Simplified Context Selection For Entropy Coding of Transform Coefficient Syntax Elements

Abstract

A method for entropy encoding syntax elements of transform coefficient levels in a video encoder to generate a compressed video bit stream is provided that includes selecting a context for a first significant coefficient flag corresponding to a first transform coefficient level in a 32×32 transform block, wherein the first transform coefficient level is at a lowest frequency position in the 32×32 transform block, entropy encoding the first significant coefficient flag using the context, selecting the context for a second significant coefficient flag corresponding to a second transform coefficient level in a 16×16 transform block, wherein the second transform coefficient level is at a lowest frequency position in the 16×16 transform block, and entropy encoding the second significant coefficient flag using the context.

1 . A method for entropy encoding syntax elements of transform coefficient levels in a video encoder to generate a compressed video bit stream, the method comprising:
 selecting a context for a first significant coefficient flag corresponding to a first transform coefficient level in a 32×32 transform block, wherein the first transform coefficient level is at a lowest frequency position in the 32×32 transform block;   entropy encoding the first significant coefficient flag using the context;   selecting the context for a second significant coefficient flag corresponding to a second transform coefficient level in a 16×16 transform block, wherein the second transform coefficient level is at a lowest frequency position in the 16×16 transform block; and   entropy encoding the second significant coefficient flag using the context.   
     
     
         2 . The method of  claim 1 , wherein all contexts for entropy encoding of all significant coefficient flags of 32×32 luma transform blocks and entropy coding of all significant coefficient flags of 16×16 luma transform blocks are shared. 
     
     
         3 . The method of  claim 1 , wherein all contexts for entropy encoding of all significant coefficient flags of 32×32 chroma transform blocks and entropy coding of all significant coefficient flags of 16×16 chroma transform blocks are shared. 
     
     
         4 . The method of  claim 1 , further comprising:
 selecting a context set for entropy coding of coefficient level flags for coefficient levels in a coefficient block, wherein the context set is for entropy coding of coefficient level flags for non-zero coefficient levels and includes no more than four contexts, each of the no more than four contexts defined for entropy coding of coefficient level flags;   selecting a context in the context set for entropy coding of a coefficient level flag for a coefficient level in the coefficient block; and   entropy encoding the coefficient level flag using the context selected from the context set.   
     
     
         5 . The method of  claim 4 , wherein a coefficient level flag indicates whether or not a corresponding non-zero coefficient in the coefficient block has an absolute value greater than one. 
     
     
         6 . The method of  claim 4 , wherein the context set consists of four contexts. 
     
     
         7 . The method of  claim 4 , wherein the context set is one of a plurality of context sets for entropy coding of coefficient level flags for non-zero coefficient levels and each context set includes no more than four contexts. 
     
     
         8 . The method of  claim 7 , wherein each context set consists of four contexts. 
     
     
         9 . A method for entropy decoding a compressed video bit stream in a video decoder, the method comprising:
 selecting, from a first plurality of contexts, a first context for entropy decoding a first bin of a first significant coefficient flag of a luma transform block encoded in the compressed video bit stream, wherein the significant coefficient flag is for a 16×16 luma transform block or a 32×32 luma transform block, and wherein the first plurality of contexts are shared for entropy decoding of all significant coefficient flags of 16×16 luma transform blocks and 32×32 luma transform blocks;   entropy decoding the bin using the first context; and   updating the first context.   
     
     
         10 . The method of  claim 9 , wherein the first significant coefficient flag corresponds to a transform coefficient level at a lowest frequency position in a 16×16 luma transform block or a 32×32 luma transform block. 
     
     
         11 . The method of  claim 9 , further comprising:
 selecting, from a second plurality of contexts, a second context for entropy decoding a second bin of a second significant coefficient flag of a chroma transform block encoded in the compressed video bit stream, wherein the significant coefficient flag is for a 16×16 chroma transform block or a 32×32 chroma transform block, and wherein the second plurality of contexts are shared for entropy decoding of all significant coefficient flags of 16×16 chroma transform blocks and 32×32 chroma transform blocks;   entropy decoding the second bin using the second context; and   updating the second context.   
     
     
         12 . The method of  claim 9 , further comprising:
 selecting a context set for entropy decoding of coefficient level flags for coefficient levels in a coefficient block, wherein the context set is for entropy decoding of coefficient level flags for non-zero coefficient levels and includes no more than four contexts, each of the no more than four contexts defined for entropy decoding of coefficient level flags;   selecting a context in the context set for entropy decoding a bin of a coefficient level flag for a coefficient level in the coefficient block; and   entropy decoding the bin using the context selected from the context set.   
     
     
         13 . The method of  claim 11 , wherein the context set consists of four contexts. 
     
     
         14 . A digital system configured to decode a compressed video bit stream, the digital system comprising:
 a memory configured to store a first context for entropy decoding of significant coefficient flags, wherein the first context is for entropy decoding of significant coefficient flags corresponding to a luma transform coefficient level at a lowest frequency position of a 32×32 luma transform block or a 16×16 luma transform block;   means for selecting the first context for entropy decoding of a first bin of a first significant coefficient flag corresponding to a first luma transform coefficient level in a 32×32 luma transform block;   means for entropy decoding the first bin using the first context;   means for selecting the first context for entropy decoding of a second bin of a second significant coefficient flag corresponding to a second luma transform coefficient level in a 16×16 luma transform block; and   means for entropy decoding the second bin using the first context.   
     
     
         15 . The digital system of  claim 13 , wherein
 the memory is configured to store a second context for entropy decoding of significant coefficient flags, wherein the second context is for entropy decoding of significant coefficient flags corresponding to a chroma transform coefficient level in a lowest frequency position of a 32×32 chroma transform block or a 16×16 chroma transform block;   means for selecting the second context for entropy decoding of a third bin of a third significant coefficient flag corresponding to a first chroma transform coefficient level in a 32×32 chroma transform block;   means for entropy decoding the third bin using the second context;   means for selecting the second context for entropy decoding of a fourth bin of a fourth significant coefficient flag corresponding to a second chroma transform coefficient level in a 16×16 chroma transform block; and   means for entropy decoding the fourth bin using the second context.   
     
     
         16 . The digital system of  claim 13 , wherein the memory is configured to store a plurality of contexts for entropy decoding of significant coefficient flags, wherein a subset of contexts in the plurality of contexts is shared for entropy decoding of all significant coefficient flags of 16×16 luma transform blocks and 32×32 luma transform blocks, wherein the first context is in the subset. 
     
     
         17 . The digital system of  claim 14 , wherein the memory is configured to store a plurality of contexts for entropy decoding of significant coefficient flags, wherein a subset of contexts in the plurality of contexts is shared for entropy decoding of all significant coefficient flags of 16×16 chroma transform blocks and 32×32 chroma transform blocks, wherein the second context is in the subset. 
     
     
         18 . The digital system of  13 , further comprising:
 means for selecting a context set for entropy decoding of coefficient level flags for coefficient levels in a coefficient block, wherein the context set is for entropy decoding of coefficient level flags for non-zero coefficient levels and includes no more than four contexts, each of the no more than four contexts defined for entropy decoding of coefficient level flags;   means for selecting a context in the context set for entropy decoding a bin of a coefficient level flag for a coefficient level in the coefficient block; and   means for entropy decoding the bin of the coefficient level flag using the context selected from the context set.   
     
     
         19 . The digital system of  17 , wherein the context set consists of four contexts. 
     
     
         20 . The digital system of  17 , wherein the context set is one of a plurality of context sets for entropy coding of coefficient level flags for non-zero coefficient levels and each context set includes no more than four contexts.