Abstract:
The method performed by an apparatus for encoding a current block, includes: generating a predicted block by predicting the current block; generating a residual block of the current block by subtracting the predicted block from the current block; partitioning the residual block into a plurality of subblocks having various sizes, and transforming each of the subblocks by using a transform unit of a size identical to each of the subblocks, to thereby generate transform blocks of the subblocks; quantizing the transform blocks; and encoding transform coefficients of each of the quantized transform blocks.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 14/687,200 filed Apr. 15, 2015, which a continuation of U.S. patent application Ser. No. 13/497,755, filed Apr. 24, 2012 (U.S. Pat. No. 9,124,901 issued on Sep. 1, 2015), which is a the National Phase application of International Application No. PCT/KR2010/006018, filed Sep. 3, 2010, which is based upon and claims the benefit of priorities from Korean Patent Application No. 10-2009-0090283, filed on Sep. 23, 2009. The disclosures of the above-listed applications are hereby incorporated by reference herein in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to a video encoding/decoding method and an apparatus that account for low frequency components. 
       BACKGROUND ART 
       [0003]    The statements in this section merely provide background information related to the present disclosure and do not constitute prior art. 
         [0004]    The inventor(s) has noted that without a compression process, video data requires extensive hardware resources including memories to store or transmit the data since the video data carries much more amount of data that audio data or still image data. Accordingly, it is typical in a video data storage or transmission to use an encoder for compressively store or transmit the video data, whereas a decoder receives the compressed video data, and decompresses the same for a reconstruction into the original video. 
         [0005]    The inventor(s) has noted that recently developed video compressing technologies attempt to effectively compress videos by using a multiple reference frame technology and a technology for encoding quantized transform coefficients through entropy coding such as Context based Variable Length Coding (CAVLC). The inventor(s) has noted that such video compression technologies issue blocking effects and the like because they involve predicting macroblocks of images to obtain a residual block which then undergoes dividing into transform units, transforming into a frequency domain, quantizing, and encoding. 
         [0006]    The inventor(s) has experienced that As the typical image compression technologies deal with the blocking effects to be removed through deblocking-filtering the images which are reconstructed after encoding and decoding thereof, deblocking filters in this operation undesirably remove even the high frequency components which are actually present in the images. 
       SUMMARY 
       [0007]    In accordance with some embodiments of the present disclosure, an apparatus for encoding a current block performs a method comprising: generating a predicted block by predicting the current block; generating a residual block of the current block by subtracting the predicted block from the current block; partitioning the residual block into a plurality of subblocks having various sizes, and transforming each of the subblocks by using a transform unit of a size identical to each of the subblocks, to thereby generate transform blocks of the subblocks; quantizing the transform blocks; and encoding transform coefficients of each of the quantized transform blocks. 
         [0008]    In accordance with some embodiments of the present disclosure, an apparatus for decoding a current block performs a method comprising: generating a predicted block of the current block to be decoded by predicting the current block; partitioning the current block to be decoded into a plurality of subblocks having various sizes; generating transform blocks of the subblocks by extracting, from a bitstream, transform coefficients corresponding to each of the subblocks having the various sizes; inversely quantizing and then inversely transforming each of the transform blocks by using a transform unit of a size identical to a corresponding subblock, and thereby reconstructing residual subblocks corresponding to the subblocks; generating a residual block of the current block by, the residual block comprised of the reconstructed residual subblocks; and reconstructing the current block by adding the predicted block and the generated residual block. 
         [0009]    In accordance with some embodiments of the present disclosure, an apparatus for decoding quantized transform coefficients in a current block performs a method comprising: identifying a plurality of subblocks partitioned from the current block to be decoded, the plurality of subblocks having various sizes; generating transform blocks of the subblocks by extracting, from a bitstream, transform coefficients corresponding to each of the subblocks having the various sizes; inversely quantizing and then inversely transforming each of the transform blocks by using a transform unit of a size identical to a corresponding subblock, and thereby reconstructing residual subblocks corresponding to the subblocks; and generating a residual block of the current block, the residual block comprised of the reconstructed residual subblocks. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is a view illustrating pictures included in a video according to at least one embodiment of the present disclosure; 
           [0011]      FIG. 2  is a block diagram schematically illustrating a video encoding apparatus according to at least one embodiment of the present disclosure; 
           [0012]      FIG. 3  is a block diagram schematically illustrating a video decoding apparatus according to at least one embodiment of the present disclosure; 
           [0013]      FIG. 4  is a block diagram schematically illustrating a video encoding apparatus according to at least one embodiment of the present disclosure; 
           [0014]      FIG. 5  is a diagram illustrating a first transform block and a second transform block according to at least one embodiment of the present disclosure; 
           [0015]      FIG. 6  is a diagram illustrating a process of selecting a low frequency transform coefficient according to at least one embodiment of the present disclosure; 
           [0016]      FIG. 7  is a diagram illustrating transform blocks generated through a combination with additional transform coefficients according to at least one embodiment of the present disclosure; 
           [0017]      FIGS. 8 and 9  are a flowchart illustrating a video encoding method according to at least one embodiment of the present disclosure; 
           [0018]      FIG. 10  is a block diagram schematically illustrating a video decoding apparatus according to at least one embodiment of the present disclosure; and 
           [0019]      FIG. 11  is a flowchart illustrating a video decoding method according to at least one embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the following description, the same elements will be designated by the same reference numerals although they are shown in different drawings. Further, in the following description of the present disclosure, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present disclosure rather unclear. 
         [0021]    Additionally, in describing the components of the present disclosure, there may be terms used like first, second, A, B, (a), and (b). These are solely for the purpose of differentiating one component from the other but not to imply or suggest the substances, order or sequence of the components. If a component were described as ‘connected’, ‘coupled’, or ‘linked’ to another component, they may mean the components are not only directly ‘connected’, ‘coupled’, or ‘linked’ but also are indirectly ‘connected’, ‘coupled’, or ‘linked’ via a third component. 
         [0022]    Some embodiments of the present disclosure provide in one aspect a method and an apparatus for encoding videos by considering low frequency components to improve the compression efficiency while reducing blocking effects due to transform and quantization operations. 
         [0023]      FIG. 1  is a view illustrating pictures included in a video. 
         [0024]    There are various video encoding methods. A typical video encoding method encodes an image in the unit of pixels through dividing the image in the unit of pixels, or encodes the image in the unit of blocks through dividing the image in the unit of blocks. When the image is encoded in the unit of blocks, one picture to be encoded among a series of pictures included in the video may be divided into macroblocks and subblocks included in the macroblock, and the block may be predicted and encoded in the unit of divided blocks. 
         [0025]    In general, since a video has a frame-rate of 30 pictures per second and, the visual difference between neighboring pictures is too small to be sensed somehow by human eyes. Accordingly, when the video outputs 30 pictures per second, the viewer recognizes that the respective pictures make a continuous motion. 
         [0026]    Accordingly, when a previous picture is visually similar to a current picture, an unknown pixel value of an image of the current picture may be predicted from a previously known pixel value of the image of the previous picture. Such a prediction method is called inter prediction. The inter prediction is performed based on a motion prediction technique. The motion prediction is performed in a scheme of referring to a previous picture or referring to both the previous picture and a future picture based on a time axis. The picture that is referenced in encoding or decoding the current picture is called a reference picture. 
         [0027]    Referring to  FIG. 1 , a video includes a series of still images. The still images are divided in the unit of Groups of Pictures (GoPs). Each still image is called a picture or a frame. One GoP includes picture I  110 , picture Ps  120 , and picture Bs  130 . Picture I  110  is a picture self-encoded without using a reference picture, picture P  120  and picture B  130  are pictures encoded through performing motion estimation and motion compensation by using the reference picture. Particularly, picture B  130  is a picture encoded by predicting a past picture and a future picture in a forward direction and a backward direction, i.e. bi-directionally. 
         [0028]    Referring to  FIG. 1 , the motion estimation and the motion compensation for encoding picture P  120  use picture I  110  or picture P  120  as a reference picture. The motion estimation and the motion compensation for encoding picture B  130  use picture I  110  and picture P  120  as reference pictures. 
         [0029]      FIG. 2  is a block diagram schematically illustrating a video encoding apparatus. 
         [0030]    The video encoding apparatus  200  includes a predictor  210 , a subtracter  220 , a transformer  230 , a quantizer  240 , an encoder  250 , an inverse quantizer  260 , an inverse transformer  270 , an adder  280 , and a memory  290 . The video encoding apparatus  200  refers to a personal computer or PC, notebook or laptop computer, personal digital assistant or PDA, portable multimedia player or PMP, PlayStation Portable or PSP, or mobile communication terminal, smart phone or such devices, and represent a variety of apparatuses equipped with, for example, a communication device such as a modem for carrying out communication between various devices or wired/wireless communication networks, a memory for storing various programs for encoding images and related data, and a microprocessor for executing the programs to effect operations and controls. Other components of the video encoding apparatus  200 , such as each of the predictor  210 , the subtracter  220 , the transformer  230 , the quantizer  240 , the encoder  250 , the inverse quantizer  260 , then inverse transformer  270 , the adder  280  is implemented by, or includes, one or more processors and/or application-specific integrated circuits (ASICs). 
         [0031]    The predictor  210  generates a predicted block by predicting a current block. That is, the predictor  210  predicts a pixel value of each pixel of a current block to be encoded in an image to generate a predicted block having a predicted pixel value of each pixel. Here, the predictor  210  may predict the current block by using the intra prediction or the inter prediction. 
         [0032]    The subtracter  220  generates a residual block by subtracting the predicted block from the current block. That is, the subtracter  220  calculates a difference between a pixel value of each pixel of the current block to be encoded and the predicted pixel value of each pixel predicted in the predictor  210 , to generate the residual block having a residual signal in the form of a block. 
         [0033]    The transformer  230  transforms the residual block. Specifically, the transformer  230  transforms a residual signal of the residual block outputted from the subtracter  220  into the frequency domain to transform each pixel value of the residual block into transform coefficients. Here, the transformer  230  may transform the residual signal to the frequency domain by using various transform methods, such as the Hadamard transform and the discrete cosine transform (DCT) based transform, for transforming an image signal in a spatial axis based on the frequency axis, wherein the residual signal transformed to the frequency domain is the transform coefficients. 
         [0034]    The quantizer  240  quantizes the transformed residual block. Specifically, the quantizer  240  quantizes the transform coefficients of the residual block outputted from the transformer  230  to output the residual block having the quantized transform coefficients. Here, the quantizer  240  may perform the quantization by using the dead zone uniform threshold quantization (DZUTQ) or the quantization weighted matrix among their various improvement options. 
         [0035]    The encoder  250  encodes the quantized residual blocks to generate a bitstream. Specifically, the encoder  250  scans quantized frequency coefficients of the residual block outputted from the quantizer  240  in a zigzag scanning or other various scanning methods to generate a quantized frequency coefficient string, and encodes the quantized frequency coefficient string by using various encoding techniques such as the entropy encoding. In this event, the encoder  250  may additionally encode various pieces of information, such as information necessary for the prediction. 
         [0036]    The inverse quantizer  260  inversely quantizes the residual block quantized by the quantizer  240 . The inverse transformer  270  inversely transforms the residual block inversely quantized by the inverse quantizer  260 . Here, the inverse quantizer  260  and the inverse transformer  270  may perform the inverse quantization and the inverse transform by inversely applying the quantization method and the transform method used in the quantizer  240  and the transformer  230 . 
         [0037]    The adder  280  adds the predicted block predicted in the predictor  210  to the residual block reconstructed by the inverse transformer  270  to reconstruct the current block. The memory  290  stores the reconstructed current block outputted from the adder  280  as a reference picture in the unit of pictures such that the reconstructed current block may be used as the reference picture when the predictor  210  encodes a next block of the current block or a different future block. 
         [0038]    Although it is not illustrated in  FIG. 2 , the video encoding apparatus  200  may additionally include a deblocking filter for deblocking-filtering the reconstructed current block, or the like. Here, the deblocking-filtering refers to an operation of reducing a block distortion generated while an image is encoded in the unit of blocks, and may optionally select one of a method of applying the deblocking filter to a block boundary and a macroblock boundary, a method of applying the deblocking filter only to the macroblock boundary, or a method of using no deblocking filter. 
         [0039]    In the meantime, since the residual block generated by subtracting the predicted block from the current block is transformed and quantized, errors are generated in the process of the transform and the quantization, and thus the bitstream generated by the encoder  250  includes the errors generated during the process of the transform and the quantization. Further, the transformed and quantized residual block is inversely quantized and inversely transformed again and added to the predicted block, to be reconstructed to the current block. In this event, the reconstructed current block includes the errors generated during the process of the transform and the quantization. 
         [0040]    That is, when it is assumed that the original current block is A and the predicted block is B, the residual block A-B, which is the difference between the current block and the predicted block, is transformed and quantized. In this event, an error component E may be generated, so that the encoded data within the finally encoded and outputted bitstream is (A−B)+E. Further, since the reconstructed current block stored in the memory  290  is generated by adding the reconstructed residual block (A−B)+E to the predicted block B, the current block A+E in which the error E is added to the original current block A is stored as the reference picture. 
         [0041]      FIG. 3  is a block diagram schematically illustrating a video decoding apparatus. 
         [0042]    The video decoding apparatus  300  includes a decoder  310 , an inverse quantizer  320 , an inverse transformer  330 , a predictor  340 , an adder  350 , and a memory  360 . The video decoding apparatus  300  may be a personal computer or PC, notebook or laptop computer, personal digital assistant or PDA, portable multimedia player or PMP, PlayStation Portable or PSP, or mobile communication terminal, smart phone or such devices, and represent a variety of apparatuses equipped with, for example, a communication device such as a modem for carrying out communication between various devices or wired/wireless communication networks, a memory for storing various programs for decoding images and related data, and a microprocessor for executing the programs to effect operations and controls. Other components of the video decoding apparatus  300 , such as each of the decoder  310 , the inverse quantizer  320 , the inverse transformer  330 , the predictor  340 , the adder  350  is implemented by, or includes, one or more processors and/or application-specific integrated circuits (ASICs). 
         [0043]    The decoder  310  decodes the residual block encoded from the bitstream to reconstruct the quantized residual block. In this event, the decoder  310  may decode the encoded residual block by inversely performing the encoding process performed in the encoder  250  of the video encoding apparatus  200 . 
         [0044]    The inverse quantizer  320  inversely quantizes the residual block reconstructed by the decoder  310 . The inverse transformer  330  inversely transforms the residual block inversely quantized by the inverse quantizer  320 . The predictor  340  predicts the current block to generate the predicted block. The adder  350  adds the residual block inversely transformed by the inverse transformer  330  to the predicted block generated by the predictor  340  to reconstruct the current block. The memory  360  stores the reconstructed current block outputted from the adder  350  as the reference picture in the unit of pictures such that the predictor  340  may use the stored current block as the reference picture. 
         [0045]    Although it is not illustrated in  FIG. 4 , the aforementioned video decoding apparatus  300  may additionally include the deblocking filter for deblocking-filtering the reconstructed current block, etc. 
         [0046]      FIG. 4  is a block diagram schematically illustrating a video encoding apparatus according to at least one embodiment of the present disclosure. 
         [0047]    The video encoding apparatus  400  may include an image encoder  410  and a low frequency encoder  430 . Other components of the video encoding apparatus  400 , such as each of the image encoder  410  and the low frequency encoder  430  is implemented by, or includes, one or more processors and/or application-specific integrated circuits (ASICs). 
         [0048]    The image encoder  410  performs a predictive encoding on a current block to generate encoded image data and a reconstructed block. Specifically, the image encoder  410  predicts a current block to generate a predicted block, subtracts the predicted block from the current block to generate a residual block, transforms and quantizes the residual block in first transform units to generate a quantized transform coefficient of the residual block, encodes the quantized transform coefficient of the residual block to generate encoded image data, inversely quantizes and inversely transforms the quantized transform coefficient of the residual block to reconstruct the residual block, and adds the reconstructed residual block to the predicted block to generate the reconstructed block. 
         [0049]    To this end, as illustrated in  FIG. 4 , the image encoder  410  includes a predictor  412 , a first subtracter  414 , a first transformer  416 , a first quantizer  418 , a first encoder  420 , a first inverse quantizer  422 , a first inverse transformer  424 , a first adder  426 , and a memory  428 . Here, the operations and the functions of the predictor  412 , the first subtracter  414 , the first transformer  416 , the first quantizer  418 , the first encoder  420 , the first inverse quantizer  422 , the first inverse transformer  424 , the first adder  426 , and the memory  428  are similar to or the same as those of the predictor  210 , the subtracter  220 , the transformer  230 , the quantizer  240 , the encoder  250 , the inverse quantizer  260 , the inverse transformer  270 , the adder  280 , and the memory  290  already described through  FIG. 2 , so their detailed description will be omitted. Other components of the image encoder  410 , such as each of the predictor  412 , the first subtracter  414 , the first transformer  416 , the first quantizer  418 , the first encoder  420 , the first inverse quantizer  422 , the first inverse transformer  424 , the first adder  426  is implemented by, or includes, one or more processors and/or application-specific integrated circuits (ASICs). 
         [0050]    However, the first transformer  416  may transform the residual block in the first transform units in operation. Such first transform units may be the transform in a form of a rectangle, i.e. P×Q transform, wherein P≠Q, such as 4×8 transform, 8×4 transform, 16×8 transform, and 8×16 transform, in which the number of transverse pixels is different from that of longitudinal pixels of the transform block. Further, the first transform units may be one of 4×4 transform, 8×8 transform, 16×16 transform, 4×8 transform, 8×4 transform, 16×8 transform, and 8×16 transform, or a combination thereof. Herein, all kinds of transform units described above have a pixel size in unit (e.g., 8×8 transform pixel size). 
         [0051]    Further, the adder  280  already described with reference to  FIG. 2  adds the reconstructed residual block outputted from the inverse transformer  270  to the predicted block outputted from the predictor  210  to reconstruct the current block and stores the reconstructed current block in the memory  428 . However, the first adder  426  adds the reconstructed residual block outputted from the first inverse transformer  424  to the predicted block outputted from the predictor  412  to output the reconstructed block, in which the reconstructed block is not stored in the memory  428 , but is outputted to the subtracter  432  of the low frequency encoder  430 . That is, the reconstructed current block outputted from the adder  280  becomes the finally reconstructed current block in the video encoding apparatus of  FIG. 2 . However, in the video encoding apparatus  400  according to the aspect of the present disclosure of  FIG. 4 , not the reconstructed block outputted from the first adder  426  but the reconstructed current block outputted from a second adder  444  of the low frequency encoder  430  becomes the finally reconstructed current block. 
         [0052]    Here, the current block means a block to be currently encoded, and may be a macroblock or a subblock. 
         [0053]    The low frequency encoder  430  encodes only a low frequency component of an error block generated by subtracting the reconstructed block from the current block to generate encoded low frequency data. Specifically, the low frequency encoder  430  may include a second subtracter  432 , a second transformer  434 , a second quantizer  436 , and a second encoder  438 , and may additionally include a second inverse quantizer  440 , a second inverse transformer  442 , and the second adder  444 . 
         [0054]    The second subtracter  432  subtracts the reconstructed block from the current block to generate an error block. Specifically, the second subtracter  432  subtracts the reconstructed block outputted from the first adder  426  of the image encoder  410  from the current block to be currently encoded in the input image to generate the error block. The reconstructed block outputted from the first adder  426  is similar to the current block because the reconstructed block outputted from the adder  426  is the block generated through encoding, decoding, and reconstructing of the current block. However, as described above, the error is generated in the process of the transform and the quantization of the residual block of the current block, and the reconstructed block generated through addition of the inversely quantized, inversely transformed, and reconstructed residual block and the predicted block also includes the error, so that the error block may be generated by subtracting the reconstructed block from the current block. 
         [0055]    The second transformer  434  transforms the error block in second transform units to generate transform coefficients of the error block. Specifically, the second transformer  434  transforms error signals of the error block outputted from the second subtracter  432  into a frequency domain to generate the error block having the transform coefficient. 
         [0056]    In this event, the second transformer  434  transforms the error signals of the error block in the second transform units. The second transform units may include at least one transform, and may be larger than the first transform units used for the predictive encoding of the current block and include a plurality of adjacent first transform units used for the predictive encoding of the current block. 
         [0057]    Hereinafter, the first transform units and the second transform units will be described with reference to  FIG. 5  which illustrates a first transform block and a second transform block according to the aspect of the present disclosure. 
         [0058]      FIG. 5  illustrates an example of the first transform block used in the transform for the generation of the encoded image data and the second transform block used in the transform for the generation of the encoded low frequency data when it is assumed that the current block is a macroblock having the size of 32×32. 
         [0059]    The first transformer  416  transforms residual signals of a residual block in the first transform units, and as illustrated in  FIG. 5 , the first transform units may be 4×4 transform or 8×8 transform. In addition, the first transform units may be the transform in a shape of a rectangle, such as 4×8 transform, 8×4 transform, and 16×8 transform, which, however, are not illustrated. 
         [0060]    The second transformer  434  transforms error signals of the error block in the second transform units, and as illustrated in  FIG. 5 , the second transform units may be 16×16 transform or 16×8 transform. In addition, the second transform units may be the transform having various sizes and shapes, such as 8×8 transform and 8×16 transform, which, however, are not illustrated. 
         [0061]    However, as illustrated in  FIG. 5 , the second transformer  434  may transform the error block by using the second transform units in the form of transform blocks including one or more transform blocks according the first transform units used in the transform of the residual block in the first transformer  416 . The left-upper end of  FIG. 5  illustrates an example, in which when the first transformer  416  uses the 4×4 transform as the first transform units, the second transformer  434  transforms the error block by using the 16×16 transform including sixteen 4×4 transforms used as the first transform unit. Further, the right-lower end of  FIG. 5  illustrates an example, in which when the first transformer  416  uses the 8×8 transform as the first transform units, the second transformer  434  transforms the error block by using the 16×8 transform including two 8×8 transforms used as the first transform units. As described above, the second transform units may include one or more transforms, may be determined larger than the first transform units, and may be the transform including one or more transforms according to the first transform unit. 
         [0062]    The second quantizer  436  quantizes a low frequency transform coefficient among the transform coefficients of the error block to generate the quantized low frequency transform coefficient. That is, the second quantizer  436  selects as many low frequency transform coefficients as a predetermined number or a number determined according to a predetermined criterion among the transform coefficients of the error block, and quantizes the selected low frequency transform coefficients to generate quantized low frequency transform coefficients. 
         [0063]    To this end, the second quantizer  436  may select the predetermined number of transform coefficients from a DC coefficient according to a zigzag scanning sequence among the transform coefficients of the transformed error block as the low frequency transform coefficients. Referring to  FIG. 6  illustrating a process of selecting the low frequency transform coefficient according to the aspect of the present disclosure, the error block transformed by the second transformer  434  may include a plurality of transform coefficients as illustrated in  FIG. 6 .  FIG. 6  illustrates an example of the error block transformed when the 8×8 transform is used as the second transform units. The error blocks transformed by using the 8×8 transform include 64 transform coefficients, i.e. one DC coefficient and 63 AC coefficients. Since the AC coefficients adjacent to the DC coefficient among the 63 AC coefficients are the low frequency transform coefficients, the second quantizer  436  may select the several number of transform coefficients from the DC coefficient (inclusive) as the low frequency transform coefficients in the zigzag scanning sequence. 
         [0064]    Here, the several number may be one (in this case, the low frequency transform coefficient is the DC coefficient) or two (in this case, the low frequency transform coefficients are the DC coefficient and AC 1  coefficient), and may have been already determined or be determined according to a predetermined criterion. Further, the selection of the several number of transform coefficients from the DC coefficient according to the zigzag scanning sequence as the low frequency transform coefficients by the second quantizer  436  is a representative method of selecting the low frequency transform coefficients, so the low frequency transform coefficients are not necessarily selected by the aforementioned method, and may be selected by other various methods. 
         [0065]    The second encoder  438  encodes the quantized low frequency transform coefficient to generate the encoded low frequency data. Specifically, the second encoder  438  encodes the quantized low frequency transform coefficient by the entropy encoding or various encoding methods to generate the encoded low frequency data. However, in  FIG. 4 , the second encoder  438  is included in the low frequency encoder  430 , but the second encoder  438  is omitted in the low frequency encoder  430  and its function may be included in the first encoder  420  of the image encoder  410 . 
         [0066]    Further, the second encoder  438  encodes flag information indicating that the low frequency components of the error block are encoded, to generate encoded flag data. That is, when the second encoder  438  generates the encoded low frequency data, the second encoder  438  may generate the flag information indicating that the low frequency components of the error block are encoded and encode the generated flag information, to generate the flag data. The flag information may be set as a flag of one bit. For example, when the flag is 1, the flag information may indicate that the low frequency components of the error block are encoded, and when the flag is 0, the flag information may indicate that the low frequency components of the error block are not encoded. 
         [0067]    Further, the second encoder  438  may encode the number of quantized low frequency transform coefficients to generate the encoded low frequency coefficient data. The encoded low frequency coefficient data may be included in the bitstream to be transmitted to the video decoding apparatus, and be used for appropriately extracting and decoding the quantized low frequency data in the video decoding apparatus. 
         [0068]    The second inverse quantizer  440  inversely quantizes the quantized low frequency transform coefficient to reconstruct the low frequency transform coefficient. Specifically, the second inverse quantizer  440  inversely quantizes the quantized low frequency transform coefficient outputted from the second quantizer  436  to reconstruct the low frequency transform coefficient. The inverse quantization process may be achieved by inversely performing the quantization process performed in the second quantizer  440 . 
         [0069]    The second inverse transformer  442  inversely transforms the reconstructed low frequency transform coefficient in the second transform units to reconstruct the error block. Specifically, the second inverse transformer  442  inversely transforms the low frequency transform coefficient reconstructed by the second inverse quantizer  440  in the second transform units to reconstruct the error block. 
         [0070]    The inverse transform process may be achieved by inversely performing the transform process performed in the second transformer  434 . Since the low frequency transform coefficients inversely quantized and reconstructed by the second inverse quantizer  440  include only the several number of transform coefficients selected from the transform coefficients generated through the transform in the second transform units, it is necessary to add the transform coefficients in accordance with the form of the block in the second transform units in order for the second inverse transformer  442  to inversely perform the transform process performed by the second transformer  434 . To this end, as illustrated in  FIG. 7  representing an example of the transform block generated through the combination of the additional transform coefficients, the second inverse transformer  442  may generate the transform blocks in the second transform units by additionally combining the low frequency transform coefficients with transform coefficients having a predetermined value, such as 0, and inversely transform the low frequency transform coefficient in the second transform units. 
         [0071]    Further, contrary to the transform process performed by the second transformer  434 , in the inverse transform process, the low frequency transform coefficients including the several number of selected transform coefficients generated through the transform in the second transform units may be individually inversely transformed. In this case, the second inverse transformer  442  may reconstruct the error block having the same size as that of the reconstructed bock by combining error signals reconstructed through the individual inverse transform of the low frequency transform coefficients with additional error signals having a predetermined value, such as 0. 
         [0072]    The second adder  444  adds the reconstructed error block to the reconstructed block to reconstruct the current block. Specifically, the second adder  444  adds the error block reconstructed by the second inverse transformer  442  to the reconstructed block generated by the first adder  426  of the image encoder  410  to reconstruct the current block. 
         [0073]    The bitstream outputted from the video encoding apparatus  400  may include the encoded image data outputted from the first encoder  420  of the image encoder  410  and the encoded low frequency data outputted from the second encoder  438  of the low frequency encoder  430 , but may include the encoded image data without including the encoded low frequency data. Specifically, the first encoder  420  or the second encoder  438  may calculate an encoding cost (hereinafter, referred to as “a first encoding cost”) when the bitstream includes the encoded image data and anpther encoding cost (hereinafter, referred to as “a second first encoding cost”) when the bitstream includes the encoded image data and the encoded low frequency data, and output the bitstream including the encoded image data without the encoded low frequency data as the final bitstream when the first encoding cost is equal to or smaller than the second encoding cost, and output the bitstream including the encoded image data and the encoded low frequency data as the final bitstream when the first encoding cost is larger than the second encoding cost. Here, the encoding cost may be a rate-distortion cost, but may be any cost if the cost is consumed for the encoding. 
         [0074]    In a video encoding method according to the aspect of the present disclosure, the video encoding apparatus  400  performs a predictive encoding on the current block to be encoded in the input image to generate the encoded image data and the reconstructed block, and encodes only the low frequency components of the error block generated by subtracting the reconstructed block from the current block, to generate the encoded low frequency data. The generated encoded image data and encoded low frequency data constitute the bitstream. 
         [0075]    Here, the current block may be a macroblock or a subblock. Specifically, the video encoding apparatus  400  may generate the encoded image data and the encoded low frequency data in the unit of macroblocks to generate the bitstream for every picture or every frame, or generate the encoded image data and the encoded low frequency data in the unit of subblocks included in the macroblock to generate the bitstream for every macroblock. 
         [0076]    A video encoding method according to an aspect of the present disclosure may be specifically implemented as illustrated in  FIGS. 8 and 9 . 
         [0077]      FIGS. 8 and 9  are a flowchart illustrating an example of the video encoding method according to at least one embodiment of the present disclosure. 
         [0078]    The video encoding apparatus  400  may predict a current block to generate a predicted block (S 802 ), subtract the predicted block from the current block to generate a residual block (S 804 ), transform and quantize the residual block in first transform units to generate quantized transform coefficients of the residual block (S 806 ), and encode the quantized transform coefficients of the residual block to generate encoded image data (S 808 ). Further, the video encoding apparatus  400  may inversely quantize and inversely transform the quantized transform coefficients of the residual block to reconstruct the residual block (S 810 ), and add the reconstructed residual block to the predicted block to generate a reconstructed block (S 812 ). 
         [0079]    Further, the video encoding apparatus  400  may subtract the reconstructed block from the current block to generate an error block (S 814 ), generate transform coefficients of the error block in second transform units (S 816 ), select low frequency transform coefficients among the transform coefficients of the error block, quantize the selected low frequency transform coefficients to generate quantized low frequency transform coefficients (S 818 ), and encode the quantized low frequency transform coefficients to generate encoded low frequency data (S 820 ). 
         [0080]    Here, the second transform unit may include one or more transforms, and may be larger than the first transform unit used for the predictive encoding of the current block and include a plurality of adjacent first transform units used for the predictive encoding of the current block. 
         [0081]    Further, in step S 818 , the video encoding apparatus  400  may select a predetermined number of transform coefficients in a zigzag scanning sequence from a DC coefficient among the transform coefficients of the transformed error block as the low frequency transform coefficients. 
         [0082]    Further, the video encoding apparatus  400  may inversely quantize the quantized low frequency transform coefficients to reconstruct the low frequency transform coefficients (S 822 ), inversely transform the reconstructed low frequency transform coefficients to reconstruct the error block (S 824 ), and add the reconstructed error block to the reconstructed block to reconstruct the current block (S 826 ). At step S 824 , the video encoding apparatus  400  may combine error signals reconstructed through the inverse transform of the reconstructed low frequency transform coefficients and additional error signals to generate the error block. 
         [0083]    Further, the video encoding apparatus  400  may encode flag information indicating that the low frequency components of the error block are encoded, to generate encoded flag data. In this case, the bitstream may include encoded image data, encoded low frequency data, and encoded flag data. 
         [0084]    As described above, the image encoded into a bitstream by the video encoding apparatus  400  may be transmitted in real time or non-real-time to the video decoding apparatus via a wired/wireless communication network including the Internet, a short range wireless communication network, a wireless LAN network, a WiBro (Wireless Broadband) also known as WiMax network, and a mobile communication network or a communication interface such as cable or USB (universal serial bus), and decoded by the video decoding apparatus to be reconstructed and reproduced as a video. 
         [0085]      FIG. 10  is a block diagram schematically illustrating a video decoding apparatus according to at least one embodiment of the present disclosure. 
         [0086]    A video decoding apparatus  1000  according to the aspect of the present disclosure includes an image decoder  1010  and a low frequency decoder  1030 . The video decoding apparatus  1000  may be a personal computer or PC, notebook or laptop computer, personal digital assistant or PDA, portable multimedia player or PMP, PlayStation Portable or PSP, or mobile communication terminal, smart phone or such devices, and represent a variety of apparatuses equipped with, for example, a communication device such as a modem for carrying out communication between various devices or wired/wireless communication networks, a memory for storing various programs for decoding images and related data, and a microprocessor for executing the programs to effect operations and controls. 
         [0087]    The image decoder  1010  performs a predictive decoding on encoded image data extracted from a bitstream to generate a reconstructed block. Specifically, the image decoder  1010  extracts encoded image data from a bitstream, decodes the encoded image data to reconstruct quantized transform coefficients, inversely quantizes and inversely transforms the reconstructed quantized transform coefficients in the first transform units to reconstruct the residual block, predicts the current block to generate a predicted block, and adds the reconstructed residual block to the predicted block to generate a reconstructed block. 
         [0088]    To this end, as illustrated in  FIG. 10 , the image decoder  1010  includes a first decoder  1012 , a first inverse quantizer  1014 , a first inverse transformer  1016 , a predictor  1018 , a first adder  1020 , and a memory  1022 . The operations and functions of the first decoder, the first inverse quantizer  1014 , the first inverse transformer  1016 , the predictor  1018 , the first adder  1020 , and the memory  1022  are similar to or the same as those of the decoder  310 , the inverse quantizer  320 , the inverse transformer  330 , the predictor  340 , the adder  350 , and the memory  360  already described with reference to  FIG. 3 , so their description will be omitted. 
         [0089]    However, the first inverse transformer  1016  may transform the residual block in the first transform units in operation. The first transform units herein may be the same as the first transform units used in the first transformer  430  of the image encoder  410 , such first transform units may be the transform in the form of a rectangle (i.e. P×Q transform, wherein P≠Q), such as 4×8 transform, 8×4 transform, 16×8 transform, and 8×16 transform, in which the number of transverse pixels is different from that of longitudinal pixels of the transform block. Further, the first transform units may be one of 4×4 transform, 8×8 transform, 16×16 transform, 4×8 transform, 8×4 transform, 16×8 transform, and 8×16 transform, or a combination thereof. 
         [0090]    Further, the adder  350  described earlier with reference to  FIG. 3  adds the reconstructed residual block outputted from the inverse transformer  330  to the predicted block outputted from the predictor  340  to reconstruct the current block and stores the reconstructed current block in the memory  360 . However, the first adder  1020  adds the reconstructed residual block outputted from the first inverse transformer  424  to the predicted block outputted from the predictor  412  to output the reconstructed block, in which the reconstructed block is not stored in the memory  428 , but is outputted to the subtracter  432  of the low frequency encoder  430 . That is, the reconstructed current block outputted from the adder  280  becomes the finally reconstructed current block in the video encoding apparatus of  FIG. 2 . However, in the video encoding apparatus  400  according to the aspect of the present disclosure of  FIG. 4 , not the reconstructed block outputted from the first adder  426  but the reconstructed current block outputted from the second adder  444  of the low frequency encoder  430  becomes the finally reconstructed current block. Here, the current block means a block to be currently encoded, and may be a macroblock or a subblock. 
         [0091]    The low frequency decoder  1030  decodes the encoded low frequency data extracted from the bitstream to reconstruct the error block, and adds the error block to the reconstructed block to reconstruct the current block. Specifically, the low frequency decoder  1030  extracts the encoded low frequency data from the bitstream, decodes the encoded low frequency data to reconstruct the quantized low frequency transform coefficients, inversely quantizes the reconstructed quantized low frequency transform coefficients to reconstruct the low frequency transform coefficients, and inversely transforms the reconstructed low frequency transform coefficients in the unit of the second transform unit to reconstruct the error block. 
         [0092]    To this end, as illustrated in  FIG. 10 , the low frequency decoder  1030  includes a second decoder  1032 , a second inverse quantizer  1034 , a second inverse transformer  1036 , and a second adder  1038 . 
         [0093]    The second decoder  1032  decodes the encoded low frequency data extracted from the bitstream to reconstruct the quantized low frequency transform coefficients. Specifically, the second decoder  1032  extracts the encoded low frequency data extracted from the bitstream and decodes the extracted low frequency data to reconstruct the quantized low frequency transform coefficients. The second decoder  1032  may decode the encoded low frequency data by inversely performing the same encoding method, such as the entropy encoding, as that used in the encoding of the quantized low frequency transform coefficients by the low frequency encoder  430 . 
         [0094]    Further, the second decoder  1032  may reconstruct information on the number of the quantized low frequency coefficients by extracting data of the number of the encoded low frequency coefficients from the bitstream and decoding the data, and appropriately extract the encoded low frequency data from the bitstream by using the number of the reconstructed quantized low frequency coefficients and decode the encoded low frequency data. 
         [0095]    The operations and functions of the second inverse quantizer  1034 , the second transformer  1036 , and the second adder  1038  are similar to or the same as those of the second inverse quantizer  440 , the second inverse transformer  442 , and the second adder  444  of the low frequency encoder  430  described earlier with reference to  FIG. 4 . However, the second adder  444  stores the reconstructed current block in the memory  428 , but the second adder  1038  not only stores the reconstructed current block in the memory  1022  but also outputs the reconstructed current block as a reconstructed image. That is, the reconstructed current blocks outputted from the second adder  1038  are accumulated in the unit of pictures to be reproduced as the reconstructed video. 
         [0096]    The second transform units may include one or more transforms, be larger than the first transform units used for the predictive decoding of the encoded image data, and include a plurality of adjacent first transform units used for the predictive decoding of the encoded image data. The second transform units have been already described with reference to  FIG. 5 , so a detailed description will be omitted. 
         [0097]    Further, the second inverse transformer  1036  may combine an error signal reconstructed through the inverse transform of the reconstructed low frequency transform coefficients and additional error signals to reconstruct the error block. Further, the second decoder  1032  may extract the encoded flag data from the bitstream and decode the extracted flag data to reconstruct the flag information, and only when the flag information reconstructed through extracting and decoding the encoded flag data indicates that only the low frequency components of the error block are encoded, the second decoder  1032  may decode the encoded low frequency data. 
         [0098]    According to a video decoding method according to an aspect of the present disclosure, the video decoding apparatus  1000  predicts and decodes the encoded image data extracted from the bitstream to generate a reconstructed block, decodes the encoded low frequency data extracted from the bitstream to reconstruct the error block, and adds the error block to the reconstructed block to reconstruct the current block. Here, the current block may be a macroblock or a subblock. 
         [0099]    The video decoding method according to the aspect of the present disclosure may be specifically implemented as illustrated in  FIG. 11 . 
         [0100]      FIG. 11  is a flowchart illustrating an implementation of the video decoding method according to the aspect of the present disclosure. 
         [0101]    The video decoding apparatus  1000  extracts the encoded image data from the bitstream and decodes the encoded image data to reconstruct the quantized transform coefficients (S 1110 ), inversely quantizes and inversely transforms the reconstructed quantized transform coefficients in the first transform units to reconstruct the residual block (S 1120 ), predicts the current block to generate a predicted block (S 1130 ), and adds the reconstructed residual block to the predicted block to generate the reconstructed block (S 1140 ). 
         [0102]    Further, the video decoding apparatus  100  extracts the encoded low frequency data from the bitstream and decodes the encoded low frequency data to reconstruct the quantized low frequency transform coefficients (S 1150 ), inversely quantizes the reconstructed quantized low frequency transform coefficients to reconstruct the low frequency transform coefficients (S 1160 ), inversely transforms the reconstructed low frequency transform coefficients in the second transform units to reconstruct the error block, and adds the error block to the reconstructed block to reconstruct the current block (S 1180 ). 
         [0103]    Here, the second transform units may include one or more transforms, be larger than the first transform units used for the predictive decoding of the encoded image data, and include a plurality of adjacent first transform units used for the predictive decoding of the encoded image data. 
         [0104]    In step S 1170 , the video decoding apparatus  1000  may reconstruct the error block by combining the error signals reconstructed through the inverse transform of the reconstructed low frequency transform coefficients and the additional error signals. 
         [0105]    Further, the video decoding apparatus  1000  may extract the encoded flag data from the bitstream and decode the encoded flag data to reconstruct the flag information. Accordingly, steps S 1150  through S 1180  may be performed only when the reconstructed flag information indicates that only the low frequency components of the error block are encoded. 
         [0106]    In the description above, although all of the components of the aspects of the present disclosure may have been explained as assembled or operatively connected as a unit, the present disclosure is not intended to limit itself to such embodiments. Rather, within the objective scope of the present disclosure, the respective components may be selectively and operatively combined in any numbers. Every one of the components may be also implemented by itself in hardware while the respective ones can be combined in part or as a whole selectively and implemented in a computer program having program modules for executing functions of the hardware equivalents. Codes or code segments to constitute such a program may be easily deduced by a person skilled in the art. The computer program may be stored in computer readable media, which in operation can realize the aspects of the present disclosure. As the computer readable media, the candidates include magnetic recording media, optical recording media, and carrier wave media. 
         [0107]    In addition, terms like ‘include’, ‘comprise’, and ‘have’ should be interpreted in default as inclusive or open rather than exclusive or closed unless expressly defined to the contrary. All the terms that are technical, scientific or otherwise agree with the meanings as understood by a person skilled in the art unless defined to the contrary. Common terms as found in dictionaries should be interpreted in the context of the related technical writings not too ideally or impractically unless the present disclosure expressly defines them so. 
         [0108]    As described above, some embodiments of the present disclosure encode videos by accounting for low frequency components, so that the compression efficiency is improved while reducing blocking effects due to the transform and quantization operations. 
         [0109]    Although exemplary aspects of the present disclosure have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the spirit and scope of the claimed invention. Specific terms used in this disclosure and drawings are used for illustrative purposes and not to be considered as limitations of the present disclosure. Therefore, exemplary aspects of the present disclosure have not been described for limiting purposes. Accordingly, the scope of the claimed invention is not to be limited by the above aspects but by the claims and the equivalents thereof.