Variable bitrate video coding method and corresponding video coder.

Abstract

The variable bitrate coding method according to the invention comprises an iterative process including a first analysis pass and a second prediction pass and followed by a last control step for adjusting said stepsize with respect to said target bitrate. According to the invention, a picture re-arrangement step is provided between the analysis and prediction steps of one iteration, in order to encode with an improved quality the picture sequence. Application: MPEG-2 encoders for storage media with limited capacity.

1 . A variable bitrate video coding method including an iterative process that comprises a first analysis step, for coding a bitstream corresponding to a picture sequence with a constant quantization stepsize, and a second prediction step, for predicting the quantization stepsize which must then be used to code said bitstream according to a predetermined target bit rate, and is followed by a final control step, for adjusting the stepsize with respect to said target bit rate, said method being characterized in that it comprises, between the analysis and prediction steps of one iteration, a picture re-arrangement step.  
     
     
         2 . A method according to    claim 1   , characterized in that said picture re-arrangement step itself comprises in series a first scene change detection sub-step, a second allocation sub-step, and a third optimum placement sub-step.  
     
     
         3 . A method according to    claim 2   , characterized in that said scene change detection sub-step comprises a correlation operation, carried out between suceeding pictures of the sequence, and a decision operation, for indicating the possible occurrence of a scene change.  
     
     
         4 . A method according to    claim 3   , characterized in that said correlation operation is based on a picture complexity estimation.  
     
     
         5 . A method according to    claim 3   , characterized in that said correlation operation is based on the comparison of the blocks of a picture with reference blocks in a previous reference picture.  
     
     
         6 . A method according to    claim 2   , characterized in that said allocation sub-step is based on the implementation of a Viterbi algorithm allowing to select a preferred size for successive groups of pictures while matching a scene change with the start of such a group of pictures.  
     
     
         7 . A method according to anyone of    claims 2    to    6   , characterized in that said optimum placement sub-step comprises a first long-term optimization operation, for finding over several groups of pictures the optimum spacing between the pictures of these groups, and a second short-term optimization operation, for finding inside a group of pictures the best places of predicted and interpolated pictures.  
     
     
         8 . A method according to anyone of    claims 2    to    7   , characterized in that an additional inter-picture prediction step is provided in case of change of the order of the picture types between two successive iterations.  
     
     
         9 . A variable bit rate video coder comprising a first coding branch, a prediction branch, and a control circuit provided for carrying out the implementation of the following operations: 
 a coding operation of a bitstream corresponding to a picture sequence, with a constant quantization stepsize;    a prediction operation, for an estimation of the quantization stepsize allowing to code said bitstream according to a specified target bitrate;    at least one repetition of said operations;    a final controlling operation, for adjusting the stepsize with respect to said target bitrate;    characterized in that said control circuit is provided for implementing, between the first coding operation and the first prediction operation, a picture re-arrangement operation.