JP2005269271A - Image processing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processing apparatus capable of enhancing the compression rate and the image quality. <P>SOLUTION: The image processing apparatus extracts an image element of a line segment from image data which have become processing objects, generates first partial image data comprising the image elements of the line segments and second partial image data except the image elements of the line segments, determines the representative color of the first partial image, limits colors of the first partial image data by using the determined representative color, determines also the representative color, even as to the second partial image data, limits the colors of the second partial image data by using the determined representative color, and uses the image data, after the colors are limited for prescribed compression processing. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image processing apparatus that separates image data according to a predetermined rule and performs compression processing.

  Raster image data (hereinafter simply referred to as “image data” unless otherwise distinguished) has many image elements with different properties such as character (text) parts and natural picture parts (design parts). Can be included. Due to the difference in properties of these image elements, it is often preferable to perform different image processing for each image element, for example, compression by a different method is suitable for compression processing.

Therefore, conventionally, image processing called T / I separation has been researched and developed, and techniques for separating and compressing characters and patterns have been developed (for example, Patent Documents 1 and 2).
JP 2002-165105 A JP 2002-175532 A

  However, in the method disclosed in Patent Document 1, for example, a non-character portion is simply subjected to irreversible compression such as JPEG (Joint Picture Experts Group), and a limited color within a contour portion or line drawing of a pattern. Since JPEG compression is performed up to the part that can be converted, there are cases where sufficient performance cannot be exhibited in terms of compression rate and image quality.

  The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an image processing apparatus capable of improving the compression rate and improving the image quality by performing limited color on a possible portion other than a character. One of the purposes.

  The present invention for solving the above-described problems of the conventional example is an image processing apparatus that extracts line segment image elements from image data to be processed, and includes a line segment image element. Determining at least one representative color based on means for generating one partial image data and second partial image data excluding the image element of the line segment, and a value of a pixel included in the first partial image data; First limited colorization means for generating first limited color image data obtained by limiting the first partial image data using the determined representative color, and pixel values included in the second partial image data A second limited colorization unit that determines at least one representative color and generates the second limited color image data by limiting the second partial image data using the determined representative color. And the first limited color image data When a second color quantization image data is characterized in that it is subjected to a predetermined compression process.

  Here, the image data to be processed may be, for example, image data obtained by removing a character portion from the original image data.

  Here, the first limited color image data calculates a significant number of pixel values included in the first partial image data, and only when the significant number does not exceed a predetermined first threshold value. At least one representative color may be determined, and first limited color image data may be generated by limiting the first partial image data using the determined representative color.

  Further, the second limited color image data is calculated only when the significant number of pixel values included in the second partial image data is calculated and the significant number does not exceed a predetermined second threshold value. At least one representative color may be determined, and second limited color image data obtained by limiting the second partial image data using the determined representative color may be generated.

  Further, based on the first limited color image data and the second limited color image data, the same color among the pixels included in the first limited color image data and the second limited color image data. The image processing apparatus may further include means for generating a plurality of color-specific image data including pixels for each color to be determined.

  Note that the image data to be processed is image data obtained by removing the character portion from the original image data, and is one of the partial image data extracted from the original image data. The first limited color image data and the second limited color image data based on the first limited color image data and the second limited color image data obtained from each of the partial image data. A plurality of color-specific image data including pixels for each color determined to be the same color may be generated.

  In addition, an image processing method according to an aspect of the present invention extracts line segment image elements from image data to be processed, the first partial image data including the line segment image elements, and the line segments. And generating at least one representative color based on the pixel value included in the first partial image data, and generating the second partial image data excluding the image element, and using the determined representative color A step of generating first limited color image data obtained by limiting the first partial image data, and determining at least one representative color based on a value of a pixel included in the second partial image data; Generating a second limited color image data obtained by limiting the second partial image data using the determined representative color, the first limited color image data obtained by the separation, 2 limited color image data is predetermined It is characterized by being subjected to a condensation process.

  Furthermore, an image processing program according to another aspect of the present invention extracts a line segment image element from image data to be processed, and first partial image data including the line segment image element. At least one representative color is determined based on the procedure for generating the second partial image data excluding the image element of the line segment and the value of the pixel included in the first partial image data, and the determined representative color Based on a procedure for generating first limited color image data in which the first partial image data is limited color using and a value of a pixel included in the second partial image data, at least one representative color is selected. And generating the second limited color image data by limiting the second partial image data using the determined representative color and executing the procedure, and performing the first limitation obtained by the separation. Colored image data and A second color quantization image data is characterized thereby subjected to a predetermined compression process.

  As shown in FIG. 1, the image processing apparatus according to the embodiment of the present invention includes a control unit 11, a storage unit 12, an image input unit 13, and an image output unit 14. The control unit 11 operates in accordance with a program stored in the storage unit 12 and performs each image processing described later. The contents of this image processing will be described in detail later.

  The storage unit 12 holds a program executed by the control unit 11. The storage unit 12 also operates as a work memory that stores various data generated during the process of the control unit 11. Specifically, the storage unit 12 can be implemented as a computer-readable recording medium and a device that writes data to or reads data from the recording medium (for example, a hard disk device or a memory device).

  The image input unit 13 is, for example, a scanner, and outputs image data obtained by optically reading a document to the control unit 11. Here, it is assumed that the value of each pixel is expressed in an RGB (red, green, blue) color space in the image data output from the image input unit 13. The image output unit 14 outputs image data in accordance with an instruction input from the control unit 11. For example, the image output unit 14 outputs the image data to an image forming unit (printer or the like) or transmits it to an external device via a network. Is what you do.

  Next, the content of the process of the control part 11 is demonstrated. As functionally shown in FIG. 2, the control unit 11 of the present embodiment sets the image data input from the image input unit 13 as a processing target, and performs predetermined preprocessing on the image data that is the processing target. A pre-processing unit 21 for performing a pattern part, a pattern candidate part specifying unit 22 for specifying a part that is a candidate for a pattern part (a pattern candidate part), a character extraction processing unit 23 for extracting characters, and a layout process for performing a layout process The unit 24, the same color region separation unit 25, the hole filling processing unit 26, and the compression processing unit 27 are configured.

  Hereinafter, each of these parts will be described in detail.

[1. Pre-processing section]
In the preprocessing unit 21, the value of each pixel of the image data (processing target image data) input from the image input unit 13 is converted from RGB to YCbCr (a value composed of luminance and color difference). Specifically, the conversion can be performed using the following equation (1). Here, the value of each component of RGB is assumed to be a value from 0x00 (“0x” indicates a hexadecimal number) to 0xFF. Further, the pre-processing unit 21 may correct the gradation of each pixel value based on the luminance and saturation of the background area. However, the gradation correction process is not always necessary.

[2. Design candidate area identification processing unit]
The pattern candidate portion identification processing unit 22 performs a process of identifying an area estimated as a pattern area from the image data expressed in the YCbCr color space output from the preprocessing unit 21 as a pattern candidate area. Specifically, the pattern candidate portion specifying processing unit 22 copies and stores the image data expressed in the YCbCr color space output from the preprocessing unit 21 in the storage unit 12. Then, referring to the value of each pixel of the copied image data (referred to as picture area specifying image data), the luminance component of the pixel value is compared with a predetermined binarization threshold value to obtain the luminance Pixels whose components are equal to or higher than the binarization threshold are black pixels (value “1”), pixels whose luminance components are less than the binarization threshold are white pixels (value “0”), and the picture area specifying image data is Convert to binary image data. Here, the larger the luminance component, the darker (black).

  Next, the pattern candidate portion specifying processing unit 22 extracts a region (connected region) in which black pixels are connected from the binary image data. For the extraction of the connected region, a process widely known as a labeling process can be used. When the labeling process is used, each connected area is given a different label identifier and stored in the storage unit 12. Next, a predetermined scale feature amount is calculated for each of the connected regions. Here, the scale feature amount includes a rectangular area defined in relation to the connected region. Then, based on the scale feature amount, it is determined whether each connected area is a pattern candidate area. As a specific example, if a character is an area (small area) less than the area (so-called minimum area) corresponding to the minimum font size (for example, 6 points) used in a general document, Since it can be determined that the pattern area does not include, such a small area is removed. In a general document, the font size rarely exceeds 24 points, for example. Therefore, when the area is equal to or larger than the maximum font size used in this general document (so-called maximum area), it is determined that characters are not included (that is, a picture area).

  The pattern candidate part specification processing unit 22 refers to the determination result for each connected area, and among the binarized pattern area specifying image data, the black pixel included in the connected area determined not to be the pattern area is set as the white pixel. Set to. In this way, image area specifying image data in which the non-image area and the background are white pixels is generated.

  Next, the pattern candidate part specification processing unit 22 paints white pixels connected to the boundary of the pattern part with a predetermined value (any color other than white and black). Here, a process of painting a closed region (inside or outside surrounded by a closed curve) as is widely known is used.

  Then, the pattern candidate portion specifying processing unit 22 sets the color of the pixels that are not painted with the predetermined value to black. A portion set as a black pixel by this processing is stored in the storage unit 12 as a pattern candidate region.

[3. Character extraction processing unit]
The character extraction processing unit 23 performs a process of extracting a character part from the image data expressed in the YCbCr color space output from the preprocessing unit 21. For this processing, for example, processing as disclosed in Patent Document 2 can be used.

  The character extraction processing unit 23 stores coordinate information of rectangles (basic rectangles) circumscribing the pixels constituting the individual characters in the storage unit 12. This information is information for specifying the character portion.

[4. Layout processing section]
The layout processing unit 24 reads, from the storage unit 12, the pattern candidate area definition information generated by the pattern candidate part specifying processing unit 22 and the coordinate information of the character circumscribing rectangle generated by the character extraction processing unit 23.

  The layout processing unit 24 performs layout analysis using different determination conditions for each of the pattern candidate area defined by the pattern candidate area definition information and the character area defined by the coordinate information of the character circumscribed rectangle. Do.

  Specifically, in the present embodiment, layout analysis processing is performed in the pattern candidate area defined by the pattern candidate area definition information read from the storage unit 12, and further character portions are extracted from the pattern candidate area. Then, a portion excluding the extracted character portion is defined as a pattern region.

  One characteristic of this embodiment is that a character portion is extracted using layout analysis in so-called T / I separation processing. As a result, the character portion included in the pattern candidate region is also extracted by the layout analysis process, and the accuracy of extracting the character portion can be improved.

  On the other hand, the layout processing unit 24 also performs layout analysis processing on the basic rectangle defined as characters by the character extraction processing unit 23. The layout processing unit 24 determines layout frames (including at least rectangles (basic rectangles) circumscribing each character) obtained as a result of the layout analysis processing, and stores information (coordinate information and the like) of these layout frames. 12.

  The layout processing unit 24 refers to the character part specifying information stored in the storage unit 12 and the basic rectangle generated by the character extraction processing unit 23 (or a layout frame as a result of layout processing for the rectangle). For the entire image data, coordinate information of a rectangular area including characters in the image data is generated. Specifically, the layout processing unit 24 integrates each piece of information of the character part extracted from the part that has become the pattern candidate and the character part extracted by the character extraction processing unit 23 to generate information that defines the character region To do.

  Then, the layout processing unit 24 generates a unique area identifier (hereinafter referred to as label data) for each combined character portion, and coordinates information (vertex coordinate of the vertex coordinates) for defining the label data and the corresponding character area. And the like are stored in the storage unit 12 as a character area database.

  In addition, the layout processing unit 24 generates rectangular information that surrounds the pattern portion and the character portion (that is, circumscribes the pixels constituting them) as layout frame information, and stores the generated information in the storage unit 12.

[5. Same color area separation unit]
The same color area separation unit 25 performs a process of separating the image data of the character part stored in the character area database and the image data of the other part into areas of different colors.

[5a. Processing for character part]
First, processing for image data of a character part will be described. The same color area separation unit 25 separates each character area stored in the character area database of the storage unit 12 into an area including only character portions of the same color. The same color region separation unit 25 reads the coordinate information of the basic rectangle (rectangle defined from both the pattern candidate region and the character region) stored in the storage unit 12 during the layout process. Then, a representative value (representative color) candidate is determined from pixel values included in each basic rectangle defined by the coordinate information in the image data (original image data) to be processed.

  As shown in FIG. 3A, the same color region separation unit 25 in the present embodiment is configured to include a representative color determination unit 51, a same color region information generation unit 52, and an inclusion image creation unit 53. ing. In the following description, the pixels in the basic rectangle may be all pixels included in the basic rectangle on the original image data, or pixels that are determined as pixels constituting the character in the basic rectangle. There may be.

  The representative color determination unit 51 refers to the coordinate information of the basic rectangle stored in the storage unit 12 and selects at least one representative pixel based on the pixel value in the target basic rectangle while sequentially selecting them as the target basic rectangle. Determine the value. Here, the representative pixel value is determined by generating a histogram (occurrence frequency) of pixel values in the target basic rectangle in the original image data, and one pixel value appearing at the highest frequency in the histogram, or a predetermined pixel value. There is a method in which at least one pixel value that appears at a frequency exceeding a threshold value (for example, 1/3 of the number of pixels in the target basic rectangle) is used as a representative pixel value. The representative color determination unit 51 stores the representative pixel value determined in association with the information specifying the target basic rectangle (identifier issued uniquely to the target basic rectangle) in the storage unit 12 as a representative pixel value database.

  The same color area information generation unit 52 groups representative pixel values stored in the representative pixel value database for each color determined to be the same. Specifically, the same color area information generation unit 52 sets representative pixel values not yet grouped among representative pixel values stored in the representative pixel value database as target representative pixel values. Different representative pixel values that have not yet been grouped are sequentially selected as comparison pixel values. If there is a comparison pixel value that can be determined to be the same color as the target representative pixel value, the at least one comparison pixel value and the target representative pixel value are determined as one group. Then, the same color area information generation unit 52 extracts the basic rectangle identifiers associated with the representative pixel values in the group from the representative pixel value database, generates a list of basic rectangle identifiers, One group representative pixel value determined based on the comparison pixel value is determined. The same color area information generation unit 52 then issues a unique group identifier, stores this group identifier, the group representative pixel value, and the generated list of basic rectangle identifiers as the same color area information database. Stored in the unit 12. Note that the group representative pixel value determination method may be, for example, a pixel value of a statistical calculation result (for example, an average) between a target representative pixel value and a comparison pixel value determined to be the same.

  On the other hand, when there is no comparison pixel value that can be determined to be the same color as the representative pixel value of interest, a group including only the representative pixel value of interest is generated. That is, the same color area information generation unit 52 extracts the identifier of the basic rectangle associated with the target representative pixel value from the representative pixel value database, and determines the target representative pixel value as the group representative pixel value as it is. The same color area information generation unit 52 issues a unique group identifier, associates the group identifier, the group representative pixel value, and the generated basic rectangle identifier with each other in the storage unit 12 as the same color area information database. Store.

  In this way, the same color area information generation unit 52 defines, for each grouped representative pixel value (group representative pixel value), the representative pixel value (group representative pixel value) and a basic area corresponding to the representative pixel value. The same color area information including the information associated with (here, the identifier of the basic area) is generated and stored in the storage unit 12 as the same color area information database.

  The included image creation unit 53 retrieves the identifier of the basic area included in the representative pixel value database while referring to the representative pixel value database. Further, with reference to the basic rectangle database, coordinate information (information defining each basic area) associated with the identifier of each extracted basic area is acquired. Then, the included image creation unit 53 generates a region that includes these basic regions. Specifically, the inclusion image creation unit 53 includes the smallest X coordinate value Xmin, the Y coordinate value Ymin, the largest X coordinate value Xmax, and the Y coordinate among the X and Y coordinate values indicated by the acquired coordinate information. Search for the value Ymax. Then, information is generated that defines a rectangle having a diagonal line (Xmin, Ymin) and (Xmax, Ymax) (a rectangle circumscribing each included basic rectangle).

  The inclusion image creation unit 53 extracts the pixels in each included basic region from the original image data, and generates this based on the background pixel values determined from the pixel values at the four corners of the original image data, for example. Binarization is performed using a threshold value, and the binarized image data is generated as inclusion image data.

  This inclusion image data is bitmap information obtained by binarizing the value of each pixel in the basic region included in the inclusion image data. Therefore, the inclusion image data includes at least one representative. Information indicating the position of the pixel to be set in the pixel value (or group representative pixel value) is included. The inclusion image creation unit 53 may compress information indicating the position of the pixel, such as bitmap information obtained by binarization, by a predetermined method (for example, MMR, JBIG2, etc.).

  Further, the same color region separation unit 25 of the control unit 11 may perform the process shown in FIG. 3B as another example of the process for the character portion. In other words, this processing is functionally configured including a representative color determination unit 51, the same color area information generation unit 52, an inclusion image creation unit 53, a color-specific inclusion image creation unit 54, and a comparison unit 55. . Here, the representative color determination unit 51, the same color area information generation unit 52, and the included image creation unit 53 perform the same operations as those already described.

  The inclusion image creation unit 54 for each color generates inclusion image data for each group. The comparison unit 55 includes character plane data (first image data) including the inclusion image data (inclusion area for each color) for each group, and character plane data (first image data including the same color area information database as the inclusion image data). 2 image data), the smaller one of the first image data and the second image data is selected, and the selected data is output as character plane data.

  In addition, the same color area information generation unit 52 determines whether two representative pixel values can be determined to be the same as the sum of squares of differences of the two representative pixel values for each component (predetermined value between representative pixel values). If the amount relating to the distance in the color space is smaller than a predetermined threshold value, it may be determined that they are the same.

  Next, the operation of the same color region separation unit 25 of the present embodiment will be described with reference to FIG. Here, an image as shown in FIG. 4A will be specifically described as an example. In the example shown in FIG. 4A, in the character string “Yesterday was rainy” on the first line, the letters “Yesterday” were red characters and the characters “Has rainy” were represented by black characters. , The whole string of the next line “Today is cloudy” is represented in black. Further, an example is shown in which there is a character string “It was a story about the weather” at a position apart from the character strings of the two lines, and only the characters “story” are red characters. In FIG. 4A, for convenience of illustration, the red character portion is surrounded by a broken line. This enclosed broken line is not actually represented.

  In this case, a rectangle (basic rectangle) circumscribing the pixel block of each character is defined by the processing of the layout processing unit 24 and the like, and the representative color determining unit 51 determines a representative color for each basic rectangle. The representative color determined by the representative color determining unit 51 is set to the same color (pixel value) for each character, for example, two characters “Yesterday” and “Day” of “Yesterday”. However, the pixel values are not always the same, and representative pixel values that are relatively close to each other but different from each other may be determined. That is, the representative color determining unit 51 determines a representative pixel value close to red for the basic rectangle circumscribing the three characters “Yesterday”, “Day”, and “Story”, and each basic character circumscribing other characters. For the rectangle, a representative pixel value close to black is determined.

  The same color area information generation unit 52 defines basic rectangles associated with representative pixel values determined to be the same as a group. As a result, one group representative pixel value (also defined as a value close to red) is associated with the information defining the basic rectangle circumscribing the three characters “Yesterday”, “Day”, and “Story”. One group representative pixel value (also defined as a value close to black) is associated with information that defines a basic rectangle that is grouped and circumscribes other characters, and is grouped.

  The inclusion image creation unit 53 generates a rectangle that includes all of the target basic rectangles (regardless of the group), and converts the pixel value in each basic rectangle into binary image data (first image). 2 image data) (FIG. 4B).

  Then, the same color region information database including at least one set of information defining the basic rectangle and the group representative pixel value is associated with the included image data and output as character plane data.

  On the other hand, the color-specific inclusion image creation unit 54 generates inclusion image data (color-specific inclusion image data) for each group (FIG. 4C). Here, the pixel values in the color-by-color inclusion image data need not be binarized. In the case of binarization, the group representative pixel values of the group corresponding to the color-specific inclusion image data are associated and included in the character plane data. Then, the comparison unit 55 selectively outputs image data having a small data size among the character plane data (first image data) generated by the color-specific inclusion image creation unit 54 and the second image data. The output result is stored in the storage unit 12 as character plane data.

[5b. Processing for non-character areas]
Next, processing for a region other than characters will be described. As shown in FIG. 5, the processing for the area other than the characters in the same color area separation unit 25 is functionally performed as shown in FIG. The identification unit 63, the line segment image element removal unit 64, the first color extraction unit 65, the second color extraction unit 66, the limited colorization unit 67, and the region integration unit 68 are configured.

  The preprocessing unit 60 reads character plane data from the storage unit 12. Then, the pixels included in the character plane data are removed from the original image data, and image data of an area other than the characters is generated. Further, the preprocessing unit 60 reads out rectangular information (layout frame information) surrounding the pattern portion and the line drawing portion from the storage unit 12. Then, the image data in the rectangle defined by the layout frame information is extracted from the image data of the area other than the character, and is stored in the storage unit 12 as the image data to be processed.

  As a result, an image of the pattern portion and the line drawing portion surrounded by the layout frame, which is the character portion removed, is generated. For example, for a graph as shown in FIG. 6A, an image in which characters are removed (FIG. 6B) is stored in the storage unit 12 as image data to be processed.

  The edge extraction unit 61 calculates a difference between the maximum luminance and the minimum luminance among the pixels adjacent to each other from the processing target image data stored in the storage unit 12 as a luminance difference, and further calculates a standard deviation of the luminance difference. To extract an edge. The line segment extraction unit 62 detects a portion where the difference between adjacent pixel values is equal to or greater than a predetermined threshold from the image data to be processed stored in the storage unit 12 and extracts a line segment. Since both the edge extraction process and the line segment extraction process are widely known, a detailed description thereof will be omitted.

  The line segment image element specifying unit 63 generates information for specifying a pixel group including pixels extracted as edges and pixels extracted as line segments, and outputs the information as line segment image element data. For example, from the example shown in FIG. 6B, when pixels constituting the edge portion or line segment are extracted, a pixel group as shown in FIG. 6C is obtained. This pixel group is line segment image element data and corresponds to the first partial image data of the present invention.

  The line segment image element removal unit 64 generates image data (line segment image element removal data) obtained by removing the pixel group included in the line segment image element data from the image data output from the preprocessing unit 60. This line segment image element removal data corresponds to the second partial image data of the present invention, and is specifically as shown in FIG.

  The first color extraction unit 65 extracts a representative color from the pixel value of each pixel included in the line segment image element data. Specifically, the first color extraction unit 65 includes a color number determination unit 71 and a representative color determination unit 72, as shown in FIG. The number-of-colors determination unit 71 color-limits the pixel values of each pixel included in the line segment image element data by a method such as median cut (median value division method), and outputs the number of colors after the color limitation To do. The color number determination unit 71 arranges pixel values of each pixel in the color space, performs clustering processing in the color space, divides the color space into a plurality of parts, and determines the number of divisions as the number of colors. You may do it. The value of the number of colors thus obtained represents the number of significant colors and corresponds to the number of significant pixel values of the present invention.

  When the number of colors output from the color number determination unit 71 does not exceed a predetermined threshold value (first threshold value), the representative color determination unit 72 has information on the number of representative colors corresponding to the number of colors. Is output. For example, when the number-of-colors determination unit 71 performs limited color by median cut, each pixel value obtained by the limited color is output as information representing the representative color as it is. When clustering is used, for each divided part, the centroid of the pixel value included in the part is calculated, and the color corresponding to the centroid is output as representative color information.

  Similar to the first color extraction unit 65, the second color extraction unit 66 extracts a representative color from the pixel values of each pixel included in the line segment image element removal data. In other words, the second color extraction unit 66 also limits the pixel value of each pixel included in the line segment image element data by a method such as median cut (median value division method), and the color after the limited color is obtained. Count the number of colors. When the number of colors does not exceed a predetermined threshold value (second threshold value), each pixel value obtained by the limited color is output as information representing a representative color as it is.

  The limited colorization unit 67 compares the pixel value of each pixel included in the line segment image element data with the information on each representative value output from the first color extraction unit 65. Then, image data (first limited colorization) in which the pixel value of each pixel of the line segment image element data is set to the representative value closest to each pixel value among the representative values output by the first color extraction unit 65. Image data). Further, the limited colorization unit 67 compares the pixel value of each pixel included in the line segment image element exclusion data with the information of each representative value output from the second color extraction unit 66. Then, the image data (second limited color) in which the pixel value of each pixel of the line segment image element exclusion data is set to the representative value closest to each pixel value among the representative values output by the second color extraction unit 66. Generated image data).

  In addition, the representative color determination unit 72 of the first color extraction unit 65 cannot perform the limited color when the number of colors output from the color number determination unit 71 exceeds a predetermined threshold value (first threshold value). Judge. In this case, the limited colorization unit 67 outputs the line segment image element data as it is and does not perform limited colorization.

  Similarly, when the number of colors included in the line segment image element exclusion data exceeds a predetermined threshold value (second threshold value), the second color extraction unit 66 determines that the limited color cannot be obtained. At this time, the limited color unit 67 outputs the line segment image element exclusion data as it is, and does not perform limited color.

  Further, at this time, the first color extraction unit 65 may determine whether or not the line segment image element data includes an area that can be partially limited in color. For this determination, for example, a histogram is calculated, and pixel values exceeding a predetermined threshold value can be determined to be substantially the same color (for example, when the difference between the pixel values is about ± 16). Judgment is made based on whether or not the number exceeds a predetermined ratio with respect to the number of pixels in the line segment image element data. If an area that can be partially limited in color is included, the first color extraction unit 65 outputs information for specifying the part and a representative color value for the part. In this case, the limited colorization unit 67 performs processing for limiting the color of the specified portion of the line segment image element data to the representative color.

  Similarly, the second color extraction unit 66 may determine whether or not a region that can be partially limited in color is included in the line segment image element exclusion data. If an area that can be partially limited in color is included, the second color extraction unit 66 outputs information for specifying the part and a representative color value for the part. In this case, the limited colorization unit 67 performs processing for limiting the color of the specified portion of the line segment image element exclusion data to the representative color.

  The region integration unit 68 includes a plurality of color-specific images including pixels for each color determined to be the same color among the pixels included in the first limited color image data and the second limited color image data. Generate data. That is, the region integration unit 68 compares the representative color information groups C1a, C1b... Output from the first color extraction unit 65 with the representative color information groups C2a, C2b. Then, representative colors that are determined to be the same color are searched. For example, each of C1a, C1b,... Is sequentially selected as the target representative color C1x. Then, the distance on the color space between the target representative color C1x and each of C2a, C2b... (The sum of squares of the differences between the components on the color space) is calculated. If there is a representative color C2y whose distance is less than a predetermined same color determination threshold value, the representative color C1x of interest and the representative color C2y are determined to be the same color.

  Next, the region integration unit 68 separates the pixel of the representative color C1x determined to be the same color from the first limited color image data. Similarly, the pixel of the representative color C2y is separated from the second limited color image data. Then, color-specific image data including pixels separated from the first limited color image data and the second limited color image data is generated and output. The region integration unit 68 repeats this process for each representative color pair determined to have the same color, and generates color-specific image data.

  Further, by this process, pixels included in each image data and determined to be the same color are removed from the first limited color image data and the second limited color image data. For example, assume that the first limited color image data includes pixels having pixel values of C1a and C1b, and the second limited color image data includes pixels having pixel values of C2a and C2b. When it is determined that C1a and C2b are the same color by the above processing, the C1a pixel of the first limited color image data and the C2b pixel of the second limited color image data are combined. The processed image data is generated as color-specific image data. Further, the C1b pixel remains in the first limited color image data. Similarly, C2a pixels remain in the second limited color image data.

  It should be noted that here, a color determined to be the same between the first limited color image data and the second limited color image data obtained from one of the plurality of layout frames extracted from the original image data is found. Yes. For this reason, when there is such a color, the pixel of the color found in the layout frame is extracted to generate color-specific image data. However, the color image data generation method is not limited to this. That is, color-specific image data including pixels for each color determined to be the same color from the entirety of the plurality of first limited color image data and the plurality of second limited color image data regarding all layout frames. It is good also as producing | generating.

  Furthermore, the region integration unit 68 may perform the following when the character-by-color inclusion image data is included in the character plane data. That is, the region integration unit 68 compares the color information associated with the color-specific inclusion image data with the representative color related to the color-specific image data. If the colors can be determined to be the same color (determined based on the distance in the color space), the color-specific image data determined to be the same color is combined with the color-specific inclusion image data.

  In this way, the same color area separation unit 25 determines the image data for each color (if any), the first limited color image data (or line segment image element data) for each layout frame, and the second limit from areas other than characters. Colored image data (or line segment image element exclusion data) is generated. Hereinafter, these image data are referred to as line segment pattern plane data.

  The control unit 11 extracts inclusion image data obtained from the character part area, the same color area information database, and coordinate information indicating the position of the inclusion image data on the original image data. Data including these pieces of information is output as character / line drawing plane data. When reproducing the original image data from the generated character / line drawing plane data, the compressed data is first decompressed to reproduce the bitmap of the included image data, which is included in the same color area information database. For each group, the pixel value of a black pixel (on pixel) on the reproduced bitmap corresponding to the basic rectangle in the group is specified, and the value of the specified pixel is used as the group representative of the group What is necessary is just to set to a pixel value.

  Here, the coordinate information that defines the basic rectangle included in the same color region information database may represent a position on the original image data, or represents a position in the included image data (for example, Coordinate information representing a relative position from the lower right coordinates of the included image data).

  The same color area information database does not necessarily include information that defines all basic rectangles. For example, among the groups, the group representative pixel value of the group having the maximum number of basic rectangles (referred to as the maximum group) is associated with the included image data shown in FIG. May be removed from the same color area information database.

  In this case, the original image data generation side first reproduces the bitmap of the inclusive image data, and associates the pixel values of the black pixels (pixels that are turned on) with the inclusive image data. Set to the group representative pixel value (the group representative pixel value of the largest group), and then turn on for each group in the same color area information database in the area corresponding to the basic rectangle included in each group on the bitmap. For the pixels that are, the group representative pixel value of each group is reset.

  In this example, the group representative pixel value of the maximum group is set, but the group representative pixel value closest to a predetermined color (for example, black) is associated with the inclusion image data shown in FIG. The group information related to the representative pixel value may be removed from the same color area information database.

  Furthermore, in consideration of the case where the pixel values in the basic rectangle vary, the same color region separation unit 25 may perform a process of determining a representative pixel value after performing a smoothing process in advance. Here, the smoothing process includes a process of sequentially specifying each pixel in each basic rectangle as a target pixel, and setting an average value of the value of the target pixel and the value of a pixel adjacent thereto as the value of the target pixel.

  Furthermore, at the time of this smoothing process, only the pixels constituting the character in the basic rectangle (for example, the portion that becomes a black pixel by the binarization process) may be selected as the target pixel. In the smoothing process, when calculating the average value, the average value may be calculated with reference to only the values of the pixels constituting the character. This prevents the representative color of the character from being influenced by the background color by referring to the pixel value of the portion other than the character.

  Here, when the representative value is determined after the smoothing process, the representative value may be corrected. Here, the correction can be performed using, for example, a tone curve (correction function) for luminance as shown in FIG. In the tone curve shown in FIG. 8, the output value (corrected value) for the input value (representative luminance before correction) from the minimum value MIN to the first threshold value TH1 is the minimum value MIN. The output value from the threshold value TH2 (TH2> TH1) to the maximum value MAX is set to be the maximum value MAX. The tone curve has a center value MID between the maximum value MAX and the minimum value MIN (for example, when the maximum value is “255” and the minimum value is “0”, the MID is “128”). The output value for this may be set to be substantially MID.

  That is, the same color region separation unit 25 corrects the luminance (Y ′) of the representative value with respect to the luminance component (Y) of the representative value by the tone curve of FIG. 8, and Y ′ and the color difference component Cb of the representative value candidate , Cr are output as corrected representative values.

  Further, although only the luminance is corrected here, the color difference component may also be corrected. Specifically, the same color region separation unit 25 performs correction to reduce the number of gradations related to the luminance component value of the representative color when each color difference component of the representative value satisfies a predetermined condition. May be determined as a representative value. Specifically, as shown in FIG. 9, the representative color difference components (a *, b *) expressed in the color space of L * a * b are different from the center value of the corresponding color difference component range. When the condition of being within a predetermined range (inside a circle defined by THa and THb in FIG. 9) is satisfied, for example, the luminance component L expressed by 256 gradations is converted to 4 gradations or 8 gradations. Etc. to a predetermined gradation. In this case, the value of the color difference component may be set to the center value. Here, the predetermined ranges THa and THb for each component may be the same value or different values.

  By this processing, when the character color is gray (including black), a representative value that reproduces the original color of the character color is set. For example, when the character color is black, the color difference component and the luminance component are originally “0”. However, depending on the characteristics of the scanner and the encoding format of the original image data (for example, JPEG), the color difference component may be “ In some cases, the luminance component is not “0” or the luminance component is not “0”. Therefore, by performing the processing relating to the color difference component shown here, the representative value can be set to the original black color.

  Here, after performing the smoothing process, the correction process is performed to determine the representative value. However, after performing the above correction process for each pixel by reversing the process order, A representative value may be determined by performing a smoothing process and calculating a histogram.

[6. Cavity processing section]
The hole filling processing unit 26 performs hole processing on the image data (line image element element data or line segment image element exclusion data) that is not limited color among the line segment pattern plane data output by the same color region separation unit 25. Perform the filling process.

  In other words, each pixel of these image data is scanned in raster scan order, and if the pixel of interest selected by the scan is not removed, the pixel value of the pixel of interest remains unchanged and the pixel value of the pixel of interest Is stored in the work memory of the storage unit 12 as the previous pixel value. If another pixel value is already stored as the previous pixel value, the stored content is overwritten.

  If the pixel of interest selected by scanning is a removed pixel, the pixel value of the pixel of interest is set to the immediately preceding pixel value stored. As a result, the pixel value of the removed portion becomes the same as the previous pixel value in the raster scan order, and the compression efficiency can be improved in many compression processes.

[7. Compression processing unit]
The compression processing unit 27 performs JPEG compression on the image data (the line segment image element data or the line segment image element exclusion data) that has not been limited color among the line segment pattern plane data stored in the storage unit 12. The first compressed line segment pattern plane data is generated. In addition, the compression processing unit 27 includes image data (color-specific image data, first limited color image data, second limited color conversion) that is limited color among line segment picture plane data stored in the storage unit 12. (Image data) is compressed by a method such as run length compression or ZIP to generate second compressed line segment pattern plane data. Further, the compression processing unit 27 also compresses the character plane data by a method such as run length compression or ZIP, and generates compressed character plane data.

  The first compressed line segment pattern plane data, the second compressed line segment pattern plane data, and the compressed character plane data obtained in this way are connected by, for example, the compression processing unit 27 and written out as PDF (Portable Document Format) data. The compression processing unit 27 stores the generated PDF data in the storage unit 12 or outputs the PDF data to the image output unit 14 to send it to an external device.

[Operation]
Next, the operation of the image processing apparatus according to this embodiment will be described. Here, as shown in FIG. 10, a document including a character portion (T1, T2), a photograph portion (P), and a map portion (M) as a line drawing portion is input from the image input unit 13, and this document A case where image data is a processing target will be described as an example. In the example of FIG. 10, a part (T2) of the character part is superimposed on the photograph part (P). The map portion (M) includes a road map and characters. Here, although shown in black and white for the sake of convenience, the road map and characters in the map portion are actually represented in different colors, and the photograph may be in color.

  The preprocessing unit 21 converts the pixel value of the image data into a value in a predetermined color space (YCbCr). The pattern candidate portion specifying processing unit 22 binarizes the image data (original image data), and generates image data from which the small portion is removed and the character portion (T1) is removed. At this time, most of the characters are removed (they may remain partially), but the characters superimposed on the photograph portion are in a state of being directly specified as the pattern candidate portion.

  The character extraction processing unit 23 binarizes the original image data and extracts a character part by a method such as specifying a small area part. At this time, the original image data is divided into a plurality of regions, and a binarization threshold value is adaptively determined for each region obtained by the division (a method disclosed in Patent Document 2). By performing binarization, characters can be extracted from a colored portion such as a map. Then, the layout processing unit 24 performs a layout analysis process in the pattern candidate part, and extracts a character part (T2) remaining in the pattern candidate part. Further, the layout processing unit 24 separates the image data for each element of the document and determines a rectangle (layout frame) circumscribing each element. Specifically, in the example shown in FIG. 10, a layout frame that defines a character portion (T1), a map portion (M), and a photograph portion (P) is defined. Since the character part (T2) is included in the photograph part (P), the layout frame may not be determined.

  The same color region separation unit 25 determines a representative color for each character for the character portion extracted by the character extraction processing unit 23 and the layout processing unit 24 (for both T1 and T2), and determines the representative color from the determined representative color. A set of representative colors determined to be the same color is searched. Next, for each set of representative colors, the characters of the representative colors belonging to the set are grouped. Then, the image data including the character portion extracted by the character extraction processing unit 23 or the layout processing unit 24 is generated, and the size of the pixel value information of the image data is binarized (or converted to gray scale). And including the group representative color for each group, coordinate information defining a basic rectangle (a rectangle circumscribing each character) included in the group, the included image data, and the included image Character plane data including coordinate information specifying the position of the data on the original image data is generated.

  On the other hand, the same color region separation unit 25 extracts partial image data in the layout frame other than the character portion from the original image data. That is, here, the map portion (M) and the photograph portion (P) are extracted. Next, the same color region separation unit 25 removes the character portion extracted by the character extraction processing unit 23 and the layout processing unit 24 from each partial image data. Thereby, the character part (T2) included in the photograph part (P) is removed. If the map part includes characters, the part is also removed.

  Next, line segment image elements (edges and line segments themselves) are detected using each partial image data as a processing target. The same color region separation unit 25, for each image data to be processed, the line segment image element data including only the detected line segment image element, and the line segment image element obtained by removing the line segment image element Generate removal data.

  Then, the same color region separation unit 25 checks whether or not the representative color can be determined for the line segment image element data. If the representative color can be determined, the representative color is determined and the line segment image element data is limited to the color. Generate data. Here, it is assumed that the line segment image element data has been limited in color for both the map portion (M) and the photograph portion (P).

  The same color region separation unit 25 also checks whether or not the representative color can be determined for the line segment image element exclusion data. If the representative color can be determined, the representative color is determined and the line segment image element exclusion data is determined. Generate limited color data. Here, it is assumed that the line segment image element exclusion data can be limited color only for the map portion (M).

  The same color region separation unit 25 extracts a set of representative colors used for the limited color for each data subjected to the limited color. Then, a search is made as to whether there is a color determined to be the same color between the representative colors included in each set. For example, if one of the representative colors for the line segment image element data of the map portion (M) and one of the representative colors for the line segment image element data of the photograph portion (P) are determined to be the same color, The pixel of the representative color is separated from the image element data to generate new image data (color-specific image data). At this time, the separated pixels are removed from each line segment image element data.

  For the line segment image element exclusion data of the photographic part (P) that has not been subjected to the limited colorization, the hole filling processing unit 26 calculates the pixel value removed as a character or the like as the nearest pixel value ( Pixel value that has not been removed).

  The compression processing unit 27 performs JPEG compression on the line segment image element exclusion data of the photographic part (P) that has not been subjected to the limited colorization, and performs color-specific image data or each line segment image element data that has undergone the limited color conversion. For the character plane data, for example, run length compression is performed. These compression results are combined to generate PDF data, which is output to the image output unit 14. The image output unit 14 outputs the PDF data to an external device.

  Here, the compression processing unit 27 may perform a process of reducing the image size (reduction process) on the line segment image element exclusion data before JPEG compression to further improve the compression rate.

  In the present embodiment, compression processing suitable for the properties of the image, such as run length, can be applied to the line segment image elements included in the pattern through processing such as limited color, so that the compression efficiency and image quality are improved. it can.

1 is a configuration block diagram illustrating an example of an image processing apparatus according to an embodiment of the present invention. It is a functional block diagram showing the processing content performed by the control part of the image processing apparatus which concerns on embodiment of this invention. It is a functional block diagram showing the example of the process with respect to the area | region of the character of the same color area separation part. It is explanatory drawing showing the process example of the same color area separation part. It is a functional block diagram showing the example of the process with respect to areas other than the character of the same color area separation part. It is explanatory drawing showing the process example of the same color area separation part. It is a functional block diagram showing the example of a color extraction part. It is explanatory drawing showing the example of the tone curve utilized in the same color area separation part 25. FIG. FIG. 11 is an explanatory diagram illustrating processing conditions for correction processing in the same color region separation unit 25. It is explanatory drawing showing the example of the image used as the process target of the image processing apparatus which concerns on embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Control part, 12 Memory | storage part, 13 Image input part, 14 Image output part, 21 Pre-processing part, 22 Design candidate part specific processing part, 23 Character extraction processing part, 24 Layout processing part, 25 Same color area separation part, 26 Hole filling processing unit, 27 Compression processing unit, 51 Representative color determination unit, 52 Same color area information generation unit, 53 Inclusive image generation unit, 54 Color-specific inclusion image generation unit, 55 Comparison unit, 60 Preprocessing unit, 61 Edge extraction , 62 line segment extraction unit, 63 line segment image element identification unit, 64 line segment image element removal unit, 65 first color extraction unit, 66 second color extraction unit, 67 limited colorization unit, 68 region integration unit, 71 Color number determination unit, 72 representative color determination unit.

Claims (6)

Extract line segment image elements from the image data to be processed, and generate first partial image data composed of the line segment image elements and second partial image data excluding the line segment image elements. Means,
First limited colorization, wherein at least one representative color is determined based on a pixel value included in the first partial image data, and the first partial image data is limited color using the determined representative color First limited colorizing means for generating image data;
Second limited colorization, wherein at least one representative color is determined based on a value of a pixel included in the second partial image data, and the second partial image data is limited color using the determined representative color Second limited colorization means for generating image data;
Including
An image processing apparatus, wherein the first limited color image data and the second limited color image data are subjected to a predetermined compression process. The image processing apparatus according to claim 1.
The first limited color image data is calculated by calculating a significant number of pixel values included in the first partial image data, and only when the significant number does not exceed a predetermined first threshold value. An image processing apparatus, comprising: determining one representative color, and generating first limited color image data obtained by limiting the first partial image data using the determined representative color. The image processing apparatus according to claim 1 or 2,
The second limited color image data is calculated by calculating a significant number of pixel values included in the second partial image data and only when the significant number does not exceed a predetermined second threshold value. An image processing apparatus that determines two representative colors and generates second limited color image data obtained by limiting the second partial image data using the determined representative colors. The image processing apparatus according to any one of claims 1 to 3,
Based on the first limited color image data and the second limited color image data,
Means for generating a plurality of color-specific image data including pixels for each color determined to be the same color among the pixels included in the first limited color image data and the second limited color image data An image processing apparatus further comprising: Extract line segment image elements from the image data to be processed, and generate first partial image data composed of the line segment image elements and second partial image data excluding the line segment image elements. Process,
First limited colorization, wherein at least one representative color is determined based on a pixel value included in the first partial image data, and the first partial image data is limited color using the determined representative color Generating image data; and
Second limited colorization, wherein at least one representative color is determined based on a value of a pixel included in the second partial image data, and the second partial image data is limited color using the determined representative color Generating image data; and
An image processing method, wherein the first limited color image data and the second limited color image data obtained by the separation are subjected to a predetermined compression process. On the computer,
Extract line segment image elements from the image data to be processed, and generate first partial image data composed of the line segment image elements and second partial image data excluding the line segment image elements. Procedure and
First limited colorization, wherein at least one representative color is determined based on a pixel value included in the first partial image data, and the first partial image data is limited color using the determined representative color A procedure for generating image data;
Second limited colorization, wherein at least one representative color is determined based on a value of a pixel included in the second partial image data, and the second partial image data is limited color using the determined representative color A procedure for generating image data;
And execute
An image processing program characterized in that the first limited color image data and the second limited color image data obtained by the separation are subjected to a predetermined compression process.

Images