JP3984813B2 - Image processing apparatus and method, computer program, and storage medium - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image processing apparatus that compresses and encodes color image data, a control method thereof, a computer program, and a computer-readable storage medium.
[0002]
[Prior art]
Conventionally, as a still image compression method, a JPEG method using discrete cosine transform and a method using Wavelet transform are often used. Since this type of encoding method is a variable length encoding method, the amount of code changes for each image to be encoded.
[0003]
In the JPEG method, which is an international standardization method, only one set of quantization matrices can be defined for an image. Accordingly, the code amount cannot be adjusted without pre-scanning, and there is a risk of memory over when used in a system that stores data in a limited memory.
[0004]
In order to prevent this, in order to adjust the code amount by changing the compression rate and re-reading the original when the code amount exceeds the planned code amount, or by estimating the code amount by pre-scanning in advance , The method of resetting the quantization parameter was taken.
[0005]
As a code amount control method for performing pre-scanning, for example, there is a method in which pre-compressed data is stored in an internal buffer memory, decompressed, changed in compression parameters, subjected to main compression, and output to an external storage. At this time, in the main compression, the compression rate is set higher than in the pre-compression.
[0006]
Further, for example, in order to obtain an allowable code amount for each pixel block and reduce the code amount, a method of performing Huffman coding on a coefficient obtained by level-shifting a DCT coefficient n times is known, and this shift amount n is an allowable code amount. Determined from.
[0007]
[Problems to be solved by the invention]
However, conventionally, a compression buffer larger than the target compression is required as the compression buffer, and in order to prevent an overflow of the buffer used intermediately, it is inevitable that the capacity is sufficient to record the original image data.
[0008]
Furthermore, in the method of repeating the encoding process, there is a problem that the speed of continuous processing is not increased because a process of decoding and recompressing all compressed data is performed.
[0009]
The present invention has been made in view of the above-described conventional example, and it is possible to generate encoded data that fits in an effectively set size by one image input, and in particular, multi-value color image data. An image processing apparatus, a control method thereof, a computer program, and a storage medium capable of minimizing deterioration in quality after compression are provided.
[0010]
[Means for Solving the Problems]
In order to solve this problem, for example, an image processing apparatus of the present invention comprises the following arrangement. That is,
Enter a luminance component and a chrominance component representing the color image, the luminance component is compressed using a first quantization step, a first compression means to compress using the second quantization step of the color difference component When,
Decodes the encoded data generated by said first compression hands stage, a second compression hands stage capable of re-compression,
Monitoring the encoded data amount generated by the first compression hands stage, the encoded data amount and a code amount monitoring means for determining whether reaches a predetermined amount,
At the first time when the code amount monitoring means determines that the predetermined amount has been reached, the second quantization step applied to the first compression means has a higher compression rate than the second quantization step. After changing to the third quantization step, the subsequent color difference component is compressed by the first compression means,
The second compression unit uses the third quantization step having a compression rate higher than that of the second quantization step for the encoded data of the color difference component previously generated by the first compression unit. To recompress
The encoded data of the color difference component obtained by the recompression is stored as encoded data of the color difference component generated by the first compression means after changing to the third quantization step,
The encoded data of the color difference component generated by the first compression unit after updating to the third quantization step is stored as subsequent encoded data,
At the second time when it is determined that the predetermined amount has been reached by the code amount monitoring means, the first quantization step applied to the first compression means has a compression rate higher than that of the first quantization step. After changing to a high fourth quantization step, the subsequently inputted luminance component is compressed by the first compression means,
The encoded data of the luminance component previously generated by the first compression means is converted into the second compression means by using the fourth quantization step having a compression rate higher than that of the first quantization step. To recompress
The encoded data of the luminance component obtained by the recompression is stored as encoded data of the luminance component generated by the first compression means after changing to the fourth quantization step,
The encoded data of the luminance component generated by the first compression unit after updating to the fourth quantization step is stored as subsequent encoded data .
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments according to the present invention will be described with reference to the accompanying drawings. First, basic portions will be described.
[0012]
FIG. 1 is a functional block configuration diagram of an image processing apparatus 100 to which the embodiment is applied. Hereafter, each part of the same figure is demonstrated easily.
[0013]
The image processing apparatus 100 includes an input unit 101 that inputs an image from an image scanner. The input unit 101 may input image data from page description language rendering or the like, or may be realized by reading an image file stored in a storage medium. Anyway.
[0014]
The encoding unit 102 encodes input image data. The encoding method uses a known JPEG encoding method, orthogonally transforms image data corresponding to 8 × 8 pixel units, and performs quantization and Huffman encoding processing using a quantization step described later. .
[0015]
The first memory control unit 103 and the second memory control unit 105 transmit the encoded data (the same encoded data) output from the encoding unit 102 to the first memory 104 and the second memory, respectively. Control is performed so that the data is stored in 106. Here, the first memory 104 is a network device or an image output device for connecting the encoded data finally determined (compressed to the data amount within the target value) outside the basic configuration of FIG. And a memory for holding the encoded data for output to a mass storage device or the like. The second memory 106 is a working memory that assists in the compression encoding process for forming the encoded data on the first memory.
[0016]
The counter 107 counts the data amount of the image data compressed and encoded by the encoding unit 102, holds the count value, and outputs the count result to the encoding sequence control unit 108 that controls the encoding sequence. To do.
[0017]
The encoding sequence control unit 108 detects whether or not the count value of the counter 107 has reached a certain set value. When it is detected that the set value has been reached (exceeded the target value), it has been stored in the memory 104. A control signal is output to the first memory control unit 103 so as to discard the data. The first memory control unit 103 discards the stored data by clearing the memory address counter or clearing the encoded data management table based on this control signal. At this time, the encoding sequence control unit 108 clears the first counter 107 to zero (the input from the input unit 101 continues), and the encoding unit 102 has a higher compression rate than before. To control encoding. That is, control is performed so that the amount of encoded data generated in the encoding process of this apparatus is finally reduced to, for example, ½. In addition, although it set to 1/2 here, it cannot be overemphasized that it can set arbitrarily.
[0018]
The encoded data after the compression rate change is also stored in the first memory 104 and the second memory 106 via the first memory control unit 103 and the second memory control unit 105, respectively, as before. The
[0019]
Furthermore, the encoding sequence control unit 108 reads out the encoded data stored in the second memory 106 so far to the second memory control unit 105 and re-encoding unit 109 serving as encoded data conversion means. A control signal is output to output the encoded data.
[0020]
The re-encoding unit 109 decodes the input encoded data, performs re-quantization to reduce the amount of data, etc., and then performs the encoding process again, and is the same as the encoding unit 102 whose compression rate has been changed. The data amount of the compression rate is output to the second counter 110.
[0021]
The encoded data output from the re-encoding unit 109 is stored in the first memory 104 and the second memory 106 via the first memory control unit 103 and the second memory control unit 105, respectively. Is done.
[0022]
The second memory control unit detects whether or not the re-encoding process has been completed. That is, when there is no more data to be read for the re-encoding process, the encoding sequence control unit 108 is notified of the end of the re-encoding process. Actually, not only the reading process of the second memory control unit 105 but also the process of the re-encoding unit 109 is completed, the encoding process is completed.
[0023]
The count value obtained by the second counter 110 is added to the counter value held by the first counter 107 after the re-encoding process is completed. This addition result represents the total amount of data in the first memory 104 immediately after the re-encoding process is completed. That is, at the time when the encoding process of the encoding unit 102 and the re-encoding unit 109 for one screen is completed, the counter value held in the first counter 107 after the addition is equivalent to one screen. Represents the total amount of data generated when is encoded (details will be described later).
[0024]
The encoding unit 102 continues the encoding process as long as image data from the input unit 101 to be encoded remains, regardless of whether the re-encoding process is completed or not.
[0025]
Whether or not the count value of the counter 107 has reached a certain set value is repeated until the encoding process (encoding and re-encoding) of one page of image data input from the input unit 101 is completed. The re-encoding process is executed under control according to the detection result obtained here.
[0026]
Also, a switching signal output from the code sequence control unit 108 is supplied to the selector 111, and the selector 111 sends the encoding symmetric signal to the re-encoding unit 109 or returns to the memory control units 103 and 105 as it is. Switch.
[0027]
FIG. 8 is a flowchart showing the process flow in the configuration shown in FIG.
[0028]
As described above, the image processing apparatus 100 of the present invention is an apparatus that compresses and encodes one page of image data input from the input unit 101 such as a scanner to a predetermined data amount or less. In order to realize the encoding process, in addition to the input unit 101, an encoding unit 102, a re-encoding unit 109, a first memory 104, a second memory 106, and the like are provided. Using these functional blocks, encoding processing is performed based on the flowchart shown in FIG.
[0029]
The flowchart of FIG. 3 is roughly divided into the following three processing phases.
(1) Encoding phase (2) Encoding / re-encoding phase (3) Transfer phase In each of the above processing phases, how image data, encoded data, etc. are processed and processed in the memory FIG. 4 to FIG. 7 show whether the data is stored visually.
[0030]
FIG. 4 shows an initial state of the encoding phase corresponding to steps S303 and S305 in the flowchart of FIG. 5 shows the processing state of the encoding / recoding phase corresponding to steps S307 to S315, FIG. 6 shows the processing state of the transfer phase corresponding to step S317, and FIG. 7 shows the encoding phase after the transfer phase. Indicates processing status. Hereinafter, each phase will be described.
[0031]
<< Encoding Phase >>
Encoding processing of image data for one page starts from initial setting of encoding parameters (step S301). Here, the upper limit value of the encoded data amount uniquely determined from the image size to be encoded (paper size read from the input unit 101 such as a scanner) or the encoding unit 102 (here, a known JPEG encoding method is used). Parameter such as a quantization step (Q1) to be applied.
[0032]
In step S303, the first counter 107 performs actual encoding processing (JPEG compression in units of 8 × 8 pixels of the image), and cumulatively counts the amount of encoded data to be output.
[0033]
Next, in step S305, it is detected whether the count value of the data amount has exceeded the upper limit value. If not, the JPEG encoding process in step S303 is continued. This is the initial encoding phase.
[0034]
The encoded data output from the encoding unit 102 is stored in both the first memory 104 and the second memory 106 as shown in FIG. A region indicated by vertical stripes represents the stored code.
[0035]
<< Encoding / Recoding Phase >>
When the encoding process of the encoding unit 102 proceeds and the count value of the data amount exceeds the set upper limit value, in step S307, the encoded data in the first memory 104 is discarded and the step In step S309, the quantization step of the encoding unit 102 is changed to Q2.
[0036]
That the count value of the data amount of the encoded data exceeds the set upper limit value means that the data amount after compression does not fall within the target value. Therefore, since it is meaningless to continue the encoding process using the same quantization step, it is changed to the quantization step Q2 having a larger quantization step width than Q1 so that the data amount is smaller than before. is there.
[0037]
After changing the quantization step, in step S311, the encoding process of the encoding unit 102 is restarted, and the encoded data is stored only in the second memory 106 as shown in FIG. In parallel with this, the re-encoding process in step S313 is performed. In the re-encoding process, the encoded data stored in the second memory 106 is read out, re-encoded by the re-encoding unit 109, and stored in the two memories 104 and 106. Then, the encoding process and the re-encoding process are continued until all the codes of the vertical stripes (1) are re-encoded. The re-encoded data output from the re-encoding unit 109 has exactly the same code as the encoded data obtained by encoding in the same quantization step as the encoded data output from the encoding unit 102 after changing the quantization step. Data.
[0038]
Specifically, in this re-encoding process, after the Huffman decoding of the encoded data, each quantized value is subjected to a bit shift process that produces a result similar to the result obtained by dividing these values by 2 ⁿ This is realized by performing Huffman coding again. This method enables high-speed re-encoding processing in that the quantization step is changed only by bit shift and that inverse orthogonal transformation or re-orthogonal transformation processing is not performed. In step 315, the end of the re-encoding process is detected.
[0039]
Since the amount of data after re-encoding is smaller than the amount of encoded data before re-encoding, as shown in FIG. 5, after re-encoding in the memory area where the code before re-encoding was stored Can be stored so as to be overwritten. When the re-encoding process is completed, the data amount of the encoded data of the vertical stripes (1) decreases to the data amount of the encoded data of the diagonal stripes (1) shown in FIG.
[0040]
Steps S307 to 315 described above are processes performed in the encoding / recoding phase.
[0041]
<< Transfer Phase >>
When the re-encoding process is completed, a transfer process is performed in step S317. In the transfer process, as shown in FIG. 6, encoded data of diagonal stripes (2) stored only in the second memory 106 in the encoding / re-encoding phase is converted into diagonal lines ▲ in the first memory 104. It is transferred to the address linked to the encoded data 1) and stored. On the other hand, the encoded data of the diagonal stripe {circle around (1)} and the encoded data of the diagonal stripe {circle around (2)} distributed on the second memory 106 are continuously stored on the first memory 104. Similarly, the encoded data of the diagonal stripe {circle around (2)} is transferred in the second memory 106 and connected. This is processing performed in the transfer phase.
[0042]
When the transfer phase ends, the process returns to the encoding phase of steps S303 and S305, and the code of the diagonal stripe (4) is output from the encoding unit 102 and stored in the two memories 104 and 106 as shown in FIG. This encoding phase is slightly different from the encoding phase in the initial state (FIG. 4), the quantization step at the time of encoding by the encoding unit 102 is changed from Q1 to Q2, and the two memories 104 and 106 The encoded data stored in is also a collection of codes processed in various phases. If these differences are ignored, the encoding phase immediately after the transfer phase and the initial encoding phase can be regarded as the same.
[0043]
Therefore, by repeating the encoding phase, the encoding / re-encoding phase, and the transfer phase, the code that finally compresses the image data of one page below the data amount setting value is stored in the first memory. I can do it. In addition, the input unit 101 simply continues input until a series of processing is completed. That is, it is not necessary to input the image again from the beginning.
[0044]
The flowchart shown in FIG. 3 describes only the processing corresponding to each phase shown in FIGS. 4, 5, and 6 so that the explanation can be easily understood. In practice, however, the input of image data for one page ends in some phase. Therefore, the correspondence after that is slightly different depending on which phase it is completed. FIG. 8 is a flowchart showing the flow considering this. The flowchart in FIG. 8 considers the relationship between the completion of input of image data for one page and the various processes described with reference to FIG. 3. Here, steps S801, S803, S805, and S807 are added to the flowchart in FIG. It has been added.
[0045]
Steps S801, S803, and S805 detect that the input of image data for one page from the input unit 101 has been completed in the encoding phase, the encoding / recoding phase, and the transfer phase, respectively.
[0046]
If it is detected that the input of image data for one page has been completed in the encoding phase and the transfer phase (steps S801 and S805), the process proceeds to step S807, where the compression encoding process for the page is ended, and the next process is performed. If there is more than one page of image data, the compression encoding process of the next one page of image data is started, and if there is no image data, a stop state is entered.
[0047]
On the other hand, when the end of input of image data for one page is detected in the encoding / re-encoding phase (step S803), the encoding unit 102 needs to stop operation until there is no image data to be re-encoded. Therefore, the encoding process of step S311 is passed, and in step S313, only the re-encoding process for suppressing the image data encoded by the encoding unit 102 so far to a predetermined encoded data amount is continued. And do it. When all the re-encoding processes are completed and the subsequent transfer process is not completed, the encoded data of the entire image data for one page is not collected on the first memory, so the input of the image data for one page is completed. After that, the re-encoding process and the subsequent transfer process need to be performed continuously. In this case, when it is detected in step S315 that all the re-encoding processes have been completed, the encoded data stored only in the second memory 106 is stored in the first memory during the encoding / re-encoding phase. After the transfer to the memory (step S317), in the next step S805, the input end of the image data for one page is detected, and the process proceeds to step S807.
[0048]
The above is the operation, and is also the operation description of FIG.
[0049]
<Modification of memory storage method>
9 and 10 are diagrams showing modifications of the memory storing method shown in the conceptual diagrams of FIGS.
[0050]
In the conceptual diagram of FIG. 5, in the encoding / re-encoding phase, the encoded data output from the encoding unit 102 is stored only in the second memory 106. However, as shown in FIG. During the re-encoding phase, the encoded data output from the encoding unit 102 is directly stored in both the first and second memories.
[0051]
When viewed from the encoding unit 102, encoded data to be encoded and output in any phase is stored in both memories. Further, unlike the conceptual diagram of FIG. 6, as shown in FIG. 10, it is not necessary to transfer data between memories in the transfer phase. In the case of this modification, the encoded data and the re-encoded data are sequentially stored in the order in which they are sent to the first memory 104 in the encoding / re-encoding phase. Therefore, there is a problem that two types of data are mixed.
[0052]
Therefore, in the case of this modification, the encoded data is divided into certain units and managed as a file or a packet in order to cope with this. Specifically, a file management table or a packet management table is created and managed separately.
[0053]
As one method, when data from the encoding unit 102 is stored in the first memory 104, an appropriate unit (for example, 8 × i (i = 1 because the unit of the orthogonal transform is an 8 × 8 block) 2) (integer) for each line), a management number is assigned from the top of the image data, and the storage start address of the encoded data corresponding to each management number and the amount of the encoded data are stored in the order of the management number. Create a management table that can be used.
[0054]
The encoding unit 102 and the re-encoding unit 109 hold the management number of the data being processed, and based on the management number, write the start address and the encoded data amount when storing the encoded data in the management table. In this way, even if the encoded data processed by the encoding unit 102 and the re-encoding unit 109 are randomly stored, the management table is accessed in the order of the management number, and the leading address and encoded data to be read at that time If the encoded data is read from the first memory 104 based on the amount, the encoded data can be read sequentially from the top of the image. Providing such a management mechanism eliminates the need to store continuous data on the image so as to be continuous on the memory.
[0055]
The encoding phase after the transfer phase in the conceptual diagram of FIG. 10 is almost the same as the two encoding phases described above (FIGS. 4 and 7), and the code storage state in the first memory is shown in FIG. It is only slightly different as shown in. Therefore, the above description and this modification are the same in that the three phases are repeated.
[0056]
Next, a second basic configuration example (the configuration described so far is referred to as a first example) for performing a characteristic encoding process in the present invention will be described with reference to FIG.
[0057]
FIG. 2 is a block diagram of the image processing apparatus 200 in the second example.
[0058]
A significant difference from the image processing apparatus 100 of FIG. 1 is that two encoding units that perform encoding first exist in parallel. The image processing apparatus 200 encodes image data input from the input unit 201 in parallel with the first encoding unit 202 and the second encoding unit 205, and two types of encoded data having different compression rates. Is generated. Also in this example, a known JPEG encoding method is used, image data corresponding to 8 × 8 pixel units is orthogonally transformed, and quantization and Huffman encoding processing using quantization steps described later are performed. It is.
[0059]
In this example, a case where the compression rate applied by the second encoding unit 205 is set higher than that of the first encoding unit 202 will be described. Specifically, the quantization step in the first encoding unit 202 is Q1, and the quantization step in the second encoding unit 205 is Q2 (= 2 × Q1).
[0060]
The encoded data output from the encoding unit 202 is stored in the first memory 204 via the first memory control unit 203. At this time, the first counter 208 counts the amount of encoded data output from the encoding unit 202, holds this, and also outputs it to the encoding sequence control unit 209.
[0061]
On the other hand, the encoded data encoded by the encoding unit 205 is stored in the second memory 207 via the second memory control unit 206. At this time, the second counter 210 counts the data amount of the encoded data output from the encoding unit 205 and holds it. Further, when the encoded data stored in the second memory 207 described later is transferred to the first memory 204, the count value is transferred to the first counter 208 at the same time.
[0062]
When the first counter 208 counts the data amount of the encoded data output from the encoding unit 202 and the count value reaches a certain set value, the encoding sequence control unit 209 As in the example, a control signal is issued to the memory control unit 203 so as to discard the data stored in the memory 204.
[0063]
Then, the encoding sequence control unit 209 reads the encoded data stored in the second memory 207, transfers it to the first memory 204, and stores it in the first memory 204. A control signal is output to the memory control unit 203. As a result, the count value of the second counter 210 is transferred to the first counter 208, and the value is loaded (overwritten) as the count value of the first counter.
[0064]
In short, since the count value of the second counter 210 represents the amount of encoded data stored in the second memory 207, the correspondence between the count value and the encoded data is changed. It can be considered that the data is copied to the first counter and the first memory as it is.
[0065]
Furthermore, the encoding sequence control 209 controls the first encoding unit 202 and the second encoding unit 205 so as to perform encoding so that encoded data is smaller than before. Put out.
[0066]
For example, the quantization step S in the first encoding unit 202 and the second encoding unit 205 is switched to twice. As a result, the first encoding unit 202 inherits the quantization step Q2 (= 2 × Q1) in the second encoding unit 205 up to immediately before, and the second encoding unit 205 further Using a large quantization step Q2 × 2, an encoding process with a higher compression ratio in preparation for the next overflow is performed.
[0067]
Here, although the magnification ratio of the quantization step is set to double, it is not limited to this, and needless to say, it can be arbitrarily set. The encoded data output from the switched encoding units 202 and 205 is stored in the corresponding memories 204 and 207 via the corresponding memory control units 203 and 206, respectively.
[0068]
Then, the encoding sequence control 209 reads out the encoded data already stored in the second memory to the memory control unit 206 and sends a control signal to send the data to the re-encoding unit 211. The re-encoding unit 211 performs re-encoding processing of encoded data in the same manner as the re-encoding unit 109 in FIG.
[0069]
The third counter 212 counts the amount of data output from the re-encoding unit 211, is reset to zero immediately before starting the re-encoding process, and counts the output data amount during the re-encoding process. When the re-encoding process is completed, the counter 212 transfers the count value obtained there to the second counter 210.
[0070]
The second counter 210 adds the transferred data amount count value to the counter value held in the second counter 210 to thereby store the code stored in the memory 207 during the re-encoding process. The total data amount of the encoded data and re-encoded data is calculated. That is, the amount of data stored in the memory 207 matches the count value of the counter 210.
[0071]
Regardless of whether the re-encoding process is completed or not, if the image data from the input unit 201 to be encoded remains, the encoding process by the two encoding units 202 and 205 is continued. Then, monitoring whether the count value of the counter 208 has reached a certain set value is repeated until the encoding process (encoding, re-encoding) of image data for one page input from the input unit 201 is completed, The encoding and re-encoding processes described above are executed under control according to the detection result obtained here.
[0072]
FIG. 12 is a flowchart showing the process flow in the configuration of FIG.
[0073]
As described with reference to FIG. 2, when there are two encoding units, image data for one page is encoded based on the flowchart shown in FIG. Note that the description of FIG. 12 is mostly similar to FIG. 8, which is a flowchart in the case of one encoding unit, and those skilled in the art can fully understand the features of the second embodiment from the above description. Since it will be understood, the processing will be described in three phases as in the case of one encoding unit, and differences from FIG. 8 will be mainly described.
[0074]
The biggest difference between the flow of FIG. 8 described above and the flow of the present embodiment is that the transfer processing in step S317 is moved between step S307 and step S309. In short, it can be regarded that the encoding / re-encoding phase and the transfer phase are switched (the exception is the discarded processing of the encoded data in step S307).
[0075]
In the initial setting of the encoding parameter in step S301, the quantization step Q1 is set in the first encoding unit 202, and the quantization step Q2 (= 2 × Q1) is set in the second encoding unit 205.
[0076]
In the encoding phase, steps S801, S303, and S305 are repeatedly executed. Steps S801 and S305 are the same processing as in the case of one encoding unit, but only the encoding processing in step S303 is different as shown in FIG.
[0077]
In order to increase the compression rate of the encoded data stored in the first memory 204 in stages, the encoded data stored first stores the data encoded in the quantization step Q1 having the lowest compression rate. The encoded data stored in the second memory stores the data encoded in the quantization step Q2.
[0078]
When the amount of data stored in the first memory 204 exceeds the set upper limit value (step S305), the encoded data held in the first memory 204 is immediately discarded (step S307). The encoded data having a high compression rate held in the second memory 207 is transferred to the first memory 204 (step S317, see FIG. 14). As a result, without waiting for the end of the first re-encoding process described in the first embodiment (FIG. 1), the encoded data of the appropriate second candidate that does not exceed the upper limit value can be quickly obtained. Can be stored in the memory 207. This is the greatest advantage of applying FIG. 2 with two encoders over FIG.
[0079]
In the second embodiment, since it is a wasteful idea to have encoded data having the same compression rate in the two memories 204 and 207, the second memory 207 stores the codes stored in the first memory 204. The encoded data having a higher compression rate than the encoded data is stored. Accordingly, the subsequent processing is also performed based on this concept, and after the processing (transfer phase) for transferring the encoded data in the second memory 207 to the first memory 204 is completed, the second processing is performed. The encoded data in the memory 207 is re-encoded so as to hold encoded data with a higher one-stage compression rate.
[0080]
Specifically, first, as shown in FIG. 15, in the encoding / re-encoding phase subsequent to the transfer phase, each quantization applied to the two encoding units 202 and 205 before the re-encoding. Steps Q1 and Q2 are changed to Q2 and Q3, respectively (step S309). If the input of one page of image data is not completed (step S803), the subsequent image data is set by a new quantization step. The input data is encoded by the two encoded units (step S311) and stored in the corresponding memories 204 and 207. The encoded data (transferred to the first memory 204) stored in the second memory in parallel with the encoding process is compressed one step higher than the encoded data in the first memory. In order to change to encoded data of rate, the re-encoding unit 211 performs re-encoding processing (S313) so that the data encoded using the quantization step Q3 is obtained, and the re-encoded data is 2 is stored again in the second memory 207.
[0081]
In the second example, as in the first example, the re-encoding process is the result of dividing these values by 2 ⁿ for each quantized value after the encoded data is once Huffman-decoded. This is realized by performing a Huffman coding again after performing a bit shift process that produces a similar result. This method is capable of high-speed re-encoding processing in that the quantization step is changed only by bit shift and the inverse orthogonal transformation or re-orthogonal transformation processing is not performed.
[0082]
When there are two encoding units as in the second embodiment, encoded data and re-encoded data are mixedly stored in the second memory 207 as shown in FIG. A situation occurs. Therefore, as described above, it is necessary for the second memory 207 to manage the encoded data as a file or a packet by dividing the encoded data into a certain unit. For this purpose, for example, a configuration similar to that of the modification of the first example may be provided.
[0083]
In FIG. 12, when the end of the re-encoding process is detected in step S315, the process proceeds to the encoding phase (steps S801 and S303) again. In the encoding phase after the encoding / re-encoding phase, as shown in FIG. 16, the encoded data held in the two memories 204 and 207 not only have different compression ratios but also a mixture of encoded data. The way (address) is also quite different. Accordingly, when the data amount of the first memory 204 again exceeds the set value, the encoded data held in the second memory 207 (the code of the horizontal stripe region of (6) + (8)) Needs to be transferred to the first memory 204. Considering these, it is necessary to manage the encoded data as a file or a packet not only in the second memory 207 but also in the first memory 204. Therefore, the first memory 204 also requires a management mechanism using the above-described management table.
[0084]
The state of the encoding phase shown in FIG. 16 is the initial state of the encoding phase (FIG. 13) except that the method of mixing the quantization step and the encoded data is different before and after the re-encoding process. Is the same. Therefore, by repeating the encoding phase, the transfer phase, and the encoding / re-encoding phase, the encoded data that is finally compressed to be equal to or less than the set upper limit value of the image data for one page is surely stored in the first memory. 204 can be stored.
[0085]
Note that since the arrangement order of the transfer phase and the encoding / re-encoding phase is reversed from the description of the first example, the input end detection of image data for one page, which has been performed after the transfer processing in FIG. (Step S805) has almost the same timing as the input end detection of image data for one page (step S803) performed in the encoding / recoding phase. The two detection processes are functionally the same as step S805 and the timing is the same as step S803. Therefore, these two steps detect the end of input of image data for a new page. Are integrated as a step to be performed and described as step S1201.
[0086]
In the first and second examples described above, the first memory and the second memory have been described as physically separate memories. This is advantageous because access to the two memories can be independent, which is a feature of the present invention. However, the case where the first memory and the second memory are not physically separate memories is also included in the scope of the present invention. Two areas corresponding to the first memory and the second memory are secured on one physical memory, the first memory is the first memory area, and the second memory is the second memory. It can be understood that the present invention can be realized with a single memory by re-reading the above description by rephrasing the area.
[0087]
Further, when the above embodiments are realized with one memory, some of the data transfer processes described in the transfer phase are not necessary. The details can be easily imagined each time, so the explanation will be omitted. However, when the two areas are used strictly separated, data transfer processing is necessary as in the case of physically having two memories. If the same data is shared between the two areas, not only the data transfer process becomes unnecessary, but also the storage capacity can be reduced.
[0088]
For example, when the encoded data held in the second memory area is transferred to the first memory area, two pieces of information of the head address and the data size in which the encoded data is stored are stored in the second memory. The same effect as that obtained by transferring the encoded data can be obtained only by transferring the data from the control unit to the first memory control unit.
[0089]
When the encoded data is stored in a file format or a packet format, the information transferred between the memory control units is slightly increased, and it is necessary to transfer the management table information related to the encoded data. Nevertheless, it is more efficient than transferring encoded data.
[0090]
Now, according to the above-described image processing apparatus, even when the input image data is encoded, even if it exceeds the target size, the processing is performed so as to keep the input within the target size. Will be able to continue. The present invention is based on the above-described structure, and is characterized by functioning particularly effectively for color image coding. A specific example will be described below as an embodiment.
[0091]
<First Embodiment>
FIG. 17 is a block diagram of a digital image processing apparatus to which this embodiment is applied.
[0092]
In the figure, reference numeral 1000 denotes a color image input port (image area information and color image data). As shown in the figure, the input port 1000 is connected to a selector 1023 for selecting either the image scanner 1020 or the rendering engine 1021 based on the print data output from the host computer (an operation panel not shown). Or automatically select the one with input). Color image data 1031 and 1032 and image area information 1033 and 1034 (information for identifying whether the pixel is in a character / line drawing area or a halftone area and whether it is color or monochrome) are output from both. Shall come. The rendering engine 1021 can generate image area information based on print data (because it is transferred as image data from a host computer computer in the case of a halftone image, and in accordance with a drawing command in the case of a character line image). . On the other hand, the image scanner unit 1020 basically needs to read a manuscript image, determine whether it is a character line image area or a halftone area, and determine whether it is color or monochrome based on the read image. Therefore, it is assumed that a circuit for generating the image area information is built in.
[0093]
Reference numeral 1001 denotes a line buffer having a plurality of lines of input images (capacity for extracting tiles described later). Reference numeral 1002 denotes a color image encoder, which corresponds to the encoding unit 102 in FIG. However, the color image encoder 1002 according to the present embodiment includes a conversion circuit that temporarily converts input color image data into a luminance signal and a color difference signal, and compresses and encodes the converted data. And As the luminance and color difference signals, there are color spaces represented by Y, Cr, Cb, etc., but YIQ may also be used. Therefore, in the embodiment, for the sake of convenience, the luminance data is expressed as Y, and the color difference signals are expressed as C1 and C2.
[0094]
Reference numeral 1003 denotes an external memory (for example, a hard disk) that stores an encoded color image. Reference numeral 1005 denotes a decoding buffer that temporarily stores the encoded image data read from the external memory 1004 in order to perform decoding processing. Reference numeral 1006 denotes a decoder. Reference numeral 1007 denotes a line buffer for temporarily storing the decoded image. Reference numeral 1008 denotes an output port for outputting an image stored in the line buffer 1008 to the connected printer unit 1009. The printer unit 1009 is provided with a conversion circuit that converts Y, C1, and C2 data into Y, M, and C (or Y, M, C, and Bk) that are recording color components. . In addition, the recording method of the printer unit 1009 may be any printing method such as a laser beam printer or a printer that ejects ink droplets.
[0095]
Reference numeral 1010 denotes a code amount monitoring unit that monitors the amount of code data stored in the internal buffer 1003. Reference numeral 1011 denotes a code conversion unit that performs encoding again.
[0096]
From the relationship with FIG. 1, the internal buffer 1003 also serves as the first memory and the second memory in FIG. 1. In addition, the code sequence control unit 108, the first counter 107, the second counter 110, and the selector 111 correspond to the code amount monitoring unit 1010, and the re-encoding unit 109 corresponds to the code conversion unit 1011.
[0097]
The color image encoder 1002 divides the image data stored in the line buffer 1001 into tiles having a size of 8 × 8 pixels (each tile is not limited to 8 × 8 but may be M × M pixels), and this 8 × Color information is encoded every 8 pixels. The color image is encoded by discrete cosine transform encoding (JPEG), and the image area information is encoded by run length encoding.
[0098]
The image area information is attached to each pixel. However, in the case of processing by DCT for each 8 × 8 block as in this embodiment, the image area flag is used for each block as a representative. It will be. As described above, the image area is divided into the character area and the photographic area of the image, either color or monochrome. However, it may be other than this, and additional components are added to this. It doesn't matter.
[0099]
The code amount monitoring unit 1010 monitors the code amount generated by the color image encoder 1002, and when it is predicted that the set amount will be exceeded, it is input to the encoder 1002 thereafter. The color image data (and attribute information) is encoded so as to be higher, and the previously encoded data is re-encoded by the code converter 1011 to perform higher encoding. Become.
[0100]
In the present embodiment, every time it is determined that the code amount exceeds the set value, the encoding efficiency is gradually increased, so that it will be described below.
[0101]
As described above, the embodiment encodes a color image. The color image data is expressed in the color space format of luminance data Y and color difference data C1 and C2.
[0102]
If the code amount monitoring unit 1010 (encoding sequence control unit 108 in FIG. 1) determines that the code amount generated by the encoding unit 102 has exceeded the target value after starting encoding of a certain page, The encoding unit 102 encodes the alternating current component (AC) obtained by orthogonally transforming the color difference components C1 and C2 with the quantization step set higher than before. As a result, the color image input after it is determined that the target value is exceeded is encoded at a higher compression rate.
[0103]
In addition, since the encoded data up to immediately before when it is determined that the target value is exceeded is stored in the second memory, the color difference component data C1 encoded with respect to the second memory control unit 105, Only C2 is output to the re-encoding unit 109, and a control command is issued to the selector 111 to output to the re-encoding unit 109. In addition, the encoding sequence control unit 108 causes the re-encoding unit 109 to decode the encoded color difference data C1 and C2, respectively, and increases the AC component (AC component) quantization step to re-encode. Encode. The recoded color difference component data is stored in the first memory 104 and the second memory 106. As a result, the encoded data before the target value is exceeded is also encoded at a high compression rate.
[0104]
In short, when the color image data of a certain page is input and encoded, when the target value is exceeded first, the quantization step is increased for the color difference components C1 and C2, and the encoding is performed. Will continue.
[0105]
If the target value is exceeded again during the processing as described above, the alternating current component of luminance Y is determined as a change target.
[0106]
Thus, every time it is determined that the target value is exceeded, the quantization step for the AC component and the DC component of each of the luminance component Y and the color difference components C1 and C2 is changed (larger), and the compression rate is gradually increased. Become.
[0107]
For this reason, the encoding sequence control unit 108 has a scenario table as shown in FIG. 18, and as the number of times the target value is exceeded for a certain page is counted, Third, control to gradually increase the compression rate.
[0108]
Therefore, when the target value is exceeded first during the encoding of a certain page, the AC component AC of the color difference components C1 and C2 is selected according to FIG. Will be done.
[0109]
FIG. 19 is an explanatory diagram of the relationship between the encoding progress status and the memory fullness rate. Hereinafter, the operation sequence will be briefly described with reference to FIG.
[0110]
In the figure, it is shown that the encoded data amount overflows at the initial value condition (encoding parameter at the initial stage) at the time of 2/8. According to the embodiment, after this time, the color image data is continuously encoded by increasing the quantization step of the AC component after orthogonal transformation of the color difference components C1 and C2, and the already encoded data is encoded. Will decode the chrominance component in it, increase the quantization step of the AC component, and start the re-encoding process.
[0111]
Further, at the time of 3/8, when it is determined that the target value is exceeded, the previous re-encoding is completed, and the transfer process to the first memory 104 is completed.
[0112]
Further, since overflow occurs again at 4/8, the re-encoding process is started. Since the overflow at this time is the second time, according to FIG. 18, the alternating current component of the luminance component is selected and processing is performed. At 5/8, this re-encoding is completed, indicating that the transfer process is complete.
[0113]
At 8/8, it indicates that the encoding of one page is complete.
[0114]
In the above example, the memory is about 25% overrun at 3/8. This is determined by the compression rate and the re-encoding time. In the actual machine, the buffer is used as a design parameter. It needs to be secured as a margin of memory, but it is not so large. As can be seen from the figure, the processing time ends within the time for encoding one image.
[0115]
As can be seen from the above description, in the embodiment, the quantization step of each DC component and AC component can be set independently for the color difference component and the luminance component, so that the first memory In 104, the value of the quantization step in each of the alternating current component and direct current component of luminance and color difference is stored.
[0116]
20A to 20D are explanatory diagrams of a DC coefficient (DC coefficient) of a DCT coefficient. FIG. 4B is an explanatory diagram of DC coefficient encoding. The difference between the 8 × 8 DCT-converted DC coefficient value (the small square at the upper left in the figure) and the DC coefficient value of the adjacent block is taken and variable-length encoded.
[0117]
FIG. 6A is an encoding block diagram, and the difference between the DC coefficient 1205 and the DC coefficient of the immediately preceding block delayed by the block delay unit 1201 is taken by the difference unit 1202. The difference value is converted into a group number SSSS by the grouping process.
[0118]
In the grouping unit, the number of additional bits corresponding to the DC difference value is determined as shown in FIG. The group number SSSS is Huffman encoded according to the one-dimensional Huffman encoder 1204 and the table shown in FIG.
[0119]
From these tables, it can be seen that the smaller the group number, the smaller the Huffman code and the number of additional bits (the same in some cases).
[0120]
Therefore, when the DC difference value is ½, the group number is decreased by one, and the variable length code portion including the Huffman code and the additional bits is shortened by 1 to 2 bits.
[0121]
FIG. 22 is an explanatory diagram of an AC coefficient (AC) coefficient encoding method of DCT coefficients.
[0122]
The AC coefficients 1301 are rearranged in the zigzag scan order, and when 0 is determined by the determiner 1302, the run length counter 1303 counts the number of consecutive 0 coefficients (run length) and outputs a run length NNNN 1306. For coefficient values other than 0, a group number 1307 and additional bits 1302 are output by the grouping unit 1304 as in FIG. The two-dimensional Huffman encoder 1305 performs Huffman encoding by combining the run length NNNN and the group number SSSS (see FIG. 23).
[0123]
When the run length NNNN exceeds 15, a necessary number of ZRLs indicating the run length 16 are output. For example, the run length 35 is converted into ZRL + ZRL + run length 3 and encoded.
[0124]
In addition, EOB (End of Block) is added after the last effective coefficient (coefficient other than 0).
[0125]
In the embodiment, since DCT processing is performed on an 8 × 8 block, the order of zigzag scanning is as shown in FIG.
[0126]
FIG. 24 shows a part of the Huffman code table subtracted from the run length and size.
[0127]
Therefore, it can be seen that when the AC coefficient value is ½, the group number is decreased by 1, and the variable-length code portion including the Huffman code and the additional bits is shortened by 1 to 2 bits.
[0128]
Furthermore, since the coefficient value becomes zero and the run length becomes longer, the number of dominant symbols to be encoded is also reduced, so that the code length is further shortened.
[0129]
<Second Embodiment>
Next, an example in which the index of the color signal selection table is set based on the re-encoding start point will be described as a second embodiment.
[0130]
FIG. 25 is an explanatory diagram of the relationship between the progress of encoding and the memory fullness rate.
[0131]
In the figure, at the time of 6/8, the encoding at the quantization step of the initial value overflows and the re-encoding process starts, and the encoding is continued by changing the quantization step. In this example, an overflow occurs near the end of 6/8, so if all the color components are re-encoded, the code will be reduced to about half from here. It is expected that the compression will be too much than the code amount. Therefore, for example, if the encoding sequence control unit performs index control based on the processing amount of one image so that only the AC component of the color difference component of the color selection table is subject to re-encoding, Subsequent over-compression can be prevented.
[0132]
<Application example>
In the above-described embodiment, the functional operation of the apparatus has been described using the apparatus that reads an image from the image scanner as an example. Most of the functions (including the encoding process) can be realized by the computer program as described above.
[0133]
Therefore, the present invention may be applied to an application program that operates on a general-purpose information processing apparatus such as a personal computer. In the case of application to an application program, a GUI may be provided such as allowing the user to specify an image file as a compression source and allowing the user to select a target size. The target value at this time can be set arbitrarily by the user, but setting with numerical values is difficult to understand, so it should be selected from an intuitive menu that takes into account the document size and image quality (high, medium, low, etc.). It would be good to make a decision.
[0134]
In addition, the quantization step has been described as an example of the encoding parameter of the encoding unit. The parameters may be used. However, for example, in the configuration of FIG. 1, in order to make the re-encoded data from the re-encoding unit 109 substantially the same as the encoded data from the encoding unit 102 after the parameter change, the above-described implementation is required. As shown in the embodiment, a method of increasing the quantization step is preferable.
[0135]
In the embodiment, an example in which a color image is compression-encoded as luminance and color difference data has been described. However, the color space expressing a color component is not limited to luminance and color difference. For example, a La * b * color space or the like may be used.
[0136]
In addition, the compression parameter is changed every time the memory is over, but it is desirable to increase the quantization step by giving priority to color components that are difficult to discern by human vision. Therefore, it is desirable to adopt a color space that matches human visual characteristics such as luminance and color difference (hue, saturation) rather than the RGB color space.
[0137]
Further, as described above, the present invention can be realized by an application program that runs on a general-purpose device, and therefore the present invention includes a computer program. In addition, since the computer program is usually performed by setting or copying or installing a storage medium such as a floppy disk or CDROM in the apparatus, such a storage medium is naturally included in the scope of the present invention.
[0138]
In the embodiment, the image data is input from the scanner. However, the present invention may be applied to a printer driver that operates on a host computer. In the case of application to a printer driver, when data to be printed is received from a higher-level process (such as an application), it is of course possible to determine whether the data is a halftone image or a character / line image. The configuration related to the area information generation processing can be omitted or simplified.
[0139]
The present invention can also be applied to a combination of a computer program and appropriate hardware (such as an encoding circuit).
[0140]
【The invention's effect】
As described above, according to the present invention, in encoding of a multi-value color image, encoding is performed so as to be within a target size without re-input, and deterioration of the quality of the color image is minimized. Is possible.
[Brief description of the drawings]
FIG. 1 is a diagram showing a first basic configuration of an image processing apparatus to which the present invention is applied.
FIG. 2 is a diagram illustrating a second basic configuration of an image processing apparatus to which the present invention is applied.
FIG. 3 is a flowchart showing a simplified process in the configuration of FIG. 1;
FIG. 4 is a diagram illustrating a data flow and memory contents in an encoding phase in an initial state.
FIG. 5 is a diagram showing a data flow and memory contents in an encoding / re-encoding phase.
FIG. 6 is a diagram illustrating a data flow and memory contents in a transfer phase.
FIG. 7 is a diagram illustrating a data flow and memory contents in an encoding phase after a transfer phase.
FIG. 8 is a flowchart showing details of processing in the configuration of FIG. 1;
9 is a diagram showing a data flow and memory contents in an encoding / re-encoding phase in the modification of the configuration of FIG. 1; FIG.
10 is a diagram showing a data flow and memory contents in a transfer phase in the modification of FIG. 9;
11 is a diagram showing a data flow and memory contents in an encoding phase after a transfer phase in the modification of FIG.
12 is a flowchart showing a processing procedure in the configuration of FIG. 2;
13 is a diagram showing a data flow and memory contents in an encoding phase in an initial state in the configuration of FIG.
14 is a diagram showing a data flow and memory contents in a transfer phase in the configuration shown in FIG.
15 is a diagram illustrating a data flow and memory contents in an encoding / re-encoding phase in the configuration of FIG.
16 is a diagram illustrating a data flow and memory contents in an encoding phase after an encoding / re-encoding phase in the configuration of FIG. 2;
FIG. 17 is a block diagram of an image processing apparatus to which the embodiment is applied.
FIG. 18 is a diagram illustrating the contents of a scenario table at the time of memory over in the embodiment.
FIG. 19 is a diagram illustrating changes in code amount in the embodiment.
FIG. 20 is a diagram for explaining the operation of the DC component encoding structure in the embodiment;
FIG. 21 is a diagram illustrating an AC component processing procedure in the embodiment;
FIG. 22 is a diagram showing a zigzag scan for DCT coefficients.
FIG. 23 is a diagram showing a table for grouping AC components.
FIG. 24 is a diagram showing a part of a Huffman code table searched from run length and size.
FIG. 25 is a diagram illustrating an example of a relationship between an encoding progress situation and a memory fullness rate according to the second embodiment.

Claims (4)

Enter a luminance component and a chrominance component representing the color image, the luminance component is compressed using a first quantization step, a first compression means to compress using the second quantization step of the color difference component When,
Decodes the encoded data generated by said first compression hands stage, a second compression hands stage capable of re-compression,
Monitoring the encoded data amount generated by the first compression hands stage, the encoded data amount and a code amount monitoring means for determining whether reaches a predetermined amount,
At the first time when the code amount monitoring means determines that the predetermined amount has been reached, the second quantization step applied to the first compression means has a higher compression rate than the second quantization step. After changing to the third quantization step, the subsequent color difference component is compressed by the first compression means,
The second compression unit uses the third quantization step having a compression rate higher than that of the second quantization step for the encoded data of the color difference component previously generated by the first compression unit. To recompress
The encoded data of the color difference component obtained by the recompression is stored as encoded data of the color difference component generated by the first compression means after changing to the third quantization step,
The encoded data of the color difference component generated by the first compression unit after updating to the third quantization step is stored as subsequent encoded data,
At the second time when it is determined that the predetermined amount has been reached by the code amount monitoring means, the first quantization step applied to the first compression means has a compression rate higher than that of the first quantization step. After changing to a high fourth quantization step, the subsequently inputted luminance component is compressed by the first compression means,
The encoded data of the luminance component previously generated by the first compression means is converted into the second compression means by using the fourth quantization step having a compression rate higher than that of the first quantization step. To recompress
The encoded data of the luminance component obtained by the recompression is stored as encoded data of the luminance component generated by the first compression means after changing to the fourth quantization step,
An image processing apparatus that stores encoded data of a luminance component generated by the first compression unit after updating to the fourth quantization step as subsequent encoded data . Enter a luminance component and a chrominance component representing the color image, the luminance component is compressed using a first quantization step, as a first compression Engineering be compressed using a second quantization step of the color difference component When,
Decodes the encoded data generated by said first compression hands stage, and as the second compression engineering that can be recompressed,
Monitoring the encoded data amount generated by the extent of the first compression Engineering, the encoded data amount and a code amount monitoring step of determining whether reaches a predetermined amount,
At the first time when it is determined that the predetermined amount is reached by the code amount monitoring step, the second quantization step applied to the first compression step is higher in compression rate than the second quantization step. After changing to the third quantization step, the subsequently input color difference component is compressed by the first compression step,
The encoded data of the color difference component previously generated by the first compression step is converted into the second compression step by using the third quantization step having a compression rate higher than that of the second quantization step. To recompress
The encoded data of the color difference component obtained by the recompression is stored as encoded data of the color difference component generated by the first compression step after changing to the third quantization step,
The coded data of the color SaNaru component generated by the first compression step after updating the third quantization step, is stored as a subsequent encoded data,
At the second time when it is determined that the predetermined amount has been reached by the code amount monitoring step, the first quantization step applied to the first compression step is more compressed than the first quantization step. After changing to a high fourth quantization step, the subsequently input luminance component is compressed by the first compression step,
The encoded data of the luminance component previously generated by the first compression step is converted into the second compression step by using the fourth quantization step having a compression rate higher than that of the first quantization step. To recompress
The encoded data of the luminance component obtained by the recompression is stored as encoded data of the luminance component generated by the first compression step after changing to the fourth quantization step,
A control method for an image processing apparatus, wherein the encoded data of the luminance component generated by the first compression step after the update to the fourth quantization step is stored as subsequent encoded data . On the computer,
Enter a luminance component and a chrominance component representing the color image, the luminance component is compressed using a first quantization step, a first compression means to compress using the second quantization step of the color difference component When,
Decodes the encoded data generated by said first compression hands stage, a second compression hands stage capable of re-compression,
Said first monitor encoded data amount generated by the compression hands stage, the encoded data amount to function as a code amount monitoring means for determining whether reaches a predetermined amount,
At the first time when the code amount monitoring means determines that the predetermined amount has been reached, the second quantization step applied to the first compression means has a higher compression rate than the second quantization step. After changing to the third quantization step, the subsequent color difference component is compressed by the first compression means,
The second compression unit uses the third quantization step having a compression rate higher than that of the second quantization step for the encoded data of the color difference component previously generated by the first compression unit. To recompress
The encoded data of the color difference component obtained by the recompression is stored as encoded data of the color difference component generated by the first compression means after changing to the third quantization step,
The encoded data of the color difference component generated by the first compression unit after updating to the third quantization step is stored as subsequent encoded data,
At the second time when it is determined that the predetermined amount has been reached by the code amount monitoring means, the first quantization step applied to the first compression means has a compression rate higher than that of the first quantization step. After changing to a high fourth quantization step, the subsequently inputted luminance component is compressed by the first compression means,
The encoded data of the luminance component previously generated by the first compression means is converted into the second compression means by using the fourth quantization step having a compression rate higher than that of the first quantization step. To recompress
The encoded data of the luminance component obtained by the recompression is stored as encoded data of the luminance component generated by the first compression means after changing to the fourth quantization step,
A computer program that causes the encoded data of the luminance component generated by the first compression means after updating to the fourth quantization step to function as stored subsequent encoded data . A computer-readable storage medium storing the computer program according to claim 3.

Images