JP2006060490A - Image compressor and image compression program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make both of the maintenance of image quality and the improvement of a compression rate highly compatible regarding an image compressor which applies data compression processing to image data, and an image compression program for operating an information processor such as a computer as the image compressor. <P>SOLUTION: The image compressor is provided with a first image compressor 510 which generates first compressed data by applying irreversible compression processing to the image data, an image decompressor 520 which generates decompressed data by decompressing the first compressed data obtained with the first image compressor 510, a difference calculator 530 which calculates difference data by difference calculation of the image data before the irreversible compression processing by the first image compressor 510 and the decompressed data obtained with the image decompressor 520, and a second image compressor 540 which generates second compressed data by applying the irreversible compression processing to the difference data obtained with the difference calculator 530. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image compression apparatus that performs data compression processing on image data, and an image compression program that causes an information processing apparatus such as a computer to operate as an image compression apparatus.

  2. Description of the Related Art Conventionally, a technique for performing data compression processing on image data has been widely adopted in order to reduce storage capacity and communication volume.

  For example, in Patent Document 1, when a representative color is selected from an original image and a CLUT (color look-up table) is configured, color numbers are assigned so that continuous color numbers have color data of close values, When a bitmap corresponding to the CLUT is created to obtain the difference in color number between adjacent pixels, and the difference takes a large value, the color number of the bitmap is changed within a range that does not cause image quality degradation, and the difference is set to a small value. A technique is disclosed in which run length encoding is applied to biased and differential data.

  Further, Patent Document 2 encodes image data configured by irreversibly compressing a plurality of data assigned to each color and assigning one of the data to a transparent color, The transparent color is reversible, and the image data is composed of an immediate value (the first value at the time of differential encoding) and a plurality of differential values (the previous value at the time of differential encoding) following the immediate value. When encoding a value by irreversible compression, the immediate value representing the transparent color and the difference value are made reversible, and the immediate value representing the transparent color is set to an intermediate value between the data values of each color, or the transparent color is changed. A technique has been proposed in which the difference value to be expressed is set to “0”.

  Patent Document 3 proposes that the number is encoded by the difference between the predicted number (s' (j)) and the actual number (s (j)).

  Further, Patent Document 4 recognizes the distribution status of the same pixel data in the sub-scanning direction and the distribution status of the same pixel data in the main scanning direction with respect to the n-th pixel data column. Based on this recognition result, there has been proposed an image compression device that determines whether to compress the same pixel data continuous in the sub-scanning direction or to compress the same pixel data continuous in the main scanning direction.

  Japanese Patent Application Laid-Open No. 2005-228561 discloses a technique for dividing the image data into small blocks in advance, and determining whether or not the attention area is included in each small block, thereby changing the compression rate between the attention area and the other areas. Is disclosed.

  Further, in Patent Document 6, the image data is separated into a plurality of planes using the result of the process on the preceding stage, and the result of the process on the preceding stage is used for the image data separated into the plurality of planes. Thus, there is disclosed a technique for compressing image data by a multi-stage process in which a subsequent process is performed.

  Further, Patent Document 7 discloses that input moving image data is decomposed into bit planes for each frame, and data of each plane group generated by the decomposition is divided into regions using correlation between frames and between frames. A technique for encoding by performing the above is disclosed.

  Here, one system to which the data compression technology is applied is introduced.

  FIG. 1 is a diagram illustrating an example of a print system to which a data compression technique is applied, and FIG. 2 is a diagram illustrating a flow of data processing in the print system.

  As shown in FIG. 1, the print system includes a host controller 100, an interface device 200, and a printer 300. A general-purpose interface cable 150 such as SCSI is used between the host controller 100 and the interface device 200. Further, the interface device 200 and the printer 300 are connected by a dedicated interface cable 250.

  In the host controller 100, as shown in FIG. 2, character and image data 11 described in various languages and formats such as PDF, PS, and TIFF are image (CT) data, characters and lines. Etc. (LW; Line Work) data, RIP (Raster Image Processing) is performed for each to generate bitmap data 12A and 13A, and further, data compression processing is performed for each, and CT is performed. Generates irreversible compressed data 14 and reversible compressed data 15 for LW. These compressed data 14 and 15 are transferred from the host controller 100 to the interface device 200 via the general-purpose interface cable 150 shown in FIG. In the interface device 200, the compressed data 14 and 15 transferred are subjected to data decompression processing, and the bitmap data 12B and 13B corresponding to the bitmap data 12A and 13A in the state before the data compression processing is performed by the host controller 100. Is generated. Here, since the CT data is subjected to lossy compression processing at the time of data compression by the host controller 100, the CT data (bitmap data 12B) after data decompression is completely CT data before data compression ( Although it does not return to the bitmap data 12A), almost the same bitmap data is restored. Since the LW data is subjected to lossless compression processing at the time of data compression by the host controller 100, the LW data (bitmap data 13B) after data expansion is the LW data (bitmap data 13A) before data compression. Is restored to the same data.

  In the interface device 200, the CT data (bitmap data 12B) after data expansion and the LW data (bitmap data 13B) are combined, and halftone dot information and the like are added as tags and sent to the printer 300. In the printer 300, an image is printed out according to the bitmap data received from the interface device 200 and the tag information added thereto.

  For example, when the host controller 100 and the interface device 200 are separated from each other, or when the interface device 200 is a system that receives image data from a plurality of host controllers, the host controller 100 and the interface device 200 are separated from each other. 2, the host controller 100 performs data compression, transfers the data to the interface device 200, and decompresses the data using the interface device, as shown in FIG. Data transfer time to the interface device 200 can be shortened, and print productivity is improved.

Here, generally, a compression method such as JPEG which is irreversible but has a high compression rate is adopted for CT data, and a reversible compression method such as PackBits is adopted for LW data.
JP-A-5-328142 Japanese Patent Laid-Open No. 10-164620 JP 2001-520822 A Japanese Patent Laid-Open No. 9-200540 JP 2000-134622 A JP 2003-348358 A JP-A-11-355770

  In the case of the data compression method shown in FIG. 2, since the CT image and the LW image are separated and different compression processing methods are applied to each, an image compression process suitable for both the CT image and the LW image is performed. On the other hand, since it is necessary to perform two systems of compression processing, there is a problem that the processing is complicated. In order to improve this, it is desirable to perform the same compression processing for both CT images and LW images. However, simply applying the same compression processing can improve image quality and improve the compression rate. There is a problem that conflicting requirements cannot be satisfied at the same time.

  SUMMARY OF THE INVENTION In view of the above circumstances, the present invention provides an image compression apparatus that performs a data compression process that highly balances both image quality maintenance and compression ratio improvement on image data, and operates the computer as such an image compression apparatus. An object of the present invention is to provide an image compression program that can be executed.

An image compression apparatus of the present invention that achieves the above object is an image compression apparatus that performs data compression processing on image data.
A first image compression unit that generates first compressed data by performing irreversible compression processing on the image data;
An image decompression unit that generates decompressed data by decompressing the first compressed data obtained by the first image compression unit;
A difference calculation unit that obtains difference data by calculating a difference between the image data before irreversible compression processing by the first image compression unit and the decompressed data obtained by the image decompression unit;
And a second image compression unit that generates second compressed data by performing irreversible compression processing on the difference data obtained by the difference calculation unit.

  The image compression apparatus of the present invention performs a lossy compression process once in the first image compression unit, performs a decompression process again, and a difference between the image data before the lossy compression process and the decompressed data obtained by the decompression process Compressed data obtained by calculating difference data by calculation and performing lossy compression processing on the difference data by the second lossy compression unit, and performing lossy compression processing by the first image compression unit. The image quality is improved as compared with the image obtained by decompressing the image, and a high compression ratio can be realized by performing the irreversible compression process in the second image compression unit.

  Here, in the image compression apparatus of the present invention, the first image compression unit may perform irreversible compression processing conforming to the JPEG standard on the image data.

  In general, JPEG compression is suitable as a compression process applied to image data.

  In the image compression apparatus of the present invention, the second image compression unit performs a compression process including a quantization process on the difference data.

  By the quantization process, a high compression rate can be realized for the difference data.

The second image compression unit outputs one or a plurality of predetermined compression target numerical values as they are in the compressed data, and for the compression target numerical value, the compression target numerical value, the compression target numerical value, A compression process including an encoding process of encoding and outputting a numerical value representing a continuous number of the same numerical value to be compressed may be performed.
In the encoding process, the second image compression unit expresses the continuous number as a unit bit number when the number of consecutive compression target numerical values is equal to or less than a predetermined number, and the continuous number exceeds the predetermined number. In some cases, the continuous number is preferably expressed by 2 unit bits.

In the image compression apparatus of the present invention, a third image compression unit that generates third compressed data by performing lossless compression processing on the image data;
A compression rate determination unit that determines whether or not the compression rate of the third compressed data obtained by the third image compression unit is equal to or higher than a predetermined compression rate,
The first image compression unit, the image decompression unit, the difference calculation unit, and the second image compression unit are compression rates at which the third compressed data by the compression rate determination unit is less than a predetermined compression rate. It is preferable to act upon receiving the determination result.

  When the reversible compression process satisfies the desired compression ratio, the desired compression ratio is achieved without causing deterioration in image quality, and no further processing is necessary. Therefore, it is preferable to perform the processing by the first image compression unit, the compression / decompression unit, the difference calculation unit, and the second image compression device only when the desired compression rate is not satisfied in the lossless compression process.

Here, the third image compression unit directly outputs one or a plurality of predetermined compression target numerical values in the data to be compressed, and for the compression target numerical value, the compression target numerical value and the compression target numerical value. It is preferable to perform a reversible compression process including an encoding process that encodes and outputs a numerical value that represents the number of consecutive compression target numerical values,
As the encoding process, the third image compression unit expresses the continuous number as a unit bit number when the number of consecutive compression target numerical values is less than a predetermined number, and the continuous number exceeds the predetermined number. In some cases, it is preferable that the continuous number is expressed by 2 unit bits.

The image compression program of the present invention that achieves the above object is an image compression program that is executed in an information processing apparatus that executes the program and that operates the information processing apparatus as an image compression apparatus that performs data compression processing on image data. There,
The information processing apparatus is
A first image compression unit that generates first compressed data by performing irreversible compression processing on the image data;
An image decompression unit that generates decompressed data by decompressing the first compressed data obtained by the first image compression unit;
A difference calculation unit that obtains difference data by calculating a difference between the image data before irreversible compression processing by the first image compression unit and the decompressed data obtained by the image decompression unit;
The image processing apparatus is operated as an image compression apparatus including a second image compression unit that generates second compressed data by performing irreversible compression processing on the difference data obtained by the difference calculation unit.

Here, also in the image compression apparatus of the present invention, it is preferable that the first image compression unit performs irreversible compression processing conforming to the JPEG standard on image data,
The second image compression unit preferably performs a compression process including a quantization process on the difference data.

In the image compression program of the present invention, the second image compression unit outputs one or a plurality of predetermined compression target numerical values as they are in the data to be compressed, and the compression target numerical values for the compression target. It is preferable to perform compression processing including encoding processing that encodes and outputs a numerical value and a numerical value that represents the number of consecutive compression target numerical values that are the same as the compression target numerical value.
In the encoding process, the second image compression unit expresses the continuous number as a unit bit number when the number of consecutive compression target numerical values is equal to or less than a predetermined number, and the continuous number exceeds the predetermined number. In some cases, the continuous number is preferably expressed by 2 unit bits.

Furthermore, the image compression program of the present invention provides the information processing apparatus,
A third image compression unit that generates third compressed data by performing reversible compression processing on the image data;
A compression rate determination unit that determines whether or not the compression rate of the third compressed data obtained by the third image compression unit is equal to or higher than a predetermined compression rate;
The first image compression unit, the image decompression unit, the difference calculation unit, and the second image compression unit are compression rates at which the third compressed data by the compression rate determination unit is less than a predetermined compression rate. It is preferable to operate as an image compression apparatus that acts upon receiving the determination result.

In this case, the third image compression unit directly outputs one or a plurality of predetermined compression target numerical values in the compressed data, and for the compression target numerical value, the compression target numerical value and the compression target numerical value It is preferable to perform a reversible compression process including an encoding process that encodes and outputs a numerical value that represents the number of consecutive compression target numerical values, and in this case,
As the encoding process, the third image compression unit expresses the continuous number as a unit bit number when the number of consecutive compression target numerical values is less than a predetermined number, and the continuous number exceeds the predetermined number. In some cases, it is preferable that the continuous number is expressed by 2 unit bits.

  As described above, according to the present invention, it is possible to achieve both high image quality maintenance and high compression ratio.

  The embodiment described below is an image compression apparatus incorporated in a host controller in the entire system shown in FIG. 1, and more specifically, CT bitmap data 12A and LW in the host controller shown in FIG. The present invention relates to a process of compressing data without distinguishing both of the bitmap data 13A. Therefore, here, the data compression processing for the CT data and LW data described with reference to FIGS. 1 and 2 is replaced with the data compression processing according to the embodiment of the present invention described below, and the data in the interface device is replaced. It is understood that the decompression (decompression) process replaces the data decompression (decompression) process corresponding to the data compression process as the embodiment of the present invention, and the entire system shown in FIG. 1 and the process flow shown in FIG. 2 are duplicated. The illustration and repeated explanation are omitted.

  FIG. 3 is a block diagram showing an embodiment of the image compression apparatus of the present invention.

  The image compression apparatus 500 shown in FIG. 3 includes a first image compression unit 510, an image decompression unit 520, a difference calculation unit 530, a second image compression unit 540, a third image compression unit 550, and a compression rate determination unit. 560. Although details of each of the units 510 to 560 will be described later, the flow of image data in the image compression apparatus 500 is as follows.

  An input image file (in this embodiment, a file storing CT data 12A or LW data 13A expanded in a bitmap as shown in FIG. 2) is stored in the first image compression apparatus 500 shown in FIG. Are input to the image compression unit 510, the difference calculation unit 530, and the third image compression unit 550.

  In the third image compression unit 550, lossless compression processing is performed on the image data in the input image file input this time.

  In the present embodiment, the third image compression unit 550 outputs one or more predetermined compression target numerical values in the compressed data as they are, and the compression target numerical values are the compression target numerical values and the compression target numerical values. A lossless compression process including an encoding process of encoding and outputting a numerical value representing a continuous number of the same numerical value to be compressed as the numerical value to be compressed is performed. In this encoding process, when the number of consecutive compression target numeric values is less than or equal to a predetermined number, the number of consecutive units is expressed by one unit bit number. When the number of consecutive units exceeds the predetermined number, the number of consecutive units is expressed by two unit bit numbers. The process of expressing with is performed. Here, in order to match the JPEG compression processing (described later) performed by the first image compression unit 510, the compression processing is performed with 8 lines of the input image as one unit. The compressed data (third compressed data referred to in the present invention) after the reversible compression processing is input to the compression rate determination unit 560. The compression rate determination unit 560 is notified of the required compression rate in advance, and the compression rate determination unit 560 compresses the compressed data input from the third image compression unit 550 to a level that satisfies the required compression rate. It is determined whether the data has been processed. This determination is also performed with the input image 8 lines as a unit. If the compressed data for 8 lines has data that satisfies the required compression rate, the compression rate is satisfied at the stage of the lossless compression process, and no further compression is required, and the data is output as it is. In outputting the compressed data, a header describing the position of the compressed data on the input image, information necessary for subsequent decompression processing, and the like is added and output.

  When the compression rate determination unit 560 determines that the compressed data sent this time from the third image compression unit 550 is data that does not satisfy the required compression rate, the determination result is sent to the first image compression unit 510. The first image compression unit 510 is notified, and irreversible compression processing is performed on the image data in the input image file that was originally input. In the present embodiment, the first image compression unit 510 employs JPEG compression processing.

  JPEG compressed data (corresponding to an example of the first compressed data referred to in the present invention) obtained by JPEG compression processing in the first image compression unit 510 is input to the image decompression unit 520, and this image decompression unit 520 Then, the input JPEG compressed data is decompressed to generate decompressed data that approximates the image data in the original input image file. The decompressed data is data approximate to the image data in the original input image file, but is subjected to irreversible compression processing (JPEG compression processing is a type of irreversible compression processing) in the first image compression unit 510. Therefore, the decompressed data is not the same as the image data in the original input image file.

  The decompressed data obtained by the decompressing process in the image decompressing unit 520 is input to the difference computing unit 530 together with the image data in the original input image file, and the difference computing unit 530 calculates the input image data and decompressed data. A difference calculation is performed for each pixel of the image to obtain difference data. The difference data is input to the second image compression unit 540.

  In the second image compression unit 540, lossy compression processing is performed on the input difference data. In this embodiment, as the compression processing in the second image compression unit 540, the quantization processing and one or more predetermined compression target numerical values in the compressed data are output as they are, and the compression target numerical values are output as they are. A compression process including a compression target numerical value and an encoding process for encoding and outputting a numerical value representing a continuous number of the same compression target numerical value as the compression target numerical value is employed. In the encoding processing here, when the number of consecutive identical numerical values to be compressed is less than or equal to a predetermined number, the number of consecutive is represented by one unit bit number, and when the number of consecutive exceeds the predetermined number, the number of consecutive is 2 Expressed in unit bits. Here, since the encoding process is a lossless compression process, the quantization process is an irreversible compression process, and thus the compression process performed by the second image compression unit 540 is an irreversible compression process as a whole.

  In this way, the compressed data generated by the second image compression unit is recorded in the header of the compressed data obtained by the first image compression unit 510 together with data necessary for decompressing the compressed data. The

  The compressed data obtained as described above in the image compression apparatus 500 shown in FIG. 3 is transferred to the interface device 200 via the general-purpose interface 150 such as SCSI shown in FIG. In the interface device 200, the received compressed data is subjected to data decompression processing, and restored to image data that is highly approximate to the image data in the original input image file.

  FIG. 4 is a hardware configuration diagram of the host controller shown in FIG.

  The host controller 100 shown in FIG. 1 is configured by a computer system having the configuration shown in FIG.

  4 includes a CPU 111, a RAM 112, a communication interface 113, a hard disk controller 114, an FD drive 115, a CDROM drive 116, a mouse controller 117, a keyboard controller 118, a display controller 119, And a communication board 120 are connected to each other via a bus 110.

  The hard disk controller 114 controls access to the hard disk 104 built in the host controller 100, and the FD drive 115 and the CDROM drive 116 are flexible disks (FD) loaded in the host controller 100 so as to be removable. 130, controls access to the CDROM 140. The mouse controller 117 and the keyboard controller 118 play a role of detecting operations of the mouse 107 and keyboard 108 provided in the host controller 100 and transmitting them to the CPU 111. Further, the display controller 119 plays a role of displaying an image on the display screen of the image display 109 provided in the host controller 100 based on the instruction of the CPU 111.

  The communication board 120 is responsible for communication conforming to a general-purpose interface protocol such as SCSI, and is responsible for transferring the compressed image data to the interface device 200 (see FIG. 1) via the interface cable 150.

  Further, the communication interface 113 is responsible for general-purpose communication such as the Internet, and the host controller 100 can also capture image data via the communication interface 113.

  A program stored in the hard disk 104 is read into the RAM 112 and expanded for execution by the CPU 111, and the program expanded in the RAM 112 is read out and executed by the CPU 111.

  FIG. 5 is a schematic configuration diagram of an image compression processing program of the present invention.

  Here, the image compression program 600 is stored in the CD ROM 140.

  The image compression program 600 includes a first image compression unit 610, an image decompression unit 620, a difference calculation unit 630, a second image compression unit 640, a third image compression unit 650, and a compression rate determination unit 660. ing. In addition to the image compression program 600 shown here, the CDROM 140 stores various programs for executing a series of processes in the host controller 100 shown in FIG. The description is omitted.

  The CD ROM 140 shown in FIG. 5 is loaded into the host controller 100 shown in FIG. 4 and accessed by the CD ROM drive 116, and the program stored in the CD ROM 140 is uploaded to the host controller 100 and stored in the hard disk 104. When the program stored in the hard disk 104 is read from the hard disk 104, loaded into the RAM 112 and executed by the CPU 111, the host controller 100 includes a process as the image compression apparatus 500 shown in FIG. It operates as an apparatus that executes various processes.

  Here, the image compression program 600 shown in FIG. 5 is installed in the host controller 100 and executed by the CPU 111, thereby realizing the image compression apparatus 500 shown in FIG. The first image compression unit 610, the image decompression unit 620, the difference calculation unit 630, the second image compression unit 640, the third image compression unit 650, and the compression rate determination unit 660 are executed by the CPU 111. The host controller 100 is included in the image compression apparatus 500 shown in FIG. 3, and includes a first image compression unit 510, an image decompression unit 520, a difference calculation unit 530, a second image compression unit 540, and a third image compression, respectively. The program component is operated as the unit 550 and the compression rate determination unit 560.

  The operations of the units 610 to 660 constituting the image compression program 600 of FIG. 5 when executed by the CPU 111 are the operations themselves of the units 510 to 560 constituting the image compression apparatus 500 of FIG. Accordingly, the description up to now and the detailed description of each of the units 510 to 560 of the image compression apparatus 500 of FIG. 3 also serve as the description of the units 610 to 660 constituting the image compression program 600 of FIG. And

  FIG. 6 is an explanatory diagram showing JPEG compression processing in the first image compression unit 510 of the image compression apparatus 500 shown in FIG.

  Here, the following data compression processing is performed for each pixel block of 8 × 8 pixels.

  A pixel column A (1 × 8 pixels in the horizontal direction) for one row in the 8 × 8 pixel block is input to the one-dimensional DCT processing unit 511. The one-dimensional DCT processing unit 511 performs a one-dimensional DCT (discrete cosine transform) process on the input pixel column A for one row, and performs one-dimensional DCT for each pixel constituting the pixel column A for one row. The coefficient A ′ is obtained, and the one-dimensional DCT coefficient A ′ is written in the first row of the memory 512 having a 64-word structure. Similarly, one-dimensional DCT processing is performed on each of the remaining seven rows of pixel columns of the 8 × 8 pixel pixel block to obtain one-dimensional DCT coefficients for each row, and the one-dimensional DCT coefficients are stored in the memory. Write to each row of 512.

  Next, the one-dimensional DCT coefficient B (vertical 8 × 1 one-dimensional DCT coefficient string) written in the memory 512 is read and input to the one-dimensional DCT processing unit 513. The one-dimensional DCT processing unit 513 performs a one-dimensional DCT process on the one-dimensional DCT coefficient B for one column to obtain a two-dimensional DCT coefficient C, and writes the two-dimensional DCT coefficient C in the first column of the 64-word memory 514. Thereafter, similarly, the one-dimensional DCT processing for the one-dimensional DCT coefficients for 2 to 8 columns written in the memory 512 is sequentially performed in the row direction, and the two-dimensional DCT coefficients are written in the memory 514.

  Further, so-called zigzag reading is performed on the two-dimensional DCT coefficient written in the memory 514. Here, the matrix of the two-dimensional DCT coefficients written in the memory 514 has a value other than “0” on the upper left side corresponding to the low spatial frequency component of the normal image, and corresponds to the high spatial frequency component. There is a high probability that '0' appears in the lower right. Therefore, zigzag reading is performed by scanning obliquely sequentially from the low frequency side to the high frequency side, that is, from the upper left end to the lower right end. Specifically, the addresses of the memory 514 are scanned in the order of numbers 0, 1, 2, 3,... Shown in FIG. 3, and the two-dimensional DCT coefficients are read in the scanned order. The two-dimensional DCT coefficients constituting the 64-word data stream D read out in this way are input to the quantization unit 515.

  The quantization unit 515 divides the input two-dimensional DCT coefficient by the corresponding coefficient in the quantization table T prepared in advance, and outputs the result to the zero run length counter 516. The zero run length counter 516 counts the continuous lengths of invalid coefficients from the quantization unit 515 in which the value of the divided two-dimensional DCT coefficient is “0”. This count value is input to the Huffman encoder 517.

  The Huffman encoder 517 performs entropy encoding processing by the Huffman encoding method. In this way, JPEG image compression is performed.

  The JPEG compressed data obtained in this way is input to the image decompression unit 520 shown in FIG. 3, and the JPEG compressed data is decompressed by performing the reverse operation to the JPEG compression processing described with reference to FIG. Generate decompressed data. The decompressed data is input to the difference calculation unit 530, and a difference operation is performed for each corresponding pixel between the image data in the original input image file and the decompressed data, and the difference data is obtained. Is input to the second image compression unit 540.

  FIG. 7 is a block diagram showing the configuration of the second image compression unit 540 of the image compression apparatus 500 shown in FIG.

  The second image compression unit 540 includes a differential encoding unit 541, a quantization processing unit 542, a run length encoding unit 543, and a Huffman encoding unit 544.

  FIG. 8 is a diagram illustrating the data structure of difference data in the input image file input to the second image compression unit 540 in FIG. 7 and the concept of difference encoding.

As shown in FIG. 8, the difference data input to the second image compression unit 540 shown in FIG. 7 has M pixels arranged in a predetermined main scanning direction. For the Nth line when taught in the sub-scanning direction perpendicular to the main scanning direction, the pixel values of the pixels arranged in the main scanning direction are as follows:
Dn _{, 1} , Dn _{, 2} ,..., Dn _{, m-2} , Dn _{, m-1} , Dn _{, m}
It is expressed.

Similarly, for the (N + 1) th line in the sub-scanning direction, the pixel values of the pixels arranged in the main scanning direction are as follows:
Dn _{+ 1,1} , Dn _{+ 1,2} ,..., Dn _{+ 1, m-2} , Dn _{+ 1, m-1} , Dn _{+ 1, m}
It is expressed.

Here, the difference encoding unit 541 constituting the second image compression unit 540 shown in FIG. 7 inputs the image data as described above, and obtains the difference between pixels adjacent in the sub-scanning direction. That is, if the difference between the Nth line and the (N + 1) th line and the jth pixel lined up in the main scanning direction is Sn _{, j} , the difference _{Sn, j} is
S _{n, j} = D _{n + 1, j} −D _{n, j} (j = _{1 to m} )
It is expressed.

  This difference calculation will be specifically described.

  FIG. 9 is a diagram illustrating a differential encoding process in the differential encoding unit 541 constituting the second image compression unit 540 of FIG.

Here, the pixel values in one vertical line arranged in the sub-scanning direction shown in FIG. 8 are “image data” in FIG.
As shown in the column
"12 01 02 FF 64 ... 40 40 3F ..."
Suppose that Here, each pixel value is represented by two hexadecimal digits (1 byte = 8 bits). Here, “line” indicates pixels arranged in the main scanning direction.

  First, the pixel value “12” on the first line is output as it is.

  Next, the pixel value “12” of the first line is subtracted from the pixel value “01” of the second line, and the result is output. Here, the result of subtracting “12” from “01” is a negative number and is expressed as “1EF” by 9 bits, but “1” that is 1 bit of the MSB is omitted and “8” is the lower 8 bits. Only “EF” is output.

  Next, the pixel value “01” of the second line is subtracted from the pixel value “02” of the third line, and the resultant value “01” is output.

  Next, the pixel value “02” of the third line is subtracted from the pixel value “FF” of the fourth line, and the resultant value “FD” is output.

  Next, the pixel value “FF” of the fourth line is subtracted from the pixel value “64” of the fifth line, and “1” that is 1 bit of the MSB is omitted from the resulting value, and the lower 8 bits. “65” is output.

  Hereinafter, by repeating the same operation, it is represented in the column of “differential encoding (lower 8 bits)” in FIG.

"(12) EF 01 FD 65 ... C0 00 FF ..."
Is output.

  In decoding the differentially encoded data, the interface device 200 shown in FIG. 1 performs the calculation shown on the right side of FIG.

  First, the pixel value of the first line remains “12”.

  The pixel value of the second line is “01” represented by the lower 8 bits of the result of adding the pixel value “12” of the first line to the difference value “EF”.

  The pixel value of the third line is “02” obtained by adding the pixel value “01” of the second line obtained above to the difference value “01”.

  The pixel value of the fourth line is “FF” obtained by adding the pixel value “02” of the third line obtained above to the difference value “FD”.

  The pixel value of the fifth line is “64” represented by the lower 8 bits of the result obtained by adding the pixel value “FF” of the fourth line obtained above to the difference value “65”.

  Thereafter, the same calculation is repeated, whereby the same data as the data before differential encoding is decoded.

  FIG. 10 is a diagram for explaining the operation by differential encoding.

  FIG. 10A shows the concept of an image. Here, the vertical direction in the figure is the main scanning direction, the horizontal direction is the sub-scanning direction, and each of the arrows A drawn in the sub-scanning direction (horizontal direction) is shown in FIG. Focus is on the pixel value of the pixel.

  On this image, a straight line L1 having a density of pixel value “63” extending in the main scanning direction and a straight line L2 having a density of pixel value “FF” are drawn. There is a CT image area to be fitted. A region to which a CT image is fitted is represented by a pixel value “00”.

  As shown in FIG. 10B, the pixel values of the pixels arranged on the arrow A in FIG. 10A are, in order from the left, first “01”, followed by “63” on the straight line L1, and again “01 ”Continues to“ FF ”on the straight line L2, returns to“ 01 ”again,“ 00 ”continues in the area where the CT image is fitted, and“ 01 ”continues again when the CT image area ends. Here, the pixel value “01” represents an area in which nothing is drawn (paper area).

  When the difference calculation is performed on the original data shown in FIG. 10B, the difference data shown in FIG. 10C is obtained, and the appearance probability of “00” is greatly increased. In the differential encoding unit 541 shown in FIG. 7 described with reference to FIG. 9, one bit (code bit) of the MSB is omitted, and the data output from the differential encoding unit 541 is shown in FIG. become that way. Even in this case, as described with reference to FIG. 9, the original data is sequentially restored by transmitting the first pixel value (the pixel value “12” in the first line in the case of FIG. 8) as it is. be able to.

  In the quantization processing unit 542 of the second image compression unit 540 shown in FIG. 7, quantization processing is performed on the post-difference data obtained as described above by the differential encoding unit 541. In this quantization process, the value of each pixel constituting the post-difference data is divided by the quantization coefficient to obtain a quotient, and the remainder is discarded. That is, for example, when 8 is used as the quantization coefficient, -7 to +7 is 0, +8 to +15 is 1, -7 to -15 is -1,. When such quantization processing is performed, the appearance probability of the same value is further increased, and the compression rate of the subsequent image compression processing can be further improved.

  The quantized data that has been quantized by the quantization processing unit 542 is then input to the run-length encoding unit 543 that forms the second image compression unit 540 shown in FIG.

  The run-length encoding unit 543 performs the encoding process only on specific numerical values among a plurality of numerical values constituting the data received from the quantization processing unit 542. For this reason, in the run length encoding unit 543, a numerical value to be encoded from the received data (here, this numerical value is referred to as a “compression target numerical value”) and a continuous number of the compression target numerical value are obtained. Detected.

  In this embodiment, as an example, three numerical values “01”, “FF”, and “00” are set as compression target numerical values.

  FIG. 11 is an explanatory diagram of encoding in the run-length encoding unit 543 shown in FIG. The upper line in FIG. 11 is the data received from the quantization processing unit 542, and the lower line is the data after the encoding process in the run length encoding unit 543 is performed.

Here, as shown in the upper line of FIG. 11, from the quantization processing unit 542,
"06 02 02 02 01 01 01 01 01 04 05 00 ..."
It is assumed that the following data is input. At this time, in the run-length encoding unit 543 in FIG. 7, the leading “06” is not a compression target numerical value, and the next “02 02 02” is not a compression target numerical value. It is detected that four consecutive “01” s are present, and next, 32767 consecutive “00” s that are compression target values are inserted between “04” and “05” that are not compression target numerical values. Is done.

  FIG. 12 is a diagram illustrating an encoding algorithm for a numerical value to be compressed in the run-length encoding unit.

  In FIG. 12, Z is a continuous number of the same numerical values to be compressed, for example, Z = 4 for “01” in the upper line of FIG. 11, and Z = 32767 for “00”.

  In FIG. 12, “YY” represents the numerical value to be compressed itself represented by two hexadecimal digits. “0” or “1” following “YY” is “0” or “1” expressed by 1 bit, and “image compression apparatus ...” following the “YY” is one bit of “X”. This “image compression apparatus...” Expresses the value of Z.

  That is, FIG. 12 shows that when the compression target numerical value “YY” continues for Z <128, the compression target numerical value “YY” is expressed by the first byte, the first bit is “0”, and the subsequent byte is the subsequent one. Express the value of Z with 7 bits, and when the compression target numerical value “YY” continues Z ≧ 128, express the compression target numerical value “YY” with the first byte, followed by 2 bytes (16 bits) The first 1 bit of “1” is expressed as “1” to express that it is expressed over 2 bytes, and the subsequent 15 bits indicate that the value of Z is expressed.

  An example of the encoding shown in FIG. 11 will be described in accordance with the rules shown in FIG.

  Since the first numerical value “06” constituting the data (upper line) input from the quantization processing unit 542 in FIG. 7 is not a compression target numerical value, it is output as “06”. Also, “02 02 02” that follows, “02” is not a compression target numerical value, and these three “02” are output as they are. Next, since “01”, which is a numerical value to be compressed, continues, it is encoded into “01 04”. Since the next “04” and “05” are not compression target numerical values, “04 05” is output as it is.

  Next, since there are 32767 consecutive “00s”, “00” is placed, the first 1 bit of the next 1 byte is set to “1”, and then 32767-128 is expressed by 15 bits. It represents that 32767 “00” s are consecutive in 3 bytes of “00 FF 7F”. That is, the consecutive number 128 is expressed as “00 00” except for the first bit “1”.

FIG. 13 is a diagram illustrating an example of an encoding process according to the number of consecutive steps in the run-length encoding unit 543 in FIG.
When “00” is 127 consecutive, it is encoded into “00 7F” using 2 bytes,
When “00” is 32767 consecutive, it is encoded into “00 FF 7E” using 3 bytes,
When “00” is 32895 consecutive, it is encoded into “00 FF FF” using 3 bytes,
When “00” is 128 consecutive, it is encoded to “00 80 00” using 3 bytes,
-When 4096 "FFs" are contiguous, they are encoded into "FF 8F 80" using 3 bytes.

  The run-length encoding unit 543 shown in FIG. 7 performs the encoding process as described above.

  According to the run-length encoding unit 543 according to the present embodiment, the maximum compression rate is improved to 3/32895 = 1 / 10,965.

  The data after the above-described encoding process is performed by the run-length encoding unit 543 in FIG. 7 is then input to the Huffman encoding unit 544 in FIG.

  In the Huffman encoding unit 544, first, the appearance frequency of the pixel value is obtained for the pixels constituting the entire image.

  Here, the appearance frequency of “A1” is the strongest, and it is assumed that “A2”, “A3”, “A4”,. These “A1”, “A2” and the like do not directly represent numerical values, but are symbols representing numerical values. That is, “A1” is, for example, a numerical value “00”, “A2” is a numerical value “FF”, and the like. Here, for simplicity, the data sent from the run-length encoding unit 543 in FIG. 4 is expressed by any one of the 16 numerical values “A1” to “A16” for all pixels. Shall be.

  FIG. 14 is a diagram illustrating an encoding process in the Huffman encoder 544 illustrated in FIG.

  Here, “A1” having the highest appearance frequency is replaced with “00” represented by 2 bits, and the next “A2” is replaced with “01” also represented by 2 bits. "A3" and further "A4" are replaced by "100" and "101", respectively, and the following "A5" to "A8" are numerical values expressed by 5 bits. In the same manner, a numerical value with a lower appearance frequency is replaced with a numerical value represented by a larger number of bits.

  FIG. 15 is a diagram illustrating an example of the Huffman table.

  This Huffman table is the same as that in FIG. 14 and is arranged so that it is replaced with a numerical value represented by a shorter number of bits as the appearance frequency is higher, and the numerical value before encoding (before replacement) and after encoding ( It is a correspondence table with numerical values after replacement.

  The interface device 200 shown in FIG. 1 performs decoding for Huffman coding by referring to the Huffman table.

  The second image compression unit 540 performs the above-described image compression processing, and the compressed data itself obtained by the second image compression unit 540 or the Huffman coding unit is added to the header of the image data (B) shown in FIG. Information necessary for data decompression processing is written, such as the Huffman table (see FIG. 15) used in 544 (see FIG. 7) and the quantization coefficient used in the quantization processing unit 542 (see FIG. 7).

  In the header of the image data file (B) shown in FIG. 3, the quantization table (see FIG. 6) used in the JPEG compression processing in the first image compression unit 510 is also written. The header in which information necessary for data decompression processing is written and the JPEG compressed data obtained by the first image compression unit 510 are combined to form an image data file (B), and the interface device 200 shown in FIG. Sent to. The interface device 200 decompresses the JPEG compressed data while referring to the header of the transmitted image data file (B), and image data sufficiently approximating the image data in the original input image file (see FIG. 3) is obtained. Played.

  In the present embodiment, the image compression performed by the first image compression unit 510, the image decompression unit 520, the difference calculation unit 530, and the second image compression unit 540 in the image compression apparatus 500 illustrated in FIG. The process is executed only when the required compression rate is not satisfied as a result of the lossless compression process in the third image compression unit 550 of the image compression apparatus 500 shown in FIG. Then, next, the lossless compression process in the 3rd image compression part 550 is demonstrated.

  FIG. 16 is a block diagram illustrating a configuration of the third image compression unit.

  The third image compression unit 550 includes a differential encoding unit 551, a run length encoding unit 552, and a Huffman encoding unit 553.

  When the third image compression unit 550 shown in FIG. 16 is compared with the image compression unit 540 shown in FIG. 7, the third image compression unit 550 shown in FIG. 16 is different from the second image compression unit 540 shown in FIG. The configuration is the same as the configuration in which the quantization processing unit 542 is omitted and the post-difference data obtained by the differential encoding unit 541 is directly input to the run-length encoding unit 543. That is, the difference encoding unit 551, the run length encoding unit 552, and the Huffman encoding unit 553 of the third image compression unit 550 illustrated in FIG. 16 are the difference codes in the second image compression unit 540 illustrated in FIG. The same processes as those of the encoding unit 541, the run length encoding unit 543, and the Huffman encoding unit 544 are performed. Therefore, the duplicate description here is omitted.

  In the compression rate determination unit 560 of the image compression apparatus 500 shown in FIG. 3, the compressed data obtained for each 8 lines by the third image compression unit 550 having the configuration shown in FIG. If the requested compression rate is not satisfied, the first image compression unit 510, the image decompression unit 520, the difference calculation unit 530, the image data for the eight lines are described. And the compression process in the 2nd image compression part 540 is performed.

  For the 8 lines determined to satisfy the required compression rate by the compression rate determination unit 560, information necessary for decompressing the compressed data obtained by the third image compression unit 550, for example, FIG. An image data file (A) to which a header to which a Huffman table used in the Huffman encoder shown is added is generated and transmitted to the interface device 200 shown in FIG. The interface device decompresses the compressed data in the image data file (A) while referring to the header of the transmitted image data file (A), and the image data in the original input image file (see FIG. 3). The same image data is reproduced.

  Next, processing examples in the first image compression unit 510, the image decompression unit 520, the difference calculation unit 530, and the second image compression unit 540 illustrated in FIG. 3 will be described.

  FIG. 17 is a diagram illustrating an example of an 8 × 8 pixel image, and FIG. 18 is a diagram illustrating each pixel value of 8 × 8 pixels illustrated in FIG. 17.

  Here, white is FF in hexadecimal, black is 00 in hexadecimal, and a numerical value in the middle indicates gray corresponding to the numerical value.

  FIG. 19 shows pixel values of an 8 × 8 pixel image obtained by performing JPEG compression processing on the 8 × 8 pixel image having the pixel values of FIG. 18 and further decompressing the data after the JPEG compression processing. FIG.

  FIG. 20 is a diagram illustrating pixel values of a difference image obtained by performing a difference operation for each pixel corresponding to the image value of 8 × 8 pixels in FIG. 18 and the image value of 8 × 8 pixels in FIG. 19. FIG. 22 is a diagram illustrating quantized data obtained by dividing the pixel value of the difference image of 8 × 8 pixels in FIG. 20 by the quantization coefficient 8; FIG. 22 is a diagram illustrating the JPEG decompressed data illustrated in FIG. It is a figure which shows the image data obtained by correct | amending with quantization data. In this correction, the JPEG decompressed data shown in FIG. 19 and the quantized data shown in FIG. 2 are added for each corresponding pixel.

  However, in FIG. 20, 0 to 127 in decimal notation is represented by 00 to 7F in hexadecimal notation, and −1 to −127 in decimal notation is represented by FF to 81 in hexadecimal notation. Decimal numbers less than -127 and +128 or more are clipped to -127 and +127, respectively.

  FIG. 23 is a diagram showing an error distribution of the JPEG decompressed image of FIG. 19 with respect to the original image shown in FIG. 18, and FIG. 24 is an image obtained by correcting the JPEG decompressed image shown in FIG. 22 with the quantized data of FIG. It is a figure which shows the error distribution with respect to the original image shown in FIG.

  As is apparent from a comparison between FIG. 23 and FIG. 24, it can be seen that the decompressed image of the present embodiment has a greatly reduced error and improved image quality than the image simply subjected to JPEG compression and decompression.

  Here, FIG. 21 shows quantized data obtained by dividing by the quantization coefficient 8 as described above. In this case, a numerical value of three or more values is shown, but a larger value (for example, this quantization coefficient) If the quantization coefficient 16) is employed, quantized data that only shows two values can be obtained. If a large value is used as the quantization coefficient, an error from the original image increases. However, if a quantization coefficient that is expressed only in binary is used in the quantized data, the quantization processing unit 542 and the run-length encoding unit illustrated in FIG. By disposing a binary encoding processing unit described below between 543 and 543, the data compression rate of the second image compression unit 540 can be further improved.

  In other words, in the binary encoding processing unit, whatever value is included in the quantized data (for example, “01” and “FF” in hexadecimal numbers), One value is converted to “00” in hexadecimal, and the other value is converted to “80” in hexadecimal. Then, in this binary encoding processing unit, first, the same difference as the differential encoding processing shown in FIG. The encoding process is executed again.

  FIG. 25 is a diagram showing a differential encoding process for binary data consisting of 00 and 80.

Here, as shown in the “binary image data” column of FIG.
"00 80 80 00 00 80 00 00 80 ..."
Suppose that Similarly to the case of FIG. 9, here, each image value is expressed by two hexadecimal digits (1 byte = 8 bits). First, the pixel value “00” on the first line is output as it is.

  Next, the pixel value “00” of the first line is subtracted from the pixel value “80” of the second line, and the resultant value “80” is output.

  Next, the pixel value “80” of the second line is subtracted from the pixel value “80” of the third line, and the resulting value “00” is output.

  Next, the pixel value “80” of the third line is subtracted from the pixel value “00” of the fourth line, and the resultant value is output. Here, the result of subtracting “80” from “00” is a negative number and is expressed as “180” with 9 bits, but “1”, which is 1 bit of the MSB, is omitted, and the lower bit 8 is “ Only 80 "is output.

  Next, the pixel value “00” of the fourth line is subtracted from the pixel value “00” of the fifth line, and the result “00” is output.

Hereinafter, by repeating the same calculation, it is represented in the “difference encoding” column of FIG.
"(00) 80 00 80 00 80 80 00 80 ..."
Is output.

  In decoding the differentially encoded data, the interface device 200 shown in FIG. 1 performs the operation shown on the right side of FIG.

  This decoding operation is the same as the decoding operation described with reference to FIG. 9 except that the numerical values to be handled are binary values “00” and “80”, and redundant description is omitted here.

  As described above, in the binary encoding processing unit, in the case of binary data, any value (for example, “01” and “FF”) constituting the binary data is “ Although converted into “00” and “80”, the compression information transmitted from the host controller 100 shown in FIG. 1 to the interface device 200 has which value (for example, “01” and “FF”) is “00”, respectively. Information such as “80” (eg, “01” assigned to “00”, “FF” assigned to “80”, etc.) is included, and the interface device displays the “decoding result” in FIG. After decoding into binary data consisting of “00” and “80” shown in the column, those “00” and “80” are returned to the original values assigned to “00” and “80”. (For example, “00” Returned to 1 "," 80 "is returned to" FF ").

  FIG. 26 is an explanatory diagram of binary encoding processing.

  In this binary encoding processing unit, binary encoding processing is performed on binary image data composed of “00” and “80” after the differential encoding processing is performed on the binary data as described above. Is done.

As shown in the column of “difference-encoded image data” in FIG. 26, the image data differentially encoded as described above is as follows.
"00 00 00 00 00 80 80 80 ..."
Suppose that

Here, these two types of numerical values “00” and “80” are identified by one bit. Here, “00” is identified by 1 bit “0”, “80” is identified by 1 bit “1”, and is shown in the “differential encoded image data” column of FIG.
"00 00 00 00 00 80 80 80 ..."
As shown in the column of “binary encoded bitstream” in FIG.
"0 0 0 0 0 1 1 1 ..."
In addition, as shown in the “byte packed byte stream” column, the bit stream is grouped in units of 8 bits.
"07 C0 03 81 ..."
Converted to a numeric column. The converted numeric string is input to the run-length encoding unit 543 in FIG.

  In the case of binary data, the amount of data can be greatly reduced by performing the above binary encoding process.

1 is a diagram illustrating an example of a print system to which a data compression technique is applied. FIG. 6 is a diagram illustrating a flow of data processing in the print system. It is a block block diagram which shows one Embodiment of the image compression apparatus of this invention. It is a hardware block diagram of the host controller shown in FIG. It is a schematic block diagram of the image compression processing program of this invention. It is explanatory drawing which shows the JPEG compression process in the 1st image compression part of the image compression apparatus shown in FIG. It is a block diagram which shows the structure of the 2nd image compression part of the image compression apparatus shown in FIG. It is a figure which shows the data structure of the image data in the input image file input into the image compression apparatus of FIG. 3, and the concept of differential encoding. It is a figure which illustrates and illustrates the difference encoding process in the difference encoding part which comprises the image compression apparatus of FIG. It is operation | movement explanatory drawing by difference encoding. It is explanatory drawing of the encoding in the run length encoding part shown in FIG. It is a figure which shows the algorithm of the encoding for the numerical value for compression in a run length encoding part. It is a figure which shows the example of the encoding process according to the continuous number in the run length encoding part of FIG. It is the figure which illustrated the encoding process in the Huffman encoding part shown in FIG. It is a figure which shows an example of a Huffman table. It is a block diagram which shows the structure of a 3rd image compression part. It is a figure which shows an example of an image of 8x8 pixel. It is a figure which shows each pixel value of 8x8 pixel shown in FIG. FIG. 19 is a diagram illustrating pixel values of an 8 × 8 pixel image obtained by performing JPEG compression processing on the 8 × 8 pixel image having the pixel values of FIG. 18 and further decompressing the data after the JPEG compression processing. It is a figure which shows the pixel value of the difference image obtained by performing a difference calculation for every pixel corresponding to the image value of 8x8 pixel of FIG. 18, and the image value of 8x8 pixel of FIG. It is a figure which shows the quantization data after dividing each pixel value of the difference image of 8x8 pixel of FIG. 20 by the quantization coefficient 8. FIG. It is a figure which shows the image data obtained by correct | amending the JPEG decompression | decompression data shown in FIG. 19 with the quantization data shown in FIG. FIG. 20 is a diagram showing an error distribution of the JPEG decompressed image of FIG. 19 with respect to the original image shown in FIG. It is a figure which shows the error distribution with respect to the original image shown in FIG. 18 of the image which correct | amended the JPEG decompression | decompression image shown in FIG. 22 with the quantization data of FIG. It is a figure which shows the difference encoding process in the case of the binary data which consists of 00 and 80. It is explanatory drawing of the binary encoding process in a binary encoding process part.

Explanation of symbols

11 Image data 12A, 12B, 13A, 13B Bitmap data 14 Lossy compression data 15 Lossless compression data 100 Host controller 140 CDROM
DESCRIPTION OF SYMBOLS 150 General-purpose interface cable 200 Interface apparatus 250 Dedicated interface cable 300 Printer 500 Image compression apparatus 510 1st image compression part 520 Image decompression | decompression part 530 Difference calculation part 540 2nd image compression part 541 Difference encoding part 542 Quantization process part 543 Run-length encoding unit 544 Huffman encoding unit 550 Third image compression unit 551 Differential encoding unit 552 Run-length encoding unit 553 Huffman encoding unit 560 Compression rate determination unit 600 Image compression program 610 First image compression unit 620 Image decompression unit 630 Difference calculation unit 640 Second image compression unit 650 Third image compression unit 660 Compression rate determination unit

Claims (16)

In an image compression apparatus that performs data compression processing on image data,
A first image compression unit that generates first compressed data by performing irreversible compression processing on the image data;
An image decompression unit that generates decompressed data by decompressing the first compressed data obtained by the first image compression unit;
A difference calculation unit for obtaining difference data by calculating a difference between the image data before irreversible compression processing by the first image compression unit and the decompressed data obtained by the image decompression unit;
An image compression apparatus comprising: a second image compression unit that generates second compressed data by performing irreversible compression processing on the difference data obtained by the difference calculation unit.   2. The image compression apparatus according to claim 1, wherein the first image compression unit performs irreversible compression processing conforming to the JPEG standard on image data.   The image compression apparatus according to claim 1, wherein the second image compression unit performs a compression process including a quantization process on the difference data.   The second image compression unit outputs one or more predetermined compression target numerical values as they are in the compressed data, and the compression target numerical values are the same as the compression target numerical values and the compression target numerical values. 2. The image compression apparatus according to claim 1, wherein a compression process including an encoding process for encoding and outputting a numerical value representing a continuous number of numerical values to be compressed is performed.   The second image compression unit expresses, as the encoding process, when the number of consecutive identical numerical values to be compressed is equal to or less than a predetermined number, the continuous number is expressed by one unit bit number, and the continuous number is the predetermined number. 5. The image compression apparatus according to claim 4, wherein when it exceeds, the continuous number is expressed by 2 unit bits. A third image compression unit that generates third compressed data by performing reversible compression processing on the image data;
A compression rate determination unit that determines whether or not the compression rate of the third compressed data obtained by the third image compression unit is equal to or higher than a predetermined compression rate;
The first image compression unit, the image decompression unit, the difference calculation unit, and the second image compression unit are compressed by the compression rate determination unit so that the third compressed data does not reach a predetermined compression rate. The image compression apparatus according to claim 1, wherein the image compression apparatus operates upon receiving a determination result that the ratio is a rate.   The third image compression unit outputs one or more predetermined compression target numerical values in the compressed data as they are, and the compression target numerical values are the same as the compression target numerical values and the compression target numerical values. 7. The image compression apparatus according to claim 6, wherein a reversible compression process including an encoding process for encoding and outputting a numerical value representing a continuous number of numerical values to be compressed is performed.   The third image compression unit represents, as the encoding process, when the number of consecutive identical numerical values to be compressed is equal to or less than a predetermined number, the continuous number is represented by one unit bit number. 8. The image compression apparatus according to claim 7, wherein when it exceeds, the continuous number is expressed by 2 unit bits. An image compression program that is executed in an information processing apparatus that executes a program and causes the information processing apparatus to operate as an image compression apparatus that performs data compression processing on image data,
The information processing apparatus;
A first image compression unit that generates first compressed data by performing irreversible compression processing on the image data;
An image decompression unit that generates decompressed data by decompressing the first compressed data obtained by the first image compression unit;
A difference calculation unit for obtaining difference data by calculating a difference between the image data before irreversible compression processing by the first image compression unit and the decompressed data obtained by the image decompression unit;
Image compression characterized by operating as an image compression apparatus including a second image compression unit that generates second compressed data by performing lossy compression processing on the difference data obtained by the difference calculation unit program.   10. The image compression program according to claim 9, wherein the first image compression unit performs irreversible compression processing conforming to the JPEG standard on image data.   10. The image compression program according to claim 9, wherein the second image compression unit performs a compression process including a quantization process on the difference data.   The second image compression unit outputs one or more predetermined compression target numerical values as they are in the compressed data, and the compression target numerical values are the same as the compression target numerical values and the compression target numerical values. 12. The image compression program according to claim 11, wherein a compression process including an encoding process for encoding and outputting a numerical value representing a continuous number of numerical values to be compressed is performed.   The second image compression unit expresses, as the encoding process, when the number of consecutive identical numerical values to be compressed is equal to or less than a predetermined number, the continuous number is expressed by one unit bit number, and the continuous number is the predetermined number. 13. The image compression program according to claim 12, wherein when the number exceeds, the continuous number is expressed by 2 unit bits. The information processing apparatus;
A third image compression unit that generates third compressed data by performing reversible compression processing on the image data;
A compression rate determination unit that determines whether or not the compression rate of the third compressed data obtained by the third image compression unit is equal to or higher than a predetermined compression rate;
The first image compression unit, the image decompression unit, the difference calculation unit, and the second image compression unit are compressed by the compression rate determination unit so that the third compressed data does not reach a predetermined compression rate. The image compression program according to claim 9, wherein the image compression program is operated as an image compression apparatus that operates upon receiving a determination result that the ratio is a rate.   The third image compression unit outputs one or more predetermined compression target numerical values in the compressed data as they are, and the compression target numerical values are the same as the compression target numerical values and the compression target numerical values. 15. The image compression program according to claim 14, wherein a lossless compression process including an encoding process for encoding and outputting a numerical value representing a continuous number of numerical values to be compressed is performed.   The third image compression unit represents, as the encoding process, when the number of consecutive identical numerical values to be compressed is equal to or less than a predetermined number, the continuous number is represented by one unit bit number. 16. The image image compression program according to claim 15, wherein when it exceeds, the continuous number is expressed by 2 unit bit number.

Images