JP2006246277A - Re-encoding apparatus, re-encoding method, and re-encoding program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a re-encoding apparatus capable of preventing compression efficiency from being reduced, by preventing a B picture from referring to a picture that is not an original reference picture, if an I picture is defined as a particular picture for enabling random access in the case of transcoding first encoded information into second encoded information using a motion vector of the first encoded information. <P>SOLUTION: In a decoding unit 100, first encoded information is decoded and in an encoding unit 200, a part of I pictures or all the I pictures in decoded image information are set to IDR pictures. If a processing target picture is a B picture between an IDR picture and the next P picture in a decoding order and forward prediction is set as a prediction mode, that prediction mode is changed and according to the set prediction mode, the decoded image information is encoded by inter-prediction and intra-prediction, thereby producing second encoded information. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a re-encoding device, a re-encoding method, and a re-encoding program. Specifically, the first encoding information obtained by encoding image information by inter prediction and intra prediction is used for the first encoding. The present invention relates to a re-encoding device, a re-encoding method, and a re-encoding program that transcode into second encoded information using a motion vector used when decoding information.

  In recent years, image information has been handled as digital data. At that time, for the purpose of efficient transmission and storage of information, compression is performed by orthogonal transform such as discrete cosine transform and motion prediction / compensation using redundancy unique to image information. An apparatus conforming to a system such as MPEG is widely used.

  In particular, MPEG2 (ISO / IEC 13818-2) is defined as a general-purpose image coding system, and is a standard that covers both interlaced and progressively scanned images, standard resolution images, and high-definition images. And widely used in a wide range of applications for consumer use. By using the MPEG2 compression method, for example, a standard resolution interlaced scanning image having 720 × 480 pixels is 4 to 8 Mbps, and a high resolution interlaced scanning image having 1920 × 1088 pixels is 18 to 22 Mbps. (Bit rate) can be assigned to achieve a high compression rate and good image quality.

  MPEG2 was mainly intended for high-quality encoding suitable for broadcasting, but did not support encoding methods with a lower code amount (bit rate) than MPEG1, that is, a higher compression rate. With the widespread use of mobile terminals, the need for such an encoding system is expected to increase in the future, and the MPEG4 encoding system has been standardized accordingly.

  Furthermore, in recent years, standardization of the H.264 (ITU-T Q6 / 16 VCEG) encoding method has been performed with the initial purpose of image encoding for video conferencing. H. H.264 is known to achieve higher encoding efficiency than a conventional encoding method such as MPEG2 or MPEG4, although a larger amount of calculation is required for encoding and decoding.

  In general, in encoding of moving images, the amount of information is compressed by reducing redundancy in the time direction and the spatial direction. Therefore, in inter prediction coding for the purpose of reducing temporal redundancy, motion detection and prediction image creation are performed in block units with reference to the front or rear picture, and the obtained prediction image and encoding target are obtained. Encoding is performed on the difference value from the picture.

  A picture that does not have a reference picture and performs intra prediction coding is called an I picture. A picture that performs inter prediction coding with reference to only one picture is called a P picture. A picture that can be subjected to inter prediction coding with reference to two pictures at the same time is called a B picture. The B picture can refer to two pictures as an arbitrary combination of display times from the front or the rear. A reference picture can be specified for each block that is a basic unit of encoding and decoding, but the reference picture described earlier in the encoded bitstream is described as the first reference picture, and is described later. Are distinguished as the second reference picture. However, as a condition for encoding and decoding these pictures, the picture to be referenced needs to be already encoded and decoded.

  Motion-compensated inter prediction coding is used for coding a P picture or a B picture. Motion compensation inter prediction coding is a coding method in which motion compensation is applied to inter prediction coding. Motion compensation is not simply predicting from the pixel value of a reference picture, but detecting the amount of motion of each part in the picture (hereinafter referred to as a motion vector) and performing prediction in consideration of the motion vector. This improves the prediction accuracy and reduces the amount of data. For example, the amount of data is reduced by detecting the motion vector of the encoding target picture and encoding the prediction residual between the prediction value shifted by the motion vector and the encoding target picture. In the case of this system, since motion vector information is required at the time of decoding, the motion vector is also encoded.

  The motion vector is detected in block units. Specifically, the block on the encoding target picture side is fixed, the reference picture block is moved within the search range, and the reference block most similar to the reference block The motion vector is detected by finding the position of.

  MPEG2 format encoding information is converted into H.264 format. When transcoding into encoded information in the H.264 format, control information (motion vector, picture type, etc.) of encoded information in the MPEG2 format is used. It is conceivable to reduce the calculation amount of the H.264 encoding process. Specifically, H.C. In the H.264 encoding process, the ratio of operations required for motion vector search is large, and MPEG2 motion vectors are used. By omitting the search for motion vectors in the H.264 encoding process, transcoding can be performed at high speed.

  H. In H.264, IDR (Instanteneous Decoding Refresh) pictures are used. This IDR picture is used for quick random access, and is the first picture in the image sequence. The state of the reference picture buffer, the frame number, POC (Picture Order Count: information indicating the output order of pictures), etc. All information necessary to decode the bitstream is reset.

  FIG. 6 is a diagram showing the relationship between the decoding order and display order of image code information in the case of Closed GOP in MPEG-2. FIG. 7 is a diagram showing the relationship between the decoding order and display order of image code information in the case of open GOP in MPEG-2. FIG. 8 is a diagram for explaining a reference picture when an IDR picture is used in the case of a Closed GOP. FIG. 9 is a diagram for explaining the reference picture list. FIG. 10 is a diagram for explaining a reference picture / area in the case of a Closed GOP (when a motion vector mv of MPEG2 is used). FIG. 11 is a diagram for explaining a reference picture / area in the case of an Open GOP. FIG. 12 is a diagram for explaining a reference picture / area in the case of an Open GOP (when a motion vector mv of MPEG2 is used).

  As shown in FIGS. 6 and 7, the image code information is composed of a plurality of groups of pictures GOP, and each group of pictures GOP is composed of a plurality of pictures. The pictures included in each group of pictures GOP are I pictures, P pictures, or B pictures, respectively. The pictures are arranged in the order in which they are encoded, that is, the order in which they are decoded, and the order in which the pictures are displayed is different from such an arrangement order.

  As shown in FIG. 7, in the case of Open GOP, a B picture between an I picture and a P picture in decoding order is before an I picture in display order, and a P picture before an I picture is encoded. And I picture are used for prediction.

  FIG. 8 is a diagram for explaining a reference picture when an IDR picture is used in the case of a Closed GOP. FIG. 9 is a diagram for explaining the reference picture list. FIG. 10 is a diagram for explaining a reference picture / area in the case of a Closed GOP (when a motion vector mv of MPEG2 is used). FIG. 11 is a diagram for explaining a reference picture / area in the case of an Open GOP. FIG. 12 is a diagram for explaining a reference picture / area in the case of an Open GOP (when a motion vector mv of MPEG2 is used).

  As shown in FIG. 8, the image code information in the open GOP shown in FIG. When transcoding to H.264 image code information, if part or all of an I picture is an IDR picture in consideration of random access without changing the picture types of P and B pictures, the decoding order B picture between I picture and P picture in H. When an IDR picture comes during H.264 encoding, the reference picture buffer is cleared, so that the P picture used for prediction cannot be referred to as shown in FIG. 11 (see, for example, Patent Document 1).

  Also, as shown in FIG. 9, since only the IDR picture is held in the reference picture list L0 and L1 in the reference picture buffer of the B picture, the motion vector of MPEG2 is set to H.264. When used for H.264 encoding, motion compensation is performed with reference to an IDR picture (future picture) with a motion vector obtained by prediction using a P picture (past picture) (see FIG. 12). Thus, since the B picture refers to a picture that is not the original reference picture, there is a problem that the pixel difference value becomes large and the compression efficiency is lowered.

JP 2004-248265 A

  The present invention has been made in view of the above, and enables random access to an I picture when transcoding to second encoded information using a motion vector of the first encoded information. When a special picture is used, a re-encoding device, a re-encoding method, and a re-encoding apparatus that can prevent a B picture from referring to a picture that is not an original reference picture and prevent a reduction in compression efficiency. An object is to provide an encoding program.

  In order to solve the above-described problems and achieve the object, the present invention uses first encoded information obtained by encoding image information by inter prediction and intra prediction when decoding the first encoded information. In the re-encoding device that transcodes the second encoded information using the motion vector, the decoding means for decoding the first encoded information, and the I picture of the decoded image information And a picture determination means for setting a part or all of a special picture for random access, and a processing target picture is a B picture between the special picture and the next P picture in decoding order. If forward prediction is set as the prediction mode, the prediction mode determining means for changing the prediction mode and the decoded image information according to the prediction mode are loaded. Encoding means for generating a second encoded information on the Coulter prediction and intra prediction, and further comprising a.

  The present invention also provides second encoding using first encoded information obtained by encoding image information by inter prediction and intra prediction, using a motion vector used when decoding the first encoded information. In a re-encoding method for transcoding information, a decoding step for decoding the first encoded information and a random access to a part or all of an I picture in the decoded image information A picture determination step for setting a special picture, and a case where the processing target picture is a B picture between the special picture and the next P picture in decoding order, and forward prediction is set as a prediction mode A prediction mode determining step for changing the prediction mode if it has been performed, and encoding the decoded image information by inter prediction and intra prediction according to the prediction mode. Characterized in that it comprises a coding step of generating a second coded information.

  The present invention also provides a computer using first encoded information obtained by encoding image information by inter prediction and intra prediction, and using a motion vector used when decoding the first encoded information. In a re-encoding program for transcoding to 2-encoded information, a decoding process for decoding the first encoded information and a part or all of an I picture in the decoded image information are randomly A picture determination step for setting a special picture for access, and a case where the processing target picture is a B picture between the special picture and the next P picture in decoding order, and as a prediction mode When forward prediction is set, a prediction mode determining step for changing the prediction mode, and the decoded image information is inter-predicted according to the prediction mode. An encoding step of generating second encoded information by encoding in the fine intra prediction, and characterized by causing a computer to execute the.

  Exemplary embodiments of a re-encoding device, a re-encoding method, and a re-encoding program according to the present invention will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments. In addition, constituent elements in the following embodiments include those that can be easily assumed by those skilled in the art or those that are substantially the same.

  FIG. 1 is a diagram illustrating a schematic configuration example of a re-encoding device 10 according to an embodiment of the present invention. In the re-encoding device 10 shown in the figure, as an example, MPEG2 format encoding information (in the case of Open GOP) is converted into H.264 format. A case of transcoding to H.264 format encoded information will be described. As shown in the figure, the re-encoding device 10 includes a decoding unit (MPEG2 decoder) 100 that decodes the first encoded information (MPEG2 bit stream), and image information decoded by the decoding unit 100. An encoding unit (H.264 encoder) 200 that encodes second encoded information (H.264 bit stream).

  The decoding unit 100 includes a variable length decoding unit 101, an inverse quantization unit 102, an inverse DCT (Discrete Cosine Transformation) unit 103, a calculation unit 104, a frame memory 105, and a motion compensation unit 106. Yes.

  The variable length decoding unit 101 performs variable length decoding on the input first encoded information (MPEG2 bit stream), converts the quantized DCT coefficients to the inverse quantization unit 102, and the motion vector MV to the motion compensation unit 205. And output to the motion detection unit 214 and the prediction mode determination unit 203 of the encoding unit 200.

  The inverse quantization unit 102 inversely quantizes the quantized DCT coefficient and outputs the DCT coefficient to the inverse DCT unit 103. The inverse DCT unit 103 performs inverse DCT conversion on the DCT coefficient and outputs the result to the arithmetic unit 104.

  The motion compensation unit 106 reads the already decoded image information stored in the frame memory 105 according to the motion vector MV input from the variable length decoding unit 101, and outputs the read image information to the calculation unit 104 as predicted image information.

  The arithmetic unit 104 adds the output information from the inverse DCT unit 103 and the predicted image information from the motion compensation unit 106, thereby decoding the original image information and outputting the decoded image information to the encoding unit 200. Along with the image information, the decoding unit 100 outputs the accompanying information such as the picture type, the prediction mode (intra prediction, inter prediction (including the prediction direction)), and the encoding order / display order to the encoding unit 200. .

  The encoding unit 200 includes a picture determination unit 201, a prediction mode determination unit 202, an intra prediction unit 203, an inter prediction unit 204, a DCT unit 205, a quantization unit 206, and a variable length encoding unit 207. I have.

  The picture determination unit 201 sets a part or all of the I picture included in the image information input from the decoding unit 100 as an IDR picture in order to specify a target to be processed as a randomly accessible GOP. It outputs to the prediction mode determination part 202.

  The prediction mode determination unit 202 is a case where the processing target picture is a B picture between the IDR picture and the next P picture in decoding order, and forward prediction is set as the prediction mode. The prediction mode is changed, and in other cases, the set prediction mode is maintained, and the image information and the prediction mode are output to the intra prediction unit 203 and the inter prediction unit 204. Details of the prediction mode changing method of the prediction mode determination unit 202 will be described later.

  In the case of image information to be intra-encoded, the intra prediction unit 203 generates predicted image information from pixel values that have already been encoded in the vicinity of the block to be encoded, and calculates a difference value from the predicted image information as a DCT unit. It outputs to 205.

  In the case of image information to be inter-encoded, the inter prediction unit 204 performs motion prediction / compensation processing on the block to be encoded to generate predicted image information, and a difference value from the predicted image information is obtained as a DCT unit. It outputs to 205.

  The DCT unit 205 performs DCT processing on the difference value input from the calculation unit 216 of the intra prediction unit 203 or the inter prediction unit 204, and outputs the DCT coefficient obtained as a result to the quantization unit 205.

  The quantization unit 205 quantizes the input DCT coefficient, and outputs the quantized DCT coefficient to the variable length coding unit 207 and the inverse quantization unit 206 of the inter prediction unit 204.

  The variable length coding unit 207 performs variable length coding on the quantized DCT coefficient, and outputs the second coded information (H.264 bit stream). The variable length coding unit 207 also performs variable length coding on the motion vector MV and the like detected by the motion detection unit 214.

  The inter prediction unit 204 includes an inverse quantization unit 210, an inverse DCT unit 211, a calculation unit 212, a frame memory 213, a motion detection unit 214, a motion compensation unit 215, and a calculation unit 216.

  The calculation unit 216 calculates a difference value between the decoded image information input from the prediction mode determination unit 202 and the predicted image information input from the motion compensation unit 215, and outputs the difference value to the DCT unit 205.

  The inverse quantization unit 210 inversely quantizes the quantized DCT coefficient input from the quantization unit 206 and outputs the obtained DCT coefficient to the inverse DCT unit 211. The inverse DCT unit 211 performs inverse DCT processing on the DCT coefficient and outputs the result to the arithmetic unit 212.

  The calculation unit 212 adds the output information from the inverse DCT unit 211 and the predicted image information from the motion compensation unit 215, and locally decodes the original image information. This locally decoded image information is output to and stored in the frame memory 213, and is used when creating predicted image information.

  The motion detection unit 214 outputs the motion vector input from the variable length coding unit 101 of the decoding unit 100 to the motion compensation unit 215 and the variable length coding unit 207, and there is a change in the prediction mode of inter prediction. In this case, a new motion vector is detected and output to the motion compensation unit 215 and the variable length coding unit 207. When detecting a motion vector, the motion vector detection unit 310 sets the current block within a predetermined search range of past encoded frame images (hereinafter referred to as “reference frames”) stored in the frame memory 123. A similar portion (hereinafter referred to as “motion prediction data”) is searched, and a two-dimensional spatial movement amount from the current block to the motion prediction data is detected as a motion vector MV.

  The motion compensation unit 215 generates predicted image information using the motion vector MV input from the variable length decoding unit 101 of the decoding unit 100 and the reference frame stored in the frame memory 213, and calculates a calculation unit. To 216.

  2 to 4 are flowcharts for explaining the processing of the prediction mode determination unit 202. In FIG. 2, it is determined whether or not the prediction mode of the processing target MB is inter prediction (step S1). If the prediction mode of the processing target MB is not inter prediction (“No” in step S1), That is, in the case of intra prediction, the prediction mode is not changed.

  When the prediction mode of the processing target MB is inter prediction (“Yes” in step S1), whether or not the picture including the processing target MB is a B picture between the IDR picture and the next P picture. Is determined (step S2). For example, when the POC (Picture Order Count) of the corresponding picture is smaller than “0”, it can be determined that the picture is a B picture between the IDR picture and the next P picture.

  If it is not a B picture between the IDR picture and the next P picture (“No” in step S2), the prediction mode is not changed. On the other hand, when it is a B picture between an IDR picture and the next P picture (“Yes” in step S2), the prediction mode of the processing target MB is only forward prediction (refers to a past picture). Is determined (step S3). When only forward prediction is performed (“Yes” in step S3), the prediction mode is changed to intra prediction only or backward prediction (step S4).

  If not only forward prediction (“No” in step S3), it is determined whether the prediction mode of the processing target MB is only backward prediction (refers to a future picture) (step S4). When only backward prediction is performed (“Yes” in step S4), the prediction mode is not changed.

  When it is not only backward prediction (“No” in step S4), that is, in the case of bidirectional prediction, the prediction mode is changed to only backward prediction or intra prediction (step S6).

  FIG. 3 is a flowchart for explaining the prediction method changing process of the forward prediction in step S4 of FIG. In FIG. 3, first, the cost of intra prediction is calculated (step S11). Next, a motion vector MV for backward prediction is calculated (step S12), and cost calculation for backward prediction is performed (step S13). The cost of intra prediction and the cost of backward prediction are compared (step S14). If the cost of intra prediction is less than the cost of backward prediction (“Yes” in step S14), the prediction mode is changed to intra prediction (step S14). S15) If the cost of intra prediction is not less than the cost of backward prediction (“No” in step S14), the prediction mode is changed to backward prediction of inter prediction (step S16). Here, the costs are compared and the prediction mode is changed to the lower cost prediction mode, but may be fixed to either intra prediction or backward prediction.

  FIG. 4 is a flowchart for explaining the prediction method change processing of bidirectional prediction in step S5 of FIG. In FIG. 4, first, the cost of backward prediction is calculated (step S21). Next, the cost of intra prediction is calculated (step S22). The cost of intra prediction and the cost of backward prediction are compared (step S23). If the cost of backward prediction is less than the cost of intra prediction ("Yes" in step S23), the prediction mode is changed to backward prediction (step S23). S24) If the cost of backward prediction is not less than the cost of backward prediction (“No” in step S23), the prediction mode is changed to intra prediction (step S25). Here, the costs are compared and the prediction mode is changed to the lower cost prediction mode, but may be fixed to either intra prediction or backward prediction.

  As described above, according to the present embodiment, the first encoded information obtained by encoding image information by inter prediction and intra prediction is used and the motion vector used when decoding the first encoded information is used. In the re-encoding device 10 that transcodes the second encoded information, the decoding unit 100 decodes the first encoded information. In the encoding unit 200, the picture determination unit 201 decodes the picture determination unit. A part or all of the I picture in the image information is set as an IDR picture (a special picture for random access), and the prediction mode determination unit 202 sets the processing target picture in the decoding order in the IDR picture. If the picture is a B picture between the picture and the next P picture and forward prediction is set as the prediction mode, that is, the B picture is an IDR picture. When prediction is performed across channels, the prediction mode is changed, and the inter prediction unit 204 and the intra prediction unit 203 encode the decoded image information by inter prediction and intra prediction according to the prediction mode. Since 2 encoded information is generated, when transcoding to the 2nd encoded information using the motion vector of the 1st encoded information, when I picture is made into IDR picture, B It is possible to prevent a picture from referring to a picture that is not the original reference picture, that is, to avoid performing inter prediction by referring to a picture different from that at the time of motion vector detection, and to prevent a reduction in compression efficiency. Become.

  Further, in the prediction mode determination unit 201, only the forward prediction is set as the prediction mode when the processing target picture is a B picture between the IDR picture and the next P picture in decoding order. In this case, since the prediction mode is changed to intra prediction or backward prediction, when only forward prediction is set as the prediction mode, the B picture refers to a picture that is not the original reference picture. This can be prevented.

  Also, in the prediction mode determination unit 201, when the processing target picture is a B picture between the IDR picture and the next P picture in decoding order, and bi-directional prediction is set as the prediction mode Since the prediction mode is changed to intra prediction or backward prediction only, when bi-directional prediction is set as the prediction mode, the B picture refers to a picture that is not the original reference picture. Can be prevented.

  In the above embodiment, MPEG2 to H.264 are used. Although the transcoding to H.264 has been described, the present invention is not limited to this, and can be applied to the case where an I picture is changed to an IDR picture. 26X or the like can be used. The first encoded information not using an IDR picture in the H.264 format is H.264. The present invention is also applicable when converting to second encoded information using IDR pictures in the H.264 format.

  Further, in the above embodiment, when it is determined that the B picture is between the IDR picture and the P picture, the determination is made based on the POC and the picture type. However, the present invention is not limited to this. By setting IDR_flag = 1 when processing an IDR picture and setting IDR_flag = 0 when processing a P picture, it is determined that the B picture when IDR_flag = 1 is a B picture that makes reference across IDR pictures It may be.

  Further, a program for realizing the function of the re-encoding device 10 is recorded on the computer-readable recording medium 500 shown in FIG. 5, and the program recorded on the recording medium 500 is shown in FIG. Each function may be realized by being read into 400 and executed.

A computer 400 shown in the figure has a CPU (Central Proc
essing Unit) 401, an input device 402 such as a keyboard and a mouse, a ROM (Read Only Memory) 403 for storing various data, and a RAM (Random for storing calculation parameters)
(Access Memory) 404, a reading device 405 for reading a program from the recording medium 500, and an output device 406 such as a display or a printer.

The CPU 401 implements the above-described functions by reading a program recorded on the recording medium 500 via the reading device 405 and then executing the program. In addition,
Examples of the recording medium 500 include an optical disk, a flexible disk, and a hard disk.

  As described above, the re-encoding device, the re-encoding method, and the re-encoding program according to the present invention are useful for various devices equipped with a re-encoding function, and are used for, for example, a DVD / HDD recorder. Is possible.

It is a figure which shows the structural example of the outline of the re-encoding apparatus which concerns on one Example of this invention. It is a flowchart for demonstrating the process of the prediction mode determination part of FIG. It is a flowchart for demonstrating the process of the prediction mode determination part of FIG. It is a flowchart for demonstrating the process of the prediction mode determination part of FIG. It is a figure which shows the structural example of the outline of the re-encoding apparatus which concerns on the other Example of this invention. It is a figure which shows the relationship between the decoding order of image code information in the case of Closed GOP in MPEG-2, and a display order. It is a figure which shows the relationship between the decoding order of image code information in the case of open GOP in MPEG-2, and a display order. It is a figure for demonstrating the reference picture at the time of using an IDR picture in the case of Closed GOP. It is a figure for demonstrating a reference picture list. It is a figure for demonstrating the reference picture / area in the case of Closed GOP (when using the motion vector mv of MPEG2). It is a figure for demonstrating the reference picture / area in the case of Open GOP. It is a figure for demonstrating the reference picture / area in the case of Open GOP (when using the motion vector mv of MPEG2).

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Re-encoding apparatus 100 Decoding part 101 Variable length decoding part 102 Inverse quantization part 103 Inverse DCT part 104 Operation part 105 Frame memory 106 Motion compensation part 200 Encoding part 201 Picture determination part 202 Prediction mode determination part 203 Intra prediction Unit 204 inter prediction unit 205 DCT unit 206 quantization unit 207 variable length coding unit 210 inverse quantization unit 211 inverse DCT unit 212 calculation unit 213 frame memory 214 motion detection unit 215 motion compensation unit 216 calculation unit 400 computer 401 CPU
402 Input device 403 ROM
404 RAM
405 Reading device 406 Output device 500 Recording medium

Claims (6)

The first encoded information obtained by encoding the image information by inter prediction and intra prediction is re-transcoded to the second encoded information using the motion vector used when decoding the first encoded information. In the encoding device,
Decoding means for decoding the first encoded information;
Picture determination means for setting a part or all of the I picture in the decoded image information as a special picture for random access;
If the target picture is a B picture between the special picture and the next P picture in decoding order, and forward prediction is set as the prediction mode, the prediction mode is changed. Prediction mode determining means for
Encoding means for encoding the decoded image information by inter prediction and intra prediction according to the prediction mode to generate second encoded information;
A re-encoding device comprising:   The prediction mode determining means is configured such that when the processing target picture is a B picture between the special picture and the next P picture in decoding order, only forward prediction is set as the prediction mode. In this case, the re-encoding apparatus according to claim 1, wherein the prediction mode is changed to intra prediction or backward prediction.   The prediction mode determination means is a case where the processing target picture is a B picture between the special picture and the next P picture in decoding order, and bi-directional prediction is set as the prediction mode The re-encoding device according to claim 1, wherein the prediction mode is changed to intra prediction or backward prediction only.   The first encoded information conforms to the MPEG2 format, and the second encoded information is H.264. The re-encoding device according to any one of claims 1 to 3, wherein the re-encoding device conforms to the H.264 format. Second encoded information having a different encoding method by using first encoded information obtained by encoding image information by inter prediction and intra prediction and using a motion vector used when decoding the first encoded information. In a re-encoding method for transcoding to:
A decoding step of decoding the first encoded information;
A picture determination step of setting a part or all of the I picture in the decoded image information as a special picture for random access;
If the target picture is a B picture between the special picture and the next P picture in decoding order, and forward prediction is set as the prediction mode, the prediction mode is changed. A prediction mode determination step to be performed;
An encoding step of generating the second encoded information by encoding the decoded image information by inter prediction and intra prediction according to the prediction mode;
A re-encoding method comprising: The computer uses the first encoded information obtained by encoding the image information by inter prediction and intra prediction, and the motion vector used when decoding the first encoded information, so that the second encoding method is different. In a re-encoding program for transcoding encoded information,
A decoding step of decoding the first encoded information;
A picture determination step of setting a part or all of the I picture in the decoded image information as a special picture for random access;
If the target picture is a B picture between the special picture and the next P picture in decoding order, and forward prediction is set as the prediction mode, the prediction mode is changed. A prediction mode determination step to be performed;
An encoding step of generating the second encoded information by encoding the decoded image information by inter prediction and intra prediction according to the prediction mode;
A re-encoding program that causes a computer to execute.

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