JP2008125020A - Image encoding device and control method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To further reduce the amount of encoded data while keeping sharpness of a character/line drawing of a multi-value image including the character/line drawing and a natural image. <P>SOLUTION: A data separator 103 extracts color information of pixels of the character/line drawing in image data of an input block and generates position information for discriminating positions of pixels of the character/line drawing and a non-characters/line drawing in the target block. A code substituting unit 106 converts the color information into a color code. Further, a reversible encoding unit 107 reversibly encodes the generated position information. A substitute value calculator 104 calculates a substitute value to substitute for values of the pixels of the character/line drawing according to the generated position information. Then a selector 105 substitutes the calculated substitute value for the values of the pixels of the character/line drawing in the block according to the position information. An irreversible encoding unit 108 irreversibly encods the block after the substitution. A multiplexing unit 109 multiplexes and outputs the color code, encoded data of the position information, and encoded data of image data of the block. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique for encoding multilevel image data.

  Conventionally, Japanese Patent Application Laid-Open No. H10-228707 has proposed a method for efficiently encoding an image in which a character line image and a natural image are mixed. This technique is briefly described as follows.

First, multi-value image data to be encoded is divided into blocks that are units of orthogonal transformation. Since the character line drawing has the same color, the most frequent pixel in the block is used as the character line drawing, and the most frequent color information is extracted (hereinafter referred to as an extracted color). Then, binary identification information for distinguishing between the pixel determined as the extracted color in the block of interest and the pixel determined as the non-extracted color is generated. Then, an average value of pixels determined as non-extracted pixels is obtained, and the pixel value determined as the extracted color is replaced with the pixel value of the average value. That is, all the pixels in the block of interest are pixels that constitute a natural image. Then, the information indicating the extracted color and the identification information are losslessly encoded, and the irreversible JPEG encoding that performs orthogonal transformation is performed on the pixel group included in the target block after replacement. Thereafter, these three encoded data are multiplexed to generate encoded data for one block. As a result, the high-frequency component and edge component of the character / line drawing are reduced in the JPEG encoding processing target block, and the encoding efficiency is increased accordingly.
JP-A-4-326669

  However, in the above-mentioned Patent Document 1, since the extracted color information is encoded as it is, there is still room for improvement. In particular, in order to facilitate random access, when decoding is completed with only information in a block, information on other blocks is not referred to, so the compression rate for the extracted color is small.

  The present invention has been made in view of such a problem, and further reduces the amount of encoded data to be generated while maintaining the sharpness of a character / line image of a multi-valued image in which characters / line images and natural images are mixed. It is intended to provide technology.

In order to solve this problem, for example, an image encoding device of the present invention has the following configuration. That is,
An image encoding device for encoding multi-value image data,
Input means for inputting image data in block units to be subjected to lossy encoding from multi-valued image data to be encoded;
Extraction means for extracting color information indicating the color of the pixel of the character / line drawing in the input block image data;
Color code conversion means for converting the color information extracted by the extraction means into a color code using a reference table stored in advance in memory;
Position information generating means for generating position information for identifying the position of the pixel of the character / line drawing and the position of the pixel of the non-character / line drawing in the target block according to the color information extracted by the extracting means;
Lossless encoding means for losslessly encoding the position information generated by the position information generating means;
A replacement value calculating means for calculating a replacement value for replacing the value of the pixel of the character / line drawing according to the position information generated by the position information generating means;
Replacement means for replacing the value of the pixel of the character / line drawing in the block of interest with the replacement value;
Irreversible encoding means for irreversibly encoding the image data of the replaced block;
And multiplexing means for multiplexing and outputting the color code, the encoded data of the position information generated by the lossless encoding means, and the encoded data of the image data generated by the lossy encoding means. .

  According to the present invention, it is possible to further reduce the amount of encoded data to be generated while maintaining the sharpness of a character / line image of a multi-value image in which a character / line image and a natural image are mixed.

  Hereinafter, embodiments according to the present invention will be described in detail with reference to the accompanying drawings.

For example, the number of colors that can be taken by a pixel in which each component of the RGB color space (which may be another color space) is represented by 8 bits (256 gradations) is approximately 2 ²⁴ ≈16 million colors. However, the inventors of the present invention, in the case of a character / line drawing mixed with a natural image (for example, an image obtained by synthesizing text such as an image date / time and a photographer's name with a captured image in a digital camera), at best, the character / line drawing. We paid attention to the fact that it is limited to a range of several colors. Therefore, a color code table that is used relatively frequently is used for pixels of character / line drawing. However, since a character / line drawing cannot always be represented by the code, a special code is assigned to a color character / line drawing that cannot be represented by the code table, and color information representing the color is subsequently added. It was supposed to be added.

  FIG. 1 is a block configuration diagram of an encoding apparatus according to the first embodiment. This apparatus includes an input terminal 101, a buffer 102, a data separation unit 103, a replacement value calculation unit 104, a selector 105, a code replacement unit 106, a lossless encoding unit 107, an irreversible encoding unit 108, and a multiplexing unit 109. Composed.

  Processing of the encoding apparatus according to the embodiment having such a configuration will be described below.

  The encoding target multi-value image data input via the input terminal 101 is temporarily stored in the buffer 102. The buffer 102 has a capacity of at least 8 lines, and outputs image data in units of blocks composed of 8 × 8 pixels to the data separation unit 103, the replacement value calculation unit 104, and the selector 105.

  The data separation unit 103 determines the most frequent color in the input block as the color of the character / line drawing. The character / line drawing color information is output to the code replacing unit 106 as extracted color information. In addition, the data separation unit 103 sets “1” for pixels determined to be an extracted color (character / line image pixel) within 8 × 8 pixels, and pixels determined to be a non-extracted color (natural image pixel). Generates binary position information of “0” and outputs it to the replacement value calculation unit 104, the selector 105, and the lossless encoding unit 107.

  The replacement value calculation unit 104 calculates an average value of each color component of pixels determined as a natural image whose position information is “0” in one block of image data, and uses the average value as a replacement value to the selector 105. Output.

  The selector 105 inputs pixel data, replacement values, and position information in the block. Then, pixel data in the block is selectively output at the pixel position indicated by position information “0”. The selector 105 selects and outputs a replacement value at the pixel position indicated by position information “1”.

  The code replacement unit 106 includes a memory that stores therein the table illustrated in FIG. In the field of “black” in the table in the figure, data “R = G = B = 0” is actually stored. Data of each RGB component similar to other colors is stored. The number of data stored in this table is at most several colors to several tens. However, it is necessary to cope with the case where the color that does not exist in this table is the most frequent. If there is a color that does not exist in the table and registration in the table is possible, the code is output after the color is registered in the table. Therefore, if 64 colors are defined including a code code for specifying a color that does not exist in the table, it is sufficient if the “code code” is 6 bits. When R, G, and B are represented by 8 bits, it means that 24-bit color information is converted to 1/4 6 bits.

  On the other hand, if registration to the table is not possible, as shown in FIG. 3B, the code code indicating the color outside the table + input color information (each component is 8 bits) is added and output. In the case of FIG. 3B, the amount of encoded data increases as much as the bit indicating the actual color is added after the code code, but such a case is rather rare.

  In any case, the code replacement unit 106 generates reversible encoded data from the input extracted color information and outputs it. When the table is updated, the table is transferred before the start of encoding. If the table cannot be transferred, the code code registered together with the registration of the table and the input color information (each component is 8 bits) are added and output. In this case, a bit indicating unregistration is included in the table of the decoding unit for each code, and a bit of the code code sent indicates unregistered (for example, 1 indicates unregistered indicates unregistered) When the bit is 1, the color information linked to the code code is registered in the portion corresponding to the code code of the table, thereby synchronizing with the encoding side.

  Further, the lossless encoding unit 107 performs lossless encoding on the position information output from the data separation unit 103, generates encoded data, and outputs the encoded data to the multiplexing unit 109. The position information of one block is 8 × 8 bits. The lossless encoding unit 107 in the embodiment performs raster scanning on the 8 × 8 binary data to perform run-length encoding. Of course, other encoding methods may be used as long as they are reversible.

  On the other hand, the irreversible encoding unit 108 performs irreversible encoding processing such as JPEG on the input 8 × 8 gradation image data. That is, the irreversible encoding unit 108 performs DCT conversion processing, quantization processing, and entropy encoding processing, generates encoded data, and outputs the encoded data to the multiplexing unit 110.

  The multiplexing unit 110 packs each piece of encoded data generated from the code replacement unit 106, the lossless encoding unit 107, and the lossy encoding unit 108 so that the encoded data can be easily stored in the subsequent memory. Is output from the output terminal 110.

  As described above, according to the present embodiment, the color of a character / line drawing mixed with a natural image is replaced with a color and a code code by using very few points as compared with that of a natural image. The amount of data occupied by the color information of the characters / line drawings in the block can be greatly reduced. Further, as in the above-described embodiment, even in the case of a natural image in which characters / line drawings are mixed, the pixels constituting the character / line images are replaced with average values determined to be natural images, thereby irreversible encoding. The value of the high frequency component generated by the processing in the unit 108 can also be made small. This means that the amount of encoded data generated by the lossy encoding unit 108 can be reduced. In addition, it is also promised that the character / line drawing can be clearly decoded in its color and edge.

  In the above embodiment, “black” in FIG. 3A is defined as R = G = B = 0. When text and ruled lines are synthesized with a natural image on a computer and encoded without an image scanner or the like, black literally has R = G = B = 0. However, when a document is read through an image scanner, the RGB components of black characters are small values, but R = G = B = 0 is not always satisfied. This is due to the accuracy and variation of the image sensor mounted on the image scanner, or the variation in the amount of ink and toner when the document is printed. Therefore, for example, an allowable value range may be provided in the field that defines the color of the table in the code replacement unit 106. For example, when the allowable value is “ε”, “black” is defined as 0 ≦ R, G, and B ≦ ε.

  In this way, the color of the character / line drawing determined as the color existing in the table cannot be faithfully reproduced in the 100% original image, but can be reproduced in a color close to that. In addition, the edge of the character / line drawing can theoretically be maintained at 100%.

<Modification of First Embodiment>
You may implement | achieve the process concerning the said embodiment with the computer program which a computer reads and executes. Since the computer is composed of a CPU, ROM, RAM, HDD, display device, keyboard, mouse, and the like mounted on a general personal computer, the description thereof is omitted. However, if the image data to be encoded is a document image, an image input device such as an image scanner is connected via an appropriate interface. In addition, when it is stored in a storage device on a network, it has a network interface for network communication. Further, if a storage medium storing the image data to be encoded is accessed, a drive or the like for accessing the medium is connected. In any case, the encoded data is stored as a file in a storage device such as an HDD.

  FIG. 4 shows a processing procedure when the CPU executes the application program in this modification. This application program is stored in the HDD, and is executed by the CPU after being loaded into the RAM.

  First, in step S1, the CPU inputs image data for one block from the image data to be encoded. The input source of the image data to be encoded is not limited.

  Next, in step S2, the CPU detects the most frequent color in the input block as an extracted color. In step S3, the extracted color code is generated with reference to the table shown in FIG. 3A and temporarily stored in the RAM. This table may be included in the program itself or may be stored in an HDD or the like.

  In step S4, position information is generated in which the position of the pixel having the extracted color in the block is “1” and the position of the pixel having a color other than the extracted color ((extracted color) is “0”. The CPU arranges the generated position information in one dimension, encodes it using a lossless encoding technique such as run-length encoding, and temporarily stores the encoded data in the RAM.

  Next, in step S6, the CPU calculates a replacement value. The replacement value is an average value of each color component of a pixel determined to have a non-extracted color (a pixel whose position information is “0”). In step S7, the color of the pixel whose position information in the block is “1” is replaced with the calculated replacement value. In step S8, lossy encoding processing such as JPEG is performed on the replaced block to generate encoded data.

  In step S9, the extracted color code, the lossless encoded data of the position information, and the lossy encoded data of the block after the replacement process are multiplexed and output as a part of the file.

  Then, in step S10, the processes in and after step S1 are repeated until it is determined that the encoding process for all blocks has been completed.

  As described above, when the computer reads and executes, it is possible to achieve the same processing and operational effects as in the first embodiment.

<Second Embodiment>
FIG. 2 shows a block configuration diagram of an image encoding device according to the second embodiment. In the figure, 201 is a data separator, and 202 is an extracted color table memory. The other components are the same as those shown in FIG. It is assumed that the extracted color table memory 202 (which is a rewritable memory) stores the data shown in FIG.

  Next, operations of parts different from those of the encoding apparatus of FIG. 1 will be described.

  The data separation unit 201 searches for a pixel having a color set in the extracted color table memory 202 in the target block. If there is a pixel having a color set in the extracted color table 202 in the target block, the code code of the color that appears most frequently is output as the extracted color code.

  If the color of the most frequent pixel in the block of interest is not the color set in the extraction color table 202, the data separation unit 201 displays the code code indicating the outside of the table and the color information subsequent thereto. Output as extracted color code.

  The rest is the same as in the first embodiment. Also, the encoded data for one block generated by the multiplexing unit 109 may have the same format as that of the first embodiment.

  Next, a method for generating a table stored in the extracted color table memory 202 will be described. The following description may be applied when generating a table to be stored in the memory in the code replacement unit 106 in the first embodiment.

  In the second embodiment, in order to facilitate random access of compressed data, a configuration is assumed in which encoding is performed without using information of other blocks so that decoding is completed with only intra-block information. Yes. Therefore, the compression rate is increased by registering colors in the extracted color table and storing only the code code (reference number) of the table. However, colors that are not in the extracted color table cannot be reproduced as they are. Therefore, only one code code indicating the outside of the table is set, and a code obtained by adding an extracted color to this code is used as an encoded code.

For example, if the number of bits of the reference number is 6 bits, 64 colors can be registered in the table as extracted colors. Since one of them indicates the outside of the table, the extracted colors that can be registered in the table are 63 colors. That is, if the number of bits of the reference number is n bits, one becomes a code outside the table, and the remaining 2 ⁿ −1 colors can be registered in the extraction color table.

  However, if the probability of occurrence of colors outside the table increases, the compression efficiency will drop rapidly. Therefore, when generating the extracted color table, it is necessary to register frequently used images from the images.

Therefore, an image to be encoded is pre-scanned with an image scanner or the like to create a color histogram of the input image and register frequently used colors. However, since it takes time to create a histogram using all images, a histogram is generated at high speed with a reduced image. As a method for reducing an image, there are various reduction methods such as linear interpolation. However, since the pixel value is changed by calculation, it cannot be used as an extracted color. Therefore, in this structure, it reduces by simple thinning. At this time, as described above, since one is a code outside the table, the upper 2 ⁿ −1 colors of the histogram are set in the extracted color table.

  Normally, frequently appearing colors such as white, black, and red may be fixed extraction colors in advance. In this way, a scanned image such as a document can be compressed without pre-scanning. In the case of the second embodiment, when one image data is encoded and stored as a file in an appropriate storage medium, the information of the table generated by the above processing is stored in the file header. . Accordingly, the larger the image data size (number of pixels in the horizontal and vertical directions), the greater the effect of the second embodiment.

  As described above, according to the second embodiment, it is possible to achieve the same functions and effects as those of the first embodiment.

  In the embodiment, one block is described as 8 × 8 pixels. However, the number of pixels in the horizontal and vertical directions may be 8n × 8m, which is an integer multiple of 8. This is because the lossy encoding unit 108 may encode the n × m blocks included in 8n × 8m.

  It is obvious that the same processing as in the second embodiment can be realized by a computer program, as in the modification of the first embodiment described above.

  In general, a computer program is stored in a computer-readable storage medium such as a CD-ROM, and the medium is set in a reading device (such as a CD-ROM drive) included in the computer and copied or installed in the system. It becomes possible. Therefore, it is clear that such a computer readable storage medium is also within the scope of the present invention.

It is a block block diagram of the image coding apparatus in 1st Embodiment. It is a block block diagram of the image coding apparatus in 2nd Embodiment. It is a figure which shows an example of the content of the table memory in embodiment, and the structure of the encoding data of an extraction color. It is a flowchart which shows the process sequence in the case of implement | achieving 1st Embodiment with a computer program.

Claims (8)

An image encoding device for encoding multi-value image data,
Input means for inputting image data in block units to be subjected to lossy encoding from multi-valued image data to be encoded;
Extraction means for extracting color information indicating the color of the pixel of the character / line drawing in the input block image data;
Color code conversion means for converting the color information extracted by the extraction means into a color code using a reference table stored in advance in memory;
Position information generating means for generating position information for identifying the position of the pixel of the character / line drawing and the position of the pixel of the non-character / line drawing in the target block according to the color information extracted by the extracting means;
Lossless encoding means for losslessly encoding the position information generated by the position information generating means;
A replacement value calculating means for calculating a replacement value for replacing the value of the pixel of the character / line drawing according to the position information generated by the position information generating means;
Replacement means for replacing the value of the pixel of the character / line drawing in the block of interest with the replacement value;
Irreversible encoding means for irreversibly encoding the image data of the replaced block;
Multiplexing means for multiplexing and outputting the color code, the encoded data of the position information generated by the lossless encoding means, and the encoded data of the image data generated by the lossy encoding means. An image encoding apparatus characterized by that.   2. The image encoding apparatus according to claim 1, wherein the extraction unit extracts the most frequent color in the block as the color of a pixel of a character / line drawing.   Further, the image data to be encoded is analyzed, and a means for storing a preset number of color information and color codes as the reference table in the memory in order of appearance frequency is provided. The image encoding device according to claim 1.   2. The image encoding apparatus according to claim 1, wherein the replacement value calculating unit calculates an average value of each component of a pixel determined to be a non-character / line drawing in a target block as a replacement value.   2. The image encoding according to claim 1, wherein the lossless encoding unit encodes position information according to run-length encoding, and the lossy encoding unit encodes block image data according to JPEG encoding. apparatus. A control method for an image encoding device for encoding multi-value image data,
An input step in which input means inputs image data in units of blocks to be subjected to lossy encoding from multi-valued image data to be encoded;
An extracting step in which the extracting means extracts color information indicating the color of the pixel of the character / line image in the image data of the input block;
A color code conversion step in which the color code conversion means converts the color information extracted in the extraction step into a color code using a reference table stored in advance in a memory;
Position information for generating position information for identifying a position of a pixel of a character / line drawing and a position of a pixel of a non-character / line drawing in the target block according to the color information extracted in the extraction step Generation process;
A lossless encoding means for losslessly encoding the position information generated in the position information generating process;
A replacement value calculating unit that calculates a replacement value for replacing the value of the pixel of the character / line drawing according to the position information generated in the position information generating step;
A replacement step in which the replacement means replaces the value of the pixel of the character / line drawing in the block of interest with the replacement value;
An irreversible encoding means for irreversibly encoding the image data of the replaced block;
Multiplexing means for multiplexing and outputting the color code, the encoded data of the position information generated by the lossless encoding process, and the encoded data of the image data generated by the lossy encoding process A control method for an image encoding device, comprising:   A computer program that causes a computer to read and execute the program to function as each unit of the image encoding device according to any one of claims 1 to 5.   A computer-readable storage medium storing the computer program according to claim 7.

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