JP2647458B2 - Halftone dot area separation device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] In order to separate a halftone dot region from other regions with high accuracy, an edge density is measured in a local region surrounding a pixel of interest, and the edge density is set to a predetermined threshold value. An edge density detection unit that determines whether or not the target pixel is a halftone dot candidate pixel based on whether the pixel exceeds the target pixel, and compares pixel information of a local region surrounding the target pixel with a predetermined halftone pixel determination pattern. A pattern comparison unit that determines whether the pixel of interest is a halftone dot candidate pixel, and determines whether the edge density detection unit and the pattern comparison unit are halftone dot candidate pixels for pixels around the pixel of interest. And a correction processing unit that determines whether the attribute of the pixel of interest is a halftone pixel or not based on the halftone determination information.

[Industrial applications]

When an image in which an image represented by a halftone dot and a line drawing such as a character are mixed is reproduced by a facsimile or a copying machine, in order to improve the quality of the reproduced image, the image is represented by a halftone dot. It is desirable to perform a smoothing process for removing moiré in an image region and a sharpening process for a line image region such as a character.

Also, in the case of transmitting an image, the efficiency of data compression can be improved by applying the encoding process according to the characteristics of each area as described above.

The present invention relates to a halftone dot region separation method for separating a halftone dot region represented by a halftone dot from another region such as a character region as preprocessing for performing such image processing.

[Conventional technology]

Japanese Patent Application Laid-Open No. 60-80365 discloses that after binarizing input image data for each pixel with a fixed threshold value, a binarized image of, for example, 5 × 5 pixels including a target pixel is stored in advance. An area separation method is described in which a halftone dot pattern is detected as a halftone dot area when the number of pixels that match the halftone dot pattern exceeds a certain value as compared with a halftone dot pattern.

[Problems to be solved by the invention]

However, in such a region separation method,
Since a fixed threshold value is used for binarization, it is difficult to identify a halftone dot pattern depending on the halftone dot ratio of a halftone dot region, and a halftone pattern in which halftone dot patterns are stored. If it is different from the dot pattern, it may not be detected as a dot area.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a halftone dot area separating apparatus for separating a halftone dot area in an image from other areas such as a character area or a photograph area with high accuracy.

[Means for solving the problem]

An edge density detection unit that measures an edge density in a local region surrounding the target pixel and determines whether the target pixel is a halftone dot candidate pixel based on whether the edge density exceeds a predetermined threshold; A pattern comparison unit that compares pixel information of a local region surrounding the target pixel with a predetermined halftone pixel determination pattern to determine whether the target pixel is a halftone dot candidate pixel; The determination of whether or not the pixel of interest is a halftone dot candidate pixel is corrected based on halftone dot determination information about pixels around the pixel of interest, and correction is performed to determine whether the attribute of the pixel of interest is a halftone pixel. The processing unit is provided in the halftone dot region separating apparatus.

(Operation)

The inventor has measured that the edge density is measured in a local region surrounding the pixel of interest, and has found that the attribute of the pixel of interest can be determined as a halftone pixel when the edge density exceeds a predetermined threshold.

In other words, most halftone dot images have the property of having an edge, and the edge density in the local region shown by the bold line surrounding the target pixel indicated by hatching in FIG. For example, it is measured by an edge extraction filter or the like, and when the density exceeds a threshold value TH, the attribute of the target pixel can be determined as a halftone dot or a halftone dot candidate. Although a character has relatively many edges, its edge density is lower than that of a halftone image having a high screen ruling as described above.

The determination of the attribute of the pixel of interest based on the edge density and the determination of the attribute of the pixel of interest based on a known pattern comparison are performed independently and in parallel, thereby increasing the accuracy of the attribute determination. By performing correction based on the halftone dot determination information for the pixels around the halftone dot region, the halftone dot region can be separated from other character regions and the like with extremely high accuracy.

〔Example〕

FIG. 1 shows a basic embodiment of the present invention, in which a block surrounded by a dashed line and indicated by 10 is an edge density detector,
A block surrounded by a dashed line and indicated by 20 is a pattern comparison unit, and 40 is a correction processing unit that performs correction based on peripheral information.

The edge density detection unit 10 and the pattern comparison unit 20,
When it is determined that the pixel of interest is a halftone dot candidate, an “H” level output is sent to the OR circuit 30.
If any one of 10 and the pattern comparison unit 20 determines that the pixel is a dot candidate, the OR circuit 30 indicates “H” indicating that the pixel is a dot candidate.
The output of the level is sent to the correction processing unit 40, and correction based on the halftone dot determination information for the peripheral pixels is performed.

More specifically, in the edge density detecting means 11 of the edge density detecting unit 10, filtering is performed on all pixels in the local region surrounding the pixel of interest by an edge extraction filter indicated by a thick frame in FIG. If the number of pixels determined to exceed the predetermined value TH, the target pixel is determined to be an edge.

FIG. 3 shows an example of an edge filter indicated by a bold line in FIG. 2, and a 3 × 3 pixel centered on a pixel for which an edge value is to be obtained (corresponding to a hatched pixel in FIG. 2). = Multiply the respective density values of the nine pixels by the coefficient shown in the figure and add the obtained pixel values to obtain the edge value of the pixel located at the center, and this edge value is a predetermined value Is exceeded, it is determined to be an edge pixel.

Block 20 is for determining a halftone pixel by known pattern matching, and in the illustrated embodiment,
After the edge emphasizing process 21 is performed, binarization 22 is performed, and it is determined whether or not the pixel of interest is a halftone dot candidate by performing pattern matching 23 with a previously prepared halftone pattern. When the judgment is made, the judgment result of the "H" level is supplied to the other input terminal of the OR circuit 30.

Therefore, from the OR circuit 30, the edge density detector
An “H” level output is obtained for a pixel determined to be a halftone dot candidate by at least one of the pattern detection unit 10 and the pattern detection unit 20, and this output is supplied to the correction processing unit 40.

The correction processing unit 40 corrects the result of determination as a halftone dot candidate based on halftone determination information on pixels around the pixel of interest, and finally determines whether or not the pixel is a halftone pixel.

FIG. 4 shows the principle of correction based on the halftone dot determination information for the surrounding pixels. When the surrounding pixels are halftone dots, the probability that the pixel of interest is also a pixel belonging to the halftone dot region is high. In order to correct the halftone dot candidate pixel or non-halftone dot candidate pixel determined as described above to a non-halftone dot candidate pixel or halftone dot candidate pixel, k × k around the target pixel indicated by hatching is corrected. If the pixel area includes a pixel determined to be a temporary halftone dot, the pixel of interest is corrected to a pixel belonging to the halftone dot area.

As a result, the halftone dot area can be accurately converted to other character areas,
It can be separated from the photographic area.

〔The invention's effect〕

In the present invention, the process of determining a halftone dot region by the edge density detection unit does not make a determination depending on the stored pattern, and is excellent in determining a halftone dot region having a high number of lines. Accordingly, it is possible to compensate for the weak point of the dot area determination processing by the pattern comparison unit.

Therefore, according to the present invention, a special effect that a halftone dot region can be separated from a character region, a photograph region, and the like with simple processing and extremely high accuracy is obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a principle embodiment of the present invention, FIG. 2 is a diagram showing a principle of edge density measurement, FIG. 3 is a diagram showing an edge filter used in the embodiment, FIG. FIG. 8 is a diagram for explaining correction based on halftone dot determination information of a peripheral region in the embodiment.

Claims (1)

(57) [Claims] 1. An edge density for measuring an edge density in a local region surrounding a pixel of interest and determining whether the pixel of interest is a halftone dot candidate pixel based on whether the edge density exceeds a predetermined threshold. A detection unit; a pattern comparison unit that compares pixel information of a local region surrounding the target pixel with a predetermined halftone pixel determination pattern to determine whether the target pixel is a halftone candidate pixel; The determination as to whether a pixel is a halftone dot candidate pixel by the density detection unit and the pattern comparison unit is corrected based on halftone dot determination information for pixels around the pixel of interest, and whether the attribute of the pixel of interest is a halftone pixel And a correction processing unit that determines whether or not the halftone dot area is separated.