Bitstream decoder

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.

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.