JP2017118433A - Image processing device, image processing method and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the enhancement of a compressibility ratio without damaging the reproducibility of a line image color.SOLUTION: According to an embodiment, an image processing device includes a line image detection part for detecting line images from an input image, a line image color processing part for calculating a line image color being the color of the line image about each of line images detected by the line image detection part, a representative color calculation part 32 for calculating a representative color of a group on the basis of a line image color calculated to remaining line images obtained by excluding line images corresponding to a prescribed condition among line images constituting a group to be a unit for unifying colors, and a line image color change part 33 for changing a line image color of a line image constituting the group to the representative color.SELECTED DRAWING: Figure 6

Description

  The present invention relates to an image processing apparatus, an image processing method, and a program.

  As an image compression technique that achieves both high compression and high image quality, a technique called high compression PDF (Portable Document Format) is known. High-compression PDF is a technology that achieves both high compression and high image quality by separating an image into line drawings such as characters and other areas and performing compression processing suitable for each.

  With high-compression PDF, for example, a line drawing color can be reproduced by providing a layer that represents the color of the line drawing (hereinafter referred to as “line drawing color”) or by expressing the line drawing with its coordinates and line drawing color. It is said. The line drawing color is generally calculated in units of line drawing. However, if the line drawing color calculated in line drawing unit is used as it is, the type (number) of line drawing colors to be reproduced may increase and the compression rate may be lowered. is there. On the other hand, characters, which are typical line drawings, often form a character line, and unifying the color for each character line not only increases the compression ratio, but also suppresses character color variations in the character line. It has the effect of improving the appearance as a line. From this point of view, it has been proposed to average the character colors of the characters constituting the character line to obtain the representative color of the character line, and to reproduce the characters constituting the character line with the representative color of the character line. (For example, refer to Patent Document 1).

  However, the conventional method simply averages the character colors of the characters that make up the character line to obtain the representative color of the character line, so that characters of a color different from other characters are included in the same character line. If this is the case, the entire character line is reproduced in a color different from the original color, and the reproducibility of the character color may be impaired.

  In order to solve the above-described problems, the present invention provides a line drawing detection unit that detects a line drawing from an image, a line drawing color calculation unit that calculates a line drawing color that is the color of the line drawing, and a group that is a unit for unifying colors. Representative color calculation means for calculating a representative color of the group based on the line drawing color calculated for the remaining line drawing excluding the line drawing that satisfies a predetermined condition among the line drawing constituting the group drawing, and the group Changing means for changing the line drawing color of the line drawing constituting the line to the representative color.

  According to the present invention, it is possible to increase the compression rate without impairing the reproducibility of the line drawing color.

FIG. 1 is a schematic diagram for explaining the processing procedure of a high-compression PDF. FIG. 2 is a block diagram illustrating a hardware configuration example of the image processing apparatus according to the embodiment. FIG. 3 is a block diagram illustrating a functional configuration example of the image processing apparatus according to the embodiment. FIG. 4 is a block diagram illustrating a specific example of the line drawing detection unit. FIG. 5 is a block diagram illustrating a specific example of the line drawing color processing unit. FIG. 6 is a block diagram illustrating a specific example of the color unification processing unit. FIG. 7 is a flowchart illustrating the flow of operations performed by the image processing apparatus according to the embodiment. FIG. 8 is a flowchart for explaining an example of processing by the first detection unit. FIG. 9 is a flowchart illustrating an example of processing by the second detection unit. FIG. 10 is a flowchart illustrating an example of processing performed by the line drawing color processing unit. FIG. 11 is a diagram illustrating a state in which the vicinity of the edge of the line drawing has changed color due to the influence of the background color. FIG. 12 is a diagram illustrating a specific example of the process of correcting the line drawing color according to the background color. FIG. 13 is a diagram illustrating a specific example of processing for correcting a line drawing color according to a background color. FIG. 14 is a diagram illustrating a specific example of processing for correcting a line drawing color according to a background color. FIG. 15 is a diagram illustrating a specific example of the process of correcting the line drawing color according to the background color. FIG. 16 is a flowchart illustrating an example of processing by the color unification processing unit. FIG. 17 is a diagram for explaining a specific example of a method for determining a group as a unit for unifying line drawing colors. FIG. 18 is a diagram illustrating a specific example of processing for calculating a representative color of a group.

  Hereinafter, specific embodiments of an image processing apparatus, an image processing method, and a program according to the present invention will be described in detail with reference to the accompanying drawings. The embodiment described below is an application example to an image processing apparatus that generates a highly compressed PDF file (one aspect of “compressed image file”). However, the present invention is not limited to this example, and can be effectively applied to various image processing apparatuses that are required to appropriately calculate line drawing colors.

<High compression PDF>
First, prior to specific description of the present embodiment, an outline of high-compression PDF will be described. High-compression PDF is an image compression technique that generates a high-compression PDF file from an image including line drawings such as characters. Here, the line drawing refers to an object represented by a character and a line that should be handled in the same manner as the character. Even if the object is represented by a line, an object that is desirably handled as a picture is included in the picture instead of the line picture. The pattern is an object other than a line drawing, that is, an object represented by a halftone dot such as a photograph, or an object such as a graphic that is not desirable to be handled in the same manner as a character.

  FIG. 1 is a schematic diagram for explaining the processing procedure of a high-compression PDF. In order to generate a highly compressed PDF file, first, from the image to be processed (hereinafter referred to as “input image”) Im0, the first image layer Im1, which is a binary image consisting only of a line drawing, and the line drawing color are changed. A second image layer Im2 that is a multi-valued image to be expressed and a third image layer Im3 that is a multi-valued image that represents a pattern and background other than a line drawing are generated. Then, the first image layer Im1 and the second image layer Im2 are subjected to a compression process suitable for line drawing compression, and the third image layer Im3 is suitable for image and background compression. Apply compression processing. Thereafter, the first image layer Im1 that has been subjected to the compression process, the second image layer Im2 that has been subjected to the compression process, and the third image layer Im3 that has been subjected to the compression process are, for example, 1 in PDF format. By integrating on one image file, a highly compressed PDF file FIm corresponding to the input image Im0 is generated.

  The compression process performed on the first image layer Im1 is, for example, a compression process using an encoding method such as MMR for a binary image. The compression process performed on the second image layer Im2 is, for example, a compression process using an encoding method such as JPEG with a lower resolution than the compression process on the third image layer Im3. Since the compression processing for the first image layer Im1 and the compression processing for the second image layer Im2 are common in that they are compression processing suitable for line drawing compression, these processings are collectively referred to as “first This is called “compression processing”. On the other hand, the compression process performed on the third image layer Im3 is, for example, a compression process using an encoding method such as JPEG for a multi-valued image with a higher resolution than the compression process for the second image layer Im2. is there. Since the compression process for the third image layer Im3 is a compression process suitable for compression of a picture and a background, hereinafter, the second compression process is distinguished from the first compression process suitable for compression of a line drawing. Call it. Note that the above-described encoding method is an example, and compression processing using an encoding method different from the above-described example may be performed.

  In the high-compression PDF, as described above, the input image Im0 to be processed is separated into a line drawing area and a pattern or background area other than that, and the first compression processing is performed on the line drawing area. The compression efficiency is enhanced by applying the second compression process to the pattern and background area other than the line drawing. Here, the compression efficiency represents how much the compression rate could be increased without impairing the image quality (reproducibility) when the image was reproduced. A high compression rate was obtained while maintaining reproducibility. If so, efficient compression has been performed.

  The above-described high compression PDF can be variously modified. For example, the first image layer Im1 described above may be divided into an image layer consisting only of a black line drawing and an image layer consisting only of a chromatic color or white line drawing. Alternatively, the second image layer Im2 that expresses the line drawing color may not be provided, but the line drawing color may be provided as other information corresponding to the coordinates of the line drawing.

<Outline of Embodiment>
In the high compression PDF, it is effective to limit the types of line drawing colors as much as possible in order to compress the second image layer Im2 expressing the line drawing colors at a high compression rate. For example, if the character color of a character which is a representative line drawing is unified in character line units, the compression rate of the second image layer Im2 can be increased as compared with the case where character characters are given in character units. In addition, by unifying the character color for each character line, it is possible to obtain an effect of improving the appearance of the character line by suppressing variations in the character color in the character line.

  From such a viewpoint, in the technique described in Patent Document 1, the character colors of the characters constituting the character line are averaged to obtain a representative color of the character line, and the characters constituting the character line are changed to the character line. Reproduced with representative colors. However, in this method, when characters with different character colors from other characters are mixed in one character line, the entire character line is reproduced in a color different from the original color, and the reproducibility of the character color is impaired. There is a concern that improvement is required.

  Therefore, in the present embodiment, the line drawings detected from the input image Im0 are grouped according to a predetermined arbitrary criterion, and the line drawing colors of the remaining line drawings excluding the line drawing that meets a predetermined condition among the line drawings constituting the group are set. Based on this, the representative color of the group is calculated, and the line drawing color of the line drawing constituting the group is changed to the representative color of the group. Here, the predetermined condition includes, for example, a condition that the number of colors representing the number of line drawing colors of the line drawing is 2 or more, a condition that the line drawing color is a predetermined specific color, and the like. Changing the line drawing color to the representative color may be performed for all line drawings that make up the group, including line drawings that have been excluded if they meet certain conditions, or may be performed for line drawings that remain without being excluded. It may be. A line drawing with two or more colors or a line drawing with a specific line drawing color is, for example, a portion where red or blue characters overlap with black characters, such as written office documents or answer sheets that have been corrected. Since it is also assumed that parts with meaningful color itself, such as the part where the seal is imprinted on the character “mark”, are detected as line drawings, such line drawings are not subject to line drawing color changes. This makes it possible to reproduce meaningful colors.

  In the present embodiment, as described above, the representative color of the group is calculated based on the line drawing colors of the remaining line drawings excluding the line drawing that satisfies a predetermined condition from among the line drawings constituting the group, and the line drawing constituting the group is calculated. Since the line drawing color is changed to the group representative color, the compression rate of the second image layer Im2 can be increased without impairing the reproducibility of the line drawing color, and the compression efficiency in the high compression PDF is improved. be able to.

<Configuration of image processing apparatus>
Next, the image processing apparatus of this embodiment will be specifically described. FIG. 2 is a block diagram illustrating a hardware configuration example of the image processing apparatus 1 according to the present embodiment. The image processing apparatus 1 can use a computer system such as a PC (personal computer) as hardware. That is, for example, as shown in FIG. 2, the image processing apparatus 1 includes a processor such as a CPU 101, a storage device such as a RAM 102, a ROM 103, and an HDD 104, and a network I / F 105 that is a communication interface connected to a network such as a LAN. And these are connected via the bus 110.

  For example, the image processing apparatus 1 according to the present embodiment acquires an image to be processed (input image Im0) from a scanner or a host computer connected to a network via the network I / F 105. The image processing apparatus 1 then processes the input image Im0 to generate a high-compression PDF file FIm. The generated high-compression PDF file FIm is stored in the HDD 104 or connected to the network via the network I / F 105. Sent to the designated host computer. The function of generating the highly compressed PDF file FIm from the input image Im0 is realized, for example, when the CPU 101 uses the RAM 102 as a work area and executes a predetermined program stored in the ROM 103, the HDD 104, or the like.

  Note that the image processing apparatus 1 of the present embodiment can also be realized as a function of an image forming apparatus including a scanner, such as a copier or a multifunction peripheral. In this case, the image forming apparatus includes a computer system as shown in FIG. Then, for example, the CPU 101 in the image forming apparatus uses the RAM 102 as a work area and executes a predetermined program stored in the ROM 103, the HDD 104, or the like, thereby performing high compression from the input image Im0 acquired through the scanner or the network. A function of generating the PDF file FIm is realized. Of the series of processes from the input image Im0 to the generation of the high-compression PDF file FIm, a part that is particularly suitable for hardware implementation is provided with dedicated hardware such as an ASIC (Application Specific Integrated Circuit). It may be a configuration realized by using.

  FIG. 3 is a block diagram illustrating a functional configuration example of the image processing apparatus 1 according to the present embodiment. As a functional component for generating a highly compressed PDF file FIm from an input image Im0, the image processing apparatus 1 includes, for example, a line drawing detection unit 10 (one form of “line drawing detection means”) as shown in FIG. The line drawing color processing unit 20 (one mode of “line drawing color calculating means”), the color unification processing unit 30, and the file generation unit 40 (one mode of “file generation means”) are provided. In the image processing apparatus 1 of the present embodiment, the input image Im0 acquired as a processing target is input to the line drawing detection unit 10, the line drawing color processing unit 20, and the file generation unit 40, and a high-compression PDF file corresponding to the input image Im0. FIm is output from the file generation unit 40.

  The line drawing detection unit 10 is a functional module that detects a line drawing from the input image Im0 to be processed. FIG. 4 is a block diagram illustrating a specific example of the line drawing detection unit 10. For example, as illustrated in FIG. 4, the line drawing detection unit 10 includes a first detection unit 11 (one aspect of “first detection means”), an edge enhancement unit 12, and a second detection unit 13 (“second detection unit”). And a third detection unit 14 (an aspect of the “third detection means”) as submodules.

  The first detection unit 11 performs processing for detecting the edge of the line drawing on the input image Im0 to be processed, and outputs the result (hereinafter referred to as “edge detection result”). The processing performed by the first detection unit 11 uses, for example, the continuity and pattern of black pixels and white pixels obtained by the ternarization of the input image Im0, as in the method described in Japanese Patent No. 3088010. This is a process of detecting edges constituting a line drawing by separating a line drawing such as a character and a halftone dot. Here, the edge detection result is, for example, coordinate data representing the coordinate position of the pixel group detected as the edge of the line drawing by the first detection unit 11 in the input image Im0. The edge detection result is input to the edge enhancement unit 12 and the third detection unit 14.

  The edge enhancement unit 12 uses the edge detection result received from the first detection unit 11 to perform processing for enhancing the edge of the line drawing included in the input image Im0 to be processed, and edge enhancement in which the edge of the line drawing is enhanced Generate an image. Since the edge enhancement processing by the edge enhancement unit 12 is performed using the edge detection result of the first detection unit 11, there is little possibility of enhancing the pattern edge included in the input image Im0. For this reason, the degree of edge enhancement can be increased, and an edge enhanced image in which the edges of the line drawing are clearly enhanced can be generated. The edge enhanced image generated by the edge enhancing unit 12 is input to the second detection unit 13.

  The second detection unit 13 performs a process of detecting a line drawing region on the edge enhanced image received from the edge enhancement unit 12 and outputs the result (hereinafter referred to as “region detection result”). The processing by the second detection unit 13 extracts, for example, connected components of black pixels and white pixels from a binarized image obtained by binarizing the edge-enhanced image, similarly to the method described in Patent Document 1. In this process, the connected components are grouped based on the size and positional relationship of the circumscribed rectangle of the connected components, and a line drawing area included in the group is detected. The method described in Patent Document 1 pays attention to the fact that characters, which are representative line drawings, often form a character line, grouping connected components that constitute a character line, and determining the character region included in the character line. Detected. Here, the region detection result is, for example, coordinate data representing the coordinate position of the pixel group included in the region detected by the second detection unit 13 in the input image Im0. The region detection result is input to the third detection unit 14.

  The third detection unit 14 performs a process of detecting a line drawing from the input image Im0 using the edge detection result received from the first detection unit 11 and the region detection result received from the second detection unit 13. The result (hereinafter referred to as “line drawing detection result”) is output. Specifically, the third detection unit 14 is detected by the first detection unit 11 by OR operation of the edge detection result by the first detection unit 11 and the region detection result by the second detection unit 13. Both the region surrounded by the edges and the region detected by the second detection unit 13 are detected as line drawings from the input image Im0. Here, the line drawing detection result is, for example, coordinate data representing the coordinate position in the input image Im0 of the pixel group constituting the line drawing detected by the third detection unit 14. The line drawing detection result is input to the line drawing color processing unit 20.

  The line drawing color processing unit 20 is a functional module that calculates a line drawing color from which the influence of the background color is removed. FIG. 5 is a block diagram illustrating a specific example of the line drawing color processing unit 20. As shown in FIG. 5, for example, the line drawing color processing unit 20 includes a line drawing color calculation unit 21, a background color calculation unit 22, a line width calculation unit 23, and a line drawing color correction unit 24 as submodules.

  The line drawing color calculation unit 21 uses the line drawing detection result received from the third detection unit 14 of the line drawing detection unit 10 to specify the positions of all line drawings included in the input image Im0 to be processed. The line drawing color calculation unit 21 calculates the number of colors and the line drawing color for each line drawing extracted from the input image Im0. In addition, the background color calculation unit 22 calculates the background color of each line drawing extracted from the input image Im0.

  Here, a specific example of the color calculation method in the present embodiment will be described. Although the calculation of the color in the image can be realized by various methods, in the present embodiment, HSV data converted from the RGB data of the input image Im0 is used as an example. HSV is a color space that expresses a color by hue (H), saturation (S), and lightness (V). Here, it is assumed that the RGB data of the input image Im0 is represented by 8 bits (value range: 0 to 255) for each color of RGB. Further, it is assumed that the S and V value ranges of the HSV data are 0 to 255 as in the RGB data. Since H in the HSV data represents a hue, it takes a value in the range of 2π (value range: 0 to 359).

If the maximum value of R, G, B of RGB data is MAX and the minimum value is MIN, RGB data can be converted into HSV data using the following equations (1) to (5).
When MAX = R: H = 60 × ((GB) ÷ (MAX−MIN)) (1)
When MAX = G: H = 60 × ((BR) ÷ (MAX−MIN)) + 120 (2)
When MAX = B: H = 60 × ((RG) ÷ (MAX−MIN)) + 240 (3)
However, 360 is added when H <0. When R = G = B, the value of H is undefined.
S = ((MAX−MIN) ÷ MAX) × 255 (4)
However, S = 0 when R = G = B.
V = MAX (5)

  In the present embodiment, for example, for chromatic colors, there are six types of red, yellow, green, cyan, blue, and magenta according to the hue (H) value, and 11 types according to the value of brightness (V). It is assumed that the colors are classified into 66 types in combination with (equal intervals) classification. As for achromatic colors, the colors are classified into 11 types (black, white and equal intervals between them) according to the value of brightness (V). Note that this classification is merely an example, and the number of color classifications can be arbitrarily set. Further, the chromatic colors may be classified only by the hue (H) value without using the lightness (V) value. Whether it is a chromatic color or an achromatic color can be determined using, for example, the value of saturation (S). For example, if S ≦ 5, an achromatic color is determined, and if S> 5, a chromatic color is determined. In addition, it is determined whether the input image Im0 is a color image or a monochrome image using a technique called existing ACS (Auto Color Selection). If the input image Im0 is a monochrome image, all colors are determined to be achromatic colors. May be.

  The line drawing color calculation unit 21 can calculate the number of line drawing colors as follows, for example. First, RGB data of each pixel constituting a line drawing (for example, a pixel block of a predetermined unit such as 2 × 2 pixels) is converted into HSV data, and an average value of the HSV data is calculated. Then, it is confirmed whether or not a pixel (hereinafter referred to as “discrete pixel”) having a difference from the average value (for example, Euclidean distance in the HSV color space) equal to or larger than a predetermined value exists in the line drawing, and there is a discrete pixel. In this case, the ratio of the number of pixels of the discrete pixels to the number of pixels of the entire line drawing is further calculated. Then, when there are no discrete pixels in the line drawing, or when the ratio of the number of discrete pixels to the total number of pixels in the line drawing is less than a predetermined value, the number of colors of the line drawing is set to 1. On the other hand, when there are discrete pixels in the line drawing and the ratio of the number of pixels of the discrete pixels to the total number of pixels in the line drawing is a predetermined value or more, the number of colors of the line drawing is estimated to be two or more. When it is estimated that the number of colors of the line drawing is 2 or more, for example, a histogram representing the distribution of discrete pixels in the HSV color space is calculated, and the number of colors of the line drawing is calculated based on the number of peaks appearing in the histogram.

  Moreover, the line drawing color calculation part 21 can calculate a line drawing color as follows, for example. That is, if the number of colors of the line drawing is 1, the average value of HSV data of each pixel constituting the line drawing is calculated as the line drawing color of the line drawing. If the number of colors of the line drawing is 2 or more, the average value of HSV data is calculated as the line drawing color for each color. Instead of the average value, a method may be used in which the peak color of the histogram or the color with the highest brightness is used as the line drawing color. The line drawing colors calculated in this way can be classified into any one of the above 66 types if they are chromatic colors, and any one of the above 11 types if they are achromatic colors. For example, when the line drawing color is a chromatic color, the hue (H) value is red if 0 ≦ H <30 or 330 ≦ H <360, yellow if 30 ≦ H <90, and 90 ≦ H <150. Line drawing colors can be classified as green if there is 150, cyan if 150 ≦ H <210, blue if 210 ≦ H <270, and magenta if 270 ≦ H <330.

  For example, the background color calculation unit 22 can calculate the background color of a line drawing as follows. That is, a region having a predetermined size and shape adjacent to the line drawing (which may have a gap of about one pixel) is selected, and RGB data of each pixel in this region is converted into HSV data, and an average value thereof Is the background color of the line drawing. Similarly to the line drawing color, this background color can be classified into any of the above-mentioned 66 types if it is a chromatic color and any of the above-mentioned 11 types if it is an achromatic color. When the input image Im0 is an image obtained by reading a document with a scanner and the image processing apparatus 1 has a function of detecting the background color of the document (original color of the document), the detected background color of the document is used as a line drawing candidate. The background color may be used.

  When the background color is chromatic or gray, the number of colors calculated by the line drawing color calculation unit 21 may increase due to the reflection of the background color. Therefore, the line drawing color calculation unit 21 recalculates the number of colors using the background color calculated by the background color calculation unit 22 for the line drawing having two or more colors. For example, the line drawing color calculation unit 21 removes pixels having a color within a predetermined Euclidean distance from the background color calculated by the background color calculation unit 22 among the pixels constituting the line drawing, and then performs the above-described method. Recalculate the number of colors in the line drawing. Further, the line drawing color calculation unit 21 predicts an intermediate color from the average value of the calculated line drawing color and the background color calculated by the background color calculation unit 22, and after excluding pixels having a color close to the intermediate color, The number of colors of the line drawing may be recalculated by the method described above. The intermediate color between the line drawing color and the background color may be predicted by using the midpoint of both obtained by conversion to another color system other than the average value. Further, the line drawing color calculation unit 21 may recalculate the number of colors by a simple method such as subtracting 1 from the number of colors calculated first. As described above, the line drawing color calculation unit 21 calculates the number of colors of the line drawing from which the influence of the background color is removed.

  The example described above is an example of a method for calculating the number of line drawing colors, the line drawing color, the background color, and the like. The number of line drawing colors, the line drawing color, the background color, and the like may be calculated by other methods. For example, the RGB data of the input image Im0 may be mapped to the Munsell color system by table conversion or the like, and the number of line drawings, the line drawing color, the background color, etc. may be calculated using the hue and brightness in the Munsell color system. .

  The line width calculation unit 23 calculates the line width of each line drawing detected from the input image Im0. The line width of the line drawing can be calculated from the distance (number of pixels) between the edges of the line drawing, for example. Further, the line width of the line drawing (whether bold or not) may be calculated from the ratio of the number of pixels in the line drawing area to the total number of pixels of the circumscribed rectangle of the line drawing.

  The line drawing color correction unit 24 corrects the line drawing color of each line drawing calculated by the line drawing color calculation unit 21 according to the background color calculated by the background color calculation unit 22. For example, if the line width of the line drawing calculated by the line width calculation unit 23 is equal to or larger than a predetermined value (for example, in the case of bold characters), the line drawing color correction unit 24 determines a predetermined value from the background color among the pixels constituting the line drawing. The line drawing color calculated by the line drawing color calculation unit 21 is corrected so that the average value of the colors of the remaining pixels excluding the pixels having colors within the Euclidean distance is set as the line drawing color. On the other hand, if the line width of the line drawing calculated by the line width calculation unit 23 is less than a predetermined value (for example, in the case of fine characters), the line drawing color calculated by the line drawing color calculation unit 21 is visually separated from the background color. Correct to color. If the line width of the line drawing calculated by the line width calculation unit 23 is equal to or larger than a predetermined value, only the vicinity of the edge of the line drawing may be corrected.

  In addition, the line drawing color correction unit 24, regardless of the line width of the line drawing, among the pixels constituting the line drawing, the average value of the colors of the remaining pixels excluding the pixels having the color within the predetermined Euclidean distance from the background color It may be configured to implement either one of a method for correcting the line drawing color to be a line drawing color or a method for correcting the line drawing color to a color far from the background color in terms of visual characteristics. In this case, the line width calculation unit 23 becomes unnecessary.

  In addition, the line drawing color correction unit 24 is a color whose line drawing color is close to black when the line drawing color calculated by the line drawing color calculation unit 21 and the background color calculated by the background color calculation unit 22 include in-phase components, for example. If so, the line drawing color may be corrected by correcting to black and correcting to white if the color is close to white.

  In addition, the line drawing color correction unit 24 performs correction according to the background color individually for each line drawing color for the line drawing having two or more colors calculated by the line drawing color calculation unit 21. For example, the line drawing color correction unit 24 performs clustering according to the line drawing color on the line drawing having two or more colors, and corrects the line drawing color by any of the methods described above for each clustering result.

  As described above, the line drawing color correction unit 24 outputs, as line drawing data (1), the result of correcting the line drawing color of each line drawing detected from the input image Im0 according to the background color. The line drawing data (1) includes, for each line drawing detected from the input image Im0, coordinate data representing the coordinate position of the pixel group constituting the line drawing in the input image Im0, the line drawing color, and the number of colors. . The line drawing data (1) is input to the color unification processing unit 30 and the file generation unit 40.

  The color unification processing unit 30 is a functional module that performs a process of unifying line drawing colors in predetermined group units. FIG. 6 is a block diagram illustrating a specific example of the color unification processing unit 30. For example, as shown in FIG. 6, the color unification processing unit 30 includes a unification target determination unit 31, a representative color calculation unit 32 (an aspect of “representative color calculation unit”), and a line drawing color change unit 33 (“change unit”). Of the above) as a submodule.

  The unification target determination unit 31 determines a group of line drawings that are targets for unifying line drawing colors. An example of a group of line drawings that is a target for unifying line drawing colors is a character line composed of characters that are representative line drawings. That is, one character line included in the input image Im0 is set as one group. However, the group that unifies the line drawing colors is not limited to this, and the group may be determined according to a predetermined criterion. For example, instead of making one character line into one group, all horizontal character lines in which characters are arranged in the horizontal direction may be made into one group, or all vertical character lines in which characters are arranged in the vertical direction are made into one group. It is good. Further, the entire character lines included in the input image Im0 may be made into one group. Further, not only the characters constituting the character line but also a set of a predetermined number of line drawings whose positions in the input image Im0 are close may be set as one group, or a set of line drawings that are densely arranged in the input image Im0 is set as one group. Also good.

  The unification object determining unit 31 can determine a group of line drawings in the input image Im0 using the coordinate data of each line drawing included in the line drawing data (1). Further, the unification target determining unit 31 may determine a group of line drawings in the input image Im0 using the result of grouping by the second detection unit 13 of the line drawing detection unit 10.

  The representative color calculation unit 32 determines a representative color for each of the groups determined by the unification target determination unit 31 using the line drawing data (1) received from the line drawing color correction unit 24 of the line drawing color processing unit 20. At this time, the representative color calculation unit 32 calculates the representative color of the group based on the line drawing colors of the remaining line drawings excluding the line drawing that satisfies a predetermined condition among the line drawings constituting the group. Here, the predetermined condition is, for example, a condition that the line drawing color is a predetermined specific color or a condition that the number of colors of the line drawing is two or more.

  That is, the representative color calculation unit 32 excludes a line drawing of a specific color (for example, red or blue) whose line drawing color is predetermined and a line drawing having two or more colors from among the line drawings constituting the group. Here, whether or not the line drawing color is a specific color is determined by, for example, the value of the hue (H) of the line drawing color calculated by the above-described method being ± 30 (degrees) from the value of the hue (H) of the specific color. When it is within the range, it can be determined by a method of setting a specific color. This determination method is an example, and it may be determined whether the line drawing color is a specific color by another method. The number of colors of the line drawing can be specified from the line drawing data (1) received from the line drawing color correction unit 24 of the line drawing color processing unit 20.

  Next, the representative color calculation unit 32 calculates the average value of the line drawing colors of the line drawings that remain without being excluded from the line drawings that form the group. Then, the representative color calculation unit 32 further excludes the line drawing of the line drawing color whose color difference exceeds the reference value with respect to the calculated average value, further calculates the average value of the line drawing colors of the remaining line drawing, and calculates this as the group value. The representative color. When calculating the average value of the line drawing colors, the representative color calculating unit 32 performs weighting such that the weight of the line drawing color of the line drawing having a larger number of pixels increases, thereby calculating the average value of the line drawing colors. desirable. In addition, when the average value of the line drawing colors is an achromatic color and the lightness is low, the representative color of the group may be black, and when the average value of the line drawing colors is an achromatic color and the lightness is low, the representative color of the group may be white.

  The line drawing color changing unit 33 changes the line drawing color of the line drawing constituting the group to the representative color calculated by the representative color calculating unit 32. At this time, the line drawing color changing unit 33 changes the line drawing colors of all the line drawings constituting the group to the representative color of the group including the line drawing excluded as the above-mentioned condition before calculating the average value of the line drawing colors. Alternatively, the line drawing colors of the remaining line drawings excluding the line drawing that is excluded when the above-described conditions are satisfied among the line drawings constituting the group may be changed to the group representative color. In addition, if the line drawing color is simply changed to the average value, it may be difficult to see the line drawing. Therefore, after adjusting the density of the representative color of the group according to the background color, the line drawing color is changed to the representative color of the group. Countermeasures may be taken.

  In addition, when the line drawing color of an excluded line drawing is not changed to the group representative color when the above conditions are met, for example, an average value of line drawing colors is calculated between the excluded line drawings, and the color difference from the average value is a predetermined value. By changing the line drawing color of the excluded line drawing by repeating the process of calculating the average value by excluding the line drawing having the line drawing color as described above, the line drawing color is unified among the excluded line drawing. Also good.

  As described above, the line drawing color changing unit 33 outputs the result of changing the line drawing color of the line drawing constituting the group for each group as line drawing data (2). This line drawing data (2) includes the line drawing color after changing the line drawing constituting the group among the line drawings detected from the input image Im0. The line drawing data (2) is input to the file generation unit 40.

  The file generation unit 40 uses the line drawing data (1) received from the line drawing color correction unit 24 of the line drawing color processing unit 20 and the line drawing data (2) received from the line drawing color change unit 33 of the color unification processing unit 30. From the input image Im0 to be processed, a first image layer Im1, which is a binary image consisting only of line drawings, a second image layer Im2, which is a multi-value image expressing line drawing colors, and a pattern and background other than line drawings And a third image layer Im3 that is a multi-valued image that expresses. At this time, in particular, the second image layer Im2 expressing the line drawing color uses the line drawing color included in the line drawing data (2) for the line drawing constituting the group, and the other line drawing is included in the line drawing data (1). Generated using the line drawing color.

  Then, the file generation unit 40 performs a first compression process suitable for line drawing compression on the first image layer Im1 and the second image layer Im2, and also applies to the third image layer Im3. Performs a second compression process suitable for compression of a pattern and a background. Then, the file generation unit 40 integrates the compressed first image layer Im1, second image layer Im2, and third image layer Im3 on one image file in the PDF format, and generates a high-level image corresponding to the input image Im0. A compressed PDF file FIm is generated.

  The compression method is not particularly limited, and a method suitable for line drawing compression is used for the first image layer Im1 and the second image layer Im2, and a pattern is used for the third image layer Im3. It is only necessary that the compression process is performed by a method suitable for compression of the background. In addition, the format of the image file that integrates the first image layer Im1, the second image layer Im2, and the third image layer Im3 is not limited to the PDF format. For example, a plurality of image layers such as JPM format can be displayed. Various formats can be used in which one image is superimposed. As described above, the highly compressed PDF file FIm generated by the file generation unit 40 is stored in, for example, the HDD 104 or transmitted to the host computer connected to the network via the network I / F 105.

<Operation of Image Processing Device>
Next, an outline of the operation of the image processing apparatus 1 of the present embodiment configured as described above will be described. FIG. 7 is a flowchart for explaining the flow of operations performed by the image processing apparatus 1 according to the present embodiment.

  When the operation of the image processing apparatus 1 of the present embodiment is started, first, in step S1, an input image Im0 to be processed is acquired. The input image Im0 is input to the line drawing detection unit 10, the line drawing color processing unit 20, and the file generation unit 40.

  Next, in step S2, the process by the first detection unit 11 of the line drawing detection unit 10 is performed on the input image Im0 acquired in step S1. Then, the edge detection result output from the first detection unit 11 is input to the edge enhancement unit 12 and the third detection unit 14.

  Next, in step S3, the edge enhancement unit 12 performs a process of enhancing the edge of the line drawing included in the input image Im0 acquired in step S1, using the edge detection result in step S2, and generates an edge enhanced image. Is done. This edge enhanced image is input to the second detection unit 13.

  Next, in step S4, processing by the second detection unit 13 is performed on the edge enhanced image generated in step S3. Then, the region detection result output by the second detection unit 13 is input to the third detection unit 14.

  Next, in step S5, the third detector 14 detects a line drawing from the input image Im0 acquired in step S1, using the edge detection result in step S2 and the region detection result in step S4. Then, the line drawing detection result output from the third detection unit 14 is input to the line drawing color processing unit 20.

  Next, in step S6, the line drawing color processing unit 20 performs processing on each line drawing included in the input image Im0 acquired in step S1, using the line drawing detection result in step S5. The line drawing data (1) output from the line drawing color processing unit 20 is input to the color unification processing unit 30 and the file generation unit 40.

  Next, in step S7, the color unification processing unit 30 performs processing using the line drawing data (1) in step S6. Then, the line drawing data (2) output from the color unification processing unit 30 is input to the file generation unit 40.

  Next, in step S8, the file generation unit 40 uses the line drawing data (1) in step S6 and the line drawing data (2) in step S7 to input the first image layer Im1 from the input image Im0 acquired in step S1. A second image layer Im2 and a third image layer Im3 are generated. Then, after performing individual compression processing on each image layer, processing for integration into one image file is performed, and a highly compressed PDF file FIm corresponding to the input image Im0 is generated.

  Finally, in step S9, the highly compressed PDF file FIm generated in step S8 is output from the file generation unit 40 and stored in, for example, the HDD 104 or connected to the network via the network I / F 105. Sent to a computer etc.

<Specific Example of Processing by First Detection Unit>
Next, a specific example of processing by the first detection unit 11 will be described with reference to FIG. As described above, the processing by the first detection unit 11 separates line images such as characters from halftone dots using the continuity and pattern of black pixels and white pixels obtained by ternization of the input image Im0. By doing so, it is a process of detecting edges constituting the line drawing. FIG. 8 is a flowchart for explaining an example of processing by the first detection unit 11.

  First, the first detection unit 11 performs MTF correction by filtering on the input image Im0 to be processed (step S101). The MTF correction is a preprocessing performed to increase the accuracy of the next ternarization. As this MTF correction filter, for example, the filter described in Japanese Patent No. 3088010 can be used.

  Next, the first detection unit 11 performs ternarization on the MTF-corrected input image Im0 (step S102), and divides the input image Im0 into black pixels, white pixels, and gray pixels.

  Next, as described in, for example, Japanese Patent No. 3088010, the first detection unit 11 extracts a black line image and a white line image from the input image Im0 by pattern matching of black connected pixels and white connected pixels. Then, the edge of the line drawing included in the input image Im0 is detected (step S103).

  Then, the first detection unit 11 outputs the processing result of step S103 as an edge detection result (step S104), and ends a series of processing.

<Specific Example of Processing by Second Detection Unit>
Next, a specific example of processing by the second detection unit 13 will be described with reference to FIG. As described above, the processing by the second detection unit 13 extracts the connected components of black pixels and white pixels from the binarized image obtained by binarizing the edge enhanced image generated by the edge enhancing unit 12, This is a process for grouping connected components based on the size of the circumscribed rectangle of the connected components, the positional relationship, and the like, and detecting a line drawing area included in the group. FIG. 9 is a flowchart illustrating an example of processing by the second detection unit 13.

  First, the second detection unit 13 performs binarization on the edge enhanced image generated by the edge enhancing unit 12 (step S201). The binarization here is a process for extracting an object having a lower luminance than the background, and an appropriate threshold value that can distinguish the low luminance object from the background is set. Also, dynamic threshold binarization may be used to increase the separation accuracy from the background.

  Next, as described in, for example, Patent Document 1, the second detection unit 13 uses, from the binarized image obtained in step S201, a black pixel line arranged in the vertical direction and a black pixel line arranged in the horizontal direction. Are connected to obtain a connected component (step S202).

  Next, the second detection unit 13 removes a connected component having a small size from the connected components acquired in Step S202 (Step S203). This process is a process for reducing the amount of calculation in the next grouping.

  Next, the second detection unit 13 combines and groups the connected components acquired in step S202 and not removed in step S203 based on the size and positional relationship of the circumscribed rectangle of the connected components (step S202). S204). The grouping here follows a predetermined rule such as grouping characters that are representative line drawings in line units. Therefore, there are connected components that are not grouped.

  Next, the second detection unit 13 performs binarization again on the edge enhanced image generated by the edge enhancing unit 12 (step S205). The binarization here is a process for extracting an object having a higher luminance than the background, and an appropriate threshold value that can distinguish the high luminance object from the background is set. Also, dynamic threshold binarization may be used to increase the separation accuracy from the background.

  Next, as described in Patent Document 1, for example, the second detection unit 13 uses white pixels arranged in the vertical direction and white pixels arranged in the horizontal direction from the binarized image obtained in step S205. Are connected to obtain a connected component (step S206).

  Next, the second detection unit 13 combines and groups the connected components acquired in step S206 based on the size and positional relationship of the circumscribed rectangle of the connected components (step S207). The grouping here also follows a predetermined rule such as grouping characters that are representative line drawings in units of lines, as in step S204. Therefore, there are connected components that are not grouped.

  Next, the second detection unit 13 determines whether or not there is a group in which the positions in the edge-enhanced image overlap between the group of connected components combined in step S204 and the group of connected components combined in step S207 ( Step S208). If there are overlapping groups (step S208: Yes), the sizes of the circumscribed rectangles of these groups are compared, and the group with the smaller size of the circumscribed rectangle is deleted (step S209).

  Next, the second detection unit 13 detects a connected component area included in the group extracted from the edge-enhanced image by the above processing as a line drawing area included in the input image Im0 (step S210). And the 2nd detection part 13 outputs the process result of step S210 as an area | region detection result (step S211), and complete | finishes a series of processes.

<Specific Example of Processing by Line Drawing Color Processing Unit>
Next, a specific example of processing by the line drawing color processing unit 20 will be described with reference to FIG. FIG. 10 is a flowchart illustrating an example of processing performed by the line drawing color processing unit 20.

  When a line drawing detection result is input from the line drawing detection unit 10 to the line drawing color processing unit 20, the line drawing color calculation unit 21 first inputs an input image to be processed based on the line drawing detection result received from the line drawing detection unit 10. One of the line drawings included in Im0 is extracted (step S301).

  Next, the line drawing color calculation unit 21 converts the RGB data of each pixel constituting the line drawing extracted in step S301 into HSV data, and calculates the line drawing color and the number of colors of this line drawing by, for example, the method described above (step). S302).

  Next, the background color calculation unit 22 selects a region having a predetermined size and shape that is adjacent to the line drawing extracted in step S301 (there may be a gap of about one pixel), and each pixel in this region The RGB data is converted into HSV data, and the background color is calculated, for example, by the method described above (step S303).

  Next, the line drawing color calculation unit 21 checks whether or not the line drawing color number calculated in step S302 is 1 (step S304). If the line drawing color number is 2 or more (step S304: No), step S303 is executed. Based on the background color calculated in step 1, the number of colors of the line drawing extracted in step S301 is recalculated by the method described above, for example (step S305).

  On the other hand, when the number of colors of the line drawing calculated in step S302 is 1 (step S304: Yes), or when the number of colors of the line drawing recalculated in step S305 is 1 (step S306: Yes), the line drawing color. The correction unit 24 corrects the line drawing color calculated in step S302 according to the background color calculated in step S303, for example, by the method described above (step S307).

  On the other hand, when the number of colors of the line drawing recalculated in step S305 is 2 or more (step S306: No), the line drawing color correction unit 24 performs clustering of each pixel constituting the line drawing according to each line drawing color. This is performed (step S308). Then, the line drawing color correction unit 24 corrects the line drawing color according to the background color for each clustering result in step S308 (step S309).

  Next, the line drawing color correction unit 24 determines whether there is an unprocessed line drawing (step S310). If there is an unprocessed line drawing (step S310: Yes), the process returns to step S301 and the subsequent processing is repeated. On the other hand, when the processing is completed for all the line drawings (step S310: No), the line drawing color correction unit 24 performs line drawing data (1) including the coordinate data, line drawing colors, and the number of colors of each line drawing included in the input image Im0. Is output (step S311), and the series of processing ends.

  In the image processing apparatus 1 according to the present embodiment, as described above, the line drawing color of each line drawing included in the input image Im0 is corrected according to the background color by the processing by the line drawing color processing unit 20, so that the line drawing is the background color. Even if it is influenced by the above, it is possible to calculate an appropriate line drawing color excluding the influence of the background color. Then, by generating the second image layer Im2 using this line drawing color, it is possible to generate a highly compressed PDF file FIm that can reproduce a high-quality line drawing.

  FIG. 11 is a diagram illustrating a state in which the vicinity of the edge of the line drawing has changed color due to the influence of the background color. For example, the line drawing included in the input image Im0 acquired by reading a document with a scanner or the like may change the vicinity of the edge due to the influence of the background color, as shown in FIG. The example of FIG. 11 shows a state in which the blue color of the background color is reflected near the edge of the character “black” which is a black line drawing.

  12A and 12B are diagrams for explaining a specific example of processing for correcting the line drawing color according to the background color. FIG. 12A shows the line drawing included in the original document, and FIG. 12B shows the vicinity of the edge due to the influence of the background color. Represents a line drawing in which the color has changed, and (c) represents a line drawing in which the line drawing color has been corrected. In the example of FIG. 12, a thick black character “black” with blue as a background color is shown as an example of a line drawing to be corrected.

  A black bold character as shown in FIG. 12A may change its color near the edge to a color close to blue, as shown in FIG. 12B, due to the influence of blue as the background color. In this case, for example, among the pixels constituting the bold character, the pixels having a color within a predetermined Euclidean distance from the background color, that is, the pixels other than the pixels near the edge close to the blue background color are excluded. By correcting the average value of the colors so as to be a line drawing color, as shown in FIG. 12C, the influence of the background color can be eliminated and the black thick character shown in FIG. 12A can be reproduced.

  FIG. 13 is a diagram for explaining a specific example of the process of correcting the line drawing color in accordance with the background color. Like the example of FIG. 12, (a) represents a line drawing included in the original document, and (b) Represents a line drawing in which the vicinity of the edge is discolored due to the influence of the background color, and (c) represents a line drawing with the line drawing color corrected. In the example of FIG. 13, a black horizontal line with blue as a background color is shown as an example of a line drawing to be corrected. Although an object represented by a line such as a horizontal line illustrated in FIG. 13 is out of the category of characters, it is an object that should be handled in the same way as characters during compression. Are detected from the input image Im0.

  A black horizontal line as shown in FIG. 13A may be changed to a color close to blue as shown in FIG. 13B due to the influence of blue as the background color. In this case, for example, of the pixels constituting the horizontal line, the pixels having a color within a predetermined Euclidean distance from the background color, that is, the remaining pixels excluding the pixels near the edge that is close to the blue background color are excluded. By correcting the average value of the colors so as to be the line drawing color, it is possible to eliminate the influence of the background color and reproduce the black horizontal line shown in FIG. 13A as shown in FIG.

  FIG. 14 is a diagram for explaining a specific example of the process of correcting the line drawing color according to the background color. FIG. 14A shows a line drawing included in the original document, and FIG. 14B shows the whole due to the influence of the background color. The line drawing which changed color is represented, (c) represents the line drawing in which the line drawing color was corrected. In the example of FIG. 14, a character string of a black character “black character” with a blue background is shown as an example of a line drawing to be corrected.

  A black fine character as shown in FIG. 14A may be discolored to a color close to blue as shown in FIG. 14B due to the influence of blue as the background color. In this case, for example, the influence of the background color is eliminated as shown in FIG. 14C by correcting the color of the fine character discolored close to blue to a color that is visually separated from the background color blue. Thus, the black thin characters shown in FIG. 14A can be reproduced.

  Note that, as in the examples of FIGS. 12 to 14 described above, when it can be determined that the line drawing color changed due to the influence of the background color is close to black, that is, the line drawing color includes a component in phase with the background color, and brightness If is lower than a predetermined value, the same effect can be obtained even if the line drawing color is simply corrected to black. In addition, when it can be determined that the line drawing color changed due to the influence of the background color is close to white, that is, when the line drawing color includes a component in phase with the background color and the lightness is higher than a predetermined value, this line drawing color. The same effect can be obtained by simply correcting the color to white.

  FIG. 15 is a diagram for explaining a specific example of the process of correcting the line drawing color according to the background color. (A) shows the line drawing included in the original document, and (b) shows the vicinity of the edge due to the influence of the background color. (C) shows a line drawing in which the line drawing color is corrected without performing clustering according to the line drawing color, and (d) shows a line drawing color corrected for each result of clustering according to the line drawing color. Represents a line drawing. The example in FIG. 15 is an example of correcting the line drawing color of a line drawing when a portion where a red stamp is stamped on a black character “mark” is detected as one line drawing. The background color is assumed to be white. This example is a specific example of processing for correcting a line drawing color for a line drawing having two or more colors.

  The line drawing as shown in FIG. 15 (a) has the effect of white as the background color, so that the black “mark” character is gray and the red stamp part is light red as shown in FIG. 15 (b). It may change color. In this case, although the number of colors of the line drawing is 2, if the line drawing color is uniformly corrected for the entire line drawing without performing clustering according to the line drawing color, as shown in FIG. Both the character “mark” and the red stamp part are expressed in a common dark red color, and the correct line drawing shown in FIG. 15A cannot be reproduced. On the other hand, clustering according to the line drawing color is performed, and for each clustering result, the line drawing color is corrected to a color visually away from the white background color as shown in FIG. In addition, the correct line drawing shown in FIG. 15A can be reproduced without the influence of the background color.

<Specific Example of Processing by Color Unification Processing Unit>
Next, a specific example of processing by the color unification processing unit 30 will be described with reference to FIG. FIG. 16 is a flowchart for explaining an example of processing by the color unification processing unit 30.

  When the line drawing data (1) is input from the line drawing color processing unit 20 to the color unification processing unit 30, first, the unification target determination unit 31 uses the coordinate data of each line drawing included in the line drawing data (1). Thus, a group of line drawings in the input image Im0 is determined (step S401).

  Next, the representative color calculation unit 32 extracts one of the groups determined in step S401 (step S402). Then, the representative color calculation unit 32 excludes a line drawing of a specific color (for example, red or blue) having a predetermined line drawing color from the line drawing configuring the group extracted in step S402 (step S403). Further, the representative color calculation unit 32 excludes line drawings having two or more colors from among the line drawings constituting the group extracted in step S402 (step S404). Then, the representative color calculation unit 32 calculates the average value of the line drawing colors of the line drawings that remain without being excluded in Step S403 and Step S404 (Step S405).

  Next, the representative color calculation unit 32 further excludes the line drawing of the line drawing color in which the color difference exceeds the reference value with respect to the average value calculated in step S405 from the line drawing configuring the group extracted in step S402 (step S406). ). Then, the representative color calculation unit 32 calculates the average value of the line drawing colors of the line drawings that remain without being excluded in step S406 as the group representative color extracted in step S402 (step S407).

  Next, the line drawing color changing unit 33 changes the line drawing color of the line drawing that remains without being excluded in Step S403, Step S404, and Step S406 to the representative color calculated in Step S407 (Step S408). The line drawing color changing unit 33 determines whether there is an unprocessed group (step S409). If there is an unprocessed group (step S409: Yes), the process returns to step S402 and the subsequent processing is repeated. On the other hand, when the processing is completed for all the groups (step S409: No), the line drawing color changing unit 33 includes the line drawing color after changing the line drawing configuring the group among the line drawings detected from the input image Im0. The line drawing data (2) is output (step S410), and the series of processing ends.

  In the image processing apparatus 1 according to the present embodiment, as described above, by the processing performed by the color unification processing unit 30, the remaining line drawings excluding line drawings that meet a predetermined condition from among line drawings that form a group that is a unit for unifying colors. A group representative color is calculated based on the line drawing color of the line drawing, and the line drawing color of the line drawing constituting the group is changed to the group representative color. Therefore, by generating the second image layer Im2 using this line drawing color, the compression ratio can be increased without impairing the color reproducibility of the line drawing.

  FIG. 17 is a diagram for explaining a specific example of a method for determining a group that is a unit for unifying line drawing colors, and an example in which a character line composed of characters that are representative line drawings is set as one group. Show. When characters are arranged in the vertical direction and the horizontal direction in the input image Im0, when these characters are grouped, one character may belong to both the vertical row and the horizontal row. In this case, for example, the vertical and horizontal character spacings are compared as shown in FIG. 17A, or the vertical and horizontal line spacings are compared as shown in FIG. 17B. Thus, the characters included in the input image Im0 can be appropriately grouped as character lines. In the example of FIGS. 17A and 17B, each character is grouped as a character constituting a horizontal line.

  FIG. 18 is a diagram illustrating a specific example of processing for calculating a representative color of a group. In this example, as shown in FIG. 18 (a), the character string "Aiue Okeki Kekko" constitutes a group, of which the character "U" is red and the character "KE" is blue. The other characters are assumed to be black. In addition, the letter “O” is overlapped with the letter “O” in green, and the symbol “# 1” and the number “green” are inserted between the letters “O” and “ka”. To do.

  First, from the character string shown in FIG. 18A, the red character “U” as a specific color and the blue character “K” as a specific color are excluded. As a result, the remaining character string is as shown in FIG.

  Next, from the character string shown in FIG. 18B, the portion where the number of colors is 2, that is, the black character “o” where the green “back” character overlaps is excluded. As a result, the remaining character string is as shown in FIG.

  Next, the average value of the color of the character string shown in FIG. 18 (c) is calculated, and the green “* 1” symbol and number whose color difference exceeds the reference value with respect to this average value are shown in FIG. 18 (c). It is excluded from the character string shown in. As a result, the remaining character string is as shown in FIG.

  Finally, the average color value of the character string shown in FIG. 18D is calculated as the representative color of this group. Then, the color of each character included in the character string shown in FIG. 18D is changed to the group representative color. Note that the color change by the representative color of the group may be performed on the entire character string shown in FIG.

<Effect of embodiment>
As described above in detail with specific examples, the image processing apparatus 1 according to the present embodiment groups the line drawings detected from the input image Im0 to be processed, and among the line drawings constituting the group, The representative color of the group is calculated based on the line drawing colors of the remaining line drawings excluding the line drawing that meets the conditions. Then, the line drawing color of the line drawing constituting the group is changed to the representative color to generate the second image layer Im2 expressing the line drawing color. Therefore, the compression rate of the second image layer Im2 can be increased without impairing the reproducibility of the line drawing color, and the compression efficiency in the high compression PDF can be improved.

  In addition, when line drawings satisfying a predetermined condition are excluded from line drawings having two or more colors, or line drawings having a specific color such as red or blue, the colors themselves are assumed to be meaningful. Therefore, by changing the line drawing color for the remaining line drawing excluding such a line drawing, a meaningful color can be reproduced while increasing the compression rate of the second image layer Im2.

<First Modification>
In the embodiment described above, the group for unifying the line drawing color is determined according to a predetermined arbitrary standard, but the number of line drawings constituting the group is switched according to the required processing speed. It is good also as a structure. Specifically, the higher the required processing speed, the smaller the number of line drawings constituting the group. For example, when trying to unify the character color of characters that are typical line drawings, if the required processing speed is high, the character color is not unified across the entire character line, but with the number of characters according to the processing speed. Are grouped, and the character color is changed to the representative color for each group. As a result, the compression rate can be improved by unifying the line drawing colors within a range in which the required processing speed can be realized, and user dissatisfaction due to the reduction in processing speed can be eliminated.

<Second Modification>
Further, a configuration may be adopted in which whether to change the line drawing color by the line drawing color changing unit 33, that is, whether to unify the line drawing color by the color unification processing unit 30, is switched according to the required compression ratio. Good. That is, if the required compression rate can be achieved without performing line drawing color unification by the color unification processing unit 30, the line unification processing unit 30 does not unify the line drawing colors and calculates in line drawing units. A second image layer Im2 is generated using the line drawing color that has been set. As a result, the reproducibility of the line drawing color can be further improved while ensuring the required compression rate, and a decrease in the processing speed can be suppressed.

<Third Modification>
In the embodiment described above, a specific example of the method for detecting a line drawing from the input image Im0 to be processed has been described. However, the method for detecting a line drawing from the input image Im0 is not limited to this example. For example, after the third detection unit 14 removes the edge of the line drawing on the color ground or the edge of the white line drawing from the edge detection result of the first detection unit 11, the edge detection result of the first detection unit 11 Alternatively, a line drawing may be detected from the input image Im0 by performing an OR operation with the area detection result of the second detection unit 13. Further, the edge detection result of the first detection unit 11 may be used only for the generation of an edge enhanced image by the edge enhancement unit 12. On the contrary, the edge detection result of the first detection unit 11 is used only for the detection of the line drawing by the third detection unit 14, and the second detection unit 13 detects the area of the line drawing from the edge enhanced image. Instead of this, the line drawing area may be detected from the input image Im0. Moreover, the 2nd detection part 13 is good also as a structure which groups the connection component for all the extracted connection components as an object, without performing the process which removes the connection component with a small size. In addition, the method for detecting a line drawing from the input image Im0 to be processed is not limited to the above example, and any existing method for detecting a line drawing such as a character from the image can be used.

<Supplementary explanation>
Functional components of the image processing apparatus 1 described in the above-described embodiments and modifications (line image detection unit 10 (first detection unit 11, edge enhancement unit 12, second detection unit 13, and third detection unit) 14), line drawing color processing unit 20 (line drawing color calculation unit 21, background color calculation unit 22, line width calculation unit 23 and line drawing color correction unit 24), color unification processing unit 30 (unification target determination unit 31, representative color calculation unit) 32, the line drawing color changing unit 33), and the file generating unit 40) can be realized, for example, by the cooperation of the hardware and software (program) shown in FIG. In this case, the program is recorded in a computer-readable recording medium such as a CD-ROM, a flexible disk (FD), a CD-R, or a DVD in a format that can be installed in the image processing apparatus 1 or an executable format. Provided. Further, the program may be provided by being stored on a computer connected to a network such as the Internet and downloaded to the image processing apparatus 1 via the network. Furthermore, the program may be configured to be provided or distributed via a network such as the Internet. Further, the program may be provided by being incorporated in advance in, for example, the ROM 103 or the HDD 104 in the image processing apparatus 1.

  In addition, the functional components of the image processing apparatus 1 described in the above-described embodiments and modifications are partly or entirely such as, for example, an ASIC (Application Specific Integrated Circuit) or an FPGA (Field-Programmable Gate Array). It can also be realized using dedicated hardware.

  In the embodiment and the modification described above, the example in which the image processing device 1 is realized as a single device is assumed. However, the functional components of the image processing device 1 are distributed to a plurality of physically separated devices. The operation as the image processing apparatus 1 may be realized by cooperation of the plurality of apparatuses.

  Although specific embodiments and modifications of the present invention have been described above, the above-described embodiments show one application example of the present invention. The present invention is not limited to the above-described embodiment as it is, and can be embodied with various modifications and changes without departing from the scope of the invention in the implementation stage.

DESCRIPTION OF SYMBOLS 1 Image processing apparatus 10 Line drawing detection part 11 1st detection part 13 2nd detection part 14 3rd detection part 20 Line drawing color processing part 30 Color unification processing part 31 Unification object determination part 31 Representative color calculation part 33 Line drawing color change 40 File generator Im0 Input image FIm High compression PDF file

Japanese Patent No. 4471202

Claims (10)

A line drawing detecting means for detecting a line drawing from the image;
Line drawing color calculation means for calculating a line drawing color which is the color of the line drawing;
A representative color of the group is calculated based on the line drawing color calculated for the remaining line drawing excluding the line drawing that satisfies a predetermined condition among the line drawing constituting a group that is a unit for unifying colors. Representative color calculating means for
An image processing apparatus comprising: changing means for changing the line drawing color of the line drawing constituting the group to the representative color.   The image processing according to claim 1, wherein the changing unit changes the line drawing color of the remaining line drawing excluding the line drawing satisfying the predetermined condition from the line drawing configuring the group to the representative color. apparatus.   The image processing apparatus according to claim 1, wherein the predetermined condition includes a condition that the number of colors of the line drawing is two or more.   The image processing apparatus according to claim 1, wherein the predetermined condition includes a condition that the line drawing color is a predetermined specific color.   The image processing apparatus according to claim 1, wherein the number of line drawings constituting the group is switched according to a required processing speed.   The image processing apparatus according to claim 1, wherein whether or not to change the line drawing color by the changing unit is switched according to a required compression rate of the line drawing. The line drawing detection means includes:
First detecting means for detecting an edge of the line drawing from the image;
Second detection means for detecting an area of the line drawing from the image in which an edge of the line drawing is emphasized using a detection result of the first detection means;
7. A third detection unit that detects the line drawing from the image using the detection result of the first detection unit and the detection result of the second detection unit. 8. An image processing apparatus according to claim 1.   The image processing apparatus according to claim 1, further comprising: a file generation unit that generates a compressed image file corresponding to the image based on the line drawing color. Detecting a line drawing from the image;
Calculating a line drawing color which is a color of the line drawing;
A representative color of the group is calculated based on the line drawing color calculated for the remaining line drawing excluding the line drawing that satisfies a predetermined condition among the line drawing constituting a group that is a unit for unifying colors. And steps to
Changing the line drawing color of the line drawing constituting the group to the representative color. On the computer,
A function to detect line drawings from images,
A function of calculating a line drawing color which is the color of the line drawing;
A representative color of the group is calculated based on the line drawing color calculated for the remaining line drawing excluding the line drawing that satisfies a predetermined condition among the line drawing constituting a group that is a unit for unifying colors. Function to
And a function for changing the line drawing color of the line drawing constituting the group to the representative color.