Abstract:
In image data coding and decoding processing, in order to improve efficiency by processing a plurality of factors in one cycle as long as possible, the factors are rearranged, in coding or decoding processing, in a predetermined scan sequence such that significant factors and 0s are paired. In addition, an appropriate scan sequence is selected in accordance with the distribution state of frequencies to further improve the efficiency.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method and apparatus for coding and decoding an image and a storage medium.  
         BACKGROUND OF THE INVENTION  
         [0002]    As a compression technique for a multilevel image, a conventional technique of segmenting a source image into blocks each constituted by a plurality of pixels, performing orthogonal transformation for each block, and quantizing the resultant data with a quantization threshold, thereby Huffman-coding the data is known. Such coding processing is used in the JPEG (Joint Photographic Experts Group) scheme. A coder/decoder implemented by forming this scheme into hardware is conventionally known.  
           [0003]    In a coder implemented as hardware, attempts have been made to realize quantization processing at a high processing rate with a minimum circuit size. For example, orthogonal transformation factors converted in the zigzag scan sequence are processed for a plurality of factors at a time. The arrangement of a conventional coder will be described below.  
           [0004]    A conventional coder performs orthogonal transformation, on a block basis, for an input source image segmented into a plurality of blocks by using an orthogonal transformer, and outputs orthogonal transformation factors. The output factors are rearranged in the zigzag scan sequence by a zigzag scan converter, and are output in twos to comparators. Corresponding quantization thresholds are also output in twos to the comparators. Each comparator compares the output factor with the corresponding quantization threshold and outputs comparison result information indicating whether the orthogonal transformat ion factor is smaller than the quantization threshold. This comparison result information is equivalent to information indicating whether the result obtained by quantizing the orthogonal transformation factor with the corresponding quantization threshold is 0.  
           [0005]    A controller outputs control signals to selectors in accordance with output results from the comparators. More specifically, if at least one of quantization results on two orthogonal transformation factors is 0, the controller outputs control signals to the selectors to select one of the quantization results which is not 0 (significant factor) (if the two quantization results are 0, outputting control signals for selecting any quantization result exerts no influence on operation). If neither of the quantization results is 0, the controller outputs control signals to the selectors to alternately select the quantization results one by one in two cycles in accordance with the zigzag scan sequence. In addition, the controller outputs a format signal to the Huffman coder in accordance with the output results from the comparators. The format signal includes information indicating “a pair of 0 and significant factor” if one of the two orthogonal transformation factors is 0, “a pair of 0 and 0” if the two factors are 0, or “only one significant factor” if the two factors are significant factors (if the two factors are significant factors, since the factors are quantized one by one in two cycles, information indicating “only one significant factor” is consecutively output in two cycles), and information indicating, if the two factors are “a pair of 0 and significant factor”, which comes first in the zigzag scan sequence.  
           [0006]    As described above, if an orthogonal transformation factor is 0, a result (i.e., 0) can be obtained without quantization processing. If, therefore, at least one of two orthogonal transformation factors is 0, control is performed to quantize the two orthogonal transformation factors substantially in one cycle. If, however, the two factors are significant as a result of comparison, since neither of the factors is 0, quantization processing is required. For this reason, a processing time of two cycles is required.  
           [0007]    In a hardware-implemented decoder, attempts have been made to perform inverse quantization processing at a high processing rate with a minimum circuit size. For example, a technique of initializing a memory by performing inverse quantization processing for only significant factors of quantized orthogonal transformation factors and writing the resultant data in the memory has been proposed. The arrangement of a conventional decoder will be described below.  
           [0008]    The conventional decoder decodes Huffman-coded data by using a Huffman decoder and outputs zero-run information indicating a quantized orthogonal transformation factor and the number of 0s preceding it. The output quantized orthogonal transformation factor is input to an inverse quantization unit to be inversely quantized by using a quantization threshold which corresponds to the quantized orthogonal transformation factor and is output from a quantization threshold table. The resultant data is output as an orthogonal transformation factor to a selector.  
           [0009]    An address generator calculates a specific position in a block as an orthogonal transformation processing unit to which the output quantized orthogonal transformation factor corresponds on the basis of the output zero-run information, and outputs a write address in a block memory which corresponds to the position and a read address in the quantization threshold table. In addition, the address generator outputs an initialization target address for initialization of the block memory before quantization processing to the block memory for each unit block, and also outputs, to a controller, a signal indicating that initialization is being performed. The initialization processing is preprocessing in which 0s are written before processing for the unit block to limit orthogonal transformation factors to be written in the block memory to significant factors (factors that are not 0) in an actual processing stage, thereby omitting write processing for insignificant factors (factors which are 0). In this case, only addresses at which significant factors are written may be initialized. For this purpose, the addresses at which the significant factors were written must be stored. Write addresses for initialization are generated on the basis of the stored address information.  
           [0010]    The quantization threshold table reads out quantization thresholds corresponding to quantized orthogonal transformation factors to be processed on the basis of outputs from the write address generator, and outputs them to the inverse quantization unit. The controller outputs a 0 value as initialization data and a sequence selection signal indicating whether to select the initialization data to the selector on the basis of a signal indicating that initialization is being performed, and also outputs a control signal for controlling write/read operation of the block memory to the read address generator. The control signal provides instructions to start read operation upon completion of write operation for a unit orthogonal transformation block, read two factors per cycle, terminate read operation when data corresponding to a unit orthogonal transformation block is read, start writing initialization data for initialization processing, and start write operation for the next processing target orthogonal transformation block upon completion of the initialization processing.  
           [0011]    During a read interval, the read address generator generates addresses so as to sequentially read out in the zigzag scan sequence data corresponding to a unit orthogonal transformation processing block, which is written in the block memory, on the basis of the write/read control signal output from the controller, and outputs the addresses to the block memory.  
           [0012]    The block memory operates in cycles of initialization of each orthogonal transformation processing unit block, write, and read in the zigzag scan sequence. The block memory operates to write an output from the selector at a write address and perform read operation according to a read address in accordance with the read/write control signal output from the controller. The read value is output to an inverse orthogonal transformer.  
           [0013]    The inverse orthogonal transformer sequentially performs inverse orthogonal transformation for the orthogonal transformation factors output from the block memory in the zigzag scan sequence, and outputs the transformation results for each unit block.  
           [0014]    With the above arrangement, write processing for the block memory requires clock cycles equal in number to the significant factors existing in a unit block. Since factors are read in twos, if the number of samples in a unit block is 64, 32 clock cycles are required. Initialization requires clock cycles equal in number to the significant factors existing in a unit block.  
           [0015]    If, for example, the number of samples in a unit block is 64 and 20 significant factors exist in the unit block to be processed, the total number of clock cycles required for processing for the processing target block is the sum of write processing=20 cycles, read processing=32 cycles, and initialization processing=20 cycle, i.e., 72 clock cycles.  
           [0016]    The number of clock cycles required to process a given unit orthogonal transformation block (8×8=64 samples) is minimized when the number of significant factors in the processing target block is 0. In this case, the total number of clock cycles is the sum of write processing=0 cycle, read processing=32 cycles, and initialization processing=0 cycle, i.e., 32 cycles. In contrast to this, the number of clock cycles required to process a given unit orthogonal transformation block is maximized when the number of significant factors in the processing target block is 64. In this case, the total number of clock cycles is the sum of write processing=64 cycles, read processing=32 cycles, and initialization processing 64 cycles, i.e., 160 cycles.  
           [0017]    According to the arrangement of the conventional coder, at least one of quantization results on a pair of orthogonal transformation factors input to the comparing means is preferably 0 from the viewpoint of processing speed. For this purpose, the respective elements of quantized orthogonal transformation factors in an orthogonal transformation block are preferably input to the comparing means in such a manner that significant factors are proportionally dispersed as much as possible. If, however, these factors are input in the zigzag scan sequence, significant factors tend to concentrate on some part of a block. This makes it difficult to increase the coding speed.  
           [0018]    According to the arrangement of the conventional decoder, if the compression ratio is high and the number of significant factors occupying each unit orthogonal transformation block is small, a high decoding speed can be obtained. However, since initialization processing is performed for each orthogonal transformation block, the processing speed decreases rapidly as the proportion of significant factors increases. As a result, the difference in time required for decoding between data with a high compression ratio and data with a low compression ratio increases.  
         SUMMARY OF THE INVENTION  
         [0019]    The present invention has been made in consideration of the above situation, and has as its object to improve the efficiency in coding/decoding process.  
           [0020]    According to the present invention, the foregoing object is attained by providing an image coder which compares a predetermined number of orthogonal transformation factors from an orthogonal transformation unit with quantization thresholds equal in number to the orthogonal transformation factors, and selectively quantizes the orthogonal transformation factors on the basis of the comparison result in coding processing, comprising a first scan converter for rearranging the orthogonal transformation factors in a first scan sequence and outputting the predetermined number of factors at a time, and a second scan converter for rearranging quantized orthogonal transformation factors in a zigzag scan sequence and outputting the factors.  
           [0021]    According to the present invention, the foregoing object is also attained by providing an image decoder which decodes an image by performing inverse quantization processing, scan sequence conversion processing, and inverse orthogonal transformation processing for input quantized orthogonal transformation factors, comprising a first scan converter for converting the quantized orthogonal transformation factors in a first scan sequence and outputting not less than two factors, a 0 determination unit for determining whether not less than two quantized orthogonal transformation factors output from the first scan converter are 0, and outputting a determination result, a first selector for selecting not less than one of not less two quantized orthogonal transformation factors, a second selector for selecting not less than one of two quantization thresholds corresponding to not less than two quantized orthogonal transformation factors, a controller for controlling the first and second selector in accordance with a result obtained by the 0 determination unit, and generating/outputting a format signal from the determination result obtained by the 0 determination unit, an inverse quantization computation unit for performing inverse quantization computation processing by using not less than one quantized orthogonal transformation factor and not less than one quantization threshold, and a second scan converter for rearranging orthogonal transformation factors output from the inverse quantization computation unit in a second scan sequence and outputting not less than two factors.  
           [0022]    Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment/embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0024]    [0024]FIG. 1 is a block diagram showing the arrangement of the first embodiment of the present invention;  
         [0025]    [0025]FIG. 2A is a view showing a zigzag scan sequence;  
         [0026]    [0026]FIG. 2B is a view showing a scan sequence in the present invention;  
         [0027]    [0027]FIG. 2C is a view showing a scan sequence in the present invention;  
         [0028]    [0028]FIG. 2D is a view showing a scan sequence in the present invention;  
         [0029]    [0029]FIG. 2E is a view showing a scan sequence in the present invention;  
         [0030]    [0030]FIG. 3 is a block diagram showing the arrangement of the second embodiment of the present invention;  
         [0031]    [0031]FIG. 4A is a view showing a quantization result example of an 8×8 block;  
         [0032]    [0032]FIG. 4B is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 4A in twos in a zigzag scan sequence;  
         [0033]    [0033]FIG. 4C is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 4A in twos in the scan sequence shown in FIG. 2B;  
         [0034]    [0034]FIG. 4D is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 4A in twos in the scan sequence shown in FIG. 2C;  
         [0035]    [0035]FIG. 4E is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 4A in twos in the scan sequence shown in FIG. 2D;  
         [0036]    [0036]FIG. 5A is a view showing a quantization result example of an 8×8 block;  
         [0037]    [0037]FIG. 5B is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 5A in twos in the zigzag scan sequence;  
         [0038]    [0038]FIG. 5C is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 5A in twos in the scan sequence shown in FIG. 2B;  
         [0039]    [0039]FIG. 5D is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 5A in twos in the scan sequence shown in FIG. 2C;  
         [0040]    [0040]FIG. 5E is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 5A in twos in the scan sequence shown in FIG. 2D;  
         [0041]    [0041]FIG. 6A is a view showing a quantization result example of an 8×8 block;  
         [0042]    [0042]FIG. 6B is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 6A in twos in the zigzag scan sequence;  
         [0043]    [0043]FIG. 6C is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 6A in twos in the scan sequence shown in FIG. 2B;  
         [0044]    [0044]FIG. 6D is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 6A in twos in the scan sequence shown in FIG. 2C;  
         [0045]    [0045]FIG. 6E is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 6A in twos in the scan sequence shown in FIG. 2D;  
         [0046]    [0046]FIG. 7 is a block diagram showing the arrangement of the fourth embodiment of the present invention;  
         [0047]    [0047]FIG. 8 is a block diagram showing the arrangement of the fifth embodiment of the present invention;  
         [0048]    [0048]FIG. 9A is a view showing an orthogonal transformation result example of an 8×8 block;  
         [0049]    In FIG. 9B is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 9A in twos in the zigzag scan sequence;  
         [0050]    [0050]FIG. 9C is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 9A in twos in the scan sequence shown in FIG. 2B;  
         [0051]    [0051]FIG. 9D is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 9A in twos in the scan sequence shown in FIG. 2C;  
         [0052]    [0052]FIG. 9E is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 9A in twos in the scan sequence shown in FIG. 2D;  
         [0053]    [0053]FIG. 9F is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 9A in twos in the scan sequence shown in FIG. 2E;  
         [0054]    [0054]FIG. 10A is a view showing an orthogonal transformation result example of an 8×8 block;  
         [0055]    [0055]FIG. 10B is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 10A in twos in the zigzag scan sequence;  
         [0056]    [0056]FIG. 10C is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 10A in twos in the scan sequence shown in FIG. 2B;  
         [0057]    [0057]FIG. 10D is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 10A in twos in the scan sequence shown in FIG. 2C;  
         [0058]    [0058]FIG. 10E is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 10A in twos in the scan sequence shown in FIG. 2D;  
         [0059]    [0059]FIG. 10F is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 10A in twos in the scan sequence shown in FIG. 2E;  
         [0060]    [0060]FIG. 11A is a view showing an orthogonal transformation result example of an 8×8 block;  
         [0061]    [0061]FIG. 11B is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 11A in twos in the zigzag scan sequence;  
         [0062]    [0062]FIG. 11C is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 11A in twos in the scan sequence shown in FIG. 2B;  
         [0063]    [0063]FIG. 11D is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 11A in twos in the scan sequence shown in FIG. 2C;  
         [0064]    [0064]FIG. 11E is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 11A in twos in the scan sequence shown in FIG. 2D;  
         [0065]    [0065]FIG. 11F is a view showing the combination of factors obtained by arranging the factors in the orthogonal transformation result example in FIG. 11A in twos in the scan sequence shown in FIG. 2E;  
         [0066]    [0066]FIG. 12A is a flow chart showing the flow of processing in the first embodiment of the present invention;  
         [0067]    [0067]FIG. 12B is a flow chart showing the flow of processing in the first embodiment of the present invention;  
         [0068]    [0068]FIG. 12C is a flow chart showing the flow of processing in the first embodiment of the present invention;  
         [0069]    [0069]FIG. 12D is a flow chart showing the flow of processing in the first embodiment of the present invention;  
         [0070]    [0070]FIG. 13A is a flow chart showing the flow of processing in the fourth embodiment of the present invention;  
         [0071]    [0071]FIG. 13B is a flow chart showing the flow of processing in the fourth embodiment of the present invention;  
         [0072]    [0072]FIG. 13C is a flow chart showing the flow of processing in the fourth embodiment of the present invention;  
         [0073]    [0073]FIG. 13D is a flow chart showing the flow of processing in the fourth embodiment of the present invention;  
         [0074]    [0074]FIG. 14A is a view showing a quantization result example of an 8×8 block;  
         [0075]    [0075]FIG. 14B is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 14A in twos in the zigzag scan sequence;  
         [0076]    [0076]FIG. 14C is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 14A in twos in the scan sequence shown in FIG. 2E;  
         [0077]    [0077]FIG. 15A is a view showing a quantization result example of an 8×8 block;  
         [0078]    [0078]FIG. 15B is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 15A in twos in the zigzag scan sequence;  
         [0079]    [0079]FIG. 15C is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 15A in twos in the scan sequence shown in FIG. 2E;  
         [0080]    [0080]FIG. 16A is a view showing a quantization result example of an 8×8 block;  
         [0081]    [0081]FIG. 16B is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 16A in twos in the zigzag scan sequence; and  
         [0082]    [0082]FIG. 16C is a view showing the combination of factors obtained by arranging the factors in the quantization result example in FIG. 16A in twos in the scan sequence shown in FIG. 2E. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0083]    Preferred embodiments of the present invention will be described in detail in accordance with the accompanying drawings.  
         [0084]    [First Embodiment] 
         [0085]    [0085]FIG. 1 shows an arrangement of a coder according to the first embodiment of the present invention. This embodiment is configured to process factors in twos. The flow charts of FIGS. 12A to  12 D show the flow of processing.  
         [0086]    Reference numeral  101  denotes an orthogonal transformer which receives the data obtained by segmenting a source image as a coding target into a plurality of blocks, performs orthogonal transformation for each block, and sequentially outputs orthogonal transformation factors to a first scan converter  102  ( S 1201 ).  
         [0087]    The first scan converter  102  receives the orthogonal transformation factors output from the orthogonal transformer  101 , rearranges the factors in a predetermined scan sequence (S 1202 ), and outputs them in twos. The first scan converter  102  is comprised of, for example, a block memory  102 ( a ) and address generator  102 ( b ). The block memory  102 ( a ) temporarily stores one-block orthogonal transformation factors output from the orthogonal transformer  101 , and performs write/read operation in accordance with the addresses indicated by the address generator  102 ( b ). The address generator  102 ( b ) generates addresses to sequentially output the orthogonal transformation factors, which are read out from the block memory  102 ( a ), in a predetermined scan sequence. In consideration of processing speed, an actual scan sequence is preferably set such that significant factors are dispersed as much as possible within each of the orthogonal transformation blocks. For example, the scan sequences shown in FIGS. 2B, 2C, and  2 D and the like are advantageous.  
         [0088]    A quantization threshold table  105  has table components rearranged in advance in correspondence with a predetermined scan sequence, and outputs, in twos, quantization thresholds corresponding to the orthogonal transformation factors output in twos from the first scan converter  102  (S 1204 ). Comparators  103  and  104  respectively receive two pairs of orthogonal transformation factors and corresponding quantization thresholds output from the first scan converter  102  and quantization threshold table  105 , and compare the orthogonal transformation factors with the quantization thresholds. Each comparator then outputs comparison result information indicating whether the orthogonal transformation factor is smaller than the quantization threshold (S 1205 ). This comparison result information is equivalent to information indicating whether the result obtained by quantizing the orthogonal transformation factor with the corresponding quantization threshold becomes 0.  
         [0089]    A controller  106  outputs control signals in accordance with the output results from the comparators  103  and  104 . More specifically, if at least one of the quantization results on the two orthogonal transformation factors output from the first scan converter  102  is 0 ( 8 - 3 ), the controller  106  outputs control signals to selectors  107  and  108  to select the factor (significant factor) the quantization result of which is not 0 (if the two quantization results are 0, outputting control signals for selecting any quantization result exerts no influence on operation). If neither of the quantization results is 0, the controller  106  outputs control signals to the selectors to alternately select the quantization results one by one in two cycles in accordance with the scan sequence of factors converted by the first scan converter  102 . In addition, the controller  106  outputs a format signal to the second scan converter  110  in accordance with the output results from the comparators  103  and  104 . The format signal includes information indicating “a pair of 0 and significant factor” if one of the two orthogonal transformation factors output from the first scan converter  102  is 0, “a pair of 0 and 0” if the two factors are 0, or “only one significant factor” if the two factors are significant factors (if the two factors are significant factors, since the factors are quantized one by one in two cycles, information indicating “only one significant factor” is consecutively output in two cycles), and information indicating, if the two factors are “a pair of 0 and significant factor”, which comes first in the scan sequence of factors converted by the first scan converter  102 .  
         [0090]    As described above, if an orthogonal transformation factor is 0, a result can be obtained (i.e., 0) without any quantization computation processing. If, therefore, at least one of two orthogonal transformation factors is 0, control is performed to quantize the two orthogonal transformation factors in one cycle.  
         [0091]    The selector  107  selects one of the two orthogonal transformation factors output from the first scan converter  102  in accordance with the control signal output from the controller  106 , and outputs the selected factor to a quantization computation unit  109 .  
         [0092]    The selector  108  selects one of the two quantization thresholds output from the quantization threshold table  105  in accordance with the control signal output from the controller  106 , and outputs the selected threshold to the quantization computation unit  109 . The quantization threshold output from the selector  108  becomes a quantization threshold that always corresponds to the orthogonal transformation factor output from the selector  107 .  
         [0093]    The quantization computation unit  109  outputs the quantization result obtained by dividing the output from the selector  107  by the output from the selector  108 , and outputs the result to a second scan converter  110 .  
         [0094]    The second scan converter  110  rearranges outputs from the quantization computation unit  109  in a zigzag scan sequence in accordance with the format signal output from the controller  106  (S 1219 ), and outputs them in twos to a Huffman coder  111  (S 1220 ). The second scan converter  110  is comprised of, for example, a block memory  110 ( a ) and address generator  110 ( b ). The block memory  110 ( a ) temporarily stores one-block outputs from the quantization computation unit  109  and performs write/read operation in accordance with the addresses indicated by the address generator  110 ( b ). The address generator  110 ( b ) generates addresses to output, in a zigzag sequence, the orthogonal transformation factors read out from the block memory  110 ( a ).  
         [0095]    The Huffman coder  111  sequentially codes the outputs from the second scan converter  110  into Huffman codes (S 1221 ). As described above, if one of the quantization results on the two orthogonal transformation factors output from the first scan converter  102  is 0, it takes only a one-cycle processing time to quantize the two factors. If either of the two factors is not 0, since quantization processing based on division is required, it takes a two-cycle processing time to quantize the two factors.  
         [0096]    Consider processing in which the following are obtained when the results obtained by quantizing the two orthogonal transformation factors output from the first scan converter  102  with quantization thresholds are arranged in the order in which the factors are output from the first scan converter  102 .  
         [0097]    Example: “0” indicates that the quantization result is 0, and “S” indicates that the quantization result is not 0 (i.e., a significant factor).  
         [0098]    Quantization result: (0, S), (0, 0), (S, 0), (S, S)  
         [0099]    Since one of the first pair (0, S) is 0 and the other is a significant factor, the controller  106  outputs control signals to the selectors  107  and  108  to select the significant factor (S 1217 ), and simultaneously outputs, to the second scan converter  110 , information indicating “a pair of 0 and significant factor” and information indicating which one of these factors comes first in the scan sequence of factors converted by the first scan converter  102  (S 1216 ).  
         [0100]    The quantization computation unit  109  outputs, to the second scan converter  110 , the quantized orthogonal transformation factor (significant factor) obtained by dividing the significant factor output from the selector  107  by the quantization thresholds output from the selector  108  (S 1218 ).  
         [0101]    Since the next pair (0, 0) are both 0, the controller  106  outputs control signals to the selectors  107  and  108  to select one of the factors (either will do) (S 1209 ). At the same time, the controller  106  outputs a format signal indicating “a pair of 0 and 0” to the second scan converter  110  (S 1208 ).  
         [0102]    The quantization computation unit  109  outputs, to the second scan converter  110 , the quantized orthogonal transformation factor (which eventually becomes an insignificant factor in this case) obtained by dividing the factor output from the selector  107  by the quantization threshold output from the selector  108  (S 1210 ).  
         [0103]    Since one of the next pair (S, 0) is a significant factor, and the other is 0, the controller  106  outputs control signals to the selectors  107  and  108  to select the significant factor (S 1217 ). At the same time, the controller  106  outputs, to the second scan converter  110 , information indicating “a pair of 0 and significant factor” and information indicating which one of the factors comes first in the scan sequence of factors converted by the first scan converter  102  (S 1216 ).  
         [0104]    The quantization computation unit  109  outputs, to the second scan converter  110 , the quantized orthogonal transformation factor (significant factor) obtained by dividing the significant factor output from the selector  107  by the quantization threshold output from the selector  108  (S 1218 ).  
         [0105]    Since neither of the next pair (S, S) is 0, the controller  106  outputs control signals to the selectors  107  and  108  to select one of them which comes first in the scan sequence of factors converted by the first scan converter  102  in the first cycle (S 1212 ). At the same time, the controller  106  outputs information indicating “only one significant factor” to the second scan converter  110  (S 1211 ). In the next cycle, the controller  106  outputs control signals to the selectors to select the factor that comes after in the scan sequence of factors converted by the first scan converter  102 , and simultaneously outputs information indicating “only one significant factor” to the second scan converter  110  (S 1214 ).  
         [0106]    The quantization computation unit  109  performs quantization computation in two cycles by using the factors and quantization thresholds respectively output from the selector  107  and selector  108  in two cycles, and outputs the results as quantized orthogonal transformation factors to the second scan converter  110  in two cycles (S 1213 , S 1215 ).  
         [0107]    As described above, in the above case, quantization processing for eight factors is performed in a total of five cycles.  
         [0108]    [Second Embodiment] 
         [0109]    [0109]FIG. 3 shows an arrangement of a coder according to the second embodiment.  
         [0110]    Reference numeral  301  denotes an orthogonal transformer which receives the data obtained by segmenting a source image as a coding target into a plurality of blocks, performs orthogonal transformation for each block, and sequentially outputs orthogonal transformation factors to a first scan converter  302 . At the same time, the orthogonal transformer  301  outputs the frequency distribution information of the processing target blocks to a sequence selection signal generator  312 . The sequence selection signal generator  312  outputs a sequence selection signal for selecting one of a plurality of scan sequences set in the first scan converter  302  on the basis of the frequency distribution information output from the orthogonal transformer  301 .  
         [0111]    Assume that disproportionally many significant factors are distributed in the upper half part, as in the case of the quantization result arrangement example shown in FIG. 5A. In this case, if scanning is performed in the scan sequences shown in FIGS. 2A to  2 D, the numbers of pairs of (S, S) are  5 ,  2 ,  1 , and  4 , respectively. If, therefore, the first scan converter  302  performs scanning in the scan sequence shown in FIG. 2C, the number of pairs of (S, S) becomes small. Assume that disproportionally many significant factors are distributed in the left half, as in the case of the quantization result arrangement example shown in FIG. 6A. In this case, if scanning is performed in the scan sequences shown in FIGS. 2A to  2 D, the numbers of pairs of (S, S) are  5 ,  2 ,  4 , and  1 , respectively. If, therefore, the first scan converter  302  performs scanning in the scan sequence shown in FIG. 2D, the number of pairs of (S, S) becomes small. As described above, the sequence selection signal generator  312  generates a sequence selection signal to select a scan sequence that is advantageous in terms of processing speed on the basis of the frequency distribution information of each processing target block.  
         [0112]    The first scan converter  302  receives the orthogonal transformation factors output from the orthogonal transformer  301 , selects one of a plurality of scan sequences prepared in advance in accordance with the sequence selection signal output from the sequence selection signal generator  312 , rearranges the factors in the selected scan sequence, and outputs the factors in twos. The first scan converter  302  is comprised of a block memory  302 ( a ), an address generator  1   302 ( b ), an address generator  2   302 ( c ), and an address generator  3   302 ( d ), and a selector  302 ( e ) for selecting one of outputs from the address generator  1   302 ( b ), address generator  2   302 ( c ), and address generator  3   302  ( d ). The block memory  302  ( a ) temporarily stores one-block orthogonal transformation factors output from the orthogonal transformer  301 , and performs read/write operation in accordance with the addresses indicated by the selector  3   302  ( e ). When orthogonal transformation factors are read out from the block memory  302 ( a ), the address generator  1   302 ( b ), address generator  2   302 ( c ), and address generator  3   302 ( d ) respectively generate addresses such that the factors are output in predetermined scan sequences like the scan sequences shown in FIGS. 2B, 2C, and  2 D.  
         [0113]    A quantization threshold table  305  outputs, in twos, quantization thresholds corresponding to the orthogonal transformation factors output in twos from the first scan converter  302  in accordance with the sequence selection signal output from the sequence selection signal generator  312 .  
         [0114]    Comparators  303  and  304  respectively receive two pairs of orthogonal transformation factors and corresponding quantization thresholds output from the first scan converter  302  and quantization threshold table  305 , and compare the orthogonal transformation factors with the quantization thresholds. Each comparator then outputs comparison result information indicating whether the orthogonal transformation factor is smaller than the quantization threshold. This comparison result information is equivalent to information indicating whether the result obtained by quantizing the orthogonal transformation factor with the corresponding quantization threshold becomes 0.  
         [0115]    A controller  306  outputs control signals in accordance with the output results from the comparators  303  and  304 . More specifically, if at least one of the quantization results on the two orthogonal transformation factors output from the first scan converter  302  is 0, the controller  306  outputs control signals to selectors  307  and  308  to select the factor the quantization result of which is not 0 (if the two quantization results are 0, outputting control signals for selecting any quantization result exerts no influence on operation). If neither of the quantization results is 0, the controller  306  outputs control signals to the selectors to alternately select the quantization results one by one in two cycles in accordance with the scan sequence selected by the first scan converter  302 . In addition, the controller  306  outputs a format signal to the second scan converter  310  in accordance with the output results from the comparators  303  and  304 . The format signal includes information indicating “a pair of 0 and significant factor” if one of the two orthogonal transformation factors output from the first scan converter  302  is 0, “a pair of 0 and 0” if the two factors are 0, or “only one significant factor” if the two factors are significant factors (if the two factors are significant factors, since the factors are quantized one by one in two cycles, information indicating “only one significant factor” is consecutively output in two cycles), and information indicating, if the two factors are “a pair of 0 and significant factor”, which of the pair of 0 and significant factor comes first in the scan sequence of factors converted by the first scan converter  302 .  
         [0116]    As described above, if an orthogonal transformation factor is 0, a result can be obtained (i.e., 0) without any quantization computation processing. If, therefore, at least one of two orthogonal transformation factors is 0, control is performed to quantize the two orthogonal transformation factors in one cycle.  
         [0117]    The selector  307  selects one of the two orthogonal transformation factors output from the first scan converter  302  in accordance with the control signal output from the controller  306 , and outputs the selected factor to a quantization computation unit  309 . The selector  308  selects one of the two quantization thresholds output from the quantization threshold table  305  in accordance with the control signal output from the controller  306 , and outputs the selected threshold to the quantization computation unit  309 . The quantization threshold output from the selector  308  becomes a quantization threshold that always corresponds to the orthogonal transformation factor output from the selector  307 .  
         [0118]    The quantization computation unit  309  outputs the quantization result obtained by dividing the output from the selector  307  by the output from the selector  308 , and outputs the result to a second scan converter  310 .  
         [0119]    The second scan converter  310  rearranges outputs from the quantization computation unit  309  in a zigzag scan sequence in accordance with the format signal output from the controller  306 , and outputs them in twos to a Huffman coder  311 . The second scan converter  310  is comprised of, for example, a block memory  310 ( a ), an address generator  4   310 ( b ), an address generator  5   310 ( c ), and an address generator  6   310 ( d ), and a selector  310 ( e ) for selecting one of outputs from the address generator  4   310 ( b ), address generator  5   310 ( c ), and address generator  6   310 ( d ) in accordance with the sequence selection signal output from the orthogonal transformer  301 . The block memory  310 ( a ) temporarily stores one-block outputs from the quantization computation unit  309  and performs write/read operation in accordance with the addresses output from the selector  310 ( e ). The address generator  4   310 ( b ), address generator  5   310 ( c ), and address generator  6   310 ( d ) respectively generate addresses to output, in a zigzag sequence, the quantized orthogonal transformation factors read out from the block memory  310 ( a ).  
         [0120]    The Huffman coder  311  sequentially Huffman-codes the output from the second scan converter  310 .  
         [0121]    [0121]FIG. 4A shows an example in which orthogonal transformation and quantization are performed for an image segmented into blocks each constituted by 8×8 pixels. Referring to FIG. 4A, “S” indicates a significant factor, and “0” indicates an insignificant factor. It is generally known that power concentrates on DC components and low-frequency components in many images upon orthogonal transformation. In addition, in many cases, large values are assigned to quantization thresholds corresponding to high-frequency components considering that the human visual characteristics are insensitive to high-frequency components. For this reason, as shown in FIG. 4A, significant factors tend to continuously concentrate on the upper left part of an orthogonal transformation block, i.e., an early part of the zigzag scan sequence.  
         [0122]    If factors are extracted and arranged in twos in the zigzag scan sequence in the quantization result arrangement example shown in FIG. 4A, the arrangement shown in FIG. 4B is obtained. In this case, since there are seven pairs of (S, S), the quantization processing time for 64 factors is 7×2+(32−7)=39 cycles. That is, in the example shown in FIG. 4A, with the conventional arrangement, a quantization processing time of 39 cycles is required.  
         [0123]    In contrast to this, with the arrangement according to the first embodiment of the present invention, the following are the processing times required for quantization processing.  
         [0124]    First of all, when the scan sequence shown in FIG. 2B is selected, and the factors in the quantization result arrangement example shown in FIG. 4A are arranged in twos, the arrangement shown in FIG. 4C is obtained. In this case, since there are two pairs of (S, S), the quantization processing time for 64 factors is 2×2+(32−2)=34 cycles.  
         [0125]    Next, when the scan sequence shown in FIG. 2C is selected, and the factors in the quantization result arrangement example shown in FIG. 4A are arranged in twos, the arrangement shown in FIG. 4D is obtained. In this case, since there is one pair of (S, S), the quantization processing time for 64 factors is 1×2+(32−1)=33 cycles.  
         [0126]    Next, when the scan sequence shown in FIG. 2D is selected, and the factors in the quantization result arrangement example shown in FIG. 4A are arranged in twos, the arrangement shown in FIG. 4E is obtained. In this case, since there are two pairs of (S, S), the quantization processing time for 64 factors is 2×2 +(32−2)=34 cycles.  
         [0127]    As described above, the difference between the number of cycles for each 8×8 block according to the conventional scan sequence and that according to the scan sequence in the present invention is five when the scan sequences shown in FIGS. 2B and 2D are selected, and six when the scan sequence shown in FIG. 2C is selected. If this difference is converted in terms of the entire image (pixel count Na), the difference (D) in the number of cycles can be given by the following equations.  
         [0128]    If the total number of pixels is 2,000,000, D=(Na/64)×5[cycles].  
         [0129]    If the scan sequence shown in FIG. 2B is selected,  
         [0130]    D=(200×10 6 /64)×5=15.6×10 6  [cycles] 
         [0131]    If the scan sequence shown in FIG. 2C is selected,  
         [0132]    D=(200×10 6 /64)×6=18.75×10 6  [cycles] 
         [0133]    [Third Embodiment] 
         [0134]    Note that a scan sequence in a raster scan longitudinal direction like that shown in FIG. 2E or the raster scan sequence may be used instead of a scan sequence based on a complex arrangement of high- and low-frequency components shown in FIGS. 2B to  2 D. A case where the scan sequence in the raster scan longitudinal direction is used will be described in detail below.  
         [0135]    [0135]FIG. 14A shows an example in which orthogonal transformation and quantization are performed for an image segmented into blocks each constituted by 8×8 pixels. As in the case shown in FIG. 4A, in the case shown in FIG. 14A, “S” indicates a significant factor, and “0” indicates an insignificant factor.  
         [0136]    In the example shown in FIG. 14A, when factors are extracted in twos in the zigzag scan sequence and arranged, the arrangement shown in FIG. 14B is obtained. In this case, since there are nine pairs of (S, S), the quantization processing time for 64 factors is 9×2+(32−9)=41 cycles. That is, in the example (the arrangement according to the prior art) shown in FIG. 14A, a quantization processing time of  41  cycles is required.  
         [0137]    In contrast to this, when the scan sequence shown in FIG. 2E is used, the following processing time is required for quantization processing. When the scan sequence shown in FIG. 2E is used, and the factors in the example shown in FIG. 14A are arranged in twos in the scan sequence, the arrangement shown in FIG. 14C is obtained. In this case, since there are six pairs of (S, S), the quantization processing time for 64 factors is 6×2+(32−6)=38 cycles.  
         [0138]    [0138]FIG. 15A shows another example in which orthogonal transformation and quantization are performed for an image segmented into blocks each constituted by 8×8 pixels. As in the example shown in FIG. 4A, in the example shown in FIG. 15A, “S” indicates a significant factor, and “0” indicates an insignificant factor.  
         [0139]    In the example shown in FIG. 15A, when factors are extracted in twos in the zigzag scan sequence and arranged, the arrangement shown in FIG. 15B is obtained. In this case, since there are eight pairs of (S, S), the quantization processing time for 64 factors is 8×2+(32−8)=40 cycles. That is, in the example (the arrangement according to the prior art) shown in FIG. 15A, a quantization processing time of 40 cycles is required.  
         [0140]    In contrast to this, when the scan sequence shown in FIG. 2E is used, the following processing time is required for quantization processing. When the scan sequence shown in FIG. 2E is used, and the factors in the example shown in FIG. 15A are arranged in twos in the scan sequence, the arrangement shown in FIG. 15C is obtained. In this case, since there are five pairs of (S, S), the quantization processing time for 64 factors is 5×2+(32−5)=37 cycles.  
         [0141]    [0141]FIG. 16A shows an example in which orthogonal transformation and quantization are performed for an image segmented into blocks each constituted by 8×8 pixels. As in the case shown in FIG. 4A, in the case shown in FIG. 16A, “S” indicates a significant factor, and “0” indicates an insignificant factor.  
         [0142]    In the example shown in FIG. 16A, when factors are extracted in twos in the zigzag scan sequence and arranged, the arrangement shown in FIG. 16B is obtained. In this case, since there are seven pairs of (S. S), the quantization processing time for 64 factors is 7×2+(32−7)=39 cycles. That is, in the example (the arrangement according to the prior art) shown in FIG. 16A, a quantization processing time of 39 cycles is required.  
         [0143]    In contrast to this, when the scan sequence shown in FIG. 2E is used, the following processing time is required for quantization processing.  
         [0144]    When the scan sequence shown in FIG. 2E is used, and the factors in the example shown in FIG. 16A are arranged in twos in the scan sequence, the arrangement shown in FIG. 16C is obtained. In this case, since there are four pairs of (S, S), the quantization processing time for 64 factors is 4×2+(32−4)=36 cycles.  
         [0145]    As described above, the number of cycles can be sufficiently reduced even by using the scan sequence shown in FIG. 2E.  
         [0146]    [Fourth Embodiment] 
         [0147]    [0147]FIG. 7 shows the fourth embodiment. Reference numeral  701  denotes a Huffman decoder for decoding Huffman-coded data and outputting the resultant data as quantized orthogonal transformation factors to a first scan converter  702  (S 1301 ).  
         [0148]    The first scan converter  702  receives the quantized orthogonal transformation factors output from the Huffman decoder  701 , rearranges them in a predetermined scan sequence (S 1302 ), and outputs them in twos (S 1303 ). The first scan converter  702  is comprised of a block memory  702 ( a ) and address generator  702 ( b ). The block memory  702 ( a ) temporarily stores one-block quantized orthogonal transformation factors output from the Huffman decoder  701  and performs write/read operation in accordance with the addresses indicated by the address generator  702 ( b ). The address generator  702 ( b ) generates addresses to sequentially output the quantized orthogonal transformation factors, which are read out from the block memory  702 ( a ), in a predetermined scan sequence. In consideration of processing speed, an actual scan sequence is preferably set such that significant factors are dispersed as much as possible within each of the orthogonal transformation blocks. For example, the scan sequences shown in FIGS. 2B, 2C, and  2 D and the like are advantageous.  
         [0149]    A quantization threshold table  704  outputs, in twos, quantization thresholds corresponding to the quantized orthogonal transformation factors output in twos from the first scan converter  702  (S 1304 ).  
         [0150]    A 0 determination unit  703  receives the two quantized orthogonal transformation factors output from the first scan converter  702 , determines whether each of the input quantized orthogonal transformation factors is 0, and outputs the determination result to a controller  705  (S 1305 ).  
         [0151]    A controller  705  outputs control signals in accordance with the output results from the 0 determination unit  703 . More specifically, if at least one of the two quantized orthogonal transformation factors output from the first scan converter  702  is 0, the controller  705  outputs control signals to selectors  706  and  707  to select the factor which is not 0 (significant factor) (if the two factors are 0, outputting control signals for selecting any factor exerts no influence on operation). If neither of the factors is 0, the controller  705  outputs control signals to the selectors to alternately select the factors one by one in two cycles in accordance with the scan sequence output from the first scan converter  702 . In addition, the controller  705  outputs a format signal to a second scan converter  709  in accordance with the output results from the 0 determination unit  703 . The format signal includes information indicating “a pair of 0 and significant factor” if one of the two quantized orthogonal transformation factors output from the first scan converter  702  is 0, “a pair of 0 and 0” if the two factors are 0, or “only one significant factor” if the two factors are significant factors (if the two factors are significant factors, since the factors are inversely quantized one by one in two cycles, information indicating “only one significant factor” is consecutively output in two cycles), and information indicating, if the two factors are “a pair of 0 and significant factor”, which of the pair of 0 and significant factor comes first in the scan sequence of factors converted by the first scan converter  702 .  
         [0152]    As described above, if a quantized orthogonal transformation factor is 0, a result can be obtained (i.e., 0) without any inverse quantization computation processing. If, therefore, at least one of two quantized orthogonal transformation factors is 0, control is performed to inversely quantize the two orthogonal transformation factors substantially in one cycle.  
         [0153]    The selector  706  selects one of the two quantized orthogonal transformation factors output from the first scan converter  702  in accordance with the control signal output from the controller  705 , and outputs the selected factor to an inverse quantization computation unit  708 . The selector  707  selects one of the two quantization thresholds output from the quantization threshold table  704  in accordance with the control signal output from the controller  705 , and outputs the selected threshold to the inverse quantization computation unit  708 . The quantization threshold output from the selector  707  becomes a quantization threshold that always corresponds to the quantized orthogonal transformation factor output from the selector  706 .  
         [0154]    The inverse quantization computation unit  708  outputs, to the second scan converter  709 , the inverse quantization results obtained by multiplying outputs from the selector  706  by outputs from the selector  707 .  
         [0155]    The second scan converter  709  rearranges the outputs from the inverse quantization computation unit  708  in a predetermined scan sequence in accordance with the format signal output from the controller  705  (S 1319 ), and outputs the results in twos to an inverse orthogonal transformer  710  (S 1320 ). The second scan converter  709  is comprised of a block memory  709 ( a ) and address generator  709 ( b ). The block memory  709 ( a ) temporarily stores one-block outputs from the inverse quantization computation unit  708  and performs write/read operation in accordance with the addresses output from the address generator  709 ( b ). The address generator  709 ( b ) generates addresses to output, in a predetermined scan sequence, the orthogonal transformation factors read out from the block memory  709 ( a ). The actual scan conversion sequence executed by the second scan converter  709  should be determined in accordance with the arrangement of the inverse orthogonal transformer  710 . For example, a raster scan sequence or a vertical raster scan sequence is generally used.  
         [0156]    The inverse orthogonal transformer  710  sequentially performs inverse orthogonal transformation processing for outputs from the second scan converter  709  (S 1321 ).  
         [0157]    As described above, the above arrangement includes a 0 determination unit like the one described above to check before inverse quantization processing whether an inverse quantization result on each quantized orthogonal transformation factor becomes 0. If at least one of the results becomes 0, there is no need to perform multiplication processing. Therefore, the two factors, i.e., this factor and the other factor, can be inversely quantized in one cycle by one inverse quantization computation unit.  
         [0158]    Processing to be performed when the two quantized orthogonal transformation factors output from the first scan converter  702  become the following factors will be described as an example.  
         [0159]    Example: “0” indicates that a quantized orthogonal transformation factor is 0, and “S” indicates that a quantized orthogonal transformation factor is not 0 (e.g., a significant factor).  
         [0160]    Quantized orthogonal transformation factors: (0, S), (0, 0), (S, 0), (S, S)  
         [0161]    First of all, since one of the first pair (0, S) is 0, and the other is a significant factor, the controller  705  outputs control signals to the selectors  706  and  707  to select the significant factor, and simultaneously outputs, to the second scan converter  709 , information indicating “a pair of 0 and significant factor” and information indicating which of the factors comes first in the scan sequence of factors converted by the first scan converter  702  (S 1316 ).  
         [0162]    The inverse quantization computation unit  708  outputs, to the second scan converter  709 , the orthogonal transformation factor (significant factor) obtained by multiplying the significant factor output from the selector  706  by the quantization threshold output from the selector  707  (S 1318 ).  
         [0163]    Since the next pair (0, 0) are both 0, the controller  705  outputs control signals to the selectors  706  and  707  to select one of the factors (either will do) (S 1314 ). At the same time, the controller  705  outputs information indicating “a pair of 0 and 0” to the second scan converter  709  (S 1313 ).  
         [0164]    The inverse quantization computation unit  708  outputs, to the second scan converter  709 , the quantized orthogonal transformation factor (which eventually becomes an insignificant factor in this case) obtained by multiplying the factor output from the selector  706  by the quantization threshold output from the selector  707  (S 1315 ).  
         [0165]    Since one of the next pair (S, 0) is a significant factor, and the other is 0, the controller  705  outputs control signals to the selectors  706  and  707  to select the significant factor (S 1317 ). At the same time, the controller  705  outputs, to the second scan converter  709 , information indicating “a pair of 0 and significant factor” and information indicating which one of the factors comes first in the scan sequence of factors converted by the first scan converter  702  (S 1316 ).  
         [0166]    The inverse quantization computation unit  708  outputs, to the second scan converter  709 , the orthogonal transformation factor (significant factor) obtained by multiplying the factor output from the selector  706  by the quantization threshold output from the selector  707  (S 1318 ).  
         [0167]    Since neither of the next pair (S, S) is 0, the controller  705  outputs control signals to the selectors  706  and  707  to select one of them which comes first in the scan sequence of factors converted by the first scan converter  702  in the first cycle (S 1308 ). At the same time, the controller  705  outputs information indicating “only one significant factor” to the second scan converter  709  (S 1309 ). In the next cycle, the controller  705  outputs control signals to the selectors to select the factor that comes after in the scan sequence of factors converted by the first scan converter  702 , and simultaneously outputs information indicating “only one significant factor” to the second scan converter  709  (S 1311 ).  
         [0168]    The inverse quantization computation unit  708  performs inverse quantization computation in two cycles by using the factors and quantization thresholds respectively output from the selector  706  and selector  707  in two cycles, and outputs the results as quantized orthogonal transformation factors to the second scan converter  709  in two cycles (S 1310  and S 1312 ).  
         [0169]    As described above, in the above example, inverse quantization processing for eight factors can be performed in a total of five cycles.  
         [0170]    [Fifth Embodiment] 
         [0171]    [0171]FIG. 8 shows an arrangement of a decoder according to the fifth embodiment of the present invention.  
         [0172]    Reference numeral  801  denotes a Huffman decoder for decoding Huffman-coded data and outputting the resultant data as quantized orthogonal transformation factors to a first scan converter  802 . The first scan converter  802  receives the quantized orthogonal transformation factors output from the Huffman decoder  801 , selects one of a plurality of predetermined scan sequences in accordance with a predetermined sequence selection signal, rearranges the factors in the selected scan sequence, and outputs the factors in twos. The first scan converter  802  is comprised of a block memory  802 ( a ), an address generator  1   802 ( b ), an address generator  2   802 ( c ), an address generator  3   802 ( d ), and a selector  802 ( e ) for selecting one of outputs from the address generator  1   802 ( b ), address generator  2   802 ( c ), and address generator  3   802 ( d ) in accordance with the sequence selection signal. The block memory  802 ( a ) temporarily stores one-block quantized orthogonal transformation factors output from the Huffman decoder  801  and performs write/read operation in accordance with the addresses output from the selector  802 ( e ). In consideration of processing speed, as the scan sequences set in the address generator  1   802 ( b ), address generator  2   802 ( c ), and address generator  3   802 ( d ), the scan sequences shown in FIGS. 2B, 2C,  2 D, and  2 E and the like are advantageous. FIG. 2E shows a raster scan longitudinal sequence. However, the raster scan sequence may be used.  
         [0173]    Assume that disproportionally many significant factors are distributed in the upper half part, as in the case of the quantization result arrangement example shown in FIG. 10A. In this case, if scanning is performed in the scan sequences shown in FIGS. 2A to  2 E, the numbers of pairs of (S, S) are  6 ,  3 ,  1 ,  5 , and  6 , respectively. If, therefore, write/read operation is performed in the scan sequence shown in FIG. 2C, the number of pairs of (S, S) becomes small. Assume that disproportionally many significant factors are distributed in the left half, as in the case of the quantization result arrangement example shown in FIG. 11A. In this case, if scanning is performed in the scan sequences shown in FIGS. 2A to  2 E, the numbers of pairs of (S, S) are  5 ,  2 ,  4 ,  1 , and  6 , respectively. If, therefore, write/read operation is performed in the scan sequence shown in FIG. 2D, the number of pairs of (S, S) becomes small. As described above, a scan sequence that is advantageous in terms of processing speed is selected on the basis of the significant factor distribution information of each processing target block.  
         [0174]    A quantization threshold table  804  outputs, in twos, quantization thresholds corresponding to the quantized orthogonal transformation factors output in twos from the first scan converter  802 . A 0 determination unit  803  receives the two quantized orthogonal transformation factors output from the first scan converter  802 , determines whether each of the input quantized orthogonal transformation factors is 0, and outputs the determination result to a controller  805 .  
         [0175]    The controller  805  outputs control signals in accordance with the output results from the 0 determination unit  803 . More specifically, if at least one of the two quantized orthogonal transformation factors output from the first scan converter  802  is 0, the controller  805  outputs control signals to selectors  806  and  807  to select the factor which is not 0 (if the two factors are 0, outputting control signals for selecting any factor exerts no influence on operation). If neither of the factors is 0, the controller  805  outputs control signals to the selectors to alternately select the factors one by one in two cycles in accordance with the scan sequence of factors converted by the first scan converter  802 . In addition, the controller  805  outputs a format signal to a second scan converter  809  in accordance with the output results from the 0 determination unit  803 . The format signal includes information indicating “a pair of 0 and significant factor” if one of the two quantized orthogonal transformation factors output from the first scan converter  802  is 0, “a pair of 0 and 0” if the two factors are 0, or “only one significant factor” if the two factors are significant factors (if the two factors are significant factors, since the factors are inversely quantized one by one in two cycles, information indicating “only one significant factor” is consecutively output in two cycles), and information indicating, if the two factors are “a pair of 0 and significant factor”, which of the pair of 0 and significant factor comes first in the scan sequence of factors converted by the first scan converter  802 .  
         [0176]    As described above, if a quantized orthogonal transformation factor is 0, a result can be obtained (i.e., 0) without any inverse quantization computation processing. If, therefore, at least one of two quantized orthogonal transformation factors is 0, control is performed to inversely quantize the two orthogonal transformation factors substantially in one cycle.  
         [0177]    The selector  806  selects one of the two quantized orthogonal transformation factors output from the first scan converter  802  in accordance with the control signal output from the controller  805 , and outputs the selected factor to an inverse quantization computation unit  808 . The selector  807  selects one of the two quantization thresholds output from the quantization threshold table  804  in accordance with the control signal output from the controller  805 , and outputs the selected threshold to the inverse quantization computation unit  808 . The quantization threshold output from the selector  807  becomes a quantization threshold that always corresponds to the quantized orthogonal transformation factor output from the selector  806 .  
         [0178]    The inverse quantization computation unit  808  outputs, to the second scan converter  809 , the inverse quantization results obtained by multiplying outputs from the selector  806  by outputs from the selector  807 .  
         [0179]    The second scan converter  809  selects a scan sequence corresponding to the scan sequence selected by the first scan converter  802  on the basis of a sequence selection signal, rearranges outputs from the inverse quantization computation unit  808  in accordance with the selected scan sequence, and outputs the factors in twos to an inverse orthogonal transformer  810 . The second scan converter  809  is comprised of a block memory  809 ( a ), an address generator  4   809 ( b ), an address generator  5   809 ( c ), an address generator  6   809 ( d ), and a selector  809 ( e ) for selecting one of outputs from the address generator  4   809 ( b ), address generator  5   809 ( c ), and block memory  6   809 ( d ) in accordance with a sequence selection signal. The block memory  809 ( a ) temporarily stores one-block outputs from the inverse quantization computation unit  808 , and performs write/read operation in accordance with the addresses output from the selector  809 ( e ). The inverse orthogonal transformer  810  sequentially performs inverse orthogonal transformation processing for outputs from the second scan converter  809 .  
         [0180]    [0180]FIG. 9A shows an example in which orthogonal transformation and quantization are performed for an image segmented into blocks each constituted by 8×8 pixels. Referring to FIG. 9A, “S” indicates a significant factor, and “0” indicates an insignificant factor.  
         [0181]    It is generally known that power concentrates on DC components and low-frequency components in many images upon orthogonal transformation. In addition, in many cases, large values are assigned to quantization thresholds corresponding to high-frequency components considering that the human visual characteristics are insensitive to high-frequency components. For this reason, as shown in FIG. 9A, significant factors tend to continuously concentrate on the upper left part of an orthogonal transformation block, i.e., an early part of the zigzag scan sequence. The quantization processing time can be minimized by using this characteristic and determining a scan sequence to disperse the significant factors as uniformly as possible.  
         [0182]    In the example shown in FIG. 9A, the following are the numbers of cycles required for processing in the arrangement of the fourth embodiment of the present invention.  
         [0183]    First of all, when the scan sequence shown in FIG. 2B is selected, and the factors in the quantization result arrangement example shown in FIG. 9A are arranged in twos, the arrangement shown in FIG. 9C is obtained. In this case, since there are two pairs of (S, S), the quantization processing time for 64 factors is 2×2+(32−2)=34 cycles.  
         [0184]    Next, when the scan sequence shown in FIG. 2C is selected, and the factors in the quantization result arrangement example shown in FIG. 9A are arranged in twos, the arrangement shown in FIG. 9D is obtained. In this case, since there is one pair of (S, S), the quantization processing time for 64 factors is 1×2+(32−2)=33 cycles.  
         [0185]    Next, when the scan sequence shown in FIG. 2D is selected, and the factors in the quantization result arrangement example shown in FIG. 9A are arranged in twos, the arrangement shown in FIG. 9E is obtained. In this case, since there are three pairs of (S, S), the quantization processing time for 64 factors is 3×2+(32−3)=35 cycles.  
         [0186]    Next, when the scan sequence shown in FIG. 2E is selected, and the factors in the quantization result arrangement example shown in FIG. 9A are arranged in twos, the arrangement shown in FIG. 9F is obtained. In this case, since there are seven pairs of (S, S), the quantization processing time for 64 factors is 7×2+(32−7)=39 cycles.  
         [0187]    The number of write cycles in the block memory  709 ( a ) therefore becomes 33 cycles in the case shown in FIG. 2C. The number of read cycles with respect to the block memory  709 ( a ) becomes 8×8÷2=32 cycles. Since no initialization cycle is required, the total number of processing cycles is 33+32=65 cycles at minimum.  
         [0188]    In the example shown in FIG. 9A, with the conventional arrangement, the following is the number of cycles required for processing.  
         [0189]    In the example shown in FIG. 9A, since the number of significant factors is 20, the total number of clock cycles required for processing for the processing target clock is 20 cycles for write processing, 32 cycles for read processing, and 20 cycles for initialization processing, i.e., a total of 72 cycles.  
         [0190]    In the example shown in FIG. 9A, therefore, the processing time is shortened by about 10% of that required in the prior art. In addition, with the conventional arrangement, the number of processing cycles required per unit block of 8×8 pixels greatly varies from 32 to 160. For this reason, there is a large difference in the time required for decoding between coded data with a high compression ratio and coded data with low compression ratio. According to this embodiment having the arrangement of the present invention, the number of processing cycles required per unit block of 8×8 pixels varies a little from 64 to 96. Therefore, there is a little difference in the time required for decoding between coded data with a high compression ratio and coded data with a low compression ratio. This fact produces an effect when a system incorporating a decoder must match the processing speed required for the decoder with the worst value of the processing speed of the decoder, and also produces an effect when many coded image data processed by a system incorporating a decoder are data with a relatively low compression ratio.  
         [0191]    As has been described above, decoding can be performed at high speed regardless of the compression ratio of an image by considering a scan sequence such that the significant factors of quantized orthogonal transformation factors input to the inverse quantization unit are dispersed as uniformly as possible, except for an initialization processing period of the block memory.  
         [0192]    As described above, according to the present invention, by using a scan sequence which is set such that the significant factors of factors quantized or inversely quantized are dispersed as uniformly as possible, the number of processing cycles can be reduced as compared with processing based on the zigzag scan sequence, thereby realizing coding/decoding processing at high speed as a whole.  
         [0193]    In addition, by preparing a plurality of scan sequences in advance and selectively using the scan sequences in accordance with a sequence selection signal, a scan sequence suitable for the distribution of input factors can be selected. This makes it possible to effectively reduce the number of processing cycles.  
         [0194]    The present invention can be applied to a system constituted by a plurality of devices (e.g., host computer, interface, reader, printer) or to an apparatus comprising a single device (e.g., copying machine, facsimile machine).  
         [0195]    Further, the object of the present invention can also be achieved by providing a storage medium storing program codes for performing the aforesaid processes to a computer system or apparatus (e.g., a personal computer), reading the program codes, by a CPU or MPU of the computer system or apparatus, from the storage medium, then executing the program.  
         [0196]    In this case, the program codes read from the storage medium realize the functions according to the embodiment/embodiments, and the storage medium storing the program codes constitutes the invention.  
         [0197]    Further, the storage medium, such as a floppy disk, a hard disk, an optical disk, a magneto-optical disk, CD-ROM, CD-R, a magnetic tape, a non-volatile type memory card, and ROM-can be used for providing the program codes.  
         [0198]    Furthermore, besides aforesaid functions according to the above embodiment/embodiments are realized by executing the program codes which are read by a computer, the present invention includes a case where an OS (operating system) or the like working on the computer performs a part or entire processes in accordance with designations of the program codes and realizes functions according to the above embodiments.  
         [0199]    Furthermore, the present invention also includes a case where, after the program codes read from the storage medium are written in a function expansion card which is inserted into the computer or in a memory provided in a function expansion unit which is connected to the computer, CPU or the like contained in the function expansion card or unit performs a part or entire process in accordance with designations of the program codes and realizes functions of the above embodiments.  
         [0200]    In a case where the present invention is applied to the aforesaid storage medium, the storage medium stores program codes corresponding to the flowcharts described in the embodiments.  
         [0201]    The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore to apprise the public of the scope of the present invention, the following claims are made.