Abstract:
An image reproducing apparatus that achieves a higher readiness, high-speed backward reproduction, and a minimum amount of data, at a time of executing a backward reproduction. An encoder-side intra predictor transcodes decode images of a front end and a back end of a GOP of an image stream. An encoder-side motion compensator transcodes a decode image other than that of the front end of the GOP of the image stream by setting a motion vector to 0 and using an immediately preceding reference image.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is entitled and claims the benefit of Japanese Patent Application No.2010-255841, filed on Nov. 16, 2010, the disclosure of which including the specifications, drawings and abstract are incorporated herein by reference in its entirety. 
       TECHNICAL FIELD  
       [0002]    The present invention relates to an image converting apparatus and an image reproducing apparatus for digitally compressed images, and in particular to a technology for use in special reproduction such as rewind reproduction. 
       BACKGROUND ART  
       [0003]    In an image reproducing apparatus that reproduces images compressed by a digital compression method such as MPEG2 or H.264, it is required to decode all pictures (I and P pictures) in a group of pictures (GOP) including the reproduced images in the case of backward reproduction (rewind reproduction), regardless of the order that the images (pictures) are reproduced. For this reason, the backward reproduction requires a large capacity of memory and thus it is generally known that the backward reproduction of digitally compressed images is relatively difficult compared with forward reproduction. Accordingly, there have been many technologies suggested so far in connection with that reproduction. 
         [0004]    For example, there is a devised technology which reencodes (transcodes) the decoded images to decrease the number of structures (pictures) of a GOP of an original encoding image and decrease the memory capacity at the time of the backward reproduction (see Patent Literature 1). 
       CITATION LIST  
     Patent Literature 
       [0005]    PTL 1: Japanese Patent Application Laid-Open No. 11-252507 (paragraphs [0041] to [0063] and FIG. 2) 
       SUMMARY OF INVENTION  
     Technical Problem 
       [0006]    However, even though the number of structures (pictures) of the GOP of the original encoding image is decreased by re-encoding (transcoding) the decoded images, the decoding needs to be performed in an amount corresponding to the number of structures of the GOP (two times when there are two pictures). For this reason, at the time of the backward reproduction of multiple screens such as 16 CHs (CH stands for channel) which are assumed to be used for monitoring for example, readiness is not still obtained. 
         [0007]    When the number of structures of the GOP is further decreased, that is, set to be a small number, one (I picture) to avoid the problem of the readiness, there occurs another problem such that the amount of data for the image increases, compared with the case of employing the P picture which uses a time correlation. 
         [0008]    An object of the present invention is to provide an image converting apparatus, an image reproducing apparatus, and an image converting method that can realize a high readiness, a high speed backward reproduction, and a minimum amount of data, so as to be suitable for the backward reproduction of a multi-screen to be used for monitoring. 
       Solution to Problem 
       [0009]    In order to achieve the above object, one aspect of an image converting apparatus according to the present invention is an image conversion apparatus which transcodes a digitally compressed image stream. The image conversion apparatus includes an intra predictor configured to convert a decoded image of the image stream into an intra prediction image and a motion compensated predictor configured to convert a decoded image of the image stream into a motion compensated prediction image. The intra predictor transcodes first and final decoded images of a GOP (Group of Pictures) of the image stream and the motion compensated predictor transcodes all decoded images except the first decoded image of the GOP of the image stream by setting a motion vector to zero and referring to only an immediately preceding decoded image. 
         [0010]    One aspect of an image reproducing apparatus of an image converting apparatus according to the present invention includes a decoder which generates a decode image of a digitally compressed image stream and the image converting apparatus transcodes the decode image. The image reproducing apparatus, when a forward reproduction is performed, transcoded images are sequentially decoded and reproduced by the decoder and the back end of the GOP is decoded and reproduced to produce a motion compensation prediction image, and, when a backward reproduction is performed, the back end of the GOP of the transcoded image is decoded and reproduced by the decode device using the intra prediction image. 
         [0011]    One aspect of an image converting method according to the present invention is an image converting method that transcodes a digitally compressed image stream. The image converting method includes converting a decoded image of the image stream into an intra prediction image and converting a decoded image of the image stream into a motion compensated prediction image. Then, converting a decoded image of the image stream into an intra prediction image transcodes first and final decoded images of a GOP (Group of Pictures) of the image stream and converting a decoded image of the image stream into a motion compensated prediction image transcodes all decoded images except the first decoded image of the GOP of the image stream by setting a motion vector to zero and referring to only an immediately preceding decoded image. 
         [0012]    Further, one aspect of an image reproducing apparatus according to the present invention includes an interface that receives a digitally compressed image stream from plural cameras through a network, a decode device that decodes the digitally compressed image stream and generates a decoded image, an image storage memory that stores the decoded image output by the decode device, a transcode device that receives the decoded image in the image storage memory and generates a transcoded stream, a storage medium that stores the digitally compressed image stream input by the interface and the transcoded stream transcoded by the transcode device, and an image combining section that combines image data stored in the image storage memory and outputs the image data to a monitoring monitor. In this case, the decode device includes an entropy decoding section that performs entropy decoding with respect to the digitally compressed image stream that is input by the interface and the storage medium, a decoder-side inverse quantizing section that performs inverse quantization with respect to entropy encoding released data that is input by the entropy decoding section, a decoder-side inverse DCT section that performs inverse DCT with respect to the data that is inversely quantized by the decoder-side inverse quantizing section, a decoder-side prediction method determining section that determines which prediction mode of intra prediction or motion compensation prediction is used, on the basis of the data where the inverse DCT is performed by the decoder-side inverse DCT section, a decoder-side intra predicting section that performs image decoding in a unit of macro block (MB) using the intra prediction, when intra prediction data is determined by the decoder-side prediction method determining section, a decoder-side motion compensating section that performs the image decoding in a unit of MB and calculates differential data, when motion compensation prediction data is determined by the decoder-side prediction method determining section, a code inverting section that determines whether inversion of positive and negative codes of the differential data calculated by the motion compensating section is performed, on the basis of a current decoding direction, and performs image decoding in a unit of MB with respect to the motion compensation data using a reference decoded image stored in a decoder-side reference image storage memory, a decoded image generating section that stores the decoded images decoded in a unit of MB in the decoder-side intra predicting section and the code inverting section and feeds back decoded peripheral MB information to the decoder-side intra predicting section, and a decoder-side reference image storage memory that stores the decoded images in the decoded image generating section, when the entire decoding in a unit of MB in one frame ends. The transcode device includes an encoder-side intra predicting section that generates an intra prediction image using the decoded image input from the image storage memory and a local decoded image obtained by the local decoded image generating section, an encoder-side motion compensating section that performs a motion compensation operation using the decoded image input from the image storage memory and the reference decoded image obtained by the encoder-side reference image storage memory, an encoder-side prediction method determining section that compares prediction images that are obtained by the encoder-side intra predicting section and the encoder-side motion compensating section and determines a prediction mode, a differential information generating section that generates differential information of an image, using a prediction method determined by the encoder-side prediction method determining section, a DCT section that performs a DC conversion with respect to the differential information obtained by the differential information generating section, a quantizing section that performs quantization with respect to data obtained by the DCT section, an entropy encoding section that performs entropy encoding with respect to the data obtained by the quantizing section and transmits the encode data to the transcoded stream combining section, an encoder-side inverse quantizing section that performs inverse quantization with respect to the data obtained by the quantizing section, an encoder-side inverse DCT section that executes an inverse DCT process with respect to the data output by the encoder-side inverse quantizing section, a local decoded image generating section that generates a local decoded image in a unit of MB, using the data output by the encoder-side inverse DCT section and the reference decoded image in the encoder-side reference image storage memory, and an encoder-side reference image storage memory that stores a reference local decoded image after the image data in a unit of MB decoded by the local decoded image generating section has been arranged by one frame. 
         [0013]    By this structure, even when the backward reproduction is performed by performing the transcoding with respect to the digitally compressed moving image and storing the transcoded data in the storage medium, a data structure where backward decoding can be performed without decoding all of reference pictures in a forward direction can be provided, and high-speed backward decoding can be performed without affecting a forward decode operation. 
       Advantageous Effects of Invention 
       [0014]    According to the present invention, when the backward reproduction is performed at the certain time by performing transcoding (re-encoding) where the motion vector is set as 0 with respect to the digitally compressed moving image, desired backward reproduction can be performed by setting only one immediately previous image as a reference image and performing only one decoding using the corresponding image, without performing decoding corresponding to the number of structure images of the GOP in the related art. 
         [0015]    Thereby, in the multi-screen reproduction such as 16 CHs which is assumed to be used for monitoring, even though the reproduction direction is switched into either the forward direction or the backward direction at the certain time, reproduction of a desired direction can be realized by only one decoding. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a block diagram showing a structure of an image reproducing apparatus in Embodiment 1 of the present invention; 
           [0017]      FIG. 2  is a diagram showing an image stream in each block of the image reproducing apparatus; 
           [0018]      FIG. 3  is a flowchart showing an operation of a decode device; 
           [0019]      FIG. 4  is a flowchart showing an operation of a transcode device (operation of a portion other than a back end of a GOP); 
           [0020]      FIG. 5  is a flowchart showing an operation of the transcode device (operation of the back end of a GOP); and 
           [0021]      FIG. 6  is a diagram showing an example of a transcoded stream. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
       [0022]    Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
       [1] Entire Structure 
       [0023]      FIG. 1  is a block diagram showing a structure of an image reproducing apparatus according to an embodiment of the present invention. Image reproducing apparatus  100  performs special reproduction such as backward reproduction. 
         [0024]    In the drawing, image reproducing apparatus  100  mostly has interface  101 , decode device  120 , image storage memory  103 , transcode device  140 , storage medium  102 , and image combining section  104 . 
         [0025]    Interface  101  receives a digitally compressed image stream from plural cameras  110   a  to  110   d  through network  111 . Decode device  120  decodes the image stream and generates a decoded image. Image storage memory  103  stores the decoded image output by decode device  120 . Transcode device  140  receives the decoded image that is stored in image storage memory  103  and generates a transcoded stream. Storage medium  102  is a removable storage medium that stores the image stream input by interface  101  and the transcoded stream transcoded by transcode device  140 . Image combining section  104  combines the image data of the plural cameras stored in image storage memory  103  and outputs the image data to monitoring monitor  112 . 
         [0026]    Decode device  120  includes entropy decoding section  121 , decoder-side inverse quantizing section  122 , decoder-side inverse DCT section  123 , decoder-side prediction method determining section  124 , decoder-side intra predicting section  125 , decoder-side motion compensating section  126 , code inverting section  127 , decoded image generating section  128 , and decoder-side reference image storage memory  129 . 
         [0027]    Entropy decoding section  121  performs entropy decoding with respect to the digitally compressed image stream that is input by interface  101  and storage medium  102 . Decoder-side inverse quantizing section  122  performs inverse quantization with respect to entropy encoding released data that is input by entropy decoding section  121 . Decoder-side inverse DCT section  123  performs inverse DCT with respect to the data that is inversely quantized by decoder-side inverse quantizing section  122 . Decoder-side prediction method determining section  124  determines which prediction mode of intra prediction or motion compensation is used, on the basis of the data where the inverse DCT is performed by decoder-side inverse DCT section  123 . When intra prediction data is determined by decoder-side prediction method determining section  124 , decoder-side intra predicting section  125  performs image decoding in a unit of macro block (MB) using the intra prediction. When motion compensation data is determined by decoder-side prediction method determining section  124 , decoder-side motion compensating section  126  performs the image decoding in a unit of MB with respect to the motion compensation data and calculates differential data. Code inverting section  127  determines whether inversion of positive and negative codes of the differential data calculated by motion compensating section  126  is performed, from a current decoding direction, and performs decoding in a unit of MB with respect to the motion compensation data using decoded image information stored in decoder-side reference image storage memory  129 . Decoded image generating section  128  stores the decoded images that are decoded in a unit of MB in decoder-side intra predicting section  125  and code inverting section  127  and feeds back decoded peripheral MB information to decoder-side intra predicting section  125 . Decoder-side reference image storage memory  129  stores the decoded images in decoded image generating section  128 , when the entire image decoding in a unit of MB in one frame ends. 
         [0028]    Transdecode device  140  includes encoder-side intra predicting section  141 , encoder-side motion compensating section  142 , encoder-side prediction method determining section  143 , differential information generating section  144 , DCT section  145 , quantizing section  146 , entropy encoding section  147 , encoder-side inverse quantizing section  148 , encoder-side inverse DCT section  149 , local decoded image generating section  150 , and encoder-side reference image storage memory  151 . 
         [0029]    Encoder-side intra predicting section  141  generates an intra prediction image using the decoded image input from image storage memory  103  and a local decoded image obtained by local decoded image generating section  150 . Encoder-side motion compensating section  142  performs a motion compensation operation using the decoded image of image storage memory  103  and the reference local decoded image obtained by encoder-side reference image storage memory  151 . Encoder-side prediction method determining section  143  compares prediction images that are obtained by encoder-side intra predicting section  141  and encoder-side motion compensating section  142  and determines the prediction mode. Differential information generating section  144  generates differential information of an image, using a prediction method determined by encoder-side prediction method determining section  143 . DCT section  145  performs a DC conversion with respect to the differential information that is obtained by differential information generating section  144 . Quantizing section  146  performs quantization with respect to data that is obtained by encoder-side DCT section  145 . Entropy encoding section  147  performs entropy encoding with respect to the data obtained by quantizing section  146  and transmits the encode data to transcoded stream combining section  152 . Encoder-side inverse quantizing section  148  performs inverse quantization with respect to the data that is obtained by quantizing section  146 . Encoder-side inverse DCT section  149  executes an inverse DCT process with respect to the data that is output by encoder-side inverse quantizing section  148 . Local decoded image generating section  150  generates a local decoded image in a unit of MB, using the data output by encoder-side inverse DCT section  149  and the reference decoded image in encoder-side reference image storage memory  151 . Encoder-side reference image storage memory  151  stores a reference local decoded image after the image data in a unit of MB decoded by local decoded image generating section  150  has been arranged by one frame. 
       [2] Operation 
       [0030]    The operation of image reproducing apparatus  100  that has the above-described structure will be described using drawings. 
         [0031]      FIG. 2  is a diagram showing an image stream in each block of image reproducing apparatus  100 . For example,  FIG. 2  shows digitally compressed input stream  200  that is output by camera  110   a,  decoded image group  210  that is obtained by decoding the input image stream by decode device  120  (decoded image reproducing section  128 ), and transcoded stream  220  that is obtained by re-encoding decoded image group  210  by transcode device  140  (output from transcoded stream combining section  152 ). 
         [0032]    In input stream  200 , I denotes an image of an I picture and P denotes an image of a P picture. In the drawing, the image streams are arranged in frame order, in encode order by the cameras from the left side. In the normal GOP, a B picture is also included. However, the B picture is not generally used for the purpose of monitoring. In this case, an image stream that does not use the B picture is used. In the GOP that is configured using the I picture and the P picture, the encode order follows display order on the monitor. For the purpose of accumulation, the image stream that is output by camera  110   a  is transmitted to storage medium  102  through interface  101  and is stored. The image stream is also transmitted to decode device  120 . 
         [0033]    Next, generation of the decoded image group by decode device  120  and generation of the transcoded stream by transcode device  140  will be described. 
         [0034]    (1) With Respect to the Generation of the Decoded Image Group 
         [0035]      FIG. 3  is a flowchart showing an operation flow of decode device  120 . In this case, a course of decoding images from image  201  of the time T 1  to image  206  of the time T 6  and generating the decoded image group from decoded image  211  to decoded image  216  will be described using the flowchart. 
         [0036]    &lt;Decode of Image  201  of the I Picture&gt; 
         [0037]    When decode device  120  receives the image  201  (I picture) of the image stream of the time T 1  from storage medium  102  (S 301 ), decode device  120  performs entropy decoding with respect to image  201  by entropy decoding section  121  (S 302 ). A process step from the following S 304  to S 312  is repeated until a decode process of all macro blocks (MB) of image  201  ends (R 303 ). 
         [0038]    Next, decode device  120  executes a quantization process with respect to the entropy decoded data by decoder-side inverse quantizing section  122  (S 304 ) and executes an inverse DCT process with respect to the inversely quantized data by the decoder-side inverse DCT section  123  to generate differential information of the image data in a unit of MB (S 305 ). 
         [0039]    Decode device  120  analyzes image information in decoder-side prediction method determining section  124  using the generated differential information and determines whether motion compensation is used in a unit of MB (B 306 ). That is, decode device  120  determines whether the corresponding block is an intra encoded macro block. When the motion compensation is used, the process proceeds to S 307  and when the motion vector is not used and the intra prediction is used, the process proceeds to S 308 . In this case, since image  201  is the I picture, the process proceeds to S 308  in all MBs. In addition, in decoder-side intra predicting section  125 , the intra prediction is performed using the already decoded MB information fed back from the decoded image generating section  128  as peripheral information and decoding performed (S 308 ). 
         [0040]    The decoded image is temporarily stored in decoded image generating section  128  until the decoding is completed with respect to all MBs in the image. The image that is temporarily stored in decoded image generating section  128  is used as peripheral information of the MBs to be decoded, in the process of S 308  as described above (S 312 ). After the process step from S 304  to S 312  ends with respect to all MBs of the image and decoded image  211  is generated, a loop process is skipped (R 313 ). Finally, decoded image  211  that is generated by decoded image generating section  128  is stored in the decoder-side reference image storage memory  129  (S 314 ) and is transmitted to image storage memory  103  (S 315 ). 
         [0041]    &lt;Decode of Image  202  of the P Picture&gt; 
         [0042]    Next, a course of generating the decoded image of image  202  (P picture) will be described using the flowchart. 
         [0043]    When decode device  120  receives image  202  (P picture) of the image stream of the time T 2  from storage medium  102  (S 301 ), decode device  120  performs entropy decoding with respect to image  202  by entropy decoding section  121  (S 302 ). A process step from the following S 304  to S 312  is repeated until a decode process of all macro blocks (MB) of image  202  ends (R 303 ). 
         [0044]    Next, an inverse quantization process is executed with respect to the entropy decoded data in a unit of MB by decoder-side inverse quantizing section  122  (S 304 ) and executes an inverse DCT process with respect to the inversely quantized data by decoder-side inverse DCT section  123  to generate differential information of the image data in a unit of MB (S 305 ). 
         [0045]    Analysis is performed on image information in decoder-side prediction method determining section  124  using the generated differential information and determines whether motion compensation is used in a unit of MB (B 306 ). That is, a determination is made on whether the corresponding block is an intra encoded macro block. When the motion compensation is used, the process proceeds to S 307  and when the motion compensation is not used and the intra prediction is used, the process proceeds to S 308 . In this case, since image  202  is the P picture, the process proceeds to either S 307  or S 308  in a unit of MB. In addition, in decoder-side motion compensating section  126 , the motion compensation data is processed in a unit of MB and differential information of the image is acquired (S 307 ). In addition, in decoder-side intra predicting section  125 , the intra prediction is performed using the already decoded MB information fed back from decoded image generating section  128  as the peripheral information and the decoding is performed (S 308 ). 
         [0046]    In this case, different from the case of the I picture, in the case of the P picture, a current decode direction is determined in the code inverting section  127  (B 309 ). The decode direction is a forward direction when the decode order follows time order and is a backward direction when the decode order follows inverse order of the time order. When the forward decoding is performed, the process proceeds to S 310 . When the backward decoding is performed, the process proceeds to S 311 . In this case, after decoding image  201  (I picture) of the image stream of the time T 1 , image  202  (P picture) of the image stream of the time T 2  is decoded. Therefore, the forward decoding is performed and the process proceeds to S 310 . Since the forward decoding is performed, the differential information that is obtained by decode-side motion compensating section  126  is used in code inverting section  127 , the motion compensation is performed using the decoded image stored in decoder-side reference image storage memory  129  as the reference image, and the decoding is performed (S 310 ). 
         [0047]    The decoded image is temporarily stored in decoded image generating section  128  until the decoding is completed on all MBs of the image. The image that is temporarily stored in decoded image generating section  128  is used as peripheral information of the MBs to be decoded, in the process of S 308  as described above (S 312 ). After the process step from S 304  to S 312  ends with respect to all MBs of the image and decoded image  212  is generated, a loop process is skipped (R 313 ). Finally, decoded image  212  that is generated by decoded image generating section  128  is stored in decoder-side reference image storage memory  129  (S 314 ) and is transmitted to image storage memory  103  (S 315 ). 
         [0048]    The course of decoding image  202  (P picture) of the image stream of the time T 2  and generating decoded image  212  is described above. However, the same process is executed with respect to following P picture images  203 ,  204 ,  205 , and  206  and decoded images  213 ,  214 ,  215 , and  216  are generated. The generated decoded images are temporarily stored in image storage memory  103  and are transmitted to transcode device  140  of a next step. 
         [0049]    (2) With Respect to the Generation of the Transcoded Stream 
         [0050]    The generation of transcoded stream  220  will be described.  FIG. 4  is a flowchart showing an operation of transcode device  140  (a portion other than the back end of the GOP). In this case, a course of receiving images from image  211  of the decoded image group of the time T 1  to image  216  of the decoded image group of the time T 6  and generating transcoded stream  221  by re-encoding where the number of images of the GOP is  6  will be described using the flowchart. The process of transcoding is mostly divided into three processes of (i) a transcode of an I picture where only the intra prediction is used, (ii) a transcode of a P picture where only the motion compensation is used, and (iii) a transcode of the back end of the GOP where both the intra prediction and the motion compensation are used. Hereinafter, each case will be described. 
         [0051]    &lt;(i) Generation of the Transcoded Image Using Only the Intra Prediction&gt; 
         [0052]    Transcode device  140  acquires decoded image  211  of the time T 1  from image storage memory  103  (S 401 ). A process step from the following S 403  to S 413  is repeated until a process of all macro blocks (MB) of image  211  ends (R 402 ). 
         [0053]    Next, in encoder-side intra predicting section  141 , peripheral information is acquired from outputs of both decoded image  211  and local decoded image generating section  150  and an intra prediction image is generated for each MB (S 403 ). Since transcoded image  231  of the generated transcoded stream of the time T 1  is the I picture, a process is not executed in encoder-side motion compensating section  142  (S 404 ). 
         [0054]    Next, in encoder-side prediction method determining section  143 , the generation result of the prediction image in each step of S 403  and S 404  and decoded image  211  input by the image storage memory  103  are compared and each prediction error is calculated (B 405 ). In addition, the calculated prediction errors are compared with each other, and the process proceeds to S 406  when the error of the prediction by the motion compensation is smaller than the error by the intra prediction and proceeds to S 407  when the error of the intra prediction is smaller than the prediction error by the motion compensation. In this case, since transcoded image  231  is the I picture, the process proceeds to S 407  in all of the macro blocks. In addition, in differential information generating section  144 , calculation of the differential information between decoded image  211  input by image storage memory  103  and the intra prediction image is performed on the basis of the determination result of B 405  (S 407 ), and in DCT section  145 , a DCT operation is performed with respect to the differential information (S 408 ). 
         [0055]    Next, quantizing section  146  performs an quantization operation with respect to the data output by DCT section  145  (S 409 ) and entropy encoding section  147  performs entropy encoding with respect to the data output by quantizing section  146  (S 410 ), and the encoding result is temporarily stored as data of 1 MB of transcoded image  231  in a memory (not shown in the drawings) of transcoded stream combining section  152 . The data that is output by quantizing section  146  is input to entropy encoding section  147  and the inverse quantization process is executed in encoder-side inverse quantizing section  148  (S 411 ). Encoder-side inverse DCT section  149  performs an inverse DCT operation with respect to the data output by encoder-side inverse quantizing section  148  (S 412 ) and local decoded image generating section  150  collects the decoded data of a unit of MB as one image data (S 413 ). Since the image data is used as the peripheral information in the intra prediction (S 403 ), the image data is output to encoder-side intra predicting section  141 . 
         [0056]    When the process step of S 403  to S 413  ends with respect to all MBs of the image, the loop process is skipped (R 414 ). After the process step ends with respect to all MBs of the image, one local decoded image is finished in local decoded image generating section  150  and the local decoded image is stored as the reference image data used at the time of next transcoding in encoder-side reference image storage memory  151  (S 415 ). Finally, the transcoded image for each MB that is stored in transcoded stream combining section  152  is collected as one transcoded image and transcoded image  231  (I picture) of the transcoded stream is generated (S 416 ). Generated transcoded image  231  is stored in storage medium  102 . 
         [0057]    &lt;(ii) Generation of the Transcoded Images Using Only the Motion Compensation Prediction&gt; 
         [0058]    Next, the transcode of the P picture using only the motion compensation will be described. Transcode device  140  acquires decoded image  212  of the time T 2  from image storage memory  103  (S 401 ). A process step from the following S 403  to S 413  is repeated until a process of all macro blocks (MB) of decoded image  212  ends (S 402 ). 
         [0059]    Next, since generated transcoded image  232  of the transcoded stream of the time T 3  is the P picture using only the motion compensation prediction, a process is not executed in encoder-side intra predicting section  141  (S 403 ). In encoder-side motion compensating section  142 , decoded image  212  of the time T 2  is acquired from image storage memory  103 , the reference image is acquired from encoder-side reference image storage memory  151 , and the prediction image based on the motion compensation is generated for each MB (S 404 ). However, a method of calculating the motion compensation in this embodiment sets a motion vector (MV) as zero and uses only an immediately previous image as time. 
         [0060]    Next, in encoder-side prediction method determining section  143 , the generation result of the prediction image in each step of S 403  and S 404  and decoded image  212  input by image storage memory  103  are compared and each prediction error is calculated (B 405 ). In addition, the calculated prediction errors are compared with each other, and the process proceeds to S 406  when the error of the prediction by the motion compensation is smaller than the error by the intra prediction and proceeds to S 407  when the error of the intra prediction is smaller than the prediction error by the motion compensation. In this case, since transcoded image  232  is the P picture using only the motion compensation prediction, the process proceeds to S 406  in all of the macro blocks. In addition, in differential information generating section  144 , calculation of the differential information between decoded image  212  input by image storage memory  103  and the prediction result by the motion compensation where the motion vector (MV) is set as zero and only the immediately previous reference image (decoded image  211 ) of T 1  is used as the time is performed on the basis of the determination result of B 405  (S 406 ). 
         [0061]    Next, DCT section  145  performs a DCT operation with respect to the differential information (S 408 ), quantizing section  146  performs a quantization operation with respect to the data output by DCT section  145  (S 409 ), and entropy encoding section  147  performs the entropy encoding with respect to the data output by quantizing section  146  (S 410 ) and temporarily stores the encoding result as data of 1 MB of transcoded image  232  in a storage memory (not shown in the drawings) of transcoded stream combining section  152 . In addition, the data that is output by quantizing section  146  is input to entropy encoding section  147  and the inverse quantization process is executed in encoder-side inverse quantizing section  148  (S 411 ). Encoder-side inverse DCT section  149  performs the inverse DCT operation with respect to the data output by encoder-side inverse quantizing section  148  (S 412 ) and local decoded image generating section  150  collects the decoded data of a unit of MB as one image data (S 413 ). 
         [0062]    When the process step of S 403  to S 413  ends with respect to all MBs of the image, the loop process is skipped (R 414 ). When the process step ends with respect to all MBs of the image, one local decoded image is finished in local decoded image generating section  150  and the local decoded image is stored as the reference image used at the time of next transcoding in encoder-side reference image storage memory  151  (S 415 ). Finally, the transcoded image for each MB that is stored in transcoded stream combining section  152  is collected as one transcoded image and transcoded image  232  (P picture) is generated (S 416 ). Generated transcoded image  232  is stored in storage medium  102 . 
         [0063]    The course of transcoding decoded image  212  (P picture) of the decoded image group of the time T 2  and generating transcoded image  232  is described above. However, the process contents are applicable to generation of transcoded images  233 ,  234 , and  235  of the P picture of the times T 3 , T 4  and T 5 . 
         [0064]    &lt;(iii) Generation of the Transcoded Image of the Back End of the GOP&gt; 
         [0065]    Next, the transcode of the back end of the GOP using both the intra prediction and the motion compensation will be described.  FIG. 5  is a flowchart showing an operation of transcode device  140  (back end of the GOP). 
         [0066]    Transcode device  140  acquires decoded image  216  of the time T 6  from image storage memory  103  (S 501 ). A process step from the following S 503  to S 510  is repeated until a decode process of all macro blocks (MB) of decoded image  216  ends and transcoded image  236   b  (I picture) of the transcoded stream is generated. 
         [0067]    First, encoder-side intra predicting section  141  acquires the peripheral information that is the information already encoded by local decoded image generating section  150  and generates the intra prediction image for each MB (S 503 ). Differential information generating section  144  calculates the differential information using decoded image  216  input by image storage memory  103  and the intra prediction result obtained in S 503  (S 504 ) and DCT section  145  performs a DCT operation on the differential information (S 505 ). 
         [0068]    Next, quantizing section  146  performs an quantization operation with respect to the data output by DCT section  145  (S 506 ) and entropy encoding section  147  performs entropy encoding with respect to the data output by quantizing section  146  (S 507 ), and the encoding result is temporarily stored as data of 1 MB of transcoded image  235   b  in a storage memory (not shown in the drawings) of transcoded stream combining section  152 . In addition, the data that is output by quantizing section  146  is input to entropy encoding section  147  and the inverse quantization process is executed in encoder-side inverse quantizing section  148  (S 508 ). Encoder-side inverse DCT section  149  performs an inverse DCT operation with respect to the data output by encoder-side inverse quantizing section  148  (S 509 ) and local decoded image generating section  150  collects the decoded data of a unit of MB as one image data (S 510 ). This data is used as the peripheral information in the intra prediction of S 503  described above. 
         [0069]    When the process step of S 503  to S 510  ends with respect to all MBs of the image, the loop process is skipped (R 511 ). At that time, the transcoded image for each MB that is stored in transcoded stream combining section  152  is collected as one transcoded image and transcoded image  236   b  of the I picture is generated. 
         [0070]    In addition, the process steps of the following S 513  to S 519  are repeated until a process of all of the macro blocks (MB) of decoded image  216  ends (R 512 ). 
         [0071]    First, encoder-side motion compensating section  142  acquires decoded image  216  of the time T 6  from image storage memory  103 , acquires the reference image from encoder-side reference image memory  151 , and generates the prediction image based on the motion compensation for each MB (S 513 ). However, a method of calculating the motion compensation sets a motion vector (MV) as zero and uses only the immediately previous local decoded image of the time T 5  stored in encoder-side reference image memory  151  as the reference image as the time. 
         [0072]    Differential information generating section  144  calculates the differential information between decoded image  216  input by image storage memory  103  and the prediction result by the motion compensation where the motion vector (MV) is set as the zero and only the reference image (decoded image  215 ) of the immediately previous reference image (decoded image  215 ) of the time T 5  is used as the time (S 514 ). 
         [0073]    Next, DCT section  145  performs the DCT operation with respect to the differential information (S 515 ), quantizing section  146  performs a quantization operation with respect to the data output by DCT section  145  (S 516 ), and entropy encoding section  147  performs the entropy encoding with respect to the data output by quantizing section  146  (S 517 ) and temporarily stores the encoding result as the data of 1 MB of transcoded image  236   a  in a storage memory (not shown in the drawings) of transcoded stream combining section  152 . Finally, transcoded stream combining section  152  collects the transcoded images of the P picture stored in a unit of MB to generate transcoded image  236   a  (P picture), collects the transcoded image of the I picture stored in a unit of MB to generate transcoded image  236   b,  combines transcoded images  236   a  and  236   b  as one image, and generates image  236  (S 519 ). 
         [0074]    The combining base is transcoded image  236   a  of the P picture and transcoded image  236   b  of the I picture is stored in an area that is not used in the normal decode, which exists in a user-defined extension area defined by the H.264. Combined image  236  is stored in storage medium  102 . 
         [0075]    (3) With Respect to the Forward Reproduction (Decode): 
         [0076]    Next, the forward reproduction of transcoded stream  221  that is stored in storage medium  102  will be described. 
         [0077]    &lt;Forward Reproduction of Transcoded Image  231  of the I Picture&gt; 
         [0078]      FIG. 6  is a diagram showing an example of the transcoded stream. Hereinafter, the forward reproduction (decode) of transcoded image  231  will be described using  FIGS. 1 ,  3 , and  6 . 
         [0079]    To decode device  120 , transcoded image  231  of the I picture of the time T 1  is input from storage medium  102  (S 301 ). Entropy decoding section  121  performs entropy decoding on transcoded image  231  (S 302 ). A process step from the following S 304  to S 312  is repeated until a process of all macro blocks (MB) of image  231  ends (R 303 ). 
         [0080]    Next, decoder-side inverse quantizing section  122  executes an inverse quantizing process on the entropy decoded data in a unit of MB (S 304 ) and decoder-side inverse DCT section  123  executes the inverse DCT process on the inversely quantized data to generate differential information of the image data in a unit of MB (S 305 ). 
         [0081]    Decoder-side prediction method determining section  124  analyzes information of the image using the generated differential information and determines whether motion compensation is used in a unit of MB (B 306 ). When the intra prediction is not used and the motion compensation is used, the process proceeds to S 307  and when the intra prediction is used, the process proceeds to S 308 . In this case, since image  231  is the I picture, the process proceeds to S 308  in all MBs. In addition, in decoder-side intra predicting section  125 , the intra prediction is performed using the already decoded MB information fed back from decoded image generating section  128  as the peripheral information and the decoding is performed (S 308 ). 
         [0082]    The decoded image is temporarily stored in decoded image generating section  128  until the decoding is completed on all MBs in the image. The image that is temporarily stored in decoded image generating section  128  is used as peripheral information of the MBs to be decoded, in the process of S 308  as described above (S 312 ). After the process step from S 304  to S 312  ends with respect to all MBs of the image and decoded image  611  is generated, and then a loop process is skipped (R 313 ). Finally, decoded image  611  that is generated by decoded image generating section  128  is stored in decoder-side reference image storage memory  129  (S 314 ) and is transmitted to image storage memory  103  (S 315 ). The image that is transmitted to image storage memory  103  is output to monitoring monitor  112  through image combining section  104  and is reproduced and displayed. 
         [0083]    &lt;Forward Reproduction of Transcoded Image  232  of the P Picture&gt; 
         [0084]    Next, the forward reproduction (decode) of transcoded image  202  will be described. 
         [0085]    To decode device  120 , transcoded image  232  of the P picture of the time T 2  is input from storage medium  102  (S 301 ). Entropy decoding section  121  performs entropy decoding with respect to transcoded image  232  (S 302 ). A process step from the following S 304  to S 312  is repeated until a process of all macro blocks (MB) of image  232  ends (R 303 ). 
         [0086]    Next, decoder-side inverse quantizing section  122  executes an inverse quantizing process with respect to the entropy decoded data in a unit of MB (S 304 ) and decoder-side inverse DCT section  123  executes the inverse DCT process with respect to the inversely quantized data to generate differential information of the image data in a unit of MB (S 305 ). 
         [0087]    Decoder-side prediction method determining section  124  analyzes information of the image using the generated differential information and determines whether motion compensation is used in a unit of MB (B 306 ). When the intra prediction is not used and the motion compensation is used, the process proceeds to S 307  and when the intra prediction is used, the process proceeds to S 308 . In this case, since entire image  232  is the P picture generated using the motion compensation prediction, the process proceeds to S 307  in all MBs. In addition, decoder-side motion compensating section  126  processes the motion compensation data in a unit of MB and acquires the differential information of the image (S 307 ). 
         [0088]    In addition, code inverting section  127  determines a current decode direction (B 309 ). The decode direction is a forward direction when the decode order follows time order and is a backward direction when the decode order follows inverse order of the time order. When the temporally forward decoding is performed, the process proceeds to S 310 . When the temporally backward decoding is performed, the process proceeds to S 311 . 
         [0089]    With respect to the transcoded image  232 , after decoding transcoded image  231  of the I picture of the time T 1 , transcoded image  232  of the P picture of the time T 2  is decoded. Therefore, the forward decoding is performed and the process proceeds to S 310 . Since the forward decoding is performed, the differential information that is obtained by motion compensating section  126  is used in code inverting section  127  without inverting the code, a motion compensation process is executed using the decoded image stored in decoder-side reference image storage memory  129  as the reference image, and the decoding is performed (S 310 ). 
         [0090]    The decoded image is temporarily stored in decoded image generating section  128  until the decoding is completed with respect to all MBs of the image. The image that is temporarily stored in decoded image generating section  128  is used as peripheral information of the MBs to be decoded, in the process of S 308  as described above (S 312 ). After the process step from S 304  to S 312  ends with respect to all MBs of the image and decoded image  612  is generated, a loop process is skipped (R 313 ). Finally, decoded image  612  that is generated by decoded image generating section  128  is stored in decoder-side reference image storage memory  129  (S 314 ) and is transmitted to image storage memory  103  (S 315 ). 
         [0091]    The course of decoding transcoded image  232  of the P picture of the time T 2  and generating decoded image  612  of the P picture is described above. However, the same process is executed with respect to transcoded images  233  to  235  of the P pictures of the following times T 3  to T 5  and forward decoded images  613  to  615  are obtained. 
         [0092]    &lt;Forward Reproduction of Transcoded Image  236  of the Back End of the GOP&gt; 
         [0093]    Next, the forward reproduction (decode) of transcoded image  236  of the back end of the GOP will be described. 
         [0094]    To decode device  120 , transcoded image  236  of the time T 6  is input from storage medium  102  (S 301 ). Entropy decoding section  121  performs entropy decoding with respect to transcoded image  236  (S 302 ). A process step from the following S 304  to S 312  is repeated until a process of all macro blocks (MB) of transcoded image  236  ends (R 303 ). In addition, the decode object in the forward decoding is only transcoded image  236   a  and transcoded image  236   b  that is stored in the user-defined extension area is discarded at that time. 
         [0095]    First, decode device  120  executes the inverse quantization process with respect to the entropy decoded data in a unit of MB by decoder-side inverse quantizing section  122  (S 304 ) and executes the inverse DCT process with respect to the inversely quantized data by decoder-side inverse DCT section  123  to generate the differential information of the image data in a unit of MB (S 305 ). 
         [0096]    Decoder-side prediction method determining section  124  analyzes image information with the generated differential information and determines whether motion compensation is used in a unit of MB (B 306 ). When the intra prediction is not used and the motion compensation is used, the process proceeds to S 307  and when the intra prediction is used, the process proceeds to S 308 . In this case, since image  236   a  is the P picture and the intra prediction is not used at the time of transcoding, the process proceeds to S 307  in all MBs. In addition, decoder-side motion compensating section  126  processes the motion compensation data in a unit of MB and acquires the differential information of the image (S 307 ). 
         [0097]    First, code inverting section  127  determines the current decode direction (B 309 ). Since transcoded image  236  is an image obtained by performing decoding with respect to transcoded image  235  of the P picture of the time T 5 , the forward decoding is performed and the process proceeds to S 310 . Since the forward decoding is performed, code inverting section  127  uses the differential information obtained by motion compensating section  126  without inverting the code, processes the motion compensation using the decoded image stored in decoder-side reference image image storage memory  129  as the reference image, and performs the decoding (S 310 ). 
         [0098]    The decoded image is temporarily stored in decoded image generating section  128  until the decoding is completed with respect to all MBs of the image. The image that is temporarily stored in decoded image generating section  128  is used as peripheral information of the MBs to be decoded, in the process of S 308  as described above (S 312 ). After the process step from S 304  to S 312  ends with respect to all MBs of the image and decoded image  616  is generated, a loop process is skipped (R 313 ). Finally, decoded image  616  that is generated by decoded image generating section  128  is stored in decoder-side reference image storage memory  129  (S 314 ) and is transmitted to image storage memory  103 , and the entire forward decoding of transcoded stream  221  is finished. The image that is transmitted to image storage memory  103  is output to the monitoring monitor  112  through image combining section  104  (S 315 ) and is reproduced and displayed. 
         [0099]    (4) With Respect to the Backward Reproduction (Decode): 
         [0100]    Next, the backward reproduction of transcoded stream  221  that is stored in storage medium  102  will be described. Since the backward reproduction (decode) is performed, according to the decode order of transcoded stream  221 , transcoded image  236  of the time T 6  is first decoded and transcoded image  231  of the time T 1  is finally decoded. 
         [0101]    Transcoded image  236  of the time T 6  includes two pieces of information, i.e., transcoded image  236   b  of the intra prediction of decoded image  216  of the time T 6  and transcoded image  236   a  using the motion compensation prediction which is the differential information between decoded images  215  and  216  of the times T 5  and T 6 , and decoded image  626  of the time T 6  and decoded image  625  of the time T 5  are obtained by decoding the images in order of transcoded images  236   b  and  236   a.    
         [0102]    &lt;Backward Reproduction of Transcoded Image  236  of the Back End of the GOP (first)&gt; 
         [0103]    To decode device  120 , transcoded image  236  of the time T 6  is input from storage medium  102  (S 301 ). Entropy decoding section  121  performs entropy decoding with respect to transcoded image  236  (S 302 ). A process step from the following S 304  to S 312  is repeated until a process of all macro blocks (MB) of image  236  ends (R 303 ). 
         [0104]    In addition, the first decode object of the backward reproduction (decode) is transcoded image  236   b  of the intra prediction that is stored in the user-defined extension area and transcoded image  236   a  using the motion compensation prediction that is used in the normal decoding is not used at that time. 
         [0105]    First, decode device  120  executes the inverse quantization process with respect to the entropy decoded data in a unit of MB by decoder-side inverse quantizing section  122  (S 304 ) and executes the inverse DCT process with respect to the inversely quantized data by decoder-side inverse DCT section  123  to generate the differential information of the image data in a unit of MB (S 305 ). 
         [0106]    Decoder-side prediction method determining section  124  analyzes image information with the generated differential information and determines whether motion compensation is used in a unit of MB (B 306 ). When the intra prediction is not used and the motion compensation is used, the process proceeds to S 307  and when the intra prediction is used, the process proceeds to S 308 . In this case, since image  236   b  is the I picture using the intra prediction, the process proceeds to S 308  in all MBs. In addition, decoder-side intra predicting section  125  performs the intra prediction using the already decoded MB information fed back from decoded image generating section  128  as the peripheral information and performs the decoding (S 308 ). 
         [0107]    The decoded image is temporarily stored in decoded image generating section  128  until the decoding is completed with respect to all MBs of the image. The image that is temporarily stored in decoded image generating section  128  is used as peripheral information of the MBs to be decoded, in the process of S 308  as described above (S 312 ). After the process step from S 304  to S 312  ends with respect to all MBs of the image and decoded image  626  is generated, a loop process is skipped (R 313 ). Finally, decoded image  626  that is generated by decoded image generating section  128  is stored in decoder-side reference image storage memory  129  (S 314 ) and is transmitted to image storage memory  103 . The image that is transmitted to image storage memory  103  is transmitted to image combining section  104 . The image that is transmitted to image storage memory  103  is transmitted to image combining section  104 , is output to monitoring monitor  120  (S 315 ), and is reproduced and displayed. 
         [0108]    &lt;Backward Reproduction of Transcoded Image  236  of the Back End of the GOP (Second)&gt; 
         [0109]    To decode device  120 , transcoded image  236  of the time T 6  is input from storage medium  102  (S 301 ). Entropy decoding section  121  performs entropy decoding with respect to transcoded image  236  (S 302 ). A process step from the following S 304  to S 312  is repeated until a process of all macro blocks (MB) of image  236  ends (R 303 ). 
         [0110]    In addition, the second decode object of the backward decoding is transcoded image  236   a  using the motion compensation prediction. Since transcoded image  236   b  of the intra prediction that is stored in the user-defined extension area is already decoded as described above, the transcoded image is not used. 
         [0111]    First, decode device  120  executes the inverse quantization process with respect to the entropy decoded data in a unit of MB by decoder-side inverse quantizing section  122  (S 304 ) and executes the inverse DCT process with respect to the inversely quantized data by decoder-side inverse DCT section  123  to generate the differential information of the image data in a unit of MB (S 305 ). 
         [0112]    Decoder-side prediction method determining section  124  analyzes image information with the generated differential information and determines whether motion compensation is used in a unit of MB (B 306 ). When the intra prediction is not used and the motion compensation is used, the process proceeds to S 307  and when the intra prediction is used, the process proceeds to S 308 . In this case, since image  236   a  is the P picture using only the motion compensation prediction, the process proceeds to S 307  in all MBs. In addition, decoder-side motion compensating section  126  processes the motion compensation data in a unit of MB and acquires the differential information of the image (S 307 ). In this case, the image to calculate the difference is decoded image  626  of transcode image  236   b  that is stored in decoder-side reference image storage memory  129 . 
         [0113]    Next, code inverting section  127  determines the current decode direction (B 309 ). When the temporally forward decoding is performed, the process proceeds to S 310  and when the temporally backward decoding is performed, the process proceeds to S 311 . With respect to transcoded image  236   a,  decoding of transcoded image  236   b  of the I picture of the time T 6  is performed, and decoded image  625  of the time T 5  is generated. Therefore, the backward decoding is performed and the process proceeds to S 311 . Since the backward decoding is performed, in code inverting section  127 , positive and negative codes of the differential information obtained by motion compensating section  126  are inverted and the differential information is used, the process of the motion compensation is executed using the decoded image stored in decoder-side reference image storage memory  129  as the reference image, and the decoding is performed (S 311 ). 
         [0114]    The decoded image is temporarily stored in decoded image generating section  128  until the decoding is completed with respect to all MBs of the image (S 312 ). After the process step from S 304  to S 312  ends with respect to all MBs of the image and decoded image  625  is generated, a loop process is skipped (R 313 ). Finally, decoded image  625  that is generated by decoded image generating section  128  is stored in decoder-side reference image storage memory  129  (S 314 ) and is transmitted to image storage memory  103 . The image that is transmitted to image storage memory  103  is output to monitoring monitor  112  through image combining section  104  (S 315 ) and is reproduced and displayed. 
         [0115]    &lt;Backward Reproduction of Transcoded Image  235  of the P Picture&gt; 
         [0116]    Next, to decode device  120 , transcoded image  235  of the time T 5  is input from storage medium  102  (S 301 ). Entropy decoding section  121  performs entropy decoding with respect to transcoded image  235  (S 302 ). A process step from the following S 304  to S 312  is repeated until a process of all macro blocks (MB) of image  235  ends (R 303 ). 
         [0117]    First, decode device  120  executes the inverse quantization process with respect to the entropy decoded data in a unit of MB by decoder-side inverse quantizing section  122  (S 304 ) and executes the inverse DCT process with respect to the inversely quantized data by decoder-side inverse DCT section  123  to generate the differential information of the image data in a unit of MB (S 305 ). 
         [0118]    Decoder-side prediction method determining section  124  analyzes image information with the generated differential information and determines whether motion compensation is used in a unit of MB (B 306 ). When the intra prediction is not used and the motion compensation is used, the process proceeds to S 307  and when the intra prediction is used, the process proceeds to S 308 . In this case, since image  235  is the P picture using only the motion compensation prediction, the process proceeds to S 307  in all MBs. In addition, decoder-side motion compensating section  126  executes the process of the motion compensation in a unit of MB and acquires the differential information of the image (S 307 ). In this case, the image to calculate the difference is decoded image  626  of transcoded image  235  that is stored in decoder-side reference image storage memory  129 . 
         [0119]    Next, code inverting section  127  determines the current decode direction (B 309 ). With respect to transcoded image  235 , the decoding of transcoded image  236   a  of the P picture of the time T 6  is performed, and decoded image  624  of the time T 4  is generated. Therefore, the backward decoding is performed and the process proceeds to S 311 . Since the backward decoding is performed, in code inverting section  127 , positive and negative codes of the differential information obtained by motion compensating section  126  are inverted and the differential information is used, the process of the motion compensation is executed using the decoded image stored in decoder-side reference image storage memory  129  as the reference image, and the decoding of the data with respect to MB is performed (S 311 ). 
         [0120]    The decoded image is temporarily stored in decoded image generating section  128  until the decoding is completed with respect to all MBs of the image (S 312 ). After the process step from S 304  to S 312  ends with respect to all MBs of the image and decoded image  625  is generated, a loop process is skipped (R 313 ). Finally, decoded image  624  that is generated by decoded image generating section  128  is stored in decoder-side reference image storage memory  129  (S 314 ) and is transmitted to image storage memory  103  (S 315 ). The image that is transmitted to image storage memory  103  is transmitted to image combining section  104 . 
         [0121]    The course of decoding transcoded image  235  of the P picture of the time T 5  and generating decoded image  624  of the P picture of the time T 4  is described above. However, by executing the same process with respect to following transcoded image  234  of the P picture of the time T 4  and transcoded image  233  of the time T 3 , decoded images  623  and  622  can be obtained. Decoded image  622  of the P picture of the time T 2  is not used in the backward decoding. 
         [0122]    &lt;Backward Reproduction of Transcoded Image  231  of the I Picture&gt; 
         [0123]    The generation order of decoded image  621  is the same as that of the forward decoding of transcoded image  231 . That is, decoded image  621  is equal to decoded image  611 . 
         [0124]    The forward decoding and the backward decoding are described separately in the above description. For example, when decoding of decoded image  614  of the time T 4  by the forward decoding is considered as a reference, if the forward decoding is performed using transcoded image  235  which is the differential information between the times T 4  and T 5 , decoded image  615  of the time T 5  is obtained. If the backward decoding is performed using transcoded image  234  which is the differential information between the times T 3  and T 4 , decoded image  615  of the time T 3  is obtained. That is, an image temporally before or after one image can be decoded by only one decode process, with respect to the decoded image at the certain time, regardless of the current GOP structure. 
       [3] Effect of the Embodiment 
       [0125]    As described above, according to this embodiment, when encoder-side intra predicting section  141  transcodes the decoded images of the front end and the back end of the GOP of the image stream and encoder-side motion compensating section  142  transcodes the decoded image other than the front end of the GOP of the image stream by setting the motion vector as 0 and using only the immediately previous reference image to perform the backward reproduction (decode) from a certain time, in the case of the GOP structure including the P picture using the time correlation, desired backward reproduction (decode) can be performed by setting one immediately previous image as the reference image and performing only one decoding using the corresponding image, without performing decoding corresponding to the number of structures of the GOP. That is, regardless of whether the decode reproduction direction is switched to the forward direction or the backward direction at the certain time, reproduction of a desired direction can be realized by one decoding without affecting a decode operation of the forward direction. 
       [4] Other Embodiment 
       [0126]    In this embodiment, the example of the case where the number of input streams is one is described. Of course, the present invention can be similarly applied to the case where streams of other arbitrary numbers are input. In addition, the example of the GOP structure that does not include the B picture in input stream  200  is described. However, the present invention can be similarly applied to the general GOP structure that includes the B picture. In addition, the example of the case where the transcoded stream generation start position is set as decoded image  211  of the I picture of the time T 1  is described. However, the present invention can be similarly applied to other arbitrary start times. In addition, the example of the case where the number of the GOP structures of the transcoded stream is  6  is described above. However, the present invention can be similarly applied to the case of the number of other arbitrary GOP structures. 
         [0127]    The present invention is not limited to the embodiment described above and the changes or applications can be made for some parts of the structure and the operation by those skilled in the art, on the basis of the description of the specification and the known technologies. 
         [0128]    The present invention can be applicable to a codec that has extension area such as mpeg2, mpeg4, H.264, and so on. 
       INDUSTRIAL APPLICABILITY  
       [0129]    In the image converting apparatus, the image reproducing apparatus, and the image converting method according to the present invention, when the backward reproduction is performed, all of the reference images do not need to be decoded in the forward direction in advance, the decoding can be performed in the backward direction, and the high-speed reproduction (decode) can be performed in both the forward direction and the backward direction. Therefore, the present invention can be applied to an image reproducing apparatus that performs special reproduction such as rewind reproduction. 
       REFERENCE SIGNS LIST  
       [0000]    
       
           100  image reproducing apparatus 
           102  storage medium 
           103  image storage memory 
           104  image combining section 
           120  decode device 
           121  entropy decoding section 
           122  decoder-side inverse quantizing section 
           123  decoder-side inverse DCT section 
           124  decoder-side prediction method determining section 
           125  decoder-side intra predicting section 
           126  decoder-side motion compensating section 
           127  code inverting section 
           128  decoded image generating section 
           129  decoder-side reference image storage memory 
           140  transcode device 
           141  encoder-side intra predicting section 
           142  encoder-side motion compensating section 
           143  encoder-side prediction method determining section 
           144  differential information generating section 
           145  DCT section 
           146  quantizing section 
           147  entropy encoding section 
           148  encoder-side inverse quantizing section 
           149  encoder-side inverse DCT section 
           150  local decoded image generating section 
           151  encoder-side reference image storage memory 
           152  transcoded stream combining section