JPH0830794A - Image compression method - Google Patents

Abstract

(57) [Abstract] [Purpose] A high compression rate is possible even if the image contains an area with low template replacement efficiency. A matching area determination unit 102 divides an input image into grid-shaped areas and determines the characteristics of the image for each area. As an example, the number of black runs is counted, and if it is a predetermined number or more, it is determined to be a halftone region. Pattern cutout 1
03 cuts out a pattern in a character area other than the halftone area. Template generation matching unit 104
Matches the cut-out pattern with a pattern registered as a template, creates symbol information, position information of each symbol, and pattern information of each template, and encodes them by the predictive coding method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image compression method for compressing a document image including a halftone area at a high compression rate.

[0002]

2. Description of the Related Art A connected component is cut out from a document image, one connected component is regarded as one pattern and registered as a template, and among the cut out patterns, a similar pattern is replaced with a template to obtain image information. There are image compression methods that reduce (see, eg, US Pat. No. 5,303,313).

FIG. 9 is a diagram for explaining the conventional image compression method described in the above publication. 901 is a document image composed of English characters, 902 is a template, 903 is position information,
Reference numeral 904 is a symbol (template number). A connected component (character pattern) is cut out from the document image 901. In the example of the drawing, first, the connected component “H” is cut out, and this is subjected to pattern matching with the pattern registered in the template 902. In this case, since nothing is registered, no matching is performed and the connected component “H” is newly registered as a template. Also, the connected component “H” is represented by the symbol “1” (template number), and the symbol 9
It is stored in the memory together with the position information 903 of 04. That is, each clipped connected component (character pattern) is represented by the symbol 904 and the symbol position information 903.

Next, the connected component "e" is cut out, and since this also does not match the pattern registered in the template, it is newly registered as a template and is represented by the symbol "2". Hereinafter, connected components “t”, “o”,
Similarly, “l”, “d”, and “m” are processed and registered as templates, symbols “3” to “7” are assigned to each, and the position information is stored in the memory.

Then, when the next "e" after "m" is cut out, this connected component "e" is not newly registered as a template because it matches the pattern "e" registered in the template. However, an average of the registered pattern and the matched pattern (or a representative value of both patterns) is created to update the template. As a result, similar patterns are updated so that their representative patterns are registered.

Also, the already assigned "2" is assigned to the symbol. In the same manner, when the symbols up to the number “9” are used in the same manner, the character “h”,
Symbols "a" and "b" are assigned to "a", respectively.

As described above, the document image includes a symbol string (template number string), symbol position information, and
It is decomposed into pattern information of each template. And
In order to improve the prediction efficiency in encoding and to obtain a high compression rate, the characters are rearranged in the reading order, and then predictive encoding is performed to compress the image with high efficiency.

[0008]

The above-mentioned compression method is
This is a method considering compression of a text image, that is, a typed character is originally the same pattern, and thus a method of obtaining a large compression ratio by eliminating redundancy in this information. Therefore, the above method has a problem that the compression efficiency is deteriorated as a whole when there is a halftone image area in the document whose replacement efficiency with the template is poor (note that a halftone image is a gradation An image that is represented by a pseudo binary image using halftone dots. This is because in the halftone area, it is not possible to match each pattern like in the character area.

In addition to the above example, individual characters are cut out,
There is an image information compression transmission system in which a typical character pattern is generated from the cut out individual characters, each character pattern is classified using this as a template, and the typical character pattern and the classification code are transmitted (JP-A-3-157064). reference). However, in this method, since the generation area of the typical character pattern is not taken into consideration, it is not possible to expect improvement in the compression efficiency as a whole because the same clipping and matching processing is performed even for the image area in which the similar pattern is difficult to appear.

It is an object of the present invention to provide an image compression method which enables a high compression rate even if an image includes an area having a low replacement efficiency with a template.

[0011]

In order to achieve the above object, according to the invention of claim 1, a connected component is cut out from a document image, and the connected component is regarded as a pattern and registered as a template, and is similar to the pattern. In the image compression method of compressing image information by replacing another pattern with the template, a region in the document image that is not replaced by the template is determined, and the region other than the determined region is replaced by the template. The feature is that the image is compressed by going.

According to a second aspect of the invention, the region is a halftone region.

According to the third aspect of the invention, the area is characterized in that the contour of the connected component cut out from the document image has a size equal to or larger than a predetermined threshold value.

According to a fourth aspect of the present invention, the area is a pattern included in a contour having a size equal to or larger than the predetermined threshold value.

According to a fifth aspect of the present invention, the area is a pattern inside the convex area when the convex area surrounding the connected component cut out from the document image has a size equal to or larger than a predetermined threshold value. It is characterized by that.

According to a sixth aspect of the present invention, in the region, the contour of the connected component cut out from the image subjected to the blurring process is subjected to the blurring process, and the contour has a size equal to or larger than a predetermined threshold value. It is characterized by being.

According to a seventh aspect of the present invention, the area is a pattern included in a contour having a size equal to or larger than the predetermined threshold value.

According to an eighth aspect of the present invention, in the area, a blurring process is performed on the document image, and a convex area surrounding a connected component cut out from the image subjected to the blurring process has a size equal to or larger than a predetermined threshold value. It is characterized in that, at one time, the pattern is inside the convex region.

[0019]

In one embodiment of the present invention, the matching area determination unit divides the input image into grid areas and determines the characteristics of the image for each area. As one determination method, the number of black runs is counted, and if it is a predetermined number or more, it is determined to be a halftone region. The pattern cutout unit cuts out a pattern from a character area or the like which is determined not to be a halftone area. The template generation matching unit matches the cut-out pattern with the pattern registered as the template, and creates the symbol information of the pattern, the position information of each symbol, and the pattern information of each template. The coding unit compresses the image at a high compression rate by coding these pieces of information by the predictive coding method.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be specifically described below with reference to the drawings. <Embodiment 1> FIG. 1 shows the configuration of Embodiment 1 of the present invention.
In the figure, 101 is an image input device such as a scanner, 1
Reference numeral 02 is a matching area determination unit that determines whether or not each lattice area is a matching target area, 103 is a pattern cutout unit, 104 is a template generation matching unit that performs template generation and matching, and 105 is extracted pattern information and An encoding unit that encodes position information and the like, 1
A storage unit 06 stores an input image and generated data.
Is a control unit for controlling the whole, and 108 is a data communication path.

FIG. 2 is a processing flowchart of the first embodiment of the present invention. The operation will be described with reference to the drawings. First, an image is input by the image input device 101 such as a scanner (step 201). Next, the matching area determination unit 102 divides the image into grid areas (for example,
(8 × 8 division), and the characteristics of the image are examined for each area (step 202). For example, as one method, the number of black runs is counted, and if there is a certain number or more, it is determined as a halftone area. The halftone region represented by the dither method is composed of fine dots, and has a larger number of black runs than the character region, and therefore can be identified by the number of runs. Note that the dither method is a method of converting a gradation image into a binary pseudo gradation image, and compares the pixel value and the element value of the threshold matrix to determine that the pixel value is small. 0 if not, 1 otherwise
It is a method of generating a binary pseudo gradation image by converting into

In a region which is determined not to be a halftone region (portion) by the matching region determination unit 102, that is, a region such as a character, the pattern cutout unit 103 cuts out a pattern (black pixel connected component) (step). 203, 204). At this time, regarding the pattern applied to the boundary of the region, it may be cut out in a form that includes all the pixels that are connected beyond the region, or if it exceeds the region, even if the pixels are connected, it is ignored, and it is only within the region. There are two methods of cutting out the pattern. In the latter case, the pattern cutout process is easier, but a part of the pattern is missing at the boundary of the area, and the degree of matching is reduced. On the other hand, in the former method, since the pattern can be completely cut out, the degree of matching does not decrease, but the processing around the boundary becomes complicated. It should be noted that whether or not the cut-out pattern covers the boundary of the area can be known by referring to the run data stored in the data storage unit 106.

Next, the template generation matching unit 10
In step 4, matching is performed between the cut-out pattern and some patterns registered as templates (step 205). As described in the prior art, if matched, the extracted pattern is represented by the symbol of the registered pattern, and if it does not match any template, it is registered as a new template. At this time, the position information of each symbol is also generated as described above, and each pattern is represented by the symbol information and the position information of each symbol and stored in the data storage unit 106.

The above matching / template generation processing is performed for all patterns in the target area.
As a result, the symbol information, the position information of each symbol, and the pattern information of each template are stored in the data storage unit 106. The encoding unit 105 encodes these pieces of information by the predictive encoding method, and thereby a high compression rate is obtained. For the halftone area, the image information inside the halftone area is encoded by a predetermined encoding method (for example, MH, JBIG, etc.) (step 206).

<Embodiment 2> FIG. 3 shows the configuration of Embodiment 2 of the present invention, which is configured by replacing the matching area determination unit of Embodiment 1 with a pattern size determination unit, and the other elements are the same as those of Embodiment 1. Similar to that of.

FIG. 4 is a processing flowchart of the second embodiment of the present invention. First, an image is input by the image input device 301 (step 401), and then the pattern cutout unit 303 cuts out a pattern (step 40).
2). The pattern size determination unit 302 determines whether or not the cut out pattern is equal to or larger than a predetermined size (step 403). If the cut-out pattern has a size larger than a predetermined size, it is determined that it is not a character but a part that constitutes a photograph or the like, and is excluded from matching.

Specifically, the contour of the cut-out pattern (the connected component of black pixels) is obtained, and when the contour (vertical and horizontal sizes or their areas) is larger than a predetermined threshold value, it is excluded from the matching target.

Therefore, as shown in FIG. 5, the character size patterns 502 and 503 included in the pattern 501 having a predetermined size or more are not subjected to matching. Further, FIG. 6 shows another example of a pattern that is excluded from the matching target. Reference numeral 601 denotes a pattern having a predetermined size or more (black pixel connected component), 602 denotes a convex region surrounding the pattern 601 having a predetermined size or more, and 603.
Is a pattern of about the character size in the convex area.
In this embodiment, the pattern 603 in the convex area surrounding the pattern having a predetermined size or more is excluded from the matching target.

The processes in and after step 404 are the same as those in the first embodiment, and the description thereof will be omitted.

<Third Embodiment> FIG. 7 shows the configuration of the third embodiment, which is configured by adding a blurring processing unit 702 to the configuration of the second embodiment. FIG. 8 is a processing flowchart of the third embodiment. When expressing the halftone part with the dither method,
Since it is difficult to connect black pixels, the pattern forming the halftone portion tends to be small.

Therefore, in this embodiment, the blurring processing unit 702 performs blurring processing on the image in advance before performing the pattern cutting processing (step 802). For example, such a blurring process is a process of setting all 64 pixels as black pixels if there is at least one black pixel in the area of 8 pixels × 8 pixels. By this blurring process, since black pixels are close to each other in the halftone portion, they are fused as a whole to form a large black pixel connected component (pattern).

On the other hand, in the character portion, adjacent characters on the same line are merged, but not between lines (blurring is set to that extent), so that the connected component after the blurring process does not become a large chunk. .

After the blurring process, the pattern cutout unit 704 cuts out the pattern as in the second embodiment (step 8).
03), the pattern size determination unit 703 determines whether or not the extracted pattern has a size equal to or larger than a predetermined size (step 804). Hereinafter, as in the second embodiment, a pattern having a size equal to or larger than a predetermined size and a pattern inside the pattern are excluded from the matching target. With respect to the area determined to be the matching target, matching / template generation is performed on the original image (step 805) and coded (step 80) as in the first and second embodiments.
6).

[0034]

As described above, according to the first and second aspects of the present invention, an area in the document image that is not replaced by the template is determined, and the area other than the determined area is replaced by the template. Since the image is compressed by performing the above process, a high compression rate can be realized by a compression method using the matching process even for an image including a halftone portion with poor matching efficiency.
Further, the halftone region can be identified by a simple process.

According to the third and fourth aspects of the present invention, the contour of the connected component has a size equal to or larger than a predetermined threshold, and the pattern included in the contour has a size equal to or larger than the predetermined threshold. Since the convex area surrounding the component has a size equal to or larger than a predetermined threshold and the pattern inside the convex area is separated from the character portion with high accuracy, the image can be efficiently compressed.

According to the sixth, seventh and eighth aspects of the invention, even if the halftone region in the document image is processed by the dither method, the blurring process is performed, so that the contour of the connected component has a predetermined threshold value. A contour having the above size, a pattern included in the contour having a size equal to or larger than a predetermined threshold, and a convex region surrounding the connected component having a size equal to or larger than the predetermined threshold and being inside the convex region. And can be separated from the character portion with high accuracy, and thus the image can be highly compressed.

[Brief description of drawings]

FIG. 1 shows a configuration of a first embodiment of the present invention.

FIG. 2 is a processing flowchart of the first embodiment of the present invention.

FIG. 3 shows a configuration of a second embodiment of the present invention.

FIG. 4 is a processing flowchart of a second embodiment of the present invention.

FIG. 5 shows an example of patterns excluded from matching.

FIG. 6 shows another example of patterns excluded from matching.

FIG. 7 shows a configuration of a third embodiment of the present invention.

FIG. 8 is a processing flowchart of a third embodiment of the present invention.

FIG. 9 is a diagram illustrating a conventional image compression method.

[Explanation of symbols]

 Reference Signs List 101 image input device 102 matching area determination unit 103 pattern cutout unit 104 template generation matching unit 105 encoding unit 106 data storage unit 107 control unit 108 data communication path

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H04N 1/411

Claims (8)

[Claims] 1. An image compression method for compressing image information by extracting a connected component from a document image, regarding the connected component as a pattern, registering it as a template, and replacing another pattern similar to the pattern with the template. An image compression method comprising: determining an area in the document image that is not replaced with the template, and replacing the area other than the determined area with the template to compress the image. 2. The image compression method according to claim 1, wherein the region is a halftone region. 3. The image compression method according to claim 1, wherein the region is a contour in which a contour of a connected component cut out from the document image has a size equal to or larger than a predetermined threshold value. 4. The image compression method according to claim 3, wherein the region is a pattern included in a contour having a size equal to or larger than the predetermined threshold value. 5. The area is a pattern inside the convex area when the convex area surrounding the connected component cut out from the document image has a size equal to or larger than a predetermined threshold value. The image compression method according to item 1. 6. The region is characterized in that a blurring process is performed on the document image, and a contour of a connected component cut out from the image subjected to the blurring process is a contour having a size equal to or larger than a predetermined threshold value. The image compression method according to claim 1. 7. The image compression method according to claim 6, wherein the region is a pattern included in a contour having a size equal to or larger than the predetermined threshold value. 8. The convex region is obtained by subjecting the document image to blur processing, and when a convex region surrounding a connected component cut out from the blur-processed image has a size equal to or larger than a predetermined threshold value. The image compression method according to claim 1, wherein the image compression pattern is an internal pattern.