Block-level window size update for arithmetic coding
This disclosure relates to video coding with a block-level window size update for arithmetic coding. A probability update rate may be a function of count of appearances for the associated syntax/context when parsing a bitstream. It may also be a function of a number of different symbol values for the associated syntax/context. The value of the count is restarted with a pre-defined value at block-level. The probability or cumulative distribution function (CDF) update rate for a particular syntax/context may be reinitialized with a pre-defined value at block level.
1. A method for window size updates when decoding a video stream, the method comprising:
receiving a coded video stream comprising a number of syntaxes and a number of symbols associated with the syntaxes;
determining a count for the number of syntaxes when updating a window size;
determining a number of symbols associated with the syntaxes;
calculating a probability update rate as a function of the determined count for an associated syntax and of the determined number of symbols associated with the syntaxes, wherein a value of the count has a pre-defined value at block-level;
restarting the value of the count with the pre-defined value at one of following levels: a prediction unit (PU) level, a transform unit (TU) level, a coding tree unit (CTU) level; and
updating a context corresponding to the associated syntax at the calculated probability update rate.
2. The method of claim 1 , further comprising:
copying probability values of the syntax from another frame to a current frame; and
restarting each value of the count for each frame with the pre-defined value.
3. The method of claim 1 , wherein the block size for the restarting is specified in high-level syntax that comprises a video parameter set (VPS), a picture parameter set (PPS), a sequence parameter setting (SPS), an adaption parameter set (APS), a picture header, a frame header, a slice header, a tile header, or a coding tree unit (CTU) header.
4. The method of claim 1 , wherein the pre-defined value of count after the restarting comprises a value of 2 n where n is an integer.
5. The method of claim 1 , wherein the restarting is dependent on a syntax and the probability update rate is controlled differently for different syntax.
6. The method of claim 5 , wherein the probability update rate applies to a particular syntax group such that the probability update rate is restarted at a different time depending on the syntax group, and the probability update rate is restarted at a different value depending on the syntax group.
7. The method of claim 5 , wherein a flag is signaled at block-level to indicate whether a selected syntax group will restart the counter for updating the probability update rate.
8. An apparatus for decoding a video bitstream, the apparatus comprising:
a memory storing instructions; and
a processor in communication with the memory, wherein, when the processor executes the instructions, the processor is configured to cause the apparatus to:
receive the video bitstream comprising syntaxes and symbols associated with the syntaxes;
determine a count of appearances of each of the syntaxes;
determine a cumulative distribution function (CDF) update rate based on the count and the associated symbols;
reinitialize the CDF update rate with a pre-defined value for the count at one of following levels: a coding unit (CU) level, a prediction unit (PU) level, a transform unit (TU) level, a coding tree unit (CTU) level; and
update a context for one of the syntaxes at the CDF update rate.
9. The apparatus according to claim 8 , wherein the block size for the reinitialization comprises a high-level syntax, wherein the high-level syntax comprises a video parameter set (VPS), a picture parameter set (PPS), a sequence parameter setting (SPS), an adaption parameter set (APS), a picture header, a frame header, a slice header, a tile header, or a coding tree unit (CTU) header.
10. The apparatus according to claim 8 , wherein the pre-defined values comprise values corresponding to a uniform distribution.
11. The apparatus according to claim 8 , wherein the pre-defined values are stored at predetermined instances during encoding or decoding.
12. The apparatus according to claim 8 , wherein the reinitialization of the CDF is dependent on the context and the CDF is controlled differently for different contexts.
13. The apparatus according to claim 12 , wherein the reinitialization occurs for a particular context group.
14. The apparatus according to claim 12 , wherein the reinitialization is at a different time depending on the context group and is at a different value depending on the context group.
15. The apparatus according to claim 12 , wherein a flag is signaled at block-level to indicate whether a selected context group will reinitialize a counter for updating the CDF.
16. The apparatus according to claim 15 , wherein the contexts are grouped into different sets, and each set can reinitialize the CDF's together and are controlled by one flag.
17. The apparatus according to claim 8 , wherein the processor is configured to cause the apparatus to:
copy the CDF update rate from another frame to a current frame; and
restart each value of the count for each frame with the pre-defined value.