JP4978934B2 - Image compression apparatus and program - Google Patents

Description

  The present invention relates to an image compression apparatus and a program for compressing image data.

In recent years, with the development of digital devices, various compression methods for image data and the like have been proposed, and JPEG (Joint Photographic Experts Group) 2000 is known as one of the compression methods. JPEG2000 defines an image compression / decompression method, and is an extension of JPEG. JPEG 2000 divides image data into a plurality of tiles and generates encoded data by performing processing such as wavelet transform, quantization, and entropy encoding for each tile, as described in Patent Document 1.
Japanese Patent Laying-Open No. 2005-167841

  In the case of such JPEG 2000, even if an arbitrary compression rate is set, there is a case where the compression is actually performed at a higher compression rate (small code amount) than the set compression rate. For example, even if a compression rate of 20% is set, a certain tile may be compressed by 18%, and another tile may be compressed by 17%. In this case, the code amount of 2%, which is the difference between 20% and 18%, is the code amount that was not used effectively. For example, if the code amount of 2% is used in other tiles, the image quality is remarkably high. In some cases, it may be improved.

  The present invention has been made in order to solve the above-described problem, and by effectively using a code amount that has not been used, while maintaining the code amount of compressed data obtained when compression is performed at a set compression rate. An object of the present invention is to provide an image compression apparatus and program for improving image quality.

  An image compression apparatus according to a first aspect of the present invention includes a dividing unit that divides an image into a plurality of tiles, and a plurality of compression rates within a predetermined range including a set compression rate. A compression unit that performs compression; a code amount calculation unit that calculates a code amount of each tile compressed at the plurality of compression rates; and an influence degree on an image quality of each tile compressed at the plurality of compression rates. Influence degree calculating means, total code amount calculating means for calculating the total code amount of each tile compressed at the set compression rate, and compression obtained when the image is compressed at the set compression rate A target code amount calculating unit that calculates a target code amount that is a code amount of data; a code amount difference calculating unit that calculates a code amount difference obtained by subtracting the total code amount from the target code amount; and If it is greater than the set number, Compression rate adjusting means for updating, as the set compression rate, a compression rate that is one step lower than the set compression rate from the tile having the highest influence degree among the tiles compressed at the set compression rate. It is characterized by.

  According to a fifth aspect of the present invention, there is provided a program according to a fifth aspect of the present invention, wherein each of the computers is divided into a plurality of compression ratios in a predetermined range including a dividing unit that divides an image into a plurality of tiles and a set compression ratio. Compression means for compressing tiles, code amount calculation means for calculating the code amount of each tile compressed at the plurality of compression rates, and the degree of influence on image quality of each tile compressed at the plurality of compression rates Influence degree calculating means, total code amount calculating means for calculating the total code amount of each tile compressed at the set compression rate, compression data obtained when the image is compressed at the set compression rate Target code amount calculating means for calculating a target code amount that is a code amount; code amount difference calculating means for calculating a code amount difference obtained by subtracting the total code amount from the target code amount; and a predetermined number of the code amount difference If greater than It functions as a compression rate adjustment unit that updates a compression rate that is one step lower than the set compression rate as the set compression rate from the tile that has the highest degree of influence among the tiles compressed at the set compression rate. It is characterized by that.

  According to these configurations, the image data is set from the total code amount obtained by compressing each tile with a plurality of compression rates including the set compression rate and integrating the code amounts of the tiles compressed at the set compression rate. The difference in code amount minus the target code amount, which is the theoretical code amount of the compressed data obtained when compression is performed at the compression rate, is used to reduce the compression rate of the high-impact tile to one lower compression rate. Thus, it is possible to improve the image quality while suppressing the total code amount to the target code amount.

  The invention according to claim 2 is the image compression apparatus according to claim 1, wherein the compression rate adjusting means is a tile having the highest influence degree from tiles compressed at the set compression rate. And detecting the difference between the code amount when the detected tile is compressed at the set compression rate and the code amount when compressed at a compression rate one step lower than the set compression rate, and the difference is When the code amount difference is equal to or less than the code amount difference, the compression rate that is one step lower than the set compression rate is updated as the set compression rate, and the difference is subtracted from the code amount difference. Repeating the process of updating the set compression rate and subtracting the difference from the code amount difference until the code amount difference is equal to or less than the predetermined number, or a predetermined number of times. Control means, It is characterized in that was example.

  According to this configuration, it is possible to improve the image quality while suppressing the total code amount of the compressed data to the target code amount.

  The invention according to claim 3 is the image compression apparatus according to claim 1 or 2, further comprising sorting means for sorting the tiles arranged in the main scanning direction as one band, and the total code The amount calculation unit, the target code amount calculation unit, the code amount difference calculation unit, and the compression rate adjustment unit perform each process for each band.

  According to this configuration, by performing the code amount difference distribution processing in units of bands, it is possible to suppress an increase in memory capacity necessary for processing work.

  According to a fourth aspect of the present invention, there is provided the image compression apparatus according to the third aspect, wherein the compression rate adjusting unit repeatedly performs the process up to a predetermined number of times under the control of the repetitive control unit. When the code amount difference is larger than the predetermined number, the compression rate adjusting means adds the code amount difference to the target code amount when adjusting the compression rate for tiles of different bands. It is a feature.

  According to this configuration, it is possible to improve the image quality by effectively using the surplus code amount by using the surplus code amount in another band by using the code amount difference distribution process of a certain band.

  According to this invention, each tile is compressed at a plurality of compression ratios including a set compression ratio, and image data is set from the total code amount obtained by integrating the code amounts of tiles compressed at the set compression rate. The difference in code amount minus the target code amount, which is the theoretical code amount of the compressed data obtained when compression is performed at a rate, is used to reduce the compression rate of tiles with high impact to one lower compression rate. Thus, it is possible to improve the image quality while suppressing the total code amount to the target code amount.

  Embodiments of an image compression apparatus and a program according to the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an electrical configuration of an image compression apparatus 1 according to the present embodiment. The image compression apparatus 1 is realized by an information processing apparatus such as a personal computer (personal computer), a workstation, or a portable information terminal. The block diagram of FIG. 1 shows a case where the image compression apparatus 1 is realized by these information processing apparatuses. Is shown as an example. In addition, it may be used by being incorporated in an image reading apparatus such as a copying machine or a scanner, or an imaging apparatus such as a digital camera, a digital video camera, or a mobile phone equipped with a digital camera.

  As shown in FIG. 1, the image compression apparatus 1 includes a control unit 11, a storage unit 12, an input operation unit 13, a display unit 14, an I / F unit 15, a network I / F unit 16, an image compression unit 17, a RAM 18, and the like. It is prepared for. The control unit 11 is configured by a CPU (Central Processing Unit) or the like, reads a program stored in the storage unit 12 in accordance with an input instruction signal or the like, executes a process, and sends the processing to each functional unit. The image compression apparatus 1 is comprehensively controlled by outputting an instruction signal, transferring data, and the like.

  The storage unit 12 stores programs, data, and the like for realizing various functions provided in the image compression apparatus 1. In the present embodiment, the storage unit 12 stores an image compression program 121. The image compression program 121 is a program for compressing the image data input by the I / F unit 15 or the network I / F unit 16 using the JPEG2000 method to generate encoded data.

  The input operation unit 13 includes various operation buttons and a pointing device such as a mouse, and outputs an operation signal to the control unit 11 when operated by a user. The user also sets the compression rate via the input operation unit 13. The display unit 14 is a display screen such as a liquid crystal display, and displays according to the content input from the input operation unit 13 or displays the processing content and processing result by the control unit 11.

  The I / F unit 15 is an interface such as IEEE1394 or USB, and can directly transmit / receive data to / from an external device. The network I / F unit 16 includes a communication module such as a LAN board, and transmits / receives various data to / from an external device via a network (not shown) connected to the network I / F unit 16.

  The image compression unit 17 executes the image compression program 121 stored in the storage unit 12 to compress the image data input via the I / F unit 15 or the network I / F unit 16 using the JPEG 2000 method. Generate encoded data. A RAM (Random Access Memory) 18 temporarily stores a control program to be executed by the control unit 11, data for executing processing, and the like. In addition, the image compression unit 17 temporarily stores encoded data, compression rate adjustment data, and the like generated when processing is performed according to the image compression program 121.

  FIG. 2 is a diagram showing a flow of image compression according to the JPEG2000 system in the present embodiment. First, the image compression unit 17 performs component conversion of the color system of the input image data from, for example, the RGB system to the YCbCr system (color space conversion 21), and the image data obtained by converting the color system is shown in FIG. Is divided into a plurality of tiles (tile division 22). Here, the image compression unit 17 classifies tiles arranged in the main scanning direction as one band, and performs processing from the discrete wavelet transform 23 to the compression rate adjustment 26 (or code stream 27) for each band. Referring to FIG. 3, tiles A to E, tiles F to K, tiles L to Q, and tiles R to V are each divided into one band, and the processing of the discrete wavelet transform 23 and the subsequent processes is performed for each band. Therefore, it is assumed that all the processes described below are performed on a band basis.

  Returning to FIG. 2, the image compression unit 17 performs discrete wavelet transform on each tile of one band and outputs coefficient data (wavelet transform 23). FIG. 4 is a schematic diagram showing the discrete wavelet transform. Discrete wavelet transform is applied to the tile image (0LL) in FIG. 4A, and is decomposed into subbands 1LL, 1HL, 1LH, and 1HH as shown in FIG. 4B. Subsequently, a discrete wavelet transform is performed on the low frequency component 1LL, and it is decomposed into subbands 2LL, 2HL, 2LH and 2HH as shown in FIG. Although FIG. 4 illustrates two-level conversion, the number of discrete wavelet conversions is not particularly limited.

  Next, the image compression unit 17 performs linear quantization on the coefficient data subjected to discrete wavelet transform (quantization 24). Here, the image compression unit 17 performs quantization according to a plurality of compression rates within a predetermined range including a compression rate arbitrarily set (hereinafter referred to as “set compression rate”). For example, when the set compression ratio is 20%, quantization is performed according to five compression ratios such as 18%, 19%, 20%, 21%, and 22%. Note that the number of compression ratios is not limited to this, but it is desirable to appropriately adjust the number of compression ratios according to the capacity of a memory (RAM 18 in the present embodiment) used for image compression processing.

  Then, the image compression unit 17 performs entropy coding on the quantized data quantized at each compression rate (entropy coding 25). Here, the processing procedure of the entropy encoding 25 will be described. FIG. 5 is a diagram showing a flow of entropy encoding. The image compression unit 17 first divides each subband coefficient into regions called precincts (precinct division 251). FIG. 6 is a diagram schematically showing precinct division. In FIG. 6, a portion 51 is obtained by frequency-converting a portion of the same region in the original image, and these portions belong to the same precinct.

  Next, the image compression unit 17 divides the precinct into smaller areas called code blocks (code block division 252). FIG. 7 is a diagram schematically showing code block division. This code block unit is a basic unit when entropy encoding is performed.

  Then, the image compression unit 17 expands the transform coefficient of the discrete wavelet transform linearly quantized for each code block for each bit (bit plane division 253). FIG. 8 is a diagram schematically showing bit plane division. In FIG. 8, 61 is an example of coefficient data of the linearly quantized discrete wavelet transform in a certain code block. 62 is a bit string representing the sign of the coefficient data 61, where value 0 means a positive value and value 1 means a negative value. 63 is a bit plane in which the absolute value of the coefficient data 61 is binary-developed from MSB (Most Significant Bit) to LSB (Least Significant Bit).

  For example, since the coefficient data 64 having a value of +12 is a positive value, the corresponding sign bit 65 is “0”. Also, since the binary representation of the absolute value of +12 is (1100), the values of the locations 66a, 66b, 66c and 66d corresponding to the bit plane are “1”, “1”, “0”, “0”, respectively. It becomes. Similarly, since the coefficient data 67 having a value of −6 is a negative value, the corresponding sign bit 68 is “1”. Since the binary representation of the absolute value of −6 is (0110), the values of the corresponding portions 69a, 69b, 69c, and 69d of the bit plane are “0”, “1”, “1”, “0”, respectively. 'Become. Bit planes that are all zero on the MSB side are called zero bit planes, and no data is stored, while the number of zero bit planes described later is counted for each code block.

  Subsequently, the image compression unit 17 further divides the bit plane into a significance propagation pass, a magnitude refinement pass, and a cleanup pass (division 254 into coding passes). FIG. 9 is a diagram schematically showing division into coding passes. As shown in FIG. 9, the bit planes 71a to 71d (hereinafter collectively referred to as “bit plane 71”) are divided as follows by division into coding paths. That is, significance propagation paths 72b to 72d (hereinafter collectively referred to as “significance propagation path 72”), magnitude refinement paths 73b to 73d (hereinafter collectively referred to as “magnitude refinement path 73”), and cleanup path 74b. Divided into ~ 74d. However, the most significant bit (MSB side) bit plane 71a is made to correspond only to the cleanup path 74a. Hereinafter, the cleanup paths 74a to 74d are collectively referred to as “cleanup path 74”.

  Each bit plane 71 and each coding pass 72 to 74 are all equal in size according to the coordinate length in the vertical and horizontal directions. Each coding pass 72 to 74 has a position where a bit value is defined and a position where it is not defined. In FIG. 9, for example, 76 and 77, the positions where bit values are defined are shaded (hatched). The bit values defined in the shaded portions of the coding paths 72 to 74 (for example, the coding paths 72b to 74b) are the bit values at the corresponding positions on the bit plane 71 (for example, the bit plane 71b) before the division. equal. Since the process of dividing the bit plane into coding passes is well known in each document, description thereof is omitted.

  Finally, the image compression unit 17 arithmetically encodes the data after the coding pass division (binary arithmetic encoding 255). After performing the entropy encoding 25 as described above, the image compression unit 17 adjusts the compression rate (compression rate adjustment 26). JPEG2000 can arbitrarily set the compression rate, but when image data is compressed at the set compression rate, conventionally, compression is performed at a compression rate higher than the set compression rate. For example, when the set compression ratio is 20%, a certain tile may be compressed by 18%, and another tile may be compressed by 17%. In this case, the code amount of 2%, which is the difference between 20% and 18%, is the code amount that was not used effectively. For example, if the code amount of 2% is used in other tiles, the image quality is remarkably high. In some cases, it may be improved. Therefore, in this compression rate adjustment 26, the compression rate of each tile is adjusted, and processing for allocating the unused code amount to other tiles is performed. Details of the compression rate adjustment procedure will be described later.

  After the compression rate adjustment is completed, the image compression unit 17 forms a code stream for writing data to the file (code stream 27). Since this code stream processing is well known from each document, the description thereof is omitted.

  10 and 11 are flowcharts showing the flow of the image compression processing executed by the image compression unit 17 in accordance with the image compression program 121. First, the image compression unit 17 performs color space conversion and tile division of the image data, and divides the divided tiles by band (step S11). Then, the image compression unit 17 substitutes 1 for a variable n (n is an integer equal to or greater than 1) (step S12), and performs discrete wavelet transform, quantization, and entropy encoding on the nth band (step S13). . Here, the image compression unit 17 performs quantization at a plurality of compression rates within a predetermined range including a set compression rate, and stores the encoded data of each tile compressed at the plurality of compression rates in the RAM 18.

  Next, the image compression unit 17 calculates the code amount of each tile and the degree of influence on the image quality (hereinafter simply referred to as “influence degree”) to create compression rate adjustment data, and this compression rate adjustment data is stored in the RAM 18. (Steps S14 and S15). FIG. 12 is a diagram showing an example of compression rate adjustment data. As shown in FIG. 12, the image compression unit 17 calculates a code amount and an influence degree for each compression rate of each tile.

  An example of how to calculate the influence level will be described. For example, assume that division into coding passes is performed as shown in FIG. The image processing unit 17 sets a higher weighting coefficient for the upper bits for the MSB to LSB bit planes. For example, weighting factors of 8, 4, 2, 1 are set for the MSB to LSB bit planes, and predetermined weighting factors are set for the significance propagation path, the magnitude refinement path, and the cleanup path, respectively. Then, the image processing unit 17 calculates and adds a numerical value obtained by multiplying the number of coding path '1's, the bit plane weighting coefficient, and the coding path weighting coefficient for each bit position, and the degree of influence in one code block. Is calculated. This influence degree is calculated over all the code blocks, and the influence degree of the tile is obtained by integrating the calculated influence degrees.

That is, with reference to FIG. 9, the image compression unit 17 determines the degree of influence of the code block 61 as follows.
Influence = (number of “1” s in cleanup path 74a × weighting coefficient (8) × weighting coefficient of cleanup path)
+ {(Number of '1's of the significance propagation path 72b) × weighting coefficient (4) × weighting coefficient of the significance propagation path 72b) + (number of'1's of the magnitude refinement path 73b) × weighting coefficient (4) × magnitude refinement path (Weighting coefficient) + (number of “1” in cleanup path 74b × weighting coefficient (4) × cleanup path weighting coefficient)}
+ {(Number of '1's of the significance propagation path 72c) × weighting coefficient (2) × weighting coefficient of the significance propagation path 72c) + (number of'1's of the magnitude refinement path 73c) × weighting coefficient (2) × magnitude refinement path (Weighting coefficient) + (number of “1” s in cleanup path 74c × weighting coefficient (2) × weighting coefficient of cleanup path)}}
+ {(Number of '1's of the significance propagation path 72d) × weighting coefficient (1) × weighting coefficient of the significance propagation path 73d) + (number of'1's of the magnitude refinement path 73d) × weighting coefficient (1) × magnitude refinement path (Weighting coefficient) + (number of “1” s in cleanup path 74d × weighting coefficient (1) × weighting coefficient of cleanup path)}}
This is calculated using the following formula. This calculation is performed over all code blocks.

  Returning to FIG. Next, the image compression unit 17 calculates the total code amount by integrating the code amounts of the tiles compressed at the set compression rate (step S16). For example, when the set compression rate is 20%, “12432” obtained by integrating the code amounts of the tiles A to E shown in FIG. Subsequently, the image compression unit 17 calculates the theoretical code amount of the compressed data obtained when the image data is compressed at the set compression rate, and sets the code amount divided by the number of bands as a target code amount (step). S17).

  Further, the image compression unit 17 calculates a code amount difference that is a value obtained by subtracting the total code amount from the target code amount (step S18). For example, when the obtained total code amount is “12432” and the target code amount is 15000, the code amount difference is “2568”. This “2568” is a code amount that has not been used effectively despite being a code amount that may be used as compressed data. Accordingly, in the following processing, the code amount that has not been used effectively is sequentially allocated from the tiles having a high degree of influence.

  First, the image compression unit 17 sets the set compression rate to a temporary compression rate. At this stage, a compression rate of 20% is set as a temporary compression rate. Then, 1 is substituted into the variable m (step S19). This variable m indicates the number of repetitions of the code amount difference allocation process.

  When the code amount difference is 1 or more (step S20; NO), the image compression unit 17 detects the tile having the highest influence from the tiles compressed at the temporary compression rate in the band. In FIG. 12, the tile having the highest influence is the tile D among the tiles compressed at the temporary compression rate (20%). Then, the image compression unit 17 sets the flag of the detected tile (for example, tile D) to “1” (step S21). This flag is set for each tile and is stored in a work memory (not shown) included in the image processing unit 17.

And the image compression part 17 is about the detected tile.
a = code amount when compressed at a temporary compression rate b = code amount when compressed at a compression rate one lower than the temporary compression rate, and substituting the code amounts corresponding to the variables a and b (step S22) ). For example, when tile D is detected in step 21, a = 1293 and b = 2723.

  Subsequently, the image compression unit 17 calculates the code amount by subtracting the code amount when compressed at the temporary compression rate from the code amount when compressed at the compression rate one lower than the temporary compression rate. It is determined whether the difference is equal to or less than the difference (step S23). That is, when a = 1293 and b = 2723, b−a = 1430, and this value is smaller than the code amount difference “2568” obtained in step S18. Therefore, the image compression unit 17 sets a compression rate one lower than the temporary compression rate in the tile D as the temporary compression rate. That is, a compression rate of 21% is set as a temporary compression rate. Then, a value obtained by subtracting the value b−a from the code amount difference is updated as a code amount difference (step S24). Accordingly, the code amount difference = 2568− (2723−1293) = 1138.

  Then, the image compression unit 17 resets the flag and adds 1 to the variable m (step S25). If the variable m is not 64 (step S28; NO), the image compression unit 17 proceeds to step S20. As described above, when the processes of steps S20 and S21 are performed, the tile detected as the tile having the highest influence among the tiles compressed at the temporary compression rate in FIG. Since a = 645 and b = 1024 (step S22), and b−a = 379, the sign amount difference is smaller than 1138 (step S23; YES). Therefore, the image compression unit 17 sets the temporary compression ratio of the tile A to 21% and updates the code amount difference to 1138-379 = 759 (step S24).

  Then, the image compression unit 17 resets the flag and adds 1 to the variable m (step S25). Since the variable m is not 64 (step S28; NO), the image processing unit 17 shifts the process to step S20 again. Here, the tile detected as the tile having the highest influence level among the tiles compressed at the temporary compression rate in step S21 is the tile A. Then, a = 1024, b = 2612 (step S22), and b−a = 1588. The value of b−a is larger than the code amount difference 759 (step S23; NO). That is, if the temporary compression rate of the tile A is lowered by one here, the total code amount exceeds the target code amount. Therefore, a tile having the highest influence level is detected from tiles other than the tile A (that is, from tiles with no flag set).

  That is, when there is a tile with no flag set (step S26; YES), the image compression unit 17 detects the tile having the highest influence from the tiles with no flag set, and sets the flag for the tile (step S27). . In the case of FIG. 12, the tile having the highest influence is the tile E among the tiles excluding the tile A compressed at the temporary compression rate. Then, the image compression unit 17 proceeds to step S22. In the case of the tile E, a = 567, b = 1045, and b−a = 478, which is smaller than the code amount difference 756 (step S23; YES). Therefore, the image compression unit 17 updates the temporary compression rate of the tile E to 21% and the code amount difference to 756-478 = 278.

  If there is no tile with no flag set in step S26 (step S26; NO), it means that the target code amount will be exceeded if the compression rate is lowered by 1 for all tiles compressed at the temporary compression rate. To do. Accordingly, the image compressing unit 17 finishes distributing the code amount difference to each tile, and proceeds to step S29.

  The above processing is repeated until the code amount becomes 0 or less or 64 times (until the variable m becomes 64). The condition for the number of repetitions is not limited to the above, and can be changed as appropriate. When the code amount becomes 0 or less (step S20; YES) and the variable m becomes 64 (step S28; YES), the image compression unit 17 officially compresses the temporary compression rate at the current stage of each tile. The rate is set (step S29).

  If the nth band is not the last band (step S30; NO), the image compression unit 17 adds 1 to the variable n (step S31), and the process proceeds to step S13. Here, for the band for which the code amount difference allocation processing has already been completed, the encoded data and compression rate adjustment data of each tile compressed at a compression rate other than the official compression rate are deleted from the RAM 18, thereby allowing the work in the RAM 18 to be performed. The area can be expanded. At this stage, if the code amount difference is larger than 0 (or a predetermined number), the code amount difference is added to the next target code amount calculated in step S17. In this way, by using the remaining code amount in the distribution process of a certain band in other bands, the remaining code amount can be used effectively to improve the image quality.

  If the n-th band is the last band (step S30; YES), the image compression unit 17 forms a code stream (step S32) and ends the process.

  As described above, when each tile is compressed at a plurality of compression rates including the set compression rate, and the image data is compressed at the set compression rate from the total code amount obtained by integrating the code amounts of tiles compressed at the set compression rate By assigning the code amount difference obtained by subtracting the target code amount, which is the theoretical code amount of the compressed data obtained to the above, to tiles with high impact that meet the conditions, the total code amount is suppressed to the target code amount and the image quality is reduced. Can be improved. Furthermore, by performing such code amount difference distribution processing in units of bands, it is possible to suppress an increase in memory capacity required for processing operations.

  The present invention is not limited to the configuration of the above embodiment, and various modifications can be made. For example, in the above embodiment, the divided tiles are divided into bands, and the code amount difference distribution process is performed for each band. This is because it is necessary to generate encoded data compressed at a plurality of compression rates in each tile, and to minimize the use area of the RAM 18 for storing these encoded data. However, if the RAM 18 has a sufficient memory capacity, it is not necessary to perform a code amount difference distribution process for each band. That is, the image compression unit 17 detects the tile having the highest influence level from all the tiles compressed at the set compression rate, and updates the temporary compression rate to the next lower compression rate when the condition is met. This process is performed over the entire tile until the code amount difference is equal to or smaller than a predetermined number, or a predetermined number of times.

1 is a block diagram showing an electrical configuration of an image compression apparatus. The figure which shows the flow of the image compression by a JPEG2000 system. The figure for demonstrating tile division | segmentation. The schematic diagram shown about discrete wavelet transform. The figure which shows the flow of entropy encoding. The figure which showed the precinct division | segmentation typically. The figure which showed code block division | segmentation typically. The figure which showed bit-plane division | segmentation typically. The figure which showed the division | segmentation to a coding pass typically. The flowchart which showed the flow of the image compression process. FIG. 11 is a flowchart continued from FIG. 10. The figure which showed an example of the data for compression rate adjustment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image compression apparatus 11 Control part 12 Storage part 121 Image compression program 13 Input operation part 14 Display part 15 I / F part 16 Network I / F part 17 Image compression part 18 RAM

Claims (5)

A dividing means for dividing the image into a plurality of tiles;
Compression means for compressing each tile at a plurality of compression ratios in a predetermined range including a compression ratio set by a user;
Code amount calculation means for calculating the code amount of each tile compressed at the plurality of compression rates;
An influence degree calculating means for calculating an influence degree on the image quality of each tile compressed at the plurality of compression ratios;
A total code amount calculating means for calculating a total code amount of each tile compressed at the set compression rate;
Target code amount calculating means for calculating a target code amount that is a code amount of compressed data obtained when the image is compressed at the set compression rate;
Code amount difference calculating means for calculating a code amount difference obtained by subtracting the total code amount from the target code amount;
If the code amount difference is larger than a predetermined number, the compression rate that is one step lower than the set compression rate is set from the tile having the highest influence level among the tiles compressed at the set compression rate. A compression rate adjusting means for updating the compressed compression rate;
An image compression apparatus comprising: The compression rate adjusting means detects a tile having the highest influence level from tiles compressed at the set compression rate, and the code amount when the detected tile is compressed at the set compression rate and the setting A difference from the code amount when compressed at a compression rate that is one step lower than the set compression rate is obtained, and when the difference is equal to or less than the code amount difference, a compression rate that is one step lower than the set compression rate is set. Update the compression rate and subtract the difference from the code amount difference.
The compression rate adjustment means updates the set compression rate and subtracts the difference from the code amount difference until the code amount difference is equal to or less than the predetermined number, or a predetermined number of times. Repetitive control means for performing repetitive control;
The image compression apparatus according to claim 1, further comprising: Further comprising sorting means for sorting the tiles arranged in the main scanning direction as one band;
3. The image compression according to claim 1, wherein the total code amount calculating unit, the target code amount calculating unit, the code amount difference calculating unit, and the compression rate adjusting unit perform each process for each band. apparatus.   When the code amount difference after the compression rate adjusting unit repeatedly performs the process up to the predetermined number of times by the control by the repetition control unit is larger than the predetermined number, the compression rate adjusting unit is different in band. The image compression apparatus according to claim 3, wherein the code amount difference is added to the target code amount when the compression rate adjustment is performed for each tile. Computer
A dividing means for dividing an image into a plurality of tiles;
Compression means for compressing each tile at a plurality of compression rates within a predetermined range including a set compression rate;
Code amount calculation means for calculating the code amount of each tile compressed at the plurality of compression rates;
An influence degree calculating means for calculating an influence degree on the image quality of each tile compressed at the plurality of compression ratios;
A total code amount calculating means for calculating the total code amount of each tile compressed at the set compression rate;
Target code amount calculating means for calculating a target code amount that is a code amount of compressed data obtained when the image is compressed at the set compression rate;
Code amount difference calculating means for calculating a code amount difference obtained by subtracting the total code amount from the target code amount;
If the code amount difference is larger than a predetermined number, the compression rate that is one step lower than the set compression rate is set from the tile having the highest influence level among the tiles compressed at the set compression rate. Compression rate adjusting means for updating as a compressed compression rate,
Program to function as.

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