Abstract:
Provided are an image encoding apparatus and an image decoding apparatus. The image encoding apparatus includes a compression unit compressing a reference image by reducing a resolution of the reference image in a resolution adjustment mode determined from among at least two resolution adjustment modes according to a distribution of values of pixels of the reference image, and providing the compressed reference image to a memory, a reconstruction unit reconstructing the reference image by increasing a resolution of the compressed reference image stored in the memory to an original resolution, a predictive encoding unit performing predictive encoding on a current image by using the reconstructed reference image, and a predictive decoding unit generating the reference image by performing decoding on the predictive encoded current image, and providing the generated reference image to the compression unit.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2008-0009682, filed on Jan. 30, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    One or more embodiments of the present invention generally relate to image encoding and decoding, and more particularly, to an image encoding apparatus and an image decoding apparatus, whereby a reference image is stored in a memory after its size is reduced in order to reduce the number of operation cycles required for a write operation and a read operation for the reference image. 
         [0004]    2. Description of the Related Art 
         [0005]    In a video codec, a reconstructed image of a previous frame has to be stored in order to perform motion prediction or motion compensation on a current frame. The reconstructed image of the previous frame is usually stored in an external memory located outside the video codec because of its large data size. It is a general feature that the number of operation cycles required for a read operation or a write operation from or to the external memory is greater than that of operation cycles required for an arithmetic operation performed within the video codec. 
       SUMMARY 
       [0006]    One or more embodiments of the present invention provide an image encoding apparatus and an image decoding apparatus, whereby the number of operation cycles required to read or write a reference image from or to a memory can be reduced. 
         [0007]    Additional aspects and/or advantages will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention. 
         [0008]    To achieve the above and/or other aspects and advantages, one or more embodiments of the present invention may include an image encoding apparatus comprising a compression unit to compress a reference image by reducing a resolution of the reference image in a resolution adjustment mode determined from among at least two resolution adjustment modes according to a distribution of values of pixels of the reference image, and to provide the compressed reference image to a memory, a reconstruction unit to reconstruct the reference image by increasing a resolution of the compressed reference image stored in the memory to an original resolution, a predictive encoding unit to perform predictive encoding on a current image by using the reconstructed reference image, and a predictive decoding unit to generate the reference image by performing decoding on the predictive encoded current image, and providing the generated reference image to the compression unit. 
         [0009]    To achieve the above and/or other aspects and advantages, one or more embodiments of the present invention may include an image decoding apparatus comprising a compression unit to compress a reference image by reducing a resolution of the reference image in a resolution adjustment mode determined from among at least two resolution adjustment modes according to a distribution of values of pixels of the reference image, and to provide the compressed reference image to a memory, a reconstruction unit to reconstruct the reference image by increasing a resolution of the compressed reference image stored in the memory to an original resolution, and a predictive decoding unit to generate the reference image by performing predictive decoding on a bitstream by using the reconstructed reference image. 
         [0010]    To achieve the above and/or other aspects and advantages, one or more embodiments of the present invention may include an image compression apparatus comprising a resolution adjustment mode determination unit to determine one of uniform quantization and dead-zone quantization as the resolution adjustment mode according to a result of comparison of a range value, for values of pixels constituting a current unit block of the reference image, with a threshold value and a result of comparison of the values of the pixels with a first sub range value and a second sub range value, a quantization unit to perform quantization in order to express each of the values of the pixels of the current unit block at a reduced bit resolution according to the determined resolution adjustment mode, and a fixed-length coding unit to perform fixed-length coding on a quantization index, a representative value, and the range value of the current unit block on which uniform quantization or dead-zone quantization is performed. 
         [0011]    To achieve the above and/or other aspects and advantages, one or more embodiments of the present invention may include an image reconstruction apparatus including a fixed-length decoding unit to reconstruct a quantization index, a representative value, and a range value of a current unit block of a compressed reference image by performing fixed-length decoding on the current unit block, a resolution adjustment mode determination unit to determine one of uniform inverse quantization and dead-zone inverse quantization as the resolution adjustment mode, according to a result of comparison of the reconstructed range value of the current unit block with a threshold value and a result of checking of a flag, and an inverse quantization unit to inversely quantize the quantization index of the current unit block in order to express each of the values of the pixels of the current unit block at the original bit resolution, according to the determined resolution adjustment mode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    These and/or other aspects and advantages will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
           [0013]      FIG. 1  is a block diagram of an image encoding apparatus according to an embodiment of the present invention; 
           [0014]      FIG. 2  is a block diagram of an image decoding apparatus according to an embodiment of the present invention; 
           [0015]      FIG. 3A  illustrates a structure of a reference image compressed according to an embodiment of the present invention; 
           [0016]      FIG. 3B  illustrates the structure of the reference image, illustrated in  FIG. 3A , in the form of a pseudo code; 
           [0017]      FIG. 4  is a block diagram of an image compression apparatus according to an embodiment of the present invention; 
           [0018]      FIG. 5  is a block diagram of an image reconstruction apparatus according to an embodiment of the present invention; 
           [0019]      FIG. 6  is a flowchart of an image compression method according to an embodiment of the present invention; 
           [0020]      FIG. 7  is a flowchart of an image reconstruction method according to an embodiment of the present invention; and 
           [0021]      FIGS. 8A and 8B  are diagrams for explaining uniform quantization and dead-zone quantization as applied to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0022]    Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. In this regard, embodiments of the present invention may be embodied in many different forms and should not be construed as being limited to embodiments set forth herein. Accordingly, embodiments are merely described below, by referring to the figures, to explain aspects of the present invention. 
         [0023]      FIG. 1  is a block diagram of an image encoding apparatus  10  according to an embodiment of the present invention. The image encoding apparatus  10  includes a motion estimation unit  101 , a motion compensation unit  102 , a spatial prediction unit  103 , a subtraction unit  104 , a transform unit  105 , a quantization unit  106 , an entropy-encoding unit  107 , an inverse quantization unit  108 , an inverse transform unit  109 , an addition unit  110 , a compression unit  111 , and a reconstruction unit  112 . 
         [0024]    Referring to  FIG. 1 , the motion estimation unit  101  estimates a motion of a current image of an image sequence based on at least one of a plurality of reference images reconstructed by the reconstruction unit  112 . More specifically, for each of a plurality of blocks, corresponding to an inter mode, from among all blocks of the current image, the motion estimation unit  101  determines a block of a reference image, which best matches a block of the current image, from among the reference images reconstructed by the reconstruction unit  112  and calculates a motion vector indicating displacement between the determined block of the reference image and the block of the current image. 
         [0025]    The motion compensation unit  102  generates a predicted image of the current image from at least one of the reference images reconstructed by the reconstruction unit  112  by using a result of the motion estimation performed by the motion estimation unit  101 . More specifically, the motion compensation unit  102  generates the predicted image of the current image by using the values of the blocks of at least one reference image, which are indicated by the motion vector of each block of the current image calculated by the motion estimation unit  101 . 
         [0026]    For each of the blocks, corresponding to the intra mode, from among all the blocks of the current image, the spatial prediction unit  103  predicts a value of the block of the current image from a value of a block located adjacent to the block of the current image, from among all blocks of a reconstructed image generated by the reconstruction unit  112 , in order to generate a predicted image of the current image. 
         [0027]    The subtraction unit  104  subtracts the predicted image generated by the motion compensation unit  102  or the predicted image generated by the spatial prediction unit  103  from the current image, in order to generate a residue image between the current image and the predicted image. 
         [0028]    The transform unit  105  transforms the residue image generated by the subtraction unit  104  from a spatial domain into a frequency domain. For example, the transform unit  105  may transform the residue image generated by the subtraction unit  104  from the spatial domain into the frequency domain by using discrete Hadamard transformation (DHT) or discrete cosine transformation (DCT). The quantization unit  106  quantizes transformation results obtained by the transform unit  105 . More specifically, the quantization unit  106  divides the transformation results obtained by the transform unit  105 , i.e., frequency component values, by a quantization step size, and approximates quantization results to integers. 
         [0029]    The entropy-encoding unit  107  performs entropy-encoding on the quantization results obtained by the quantization unit  106 , in order to generate a bitstream. For example, the entropy-encoding unit  107  may perform entropy-encoding on the quantization results obtained by the quantization unit  106  by using context-adaptive variable-length coding (CAVLC) or context-adaptive binary arithmetic coding (CABAC). In particular, the entropy-encoding unit  107  entropy-encodes information required for moving image decoding, e.g., index information of a reference image used for inter-prediction, motion vector information, and position information of a block of a reconstructed image used for intra-prediction, in addition to the quantization results obtained by the quantization unit  106 . According to the current embodiment of the present invention, the entropy-encoding unit  107  may also entropy-encode resolution adjustment information. 
         [0030]    The inverse quantization unit  108  performs inverse quantization on the quantization results obtained by the quantization unit  106 . More specifically, the inverse quantization unit  108  reconstructs frequency component values by multiplying the integers approximated by the quantization unit  106  by the quantization step size. The inverse transform unit  109  transforms inverse-quantization results obtained by the inverse quantization unit  108 , i.e., the frequency component values, from the frequency domain into the spatial domain, in order to reconstruct the residue image between the current image and the predicted image. The addition unit  110  adds the residue image reconstructed by the inverse transform unit  109  to the predicted image generated by the motion compensation unit  102  or the spatial prediction unit  103 , in order to generate a reconstructed image of the current image. 
         [0031]    The compression unit  111  compresses the reconstructed image generated by the addition unit  110  by reducing a resolution, e.g., a bit resolution, of the reconstructed image, and stores the compressed reconstructed image (or compression image) in a memory  113 . More specifically, the compression unit  111  expresses the reconstructed image generated by the addition unit  110  with a bit resolution of each of the pixels constituting the reconstructed image in predetermined block units by referring to resolution adjustment information, in order to compress the reconstructed image. 
         [0032]    In the current embodiment of the present invention, the term “bit resolution” means the number of bits that express a value of each pixel. Throughout embodiments of the present invention, it can be easily understood by one of ordinary skill in the art that a bit resolution can be replaced with other terms such as a bit depth or a color depth. In other words, the compression unit  111  compresses the reconstructed image generated by the addition unit  110  by expressing a value of each pixel of the reconstructed image with a reduced bit resolution. 
         [0033]    In general, a basic unit of access to the memory  113 , i.e., the smallest unit of a read or write operation from or to the memory  113 , is 8 bits, i.e., 1 byte. The compression unit  111  reduces the bit resolution of each of the pixels of the reconstructed image in predetermined block units, e.g., in units of 4×4 blocks. The total amount of data of a 4×4 block is 16 bytes when a value of a single pixel is expressed with 8 bits. For example, the total amount of data of a 4×4 block is 8 bytes when a value of a single pixel is expressed with 4 bits, the total amount of data of a 4×4 block is 10 bytes for a pixel value expressed with 5 bits, the total amount of data of a 4×4 block is 12 bytes for a pixel value expressed with 6 bits, and the total amount of data of a 4×4 block is 14 bytes for a pixel value expressed with 7 bits. When a value of each pixel of the reconstructed image generated by the addition unit  110  is composed of a Y value of 8 bits, a Cb value of 8 bits, and a Cr value of 8 bits, the compression unit  111  reduces the number of bits expressing each of the Y value, the Cb value, and the Cr value of each pixel of the reconstructed image, i.e., 8, to 6, 5, or 4. Thus, the Y value of 8 bits, the Cb value of 8 bits, and the Cr value of 8 bits are expressed as a Y value of 4, 5, or 6 bits, a Cb value of 4, 5, or 6 bits, and a Cr value of 4, 5, or 6 bits. 
         [0034]    Although a bit resolution of each of pixels constituting an image is adjusted in 4×4 block units in the current embodiment of the present invention, it can be easily understood by one of ordinary skill in the art that a bit resolution of each of the pixels constituting an image can also be adjusted in various block units such as 2×2 block units, 8×8 block units, and 16×16 block units. 
         [0035]    The reconstruction unit  112  reads the compression image stored in the memory  113  and expresses a value of each pixel of the compression image at the original bit resolution, in order to generate a final reconstructed image of the current image. More specifically, the reconstruction unit  112  expresses a value of each of the pixels constituting the compression image stored in the memory  113  in units of 4×4 blocks at the original bit resolution by referring to the resolution adjustment information, in order to generate the final reconstructed image of the current image. 
         [0036]    In other words, the compression unit  111  expresses a pixel value at the original bit resolution with a pixel value at a reduced bit resolution, and the reconstruction unit  112  expresses the pixel value at the reduced bit resolution with the pixel value at the original bit resolution. For example, when the original bit resolution is 8 bits and a value of each pixel constituting the compression image stored in the memory  113  is composed of a Y value of 4, 5, or 6 bits, a Cb value of 4, 5, or 6 bits, and a Cr value of 4, 5, or 6 bits, the reconstruction unit  112  expresses all of the Y value, the Cb value, and the Cr value of each pixel of the compression image with 8 bits. The final reconstructed image generated by the reconstruction unit  112  is used as a reference image for future images following the current image or past images preceding the current image. 
         [0037]      FIG. 2  is a block diagram of an image decoding apparatus  20  according to an embodiment of the present invention. Referring to  FIG. 2 , the image decoding apparatus  20  includes an entropy-decoding unit  201 , an inverse quantization unit  202 , an inverse transform unit  203 , a motion compensation unit  204 , a spatial prediction unit  205 , an addition unit  206 , a compression unit  207 , and a reconstruction unit  208 . An image reconstruction process performed by the image decoding apparatus  20  is the same as that performed by the image encoding apparatus  10  illustrated in  FIG. 1 . Thus, although not provided below, a description for the image encoding apparatus  10  illustrated in  FIG. 1  is also applied to the description for the image decoding apparatus  20  according to the current embodiment of the present invention. 
         [0038]    The entropy-decoding unit  201  entropy-decodes a bitstream output from the image encoding apparatus  10  illustrated in  FIG. 1 , in order to reconstruct information required to decode an image and integers of image data. The inverse quantization unit  202  inversely quantizes the integers reconstructed by the entropy-decoding unit  201  in order to reconstruct frequency component values. The inverse transform unit  203  transforms the frequency component values reconstructed by the inverse quantization unit  202 , from a frequency domain into a spatial domain, in order to reconstruct a residue image between a current image and a predicted image. 
         [0039]    The motion compensation unit  204  generates a predicted image of the current image from at least one of a plurality of reference images generated by the reconstruction unit  208 , by using a motion vector. For each of a plurality of blocks, corresponding to an intra mode, from among all blocks constituting the current image, the spatial prediction unit  205  predicts a value of the block of the current image from a value of a block of a reconstructed image located adjacent to the block of the current image, from among all blocks of a reconstructed image generated by the reconstruction unit  208 , in order to generate a predicted image of the current image. The addition unit  206  adds the residue image reconstructed by the inverse transform unit  203  to the predicted image generated by the motion compensation unit  204  or the predicted image generated by the intra-prediction unit  205  in order to generate a reconstructed image of the current image. 
         [0040]    The compression unit  207  compresses the reconstructed image generated by the addition unit  206  by expressing a value of each pixel of the reconstructed image at a reduced bit resolution and stores the compressed reconstructed image (or compression image) in a memory  209 . More specifically, the compression unit  207  expresses a value of each pixel of the reconstructed image generated by the addition unit  206  at a reduced bit resolution in units of 4×4 blocks by referring to resolution adjustment information in order to compress the reconstructed image. 
         [0041]    The reconstruction unit  208  reads the compression image stored in the memory  209  and expresses a value of each pixel of the compression image at the original bit resolution in order to generate a final reconstructed image of the current image. More specifically, the reconstruction unit  208  expresses a value of each pixel of the compression image stored in the memory  209  at the original bit resolution in units of 4×4 blocks by referring to resolution adjustment information, in order to generate the final reconstructed image of the current image. 
         [0042]    According to the foregoing embodiments of the present invention, a reference image is compressed by reducing the bit resolution of the reference image and the compressed reference image is stored in an external memory, thereby reducing the amount of data of the reference image stored in the external memory. Thus, the number of cycles required for an image encoding apparatus or an image decoding apparatus to read or write a reference image from or to an external memory can be reduced. Such a reduction in the number of cycles required to access the external memory leads to a reduction in the number of operation cycles taken for the entire image encoding/decoding process, thereby implementing a low-power video codec. 
         [0043]      FIG. 3A  illustrates a structure of a reference image compressed according to an embodiment of the present invention. Referring to  FIG. 3A , the compressed reference image includes a MIN field, a RANGE field, and a PIXEL field. Here, the MIN field may be replaced with a MAX field. In particular, the reference image illustrated in  FIG. 3A  is structured in such a manner that different fields are repeated. To reflect this structure, the structure of the reference image is illustrated in the form of a flowchart. 
         [0044]    A minimum value (MIN) or a maximum value (MAX) among values of pixels included in each unit block, e.g., each 4×4 block, is recorded in the MIN field or the MAX field. A range value indicating a difference between the minimum value and the maximum value of each 4×4 block is recorded in the RANGE field. Values of 16 pixels of each 4×4 block are recorded in the PIXEL field. In other words, values of pixels expressed at a reduced bit resolution, i.e., quantization indices are recorded in the PIXEL field. 
         [0045]      FIG. 3B  illustrates the structure of the reference image, illustrated in  FIG. 3A , in the form of a pseudo code. Among the items of the table illustrated in  FIG. 3B , a “bit depth” indicates the number of bits expressing each field and a “reference number” indicates matches to numbers within the brackets “( )” illustrated in  FIG. 3A . For example, “(2)” illustrated in  FIG. 3A  indicates that the PIXEL field is repeated for each of the 16 pixels of each 4×4 block and such repetition can be expressed as a portion corresponding to the reference number “(2)” of  FIG. 3B  in the form of a pseudo code. “BIT_DEPTH_PIXEL” of the PIXEL field indicates a reduced bit resolution and can be included in a header of a bitstream. A threshold value, a first sub range value and a second sub range value may also be included in the header in order to constitute resolution adjustment information together with the reduced bit resolution. 
         [0046]      FIG. 4  is a block diagram of an image compression apparatus according to an embodiment of the present invention. The image compression apparatus illustrated in  FIG. 4  corresponds to the compression unit  111  illustrated in  FIG. 1  and the compression unit  207  illustrated in  FIG. 2 . Referring to  FIG. 4 , the image compression apparatus according to the current embodiment of the present invention, i.e., each of the compression unit  111  illustrated in  FIG. 1  and the compression unit  207  illustrated in  FIG. 2 , includes a resolution adjustment mode determination unit  410 , a quantization unit  430 , and a fixed-length coding unit  450 , and can be implemented as at least one single processor. 
         [0047]    The resolution adjustment mode determination unit  410  determines a resolution adjustment mode by considering the distribution of values of pixels of a current unit block of a reconstructed image. At this time, a threshold value, the first sub range value, and the second sub range value or the threshold value may be input to the resolution adjustment mode determination unit  410 . The threshold value, the first sub range value, and the second sub range value may be previously stored or may be set for each unit block variably according to the characteristics of the reconstructed image. 
         [0048]    More specifically, the resolution adjustment mode determination unit  410  obtains a minimum value and a maximum value among values of pixels of a unit block, e.g., a 4×4 block, of the reconstructed image. At this time, when a value of each pixel is composed of a Y value, a Cb value, and a Cr value, a maximum value and a minimum value for each of the Y value, the Cb value, and the Cr value or only for the Y value may be obtained. The resolution adjustment mode determination unit  410  obtains a range value indicating a difference between the maximum value and the minimum value of the current unit block and compares the range value with the threshold value. If the range value is less than the threshold value, the resolution adjustment mode determination unit  410  determines a resolution adjustment mode of the current unit block as a uniform adjustment mode, e.g., a uniform quantization mode. When the range value is less than the threshold value, a value of each pixel of a unit block has high correlation with values of its neighboring pixels and thus a quantization error can be minimized with uniform quantization. On the other hand, when the range value is greater than or equal to the threshold value, the unit block is highly likely to include an edge component composed of at least two regions and thus denser quantization is performed in a region where the edge component exists. 
         [0049]    Thus, when the range value is greater than or equal to the threshold value, the resolution adjustment mode determination unit  410  determines whether the remaining values of pixels of the current unit block, except for the maximum value and the minimum value, fall within the first sub range value from the minimum value or within the second sub range value from the maximum value. If the remaining values of pixels of the current unit block, except for the maximum value and the minimum value, fall within the first sub range value from the minimum value or within the second sub range value from the maximum value, the resolution adjustment mode determination unit  410  determines that the current unit block includes an edge component. Thus, in this case, the resolution adjustment mode determination unit  410  determines the resolution adjustment mode of the current unit block as a non-uniform adjustment mode, e.g., a dead-zone quantization mode, and sets a flag to ‘1’. If the remaining values of pixels of the current unit block, except for the maximum value and the minimum value, fall between the maximum value and the minimum value, the resolution adjustment mode determination unit  410  determines that the current unit block has no edge component. Thus, in this case, the resolution adjustment mode determination unit  410  determines the resolution adjustment mode of the current unit block as the uniform adjustment mode, i.e., the uniform quantization mode, and sets a flag to ‘0’. 
         [0050]    Flag information is expressed as a least significant bit (LSB) in the range value and provided to the quantization unit  430 . In other words, when the range value is greater than the threshold value, a LSB of ‘0’ in the range value indicates a first resolution adjustment mode and a LSB of ‘1’ in the range value indicates a second resolution adjustment mode. 
         [0051]    The quantization unit  430  quantizes the current unit block according to the resolution adjustment mode determined by the resolution adjustment mode determination unit  410 . A quantization process performed according to the determined resolution adjustment mode will now be described in more detail below. 
         [0052]    When the resolution adjustment mode determined by the resolution adjustment mode determination unit  410  is the uniform adjustment mode, the quantization unit  430  performs uniform quantization on the current unit block. To this end, a previously-determined reduced bit resolution, one of a minimum value (MIN) and a maximum value (MAX), and a range value are input to the quantization unit  430 , and the quantization unit  430  uniformly divides an interval between the minimum value and the maximum value for expressing at the reduced bit resolution and determines a boundary value that is closest to a value of each pixel of the current unit block from among boundary values obtained by uniform division as a quantization index of the value. When the original bit resolution of a reference image is 8 bits, each pixel of the reference image has a value between 0 and 255. 
         [0053]    For a reduced bit resolution of 5 bits as illustrated in  FIG. 8A , when the current unit block has a maximum value (MAX) of 167 and a minimum value (MIN) of 103, 32 boundary values B 1 -B 32 , i.e., 103, 105, 107, . . . , 167, are obtained by uniform division in order to express a pixel value range between the maximum value and the minimum value with 5 bits, and quantization indices ‘00000’-‘11111’ are assigned. Next, a value of each pixel of the current unit block is compared with the 32 boundary values and a boundary value that is closest to the value is determined as a quantization index of the value. 
         [0054]    When the resolution adjustment mode determined by the resolution adjustment mode determination unit  410  is the non-uniform adjustment mode, the quantization unit  430  performs dead-zone quantization on the current unit block. To this end, a previously-determined reduced bit resolution, one of a minimum value (MIN) and a maximum value (MAX), a first sub range value, and a second range value are input to the quantization unit  430 , and the quantization unit  430  uniformly divides an interval between the minimum value and the first sub range value and an interval between the maximum value and the second sub range value for expressing at a reduced bit resolution and determines a boundary value that is closest to a value of each pixel of the current unit block from among boundary values obtained by uniform division as a quantization index of the value. Here, an interval between the first sub range value and the second sub range value is a dead zone where no pixel value exists. 
         [0055]    It is assumed that the original bit resolution of a reference image is 8 bits and a reduction in the bit resolution is set to 5 bits as illustrated in  FIG. 8B . When the current unit block has a maximum value (MAX) of 183, a minimum value (MIN) of 87, a first sub range value (SR 1 ) of 32, and a second sub range value (SR 2 ) of 32, 32 boundary values B 1 -B 32 , i.e., 87, 89, . . ., 117, 119, 151, 153, . . . , 181, 183, are obtained by uniform division in order to express a pixel value range between the maximum value and the first sub range value and a pixel value range between the minimum value and the second sub range value with 5 bits, and quantization indices ‘00000’-‘11111’ are assigned. A value of each pixel of the current unit block is compared with the 32 boundary values and a boundary value that is closest to the value is determined as a quantization index of the value. Here, an interval between a boundary value of 119, corresponding to the first sub range value, and a boundary value of 151, corresponding to the second sub range value, is a dead zone. 
         [0056]    Here, the reduced bit resolution of the reference image is previously determined by simulation based on the amount of computation required for quantization and a quantization error and is provided through at least one of a sequence header, a group-of-pictures (GOP) header, a picture header, and a macroblock header or is previously stored in the quantization unit  430  or the memory  117  or  209 . The threshold value is previously set to an optimal value by simulation based on a quantization error generated according to selection of uniform quantization or dead-zone quantization and is provided through a header or is previously stored in the quantization unit  430  or the memory  117  or  209 . The first sub range value and the second sub range value are previously set to optimal values by simulation based on a quantization error generated during dead-zone quantization or are set variably according to the range value of the unit block. 
         [0057]    A total of 128 bits are required to store a single 4×4 block in the memory  113  or  209  prior to quantization, whereas by performing uniform quantization or dead-zone quantization, a total of 96 bits including 8 bits for the minimum value or the maximum value, 8 bits for the range value, and 80 bits for quantization indices of 16 pixels are required. Thus, the number of operation cycles required to access data of an external memory can be reduced. In particular, by selecting dead-zone quantization based on the characteristics of an image according to pixel value distribution, for a unit block including an edge component, an interval where pixel values exist is more precisely divided to determine quantization indices, thereby minimizing a quantization error in spite of bit resolution reduction. Consequently, improvement can be achieved in a peak signal-to-noise ratio (PSNR), which is a barometer of objective display quality as well as in subjective display quality. 
         [0058]    The fixed-length coding unit  450  performs fixed-length coding on a range value and a representative value obtained by the resolution adjustment mode determination unit  410  and a quantization index for a value of each pixel of the current unit block on which uniform quantization or dead-zone quantization is performed, in such a way to correspond to a basic access unit of the memory  113  or  209 . The representative value is one of a minimum value and a maximum value, and the minimum value will be used as the representative value as an example herein. 
         [0059]      FIG. 5  is a block diagram of an image reconstruction apparatus according to an embodiment of the present invention. In particular, the image reconstruction apparatus illustrated in  FIG. 5  corresponds to the reconstruction unit  112  illustrated in  FIG. 1  and the reconstruction unit  208  illustrated in  FIG. 1 . Referring to  FIG. 5 , the image reconstruction apparatus according to the current embodiment of the present invention includes a fixed-length decoding unit  510 , a resolution adjustment mode determination unit  530 , and an inverse quantization unit  550 , and may be implemented as at least one single processor. 
         [0060]    The fixed-length decoding unit  510  reads a compression image stored in the memory  113  or  209  in units of unit blocks, e.g., 4×4 blocks, extracts a fixed-length encoding value of each pixel of a read 4×4 block, and performs fixed-length decoding on the extracted fixed-length encoding value, in order to reconstruct a quantization index, a representative value, i.e., a minimum value, and a range value for a value of each pixel. 
         [0061]    The resolution adjustment mode determination unit  530  compares the range value reconstructed by the fixed-length decoding unit  510  with a threshold value. If the range value is greater than or equal to the threshold value, the resolution adjustment mode determination unit  530  checks a flag, i.e., a LSB of the range value in order to determine a resolution adjustment mode. At this time, the threshold value, a first sub range value, and a second sub range value, or the threshold value may be input to the resolution adjustment mode determination unit  530 , the first sub range value and the second sub range value may be previously stored or may be set for each unit block variably according to the characteristics of the reconstructed image. 
         [0062]    More specifically, if the range value is less than the threshold value, the resolution adjustment mode determination unit  530  determines the resolution adjustment mode as a uniform adjustment mode, i.e., a uniform inverse quantization mode. If the range value is greater than or equal to the threshold value, the resolution adjustment mode determination unit  530  checks the LSB of the range value. If the LSB of the range value is ‘0’, it means that the flag is set to ‘0’ and thus the uniform adjustment mode, i.e., the uniform inverse quantization mode is determined as the resolution adjustment mode. If the LSB of the range value is ‘1’, it means that the flag is set to ‘1’ and thus a non-uniform adjustment mode, i.e., a dead-zone inverse quantization mode is determined as the resolution adjustment mode. 
         [0063]    The inverse quantization unit  550  inversely quantizes the quantization index for the value of each pixel of the current unit block, which is reconstructed by the fixed-length decoding unit  510 , according to the resolution adjustment mode determined by the resolution adjustment mode determination unit  530 . An inverse quantization process performed according to the determined resolution adjustment mode will now be described in more detail below. 
         [0064]    When the resolution adjustment mode determined by the resolution adjustment mode determination unit- 530  is the uniform adjustment mode, the inverse quantization unit  550  performs uniform inverse quantization on the current unit block. To this end, the reconstructed minimum value and range value may be input to the inverse quantization unit  550  and the inverse quantization unit  550  obtains a maximum value from the range value. The inverse quantization unit  550  uniformly divides an interval between the minimum value and the maximum value for expressing at a reduced bit resolution and determines a boundary value, corresponding to the quantization index for the value of each pixel of the current unit block, from among boundary values obtained by uniform division as a reconstruction value of the pixel. 
         [0065]    When the resolution adjustment mode determined by the resolution adjustment mode determination unit  530  is the non-uniform adjustment mode, the inverse quantization unit  550  performs dead-zone inverse quantization on the current unit block. To this end, the reconstructed minimum value and range value is input to the inverse quantization unit  550  and the inverse quantization unit  550  obtains a maximum value from the range value. A first sub range value and a second sub range value are input to the inverse quantization unit  550 , and the inverse quantization unit  550  uniformly divides an interval between the minimum value and the first sub range value and an interval between the maximum value and the second sub range value for expressing at a reduced bit resolution and determines a boundary value, corresponding to the quantization index for the value of each pixel of the current unit block, from among boundary values obtained by uniform division as a reconstruction value of the pixel. 
         [0066]      FIG. 6  is a flowchart of an image compression method according to an embodiment of the present invention. 
         [0067]    Referring to  FIG. 6 , the values of pixels of a unit block of a reference image are input in operation  611 , and a minimum value and a maximum value from among the values of the pixels of the unit block are obtained and a range value indicating a difference between the minimum value and the maximum value is obtained in operation  613 . 
         [0068]    The range value is compared with a threshold value in operation  615 . If the range value is less than the threshold value, the unit block is quantized in a first resolution adjustment mode, in operation  621 . Otherwise, if the range value is greater than or equal to the threshold value, it is determined whether the value of each pixel of the unit block falls within a specific range from the minimum value or the maximum value, in operation  617 . 
         [0069]    If the value of each pixel of the unit block does not fall within the specific range from the minimum value or the maximum value, a LSB of the range value is set to ‘0’ in order to set a flag to ‘0’, in operation  619 , and the unit block is quantized in the first resolution adjustment mode, in operation  621 . Otherwise, if the value of each pixel of the unit block falls within the specific range from the minimum value or the maximum value, the LSB of the range value is set to ‘1’ in order to set the flag to ‘1’, in operation  623 , and the unit block is quantized in a second resolution adjustment mode, in operation  625 . 
         [0070]    In operation  627 , fixed-length coding is performed on the quantization index, the representative value, and the range value for a value of each pixel of each unit block of a reference image compressed at a reduced bit resolution according to the first resolution adjustment mode or the second resolution adjustment mode, and a fixed-length coding value is stored in the memory  113  or  209 . 
         [0071]    Here, the first resolution adjustment mode is a uniform adjustment mode, the second resolution adjustment mode is a non-uniform adjustment mode, and the representative value is one of the minimum value and the maximum value. Herein, the minimum value is used as the representative value as an example. 
         [0072]      FIG. 7  is a flowchart of an image reconstruction method according to an embodiment of the present invention. 
         [0073]    Referring to  FIG. 7 , the values of pixels of a unit block of a reference image read from the memory  113  or  209  are input, in operation  711 , and fixed-length decoding is performed on the values of the pixels, in operation  713 . A quantization index, a representative value, and a range value of each of the pixels are obtained from fixed-length decoding values, in operation  715 . 
         [0074]    The range value is compared with a threshold value in operation  717 . If the range value is less than the threshold value, inverse quantization is performed on the unit block in a first resolution adjustment mode, in operation  721 . Otherwise, if the range value is greater than or equal to the threshold value, a LSB of the range value is checked in order to check a flag, in operation  719 . 
         [0075]    If the flag is set to ‘0’, inverse quantization is performed on the unit block in the first resolution adjustment mode, in operation  721 . Otherwise, if the flag is set to ‘1’, inverse quantization is performed on the unit block in a second resolution adjustment mode, in operation  723 . 
         [0076]    Here, the first resolution adjustment mode is a uniform adjustment mode, the second resolution adjustment mode is a non-uniform adjustment mode, and the representative value is one of the minimum value and the maximum value. Herein, the minimum value is used as the representative value as an example. 
         [0077]    In addition to the above described embodiments, embodiments of the present invention can also be implemented through computer readable code/instructions in/on a medium, e.g., a computer readable medium, to control at least one processing element to implement any above described embodiment. The medium can correspond to any medium/media permitting the storing and/or transmission of the computer readable code. 
         [0078]    The computer readable code can be recorded/transferred on a medium in a variety of ways, with examples of the medium including recording media, such as magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.) and optical recording media (e.g., CD-ROMs, or DVDs), and transmission media such as media carrying or including carrier waves, as well as elements of the Internet, for example. Thus, the medium may be such a defined and measurable structure including or carrying a signal or information, such as a device carrying a bitstream, for example, according to embodiments of the present invention. The media may also be a distributed network, so that the computer readable code is stored/transferred and executed in a distributed fashion. Still further, as only an example, the processing element could include a processor or a computer processor, and processing elements may be distributed and/or included in a single device. 
         [0079]    While aspects of the present invention has been particularly shown and described with reference to differing embodiments thereof, it should be understood that these exemplary embodiments should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in the remaining embodiments. 
         [0080]    Thus, although a few embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.