Bitstream decoder

Abstract

The present disclosure relates to: a method of decoding, in a decoder, a bitstream representing an image, wherein the bitstream comprises a plurality of blocks of residual samples, and one or more reconstructed image frames of the image are obtained by adding a respective block of residual samples to a combined hypotheses block of intra-predictions, the combined hypotheses block of intra-predictions being formed by applying a method comprising: receiving the bitstream; for each block of residual samples: determining if a combined intra-prediction hypothesis is indicated and/or inferred from the bitstream and, if so: for each block of residual samples of a block for which a combined intra- prediction hypothesis is indicated and/or inferred: determining a first intra-prediction mode from information extracted from the bitstream; determining or inferring at least a second, different, intra-prediction mode; applying the first intra-prediction mode to at least one reference sample to form a first hypothesis prediction sample; applying the at least one second intra-prediction mode to said at least one reference sample to form a second hypothesis prediction sample; and forming a said combined hypotheses block of predictions by combining the first and second hypothesis prediction samples formed for the block.

1 . A method of decoding, in a decoder, a bitstream representing an image, wherein the bitstream comprises a plurality of blocks of residual samples, and one or more reconstructed image frames of the image are obtained by adding a respective block of residual samples to a combined hypotheses block of intra-predictions, the combined hypotheses block of intra-predictions being formed by applying a method comprising:
 receiving the bitstream;   for each block of residual samples: 
 determining if a combined intra-prediction hypothesis is indicated by and/or can be inferred from the bitstream and, if so: 
 for each block of residual samples of a block for which a combined intra-prediction hypothesis is indicated and/or can be inferred: 
 determining a first intra-prediction mode from information extracted from the bitstream; 
 inferring at least a second, different, intra-prediction mode based upon the first intra-prediction mode; 
 applying the first intra-prediction mode to at least one reference sample to form a first hypothesis prediction sample; 
 applying the at least one second intra-prediction mode to said at least one reference sample to form a second hypothesis prediction sample; and 
 forming a said combined hypotheses block of predictions by combining the first and second hypothesis prediction samples formed for the block. 
 
   
     
     
         2 . The method of  claim 1 , wherein, for one or more blocks of residual samples of a block for which a combined intra-prediction hypothesis is indicated and/or inferred, the method of forming a combined hypothesis block of intra-predictions comprises, for each residual sample of the block, applying the first intra-prediction mode to a reference sample, applying the second intra-prediction mode to the same reference sample, and combining the resultant first and second hypothesis intra-prediction samples to form a combined hypotheses prediction sample, the method further comprising combining the combined hypotheses prediction samples thus obtained for the block to form said combined hypothesis block of intra-predictions. 
     
     
         3 . The method of  claim 1 , wherein, for one or more blocks of residual samples for which a combined intra-prediction hypothesis is indicated and/or inferred, the method of forming a combined hypotheses block of intra-predictions comprises applying said first intra-prediction mode to all of the reference samples of the said block to form a plurality of first hypothesis prediction samples, applying the second intra-prediction mode to all of the reference samples of the said block to form a plurality of second hypothesis prediction samples, and combining the first and second hypothesis prediction samples to form said combined hypotheses block of intra-predictions. 
     
     
         4 . The method of  claim 1 , comprising inferring the second intra-prediction mode based upon an indicator within the bitstream. 
     
     
         5 . The method of  claim 1 , wherein combining the first and second combined hypothesis intra-prediction samples to form said combined hypotheses block of predictions comprises adding said first and second hypothesis prediction samples to respective said residual samples. 
     
     
         6 . The method of  claim 1 , wherein combining the first and second combined hypothesis intra-prediction samples to form said combined hypotheses block of predictions comprises computing a weighted average of the first and second combined hypotheses intra-prediction samples. 
     
     
         7 . The method of  claim 1 , wherein combining the first and second combined hypothesis intra-prediction samples to form said combined hypotheses block of predictions comprises adding a weighted combination thereof to respective residual samples. 
     
     
         8 . The method of  claim 1 , wherein combining the first and second combined hypothesis intra-prediction samples to form said combined hypotheses block of predictions comprises determining weightings to be applied to the first and/or second hypothesis samples. 
     
     
         9 . The method of  claim 8 , comprising determining the weightings to be applied to the first and/or second hypothesis intra-prediction samples based upon the values of bits within the bitstream. 
     
     
         10 . The method of  claim 8 , comprising determining the weightings to be applied to the first and/or second hypothesis intra-prediction samples by querying a lookup table. 
     
     
         11 . The method of  claim 8 , comprising at least partially inferring the weightings to be applied to the first and/or second hypothesis intra-prediction samples using information extracted from the neighbouring blocks. 
     
     
         12 . The method of  claim 1 , wherein at least one of the first intra-prediction mode and the second intra-prediction mode is a planar intra-prediction mode. 
     
     
         13 . The method of  claim 1 , wherein at least one of the first intra-prediction mode and the second intra-prediction mode is a DC intra-prediction mode. 
     
     
         14 . The method of  claim 1 , wherein at least one of the first intra-prediction mode and the second intra-prediction mode is a directional intra-prediction mode. 
     
     
         15 . The method of  claim 1 , wherein the bitstream comprises one or more bits indicating whether a second directional intra-prediction mode is to be applied. 
     
     
         16 . The method of  claim 1 , wherein the bitstream comprises one or more bits indicating the first intra-prediction mode and/or the second intra-prediction mode to be applied. 
     
     
         17 . The method of  claim 16 , wherein determining the first intra-prediction mode and/or the second intra-prediction mode comprises querying a lookup table. 
     
     
         18 . The method of  claim 1 , further comprising, in respect of a block for which a combined intra-prediction hypothesis is indicated and/or inferred: determining a third intra-prediction mode; and applying the third intra-prediction mode to one or more reference samples to form a third hypothesis intra-prediction sample, and combining the first, second and third hypothesis prediction samples formed for a block, to create a said combined hypotheses block of predictions. 
     
     
         19 . A method of decoding a video bitstream comprising the steps of:
 receiving a bitstream representing blocks of residual samples, and data indicative of intra-prediction hypotheses used in forming the prediction samples;   using said data indicative of said intra-prediction hypotheses to form respective blocks of prediction samples; and   adding said blocks of prediction samples to respective said blocks of residual samples to reconstruct images;   characterized by:   for each block of residual samples: 
 determining if a combined intra-prediction hypothesis is indicated by and/or can be inferred from the bitstream and, if so: 
 for each block of residual samples of a block for which a combined intra-prediction hypothesis is indicated and/or can be inferred: 
 determining a first intra-prediction mode from information extracted from the bitstream; 
 inferring at least a second, different, intra-prediction mode based upon the first intra-prediction mode; 
 applying the first intra-prediction mode to at least one reference sample to form a first hypothesis prediction sample; 
 applying the at least one second intra-prediction mode to said at least one reference sample to form a second hypothesis prediction sample; and 
 forming a said combined hypotheses block of predictions by combining the first and second hypothesis prediction samples formed for the block. 
 
   
     
     
         20 . A method of encoding, a video bitstream comprising the steps of:
 forming blocks of prediction samples using blocks of intra-predictions and reconstructed image samples;   subtracting blocks of prediction samples from blocks of image samples to form blocks of residual samples; and   forming a bitstream representing blocks of residual samples and data representative of blocks of intra-predictions used to form the respective prediction samples;   wherein at least one of the blocks of prediction samples is formed using a combined hypotheses block of predictions formed by: 
 applying a first intra-prediction mode to the reconstructed image samples of a respective block to form first prediction samples and a plurality of respective first residual samples; 
 applying a second, different, intra-prediction mode to the reconstructed image samples of said respective block to form second prediction samples and a plurality of respective second residual samples; and 
 forming a block of residual samples by combining the first and second residual samples 
   wherein the second intra-prediction mode is not explicitly signalled in the bitstream, whereby a decoder is able to infer the second intra-prediction mode based on the first intra-prediction mode.