of interest, and determining from the count value whether the input binary image belongs to the error diffusion image or the character image. Having, before First, second and third determination step input binary image data from a combination of the determination results of and judging whether belonging to the pseudo-halftone regions belonging to the character region.

Embodiment An embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a configuration example of an apparatus for implementing the image area determination method of the present invention. The data of one line in the main scanning direction input to the shift register 1 is stored in the line memory group 2 for outputting parallel data of six pixels in the sub-scanning direction.
Input to the two line memories 2a to 2f,
The output from 2f is written to six 6-bit registers 3a to 3f of the shift register group 3. At this time, the data is shifted in the main scanning direction to 3a to 3f, pixel density value data is extracted by a matrix of 6 lines × 6 pixels, and is latched into 36 latches of the latch group 4. The 6 × 6 matrix is input to the image area separation circuit 5, and it is determined whether the center pixel of interest is included in the character area or the pseudo halftone area, and is output.

Hereinafter, the algorithm of the image area separation will be described in detail.

<Determination Based on Spatial Frequency> In a binary photographic image, the density of a pixel changes drastically and there is no directionality, but in a character image, the number of density changes is relatively small and appears only in a direction orthogonal to the contour. The image area can be separated by the difference in the occurrence of the density change.

In facsimile, halftone dither is usually 4x
Since the matrix of Fig. 4 is used, Fig. 2 (a),
As shown in (b), first, the image signal is divided into 4 × 4 blocks, and the boundaries of the pixels in the blocks are divided by 12 times in the main scanning direction and the sub-scanning direction in the number of times I _X ,
When I _Y is obtained, there is a strong correlation between the image area of each image and I _X and I _Y.

However, there are considerable portions where the character image and the photographic image overlap, and even if they are divided by a certain threshold value, they cannot be completely separated from each other, and it is considered that this method alone is not sufficient. Here, it is determined that the photographic image area portion once the _{I X + I Y ≧ I S} .

If I _S = 8, the outline of the character is almost erroneously determined to be a photograph. When I _S = 11, the misjudgment of the character portion is considerably reduced, but the spiral dither image can hardly be judged as the photographic portion. In the Bayer-type, halftone-type, and error-diffusion images, the image is determined to be a photographic part to a certain extent. However, the gray part close to black and the gray part close to white have low spatial frequencies, and are erroneously determined to be character parts. I will. However, it has been found that the case where I _S = 11 has the least number of erroneous determinations and is appropriate.

Since there is quite a misjudgment in the above 4 × 4 block, the block is extended a little more and the same decision is made in the 6 × 6 block to obtain the number of times of density inversion I _X and I _Y.
Since a correlation similar to that obtained by dividing by 4 blocks appears more strongly, erroneous determination is reduced by dividing by 6 × 6 blocks.

As a result of performing image area separation in the range of I _S = 20 to 30, I _S = 26
Was found to be the most appropriate with the least misjudgment. However, the gray part close to black and the gray part close to white are simple 2
Since the region is determined to be a valued region, the following method is further used to prevent this portion from being erroneously determined.

<Determination Based on Periodicity> This method is for dither image determination, and identifies a dither-processed image from a character image based on its specific periodicity. First, after dividing the pixel into 4 × 4 blocks, the density values are compared for each of the 16 pixels in the upper, lower, left and right blocks. In a dither image, there is almost no high-frequency component in most parts other than the edge part, and the average gray level rarely changes abruptly in block units. So 16
The densities of the pixels and the corresponding pixels are compared, and all pixels are assumed to be equal or change in one gradation, and a case where only one pixel differs is considered as a dither image. In this method, the matrix size is 4 × 4 regardless of the type of dither pattern.
Is applicable. In a character image, it is extremely rare that patterns in blocks coincide with each other by chance. In addition, in this method, the solid portion and the solid black portion also have periodicity and are determined to be a photographic portion, so this case is excluded.

Specifically shown, FIG. 3 (a) a block of 4 × 4 as shown in vertical and horizontal directions and a total of 5 Tsutori, respectively compare the corresponding 5 pixels as A ₀ to A _5, equal total It can be said that A has periodicity. Similarly, for B to P, the corresponding five pixel density values are compared to detect the periodicity.
Only when all 15 or more of the 16 types of A to P are equal,
This area is determined to be a pseudo halftone image portion. or,
This as a process of the block unit, for example, 4 × of A ₀ to A ₅
While the block of 4 pixels judgment result can also be considered as area shown here, made to correspond to one of the determination results to one pixel for example K _0, and thereby the pixel to perform a determination. By doing so, image area separation with less error can be achieved.

As shown in FIG. 3 (b), when the comparison is performed within the range of 8 × 8, the number of erroneous determinations increases to some extent, but it is still possible to make a considerable determination. Also in this case, the periodicity is checked by comparing the density values of the corresponding pixels in the range shown in FIG. 3B with respect to AP similarly to FIG. 3A.

As shown in FIG. 3 (c), if the comparison is performed in the same manner in the 6 × 6 range, this alone will not be used because of many erroneous determinations, but by using together with the density value comparison between the lines shown below,
More accurate judgment can be made.

A 4 × 4 matrix is generally used for dithering used in facsimile machines and the like. There are Bayer type, halftone type, spiral type, etc., but in any system, 16 pixels in one matrix are used. When pixel density values are compared in the main scanning direction and the sub-scanning direction, there is no line in which all four pixels are black and all four pixels are white in four lines. For example, counting the number of black pixels in the main scanning direction as shown in FIG.
When the number of black pixels in the sub-scanning direction is 2,0,
2,0. Thus, in the case of a dither image, if there is a line in which all four pixels are white in the main scanning direction, there is no line in which all four pixels are black in the main scanning direction. Similarly, when there is a line in which all four pixels are white in the sub-scanning direction, there is no line in which all four pixels are black in the sub-scanning direction. Therefore, if there are lines where all four pixels are black in both the main scanning direction and sub-scanning direction and there are lines where all four pixels are white, it is determined that the image is not a dither image even if it has periodicity.

<Determination Based on Pixel Isolation> This method aims to determine an image by an error diffusion method. The image based on the error diffusion method does not have a specific periodicity unlike the dither image, and thus the above-described determination method based on the periodicity cannot be used. The intermediate gray level portion can be covered by the determination method based on the spatial frequency, but another method is necessary for the gray portion close to black (high density portion) and the gray portion close to white (highlight portion).

Therefore, attention is paid to the fact that dots are dispersed and the number of continuous dots is small in an image obtained by the error diffusion method, and discrimination based on dot isolation is performed. There are almost no isolated pixels in character images,
Limited to noise and faint thin lines.

First, the block is divided by a block of 4 × 4 pixels.
Count the number of isolated pixels in the pixel. A value with the number of isolated pixels
If I ₀ or more, the block is determined to be an image by the error diffusion method. Here, an isolated pixel is defined as 4
It is defined that all the pixels are different from the target pixel density. It can be seen that the image can be discriminated well by the error diffusion method, and that there is almost no erroneous judgment in the character image. I ₀ = 2 is appropriate.

<Image Area Separation Combining Three Methods> By combining the three methods described above, a method for accurately determining any image can be provided. In order to make an effective judgment in a narrow range to be effective, the above three methods are used with a matrix of 6 lines × 6 pixels, (1) Judgment by spatial frequency (2) Judgment by periodicity (3) Pixel isolation It is appropriate to combine the judgments with, because accurate judgment can be made.

Next, using this image area separation method, a character resolution conversion method was applied to the part determined to be characters, and a simulation was performed so that nothing was converted to the part determined to be a photograph. The conversion effect was exactly the same as when the resolution conversion was performed without any intervention, and it was confirmed that the photograph had almost no deterioration.

According to the algorithm described above, this is calculated as 6 × 6 = 36
A logic circuit is configured to determine whether the central pixel belongs to a simple binary area or a pseudo-halftone portion with respect to a parallel input of pixels, and the image area separating circuit 5 in FIG. 1 is formed. This output result is converted into a simple binary image (characters and line drawings) and a pseudo intermediate image according to the characteristics of each of them, for example, by performing a pixel-by-pixel control switch to a logic circuit that performs high image density and interpolation. Processing without deterioration can be performed. It is preferable that the image area separating circuit 5 is constituted by a ROM having a 36-bit address in terms of hardware scale and processing speed.

[Effects of the Invention] As described above, according to the present invention, the input 2 is obtained from the count value of the number of times that the density is inverted at the adjacent pixels in the block.
It is determined whether the value image belongs to a photographic image or a character image. Further, a dither image that cannot be determined based on the count value of the number of inversions is determined by examining the periodicity of the input binary image data. Since the error diffusion image is also determined from the count value of the isolated pixels in, it is possible to determine with extremely high accuracy whether the input binary image data belongs to the character area or the pseudo halftone area.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an apparatus for realizing an image area separating direction according to the present invention, FIGS. 2 (a) and 2 (b) are diagrams for explaining determination based on a spatial frequency, and FIGS. c) is a diagram showing a comparison pattern for making a determination based on periodicity, and FIG. 4 is a diagram showing a comparison of the number of black pixels on each line in a 4 × 4 pixel dither pattern. In the figure, 1 ... shift register, 2 ... line memory group of 6 lines, 3 ... 6 bit shift register group, 4 ... latch group,
5: image area separation circuit

Claims (1)

(57) [Claims] In a binary image image area determining method for determining an image area of input binary image data, the number of times that the density is inverted at adjacent pixels in a block including a plurality of pixels is counted, and the count is performed. Input 2 from value
A first determining step of determining whether the value image belongs to a photographic image or a character image; and a periodicity of the input binary image data, the input binary image belongs to a dither image or a character image. A second discriminating step of discriminating whether or not the pixel belongs to, within a block of a plurality of pixels, counting the number of isolated pixels in which all pixels above, below, left, and right of a pixel of interest differ from the density of the pixel of interest, and input the count value. And a third determining step of determining whether the binary image belongs to the error diffusion image or the character image. An input is performed based on a combination of the determination results of the first, second, and third determining steps. A method for determining the image area of a binary image, comprising determining whether the binary image data belongs to a character area or a pseudo halftone area.