JP2003153011A - Apparatus and method of determining compression parameters of image data compression process, storage medium and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem, for recompressing once expanded compressed image data for obtaining recompressed image data, the data are recompressed, using compressing parameters independent of compressing parameters used for the compression of the image data, so that the quantity of the recompressed image data differing from that of the compressed data by a larger amount. SOLUTION: A means is provided for determining compression parameters for recompression processing, based on compression parameters used for compressing original compressed image data and transform functions, thereby reducing the difference in the quantity of the recompressed image data and the quantity of the original image data to be set below a specified reference value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compression parameter determination method for image data compression. More specifically, the present invention relates to a compression parameter determination method for image data compression processing in which compressed compressed image data is decompressed and recompressed to obtain recompressed image data.

[0002]

2. Description of the Related Art At present, compression processing of image data such as black and white image data, gray scale image data and full color image data is widely used in digital devices such as computers, facsimiles and digital cameras. There are various formats for compression processing of image data, for example, J
PEG format, LZW format, MH format, MR format or MM
R form etc. are mentioned.

FIG. 1 shows a process chart of image data compression processing. The image data 10 is compressed by a pre-compression process, a compression process and a post-compression process. In the pre-compression processing, the number of pixels of the image data 10, the resolution, etc. are analyzed and the color space is converted. Next, the compression process is performed by one or more compression steps. In each compression step, compression is performed using compression parameters. Examples of the compression parameter include a subsampling factor and a quantization table in the JPEG format compression process. In the post-compression process, the image attributes and the compression parameters used in the compression process are stored in the header or the table, and the compressed image data 10 is combined to form the compressed image data 12.

As shown in FIG. 2, compressed image data 22
Is often used when transmitting / transferring the image data 10 through a telecommunication line 14 such as a telephone line.
The compressed image data 22 compressed by the first digital device 16 is transmitted to the second digital device 18 through the telecommunication line 14. In addition, the second digital device 18
The compressed image data 22 received in step 3 may be transferred to the third digital device 20 through the telecommunication line 14. When the compressed image data 22 transmitted from the first digital device 16 is in a compression format that cannot be used by the third digital device 20, the second digital device 18
In, the compressed image data 22 is decompressed and recompressed to recompressed image data 24 that is converted into a different compression format or different color space from the compressed image data 22 usable in the third digital device 20.

FIG. 3 shows the first digital device 16 to the second digital device 16.
To the digital device 18 and the second digital device 1
8 is a process diagram of a compression process performed by transfer from 8 to the third digital device 20. The first digital device 16 compresses the compressed image data 22 according to the compression format 1 including the compression process A and the compression process B. In the compression process A and the compression process B, compression is performed using the compression parameter A and the compression parameter B, respectively. The number of compression steps and the number of compression parameters at this time differ depending on the compression format. Then, from the first digital device 16 to the second digital device 1
The compressed image data 22 is transmitted to the server 8. The second digital device 18 expands the compressed image data 22 by the expansion processing of the compression format 1. Third digital device 20
In the above, if only the compression format 2 different from the compression format 1 can be used, the second digital device 18 recompresses the image data 24 with the compression format 2 including the compression steps a and b different from the compression steps A and B. Re-compression processing is performed. In the compression steps a and b, compression parameters A and B
The compression process is performed using recompression parameters a and b different from Subsequently, the recompressed image data 24 is transferred from the second digital device 18 to the third digital device 20, and the third digital device 20 expands the recompressed image data 24 by decompressing the compression format 2. Extend.

For example, in FIG. 2, when the first digital device 16 and the second digital device 18 are facsimile terminals and the third digital device 20 is a personal computer, the first digital device 16 and the second digital device 16 are the same. Transmission between the digital devices 18 is normally performed with compressed image data in the JPEG format (hereinafter, referred to as Lab-JPEG format) in the Lab color space. Second digital device 18 and third
Between the digital devices 20 of (1) and (2), compressed image data of JPEG format (hereinafter, referred to as YCbCr-JPEG format) in the YCbCr color space is normally transmitted. for that reason,
When transferring the compressed image data 22 transmitted from the first digital device 16 to the second digital device 18 from the second digital device 18 to the third digital device 20, the second digital device 18 The original compressed image data in the Lab-JPEG format is decompressed, and the recompressed image data 24 recompressed in the YCbCr-JPEG format is transferred.

[0007]

However, in the conventional compression processing, the compression processing in the first digital device 16 and the recompression processing in the second digital device 18 are performed before the compression processing. Since the fixed compression parameter defined in step 1 is used, the compressed image data 22 compressed by the first digital device 16 and the second digital device 18 are used.
The original compressed image data may be decompressed with and the recompressed image data 24 may have a significantly different data amount.

Particularly, in the lossy compression format such as the JPEG format, for example, the information of the frequency component is lost in the process of division by the quantization table.
When compression is performed using a specific compression parameter, information is truncated according to the parameter. Therefore, once the compressed image data 22 is obtained by performing the compression processing with high compression, even if the compressed image data 22 is expanded, it is not possible to obtain a high image quality like the original image. However, when a fixed compression parameter is used for the recompression process and the parameter is a parameter for compressing with high image quality, the recompressed image data 24 unnecessarily increases the data amount.

For example, as shown in FIG. 4, the image data of 8 × 8 pixels, which is one minimum conversion element (MCU) 26, is 4: 1: 1 in the Lab-JPEG format of the first digital device 16. The YCbCr-JPEG of the second digital device 18 is subjected to compression processing using a fixed sub-sampling factor (hereinafter referred to as SS factor) 28.
When the compression process is performed using the fixed SS factor 30 of 2: 1: 1 in the format, the data amount of the sub-sampling result 32 of the Lab-JPEG format is different from that of the sub-sampling result 34 of the YCbCr-JPEG format. (In this case, the amount of data before quantization and encoding is doubled). Further, as shown in FIG.
The CU 36 is the Lab-JPE of the first digital device 16.
In the G format, division processing is performed using the fixed L component quantization table 38, and YCbCr-J of the second digital device 18 is performed.
When division processing is performed using the Y component quantization table 40 that is fixed even in the PEG format, the L component quantized data 42 and the Y component quantized data 44 have different original images due to color conversion and different quantization table coefficients. Will generally give different results. As compared with the L component quantization table 38, if the Y component quantization table 40 is used with a small value of each element of the quantization table aiming at compression with high image quality, as a result, the Y component quantization table is obtained. It can be seen that each element of the digitized data 44 tends to be large. Therefore, when these are finally subjected to Huffman encoding or the like, the resulting data amount increases and the compression rate deteriorates.
Similarly, quantization is performed on the Cb component and Cr component with respect to the a component and the b component, and the data compression rate for each MCU may differ. MCU26 of the whole image
However, since the sub-sampling process and the quantization process are repeated, the data amount of the final compressed image data by the Lab-JPEG compression and the compressed image data by the YCbCr-JPEG compression significantly increase. In some cases, it will happen.

On the contrary, when the compressed image data 22 is compressed by the compression parameter of high image quality, it is considered not preferable to increase the compression rate more than necessary and to deteriorate the image quality of the original image. However, the compressed image data 2
If a fixed compression parameter is used when decompressing 2 and performing recompression processing, if the compression parameter is for high compression, it becomes difficult to maintain sufficient image quality in exchange for reduction of the data amount. There are also problems.

For example, when the compressed image data 22 obtained by compressing the image data having a sharp edge such as character information in a lossy compression format such as JPEG is transmitted, the compressed image data 22 is decompressed and further non-compressed. When the recompressed image data 24 is obtained by recompressing in the lossless compression format, even if the recompressed image data 24 is expanded, the image quality sufficient for discriminating the image may not be reproduced in some cases.

As described above, in the conventional image data recompression processing, the data amount of the recompressed image data 24 becomes unnecessarily large, and the image quality of the original image data cannot be sufficiently reproduced. Furthermore, when a compression parameter for high image quality is unnecessarily used for the recompression process, there is a problem that the transmission time at the time of transfer becomes longer as the data amount of the recompressed image data 24 increases.

Although it is possible to use the compression parameter of the original compressed image as it is as the recompression parameter, when the compressed image is decompressed and subjected to processing such as color conversion and recompressed, the image data will be different, so Therefore, it is impossible to obtain the same amount of compressed image data.

For example, when color conversion is performed from the Lab color space to the RGB color space, it is generally not appropriate to perform sub-sampling on specific RGB components, so that the compressed image in the original Lab color space is used. When ab is sub-sampled, the amount of data after compression tends to increase with the same compression parameter.

[0015]

According to a first aspect of the present invention, there is provided a compression parameter determining method for recompressing decompressed image data obtained by decompressing compressed image data compressed using a predetermined compression parameter into recompressed image data. A compression parameter determination method for determining a recompression parameter used for compression, wherein the recompression parameter is determined based on the compression parameter.

The compression parameter determination method of the second invention is
A decompressed image data obtained by decompressing compressed image data compressed using a predetermined compression parameter, a compression parameter determination method for determining a recompression parameter used to recompress to recompressed image data, Based on the compression parameter, a step of obtaining a conversion function that makes the recompressed image data satisfy a predetermined criterion, and a step of determining the recompression parameter using the compression parameter and the conversion function are included.

The compression parameter determining method of the third invention is
Image data of a predetermined image type is compressed to decompress compressed image data compressed with a predetermined compression parameter, and decompressed image data is used to determine recompression parameters used to recompress recompressed image data. A method for determining a parameter, the method comprising: determining a conversion function that makes the recompression parameter satisfy a predetermined criterion based on the compression parameter and the image type; and re-determining using the compression parameter and the conversion function. And a step of determining a compression parameter.

The compression parameter determination method of the fourth invention is
In the second or third invention, the reference includes that a difference in data amount between the original compressed image data and the recompressed image data is within a predetermined reference range.

The compression parameter determination method of the fifth invention is
In any one of the first to fourth inventions, the recompression parameter includes at least one of a subsampling factor and a quantization table.

The compression parameter determination device of the sixth invention is
A decompression image data obtained by decompressing compressed image data compressed using a predetermined compression parameter, a compression parameter determination device for determining a recompression parameter used to recompress to recompressed image data, It has a parameter determination means for determining the recompression parameter based on the compression parameter.

The compression parameter determination device of the seventh invention is
A decompression image data obtained by decompressing compressed image data compressed using a predetermined compression parameter, a compression parameter determination device for determining a recompression parameter used to recompress to recompressed image data, The recompressed image data includes a conversion function acquisition unit that obtains a conversion function that satisfies a predetermined criterion, and a parameter determination unit that determines the recompression parameter using the compression parameter and the conversion function.

The compression parameter determination device of the eighth invention is
Image data of a predetermined image type is compressed to decompress compressed image data compressed with a predetermined compression parameter, and decompressed image data is used to determine recompression parameters used to recompress recompressed image data. A parameter determining device, comprising: a conversion function acquisition unit that obtains a conversion function that makes the recompression parameter satisfy a predetermined criterion based on the compression parameter and the image type; and the compression parameter and the conversion function. And a parameter determining means for determining a recompression parameter by using the parameter.

The compression parameter determination device of the ninth invention is
In the seventh or eighth invention, the reference includes that a difference in data amount between the original compressed image data and the recompressed image data is within a predetermined reference range.

A compression parameter determination device of a tenth aspect of the invention is the compression parameter determination device of any of the sixth to ninth aspects, wherein the recompression parameter includes at least one of a subsampling factor and a quantization table.

An eleventh invention is a recompression used for recompressing decompressed image data obtained by decompressing compressed image data compressed using a predetermined compression parameter into recompressed image data by a computer. A computer-readable storage medium that stores a program for determining the recompression parameter based on the compression parameter when determining the parameter.

The twelfth invention is a recompression used for recompressing decompressed image data obtained by decompressing compressed image data compressed using a predetermined compression parameter into recompressed image data by a computer. It is a program for determining the recompression parameter based on the compression parameter when determining the parameter.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1. In this embodiment of the present invention, the difference in the data amount of the compressed image data between the compressed image data 22 compressed by the first digital device 16 and the recompressed image data 24 recompressed by the second digital device 18 is A method of determining the recompression parameter used in the recompression process for setting the value to a predetermined reference value or less will be described.

FIG. 6 shows a method of acquiring the conversion coefficient of the compression parameter according to this embodiment of the present invention. For simplification of description, two compression parameters are shown for the compression processing of the compression format 1 of the first digital device 16 and the compression format 2 of the second digital device 18, respectively. Here, two compression parameters are used, but the compression parameters may be changed according to the compression format.

The standard image data 46 is compressed by the compression steps (A and B) of the compression format 1 using A and B of the compression parameter (table) 48. Compression format 1
The compressed image data 22 obtained in step 1 is decompressed, and the compression format 2 is obtained using a and b of the recompression parameter (table) 50.
The recompression process is performed by the compression steps a and b. At this time, the recompression parameters a and b are calculated by calculating the compression parameters A and B and the conversion coefficients α and β using the functions f and g of the formulas (1) and (2).

[0030]

[Equation 1]

Compressed image data 22 obtained in compression format 1
And the data amounts of the recompressed image data 24 obtained in the compression format 2 are compared, and the conversion coefficients α and β are determined so that the difference becomes equal to or smaller than a predetermined reference value.

FIG. 7 shows a step diagram of the method of acquiring the conversion coefficient of the compression parameter according to the present embodiment. In ST1, the standard image data 46 is compressed using the compression parameters A and B according to the compression format 1. In ST2, α and β
Is initialized. In ST3, the compression parameters A and B used in the compression format 1 of ST1 and α and β initialized in ST2 are calculated using predetermined functions f and g, and the recompression parameters a and B used in compression format 2 are calculated. Calculate b.
In ST4, the compressed image data compressed in ST1 is expanded in the format 1. In ST5, the decompressed image data is recompressed by the compression format 2 using the recompression parameters a and b of the compression format 2 calculated in ST2.
In ST6, the data amounts of the compressed image data compressed by the compression format 1 and the compression format 2 are compared, and if the difference is less than a predetermined reference value, α and β at that time are determined as conversion coefficients. . If the difference in the amount of data is more than the standard value,
In ST7, α and β are changed, and the process is repeated from ST3.

FIG. 8 shows an example of a conversion coefficient table for recompressing parameters in the second digital device 18. The table of FIG. 8 indicates that various conversion coefficients for calculating the recompression parameters a to z used in the compression format of the second digital device 18 may be transmitted from the first digital device 16. The calculation result for the standard image data 46 corresponding to the compression format, the color, and the content of the color conversion processing performed by the second digital device 18 is shown.

With respect to the compressed image data 22 transmitted in each compression format, the compression parameter used for compression into the compressed image data 22 and the corresponding conversion coefficient are calculated by the predetermined functions f and g. As a result, it is possible to obtain the recompression parameter of the recompression process in which the compressed image data 22 and the recompressed image data 24 have the same data amount.

Further, each step of the compression parameter determination method can be processed as a program by using a computer. Further, each step of the compression parameter determination method can be stored as a program in a computer-readable storage medium, and the program can be read from the storage medium to the computer and processed.

Specific example of the present embodiment. The compressed image data 22 compressed in the Lab-JPEG format is received from the first digital device 16, and YCb is received by the second digital device 18.
Recompressed image data 24 recompressed in Cr-JPEG format
In the case of transferring, a concrete example will be shown of a method of determining the SS factor and the quantization table for keeping the data amounts of the compressed image data 22 and the recompressed image data 24 equal.

Of the compression parameters, the SS factor is the LabSS factor of the original image to the YCbCr of the recompressed image.
Obtain the compression parameters using the table to convert to SS factor. In this example, Lab to YCbCr
It is considered that it is appropriate to match the sub-sampling information of the L component and the Y component which are the lightness / luminance components and the ab component and the CbCr component which are the color difference components because the conversion is to the Lab component. Use the table to be used. Further, the quantization table for the Y component and the quantization table for the CbCr component are represented by the conversion functions of the mathematical expressions (3) and (4).

[0038]

[Equation 2]

FIG. 9 shows the process of acquiring the conversion coefficient for the reference image data 46. First, standard image data 46 in the Lab color space is prepared. The image data converted into the Lab color space is compressed using the SS factor and the quantization table for the L component, the a component, and the b component used in the Lab-JPEG format, and the compressed image data 22 is obtained. Next, using the Lab-JPEG format, the compressed image data 22
To stretch. The expanded image data is converted to YCbCr
Convert to color space. Here, the SS factors and the quantization tables for the L component, the a component, and the b component and α and β initialized are calculated using predetermined functions f and g, and Y is calculated.
The SS factor and the quantization table for the Y component, the Cb component, and the Cb component used in the CbCr-JPEG format are calculated. The YCbCr-JPE of the second digital device 18 is applied to the YCbCr color space converted image data.
Y component, Cb component and C used for recompression in G format
Recompression processing is performed using the SS factor and the quantization table for the b component to obtain recompressed image data 24.
Compressed image data 22 obtained in Lab-JPEG format
And the data amount of the recompressed image data 24 obtained in the YCbCr-JPEG format are compared, and if the difference is less than or equal to a predetermined reference value, α and β at that time are determined as conversion coefficients. If the difference is greater than or equal to the predetermined reference value, the values of α and β are changed and the process is performed again.

FIG. 10 shows a table of conversion coefficients obtained in this example. As shown in the table, by preparing conversion coefficients for various standard image data 46 and conversion formats, the compressed image data obtained by the Lab-JPEG compression processing of the first digital device 16 and the 2 digital devices 1
It is possible to determine the compression parameter that makes the difference in the data amount of the recompressed image data obtained by the compression processing of YCbCr-JPEG format of No. 8 below a predetermined reference value. At the time of actual transmission / transfer, the conversion coefficient for the compressed image data 22 transmitted in each compression format is obtained from the table, and the compression parameter used for compression to the compressed image data 22 and its conversion coefficient are determined by a predetermined function f and By calculating with g,
It is possible to obtain the recompression parameter of the recompression process in which the compressed image data 22 and the recompressed image data 24 have the same data amount.

Embodiment 2. In the present embodiment, a case will be described in which the compression parameter used for recompression is changed depending on the image type transmitted together with the compressed image data. Here, the image type means an index indicating that the original image before the compressed image data is compressed is a character or a photograph.

For example, in the case where an index indicating that the image type of the original image is a photograph is transmitted together with the compressed image data, the image quality of the image can be improved even when the compression rate is increased during the recompression in the JPEG format. Since the deterioration is not noticeable, the compression parameter can be set to increase the compression rate.

Specifically, instead of ST6 in the step diagram of FIG. 7, for compressed image data compressed in format 1,
A conversion coefficient satisfying the criterion that the data amount of the image data compressed in the format 2 is in a range smaller by a predetermined ratio is obtained. As a result, it is possible to obtain an image type conversion coefficient that can increase the compression rate. FIG. 11 shows an example of a conversion coefficient table obtained in the second specific example.

By thus obtaining the conversion coefficient which differs depending on the image type, the compression parameter can be determined so that the data amount is positively reduced in the case of the image type in which the deterioration of the image quality is not noticeable.

Further, by applying this, the compression parameter is controlled not only by the original type but also by utilizing the information contained in the compressed data of the original image such as the image size, resolution and compression rate. It can also be extended to

Embodiment 3. FIG. 12 shows a block diagram of a compression parameter determination device according to the present embodiment of the present invention.

The compression processing function 52 in the compression format 1 compresses the standard image data 46 using the compression parameters A and B in the compression format 1. Conversion coefficient setting function 5
At 6, α and β are initialized. In the recompression parameter determination function, the compression parameters A and B used in the compression processing function 52 in the compression format 1 and α and β initially set in the recompression parameter determination function 58 are determined using predetermined functions f and g. Recompression parameter a calculated and used in compression format 2
And b are calculated. With the decompression processing function 54 in format 1,
The compressed image data compressed by the compression processing function 52 is expanded. The compression processing function 60 in the compression format 2 uses the calculated recompression parameters a and b to expand the decompression processing function 54.
The image data expanded by is recompressed. The comparison / determination function 62 compares the data amounts of the compressed image data compressed in each of the compression format 1 and the compression format 2, and if the difference is equal to or larger than a predetermined reference value, feeds the feedback to the conversion coefficient setting function 56. The multiplication α and β are changed, and the above operation is repeated. If the difference in data amount is equal to or larger than the reference value, the discriminator 64 outputs the recompression parameters a and b obtained by the recompression parameter determination function 58.

As described above, the original compressed image data and the re-compressed image data are equivalent to each other by calculating the compression parameter used for compression to the compressed image data and the corresponding conversion coefficient by a predetermined function. The recompression parameter of the recompression process as the quantity can be obtained.

[0049]

According to the compression parameter determining method of the present invention, when the compressed original compressed image data is expanded and recompressed to obtain the recompressed image data, the recompressed data has the same data amount as the original compressed image data. The compression parameters for obtaining the image data can be determined.

Further, the compression parameter used for the recompression process can be determined according to the image type of the original image data. Further, by applying this, it is possible to control the compression parameter by utilizing not only the original type but also the information included in the compressed data of the original image such as the image size, the resolution, and the compression rate. It can be extended.

[Brief description of drawings]

FIG. 1 is a diagram showing a process of image data compression processing.

FIG. 2 is a diagram illustrating transmission / transfer of compressed image data.

FIG. 3 is a diagram showing steps of compression / expansion processing in transmission and recompression / expansion processing in transfer.

FIG. 4 is a diagram showing a conventional sub-sampling process of Lab-JPEG format for transmission and YCbCr-JPEG format for transfer.

FIG. 5 is a diagram showing quantization processing in a Lab-JPEG format for transmission and a YCbCr-JPEG format for transfer in the related art.

FIG. 6 is a diagram showing a method of acquiring a conversion function according to the first embodiment of the present invention.

FIG. 7 is a step diagram of a method of acquiring transform coefficients according to the first embodiment of the present invention.

FIG. 8 is a diagram showing an example of a conversion coefficient table obtained in the first embodiment of the present invention.

FIG. 9 is a diagram showing a method of acquiring a conversion coefficient in the specific example of the first embodiment.

FIG. 10 is a diagram showing a table of conversion coefficients obtained in the specific example of the first embodiment.

FIG. 11 is a diagram showing a table of conversion coefficients obtained in the second embodiment of the present invention.

FIG. 12 is a configuration diagram of a compression parameter determination device according to a third embodiment of the present invention.

[Explanation of symbols]

10 image data, 12 compressed image data, 14 telecommunication line, 16 first digital device, 18 second digital device, 20 third digital device, 22 original compressed image data, 24 recompressed image data, 26 minimum conversion Element (MCU), 28 Lab-JPEG format subsampling factor, 30 YCbCr-JPEG format subsampling factor, 32 Lab-JPEG format subsampling result, 34 YCbCr-JPE
G format sub-sampling result, 36 1 block of image data (RGB), 38 L component quantization table, 4
0 Y component quantization table, 42 L component quantized data, 44 Y component quantized data, 46 standard image data, 48 compression format 1 compression parameter, 50 compression format 2 compression parameter, 52 compression format 1 compression process Function, 54 expansion processing function, 56 conversion coefficient setting function,
58 recompression parameter determination function, 60 compression processing function in compression format 2, 62 comparison / judgment function, 64 discriminator.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toruharu Tsuruma             3-2-1 Sakado, Takatsu-ku, Kawasaki City, Kanagawa Prefecture               KSP R & D Business Park Building             Within Fuji Xerox Co., Ltd. (72) Inventor Minoru Yusa             3-2-1 Sakado, Takatsu-ku, Kawasaki City, Kanagawa Prefecture               KSP R & D Business Park Building             Within Fuji Xerox Co., Ltd. (72) Inventor Takanori Sato             3-2-1 Sakado, Takatsu-ku, Kawasaki City, Kanagawa Prefecture               KSP R & D Business Park Building             Within Fuji Xerox Co., Ltd. F-term (reference) 5C057 AA11 DA06 EA06 ED08 EM00                       EM16 FB03                 5C059 KK41 LB05 MA00 MC14 PP01                       PP16 PP20 SS20 TA47 TB04                       TC24 TC25 TC38 TD05 UA39                 5C078 AA09 BA53 CA22 CA27 DA01                       DA06

Claims (12)

[Claims] 1. Decompressed image data obtained by decompressing compressed image data compressed using a predetermined compression parameter,
A compression parameter determination method for determining a recompression parameter used for recompression into recompressed image data, wherein the recompression parameter is determined based on the compression parameter. 2. Decompressed image data obtained by decompressing compressed image data compressed using a predetermined compression parameter,
A compression parameter determining method for determining a recompression parameter used for recompressing to recompressed image data, wherein a conversion function that makes the recompressed image data satisfy a predetermined criterion based on the compression parameter. A method of determining a compression parameter, comprising: a step of obtaining and a step of determining a recompression parameter using the compression parameter and the conversion function. 3. Recompression used for recompressing decompressed image data obtained by decompressing compressed image data compressed by a predetermined compression parameter into image data of a predetermined image type. A compression parameter determination method for determining a parameter, the method comprising: determining, based on the compression parameter and the image type, a conversion function that makes the recompression parameter satisfy a predetermined criterion; and the compression parameter and the conversion function. And a step of determining a recompression parameter by using the compression parameter determination method. 4. The compression parameter determination method according to claim 2, wherein the reference includes that a difference in data amount between the compressed image data and the recompressed image data is within a predetermined reference range. The method of determining compression parameters. 5. The compression parameter determination method according to claim 1, wherein the recompression parameter is a subsampling factor,
A compression parameter determination method comprising any one or more of quantization tables. 6. Decompressed image data obtained by decompressing compressed image data compressed using a predetermined compression parameter,
A compression parameter determination device for determining a recompression parameter used for recompressing to recompressed image data, comprising a parameter determination means for determining the recompression parameter based on the compression parameter. Compression parameter determination device. 7. Decompressed image data obtained by decompressing compressed image data compressed using a predetermined compression parameter,
A compression parameter determination device for determining a recompression parameter used to recompress to recompressed image data, wherein the recompressed image data obtains a conversion function that satisfies a predetermined criterion, A compression parameter determination device comprising: a parameter determination unit that determines a recompression parameter using the compression parameter and the conversion function. 8. Recompression in which image data of a predetermined image type is used to recompress decompressed image data obtained by decompressing compressed image data compressed by a predetermined compression parameter into recompressed image data. A compression parameter determination device for determining a parameter, wherein, based on the compression parameter and the image type, a conversion function acquisition unit that obtains a conversion function such that the recompression parameter satisfies a predetermined criterion, and the compression parameter. And a parameter determining unit that determines a recompression parameter using the conversion function. 9. The compression parameter determination device according to claim 7, wherein the reference includes that a difference in data amount between the original compressed image data and the recompressed image data is within a predetermined reference range. Compression parameter determination device. 10. The compression parameter determination device according to claim 6, wherein the recompression parameter is a subsampling factor,
A compression parameter determination device comprising any one or more of quantization tables. 11. A recompression parameter used for recompressing decompressed image data obtained by decompressing compressed image data compressed using a predetermined compression parameter into recompressed image data by a computer. In this case, a computer-readable storage medium storing a program for determining the recompression parameter based on the compression parameter. 12. A recompression parameter used for recompressing decompressed image data obtained by decompressing compressed image data compressed using a predetermined compression parameter into recompressed image data by a computer. In that case, the program which determines the said recompression parameter based on the said compression parameter.