Reducing context coded and bypass coded bins to improve context adaptive binary arithmetic coding (CABAC) throughput
Techniques for context-adaptive binary arithmetic coding (CABAC) coding with a reduced number of context coded and/or bypass coded bins are provided. Rather than using only truncated unary binarization for the syntax element representing the delta quantization parameter and context coding all of the resulting bins as in the prior art, a different binarization is used and only part of the resulting bins are context coded, thus reducing the worst case number of context coded bins for this syntax element. Further, binarization techniques for the syntax element representing the remaining actual value of a transform coefficient are provided that restrict the maximum codeword length of this syntax element to 32 bits or less, thus reducing the number of bypass coded bins for this syntax element over the prior art.
1. A method for encoding a video sequence in a video encoder to generate a compressed video bit stream, the method comprising:
determining a value of a syntax element for a remaining actual value of a transform coefficient;
binarizing the value using a variable length code (VLC) to generate a sequence of bins, wherein a maximum codeword length of the VLC is 32 bits or less; and
bypass coding the sequence of bins into the compressed video bit stream.
2. The method of claim 1 , wherein the VLC is a code that adapts based on a value of a parameter R, wherein codewords for values less than 4×2 R have a fixed length suffix of R bits, prefixes of codewords are unary coded, suffix length of codewords increases by 1 bit for each 1 bit increase in prefix length, and the maximum codeword length is 32 bits.
3. The method of claim 2 , wherein the value of R is one selected from a group consisting of 0, 1, 2, 3, and 4.
4. The method of claim 1 , wherein the VLC is a code that adapts based on a value of a parameter R, wherein codewords for values less than 4×2 R have a fixed length suffix of R bits, prefixes of codewords are truncated unary coded, suffix length of codewords increases by 1 bit for each 1 bit increase in prefix length, and the maximum codeword length is 31 bits.
5. The method of claim 1 , wherein the VLC is a code that adapts based on a value of a parameter R, wherein codewords for values less than 5×2 R have a fixed length suffix of R bits, prefixes of codewords are truncated unary coded, suffix length of codewords increases by 1 bit for each 1 bit increase in prefix length, and the maximum codeword length is 32 bits.
6. The method of claim 1 , wherein the VLC is a code that adapts based on a value of a parameter R, wherein codewords for values less than 8×2 R have a fixed length suffix of R bits, prefixes of codewords are unary coded, suffix length of codewords increases by 1 bit for each 1 bit increase in prefix length for syntax element values less than 263×2 R , suffix length of codewords for syntax element values greater than or equal to 263×2 R is 15 bits, and the maximum codeword length is 32 bits.
7. The method of claim 1 , wherein the VLC is a code that adapts based on a value of a parameter R, wherein codewords for values less than 8×2 R have a fixed length suffix of R bits, prefixes of codewords are unary coded, suffix length of codewords increases by 2 bits for each 1 bit increase in prefix length, and the maximum codeword length is 32 bits.
8. The method of claim 1 , wherein the VLC is a code that adapts based on a value of a parameter R, wherein codewords for values less than 6×2 R have a fixed length suffix of R bits, prefixes of codewords are unary coded, suffix length of codewords increases by 2 bits for each 1 bit increase in prefix length, and the maximum codeword length is 30 bits.
9. The method of claim 1 , wherein the VLC is a code that adapts based on a value of a parameter R, wherein codewords for values less than 6×2 R have a fixed length suffix of R bits, prefixes of codewords are unary coded, suffix length of codewords increases by 2 bits for each 1 bit increase in prefix length, and the maximum codeword length is 29 bits.
10. A method for decoding a compressed video bit stream in a video decoder, the method comprising:
bypass decoding a sequence of bins from the compressed video bit stream, the sequence of bins comprising a prefix and a suffix of a binarized value of a syntax element for a remaining actual value of a transform coefficient; and
debinarizing the sequence of bins using a variable length code (VLC) to determine a value of the syntax element, wherein a maximum codeword length of the VLC is 32 bits or less.
11. The method of claim 10 , wherein the VLC is a code that adapts based on a value of a parameter R, wherein codewords for values less than 4×2 R have a fixed length suffix of R bits, prefixes of codewords are unary coded, suffix length of codewords increases by 1 bit for each 1 bit increase in prefix length, and the maximum codeword length is 32 bits.
12. The method of claim 10 , wherein the value of R is one selected from a group consisting of 0, 1, 2, 3, and 4.