Context initialization in entropy coding
A decoder includes an entropy decoder configured to derive a number of bins of the binarizations from the data stream using binary entropy decoding by selecting a context among different contexts and updating probability states associated with the different contexts, dependent on previously decoded portions of the data stream; a desymbolizer configured to debinarize the binarizations of the syntax elements to obtain integer values of the syntax elements; a reconstructor configured to reconstruct the video based on the integer values of the syntax elements using a quantization parameter, wherein the entropy decoder is configured to distinguish between 126 probability states and to initialize the probability states associated with the different contexts according to a linear equation of the quantization parameter, wherein the entropy decoder is configured to, for each of the different contexts, derive a slope and an offset of the linear equation from first and second four bit parts of a respective 8 bit initialization value.
1 . A decoder for decoding a video encoded in a data stream comprising syntax elements encoded therein, the decoder comprising:
an entropy decoder configured to decode the syntax elements based on context-adaptive binary arithmetic coding (CABAC), wherein the entropy decoder is configured to:
initialize at least one context of a plurality of contexts based on a slope value of an initialization equation and an offset value of the initialization equation, wherein:
the slope value is computed by applying a first arithmetic operation on an 8-bit initialization value associated with the at least one context, and
the offset value is computed by applying a second arithmetic operation on the 8-bit initialization value.
2 . The decoder according to claim 1 , wherein:
the data stream comprises binarizations of the syntax elements, and
the entropy decoder is configured to decode the binarizations based on the CABAC, the decoder further comprises a desymbolizer configured to debinarize the binarizations of the syntax elements to obtain integer values of the syntax elements.
3 . The decoder according to claim 1 , wherein the entropy decoder is configured to select a context from the plurality of contexts to decode a syntax element based on one or more of:
a bin position of a bin of the syntax element,
a syntax element type of the syntax element, and
a value associated with a syntax element previously decoded.
4 . The decoder according to claim 1 , wherein the entropy decoder is configured to update probability states associated with each of the plurality of contexts by transitioning from a current probability state associated with the context to a new probability state among 126 probability sates based on a portion of a syntax element being currently decoded.
5 . The decoder according to claim 1 , wherein the entropy decoder is configured to decode a bin of a syntax element being currently decoded by quantizing a current probability interval width value representing a current probability interval to acquire a probability interval index, and performing an interval subdivision by indexing a table entry among tables entries using the probability interval index and a probability state index depending on a current probability state associated with the context selected for the bin, to acquire a sub-division of the current probability interval into two partial intervals.
6 . The decoder according to claim 5 , wherein the entropy decoder is configured to use an 8-bit representation for the current probability interval width value and to obtain 2 or 3 most significant bits of the 8-bit representation in quantizing the current probability interval width value.
7 . The decoder according to claim 5 , wherein the entropy decoder is configured to:
select one of the two partial intervals based on an offset state value from an interior of the current probability interval;
update the current probability interval width value and the offset state value;
infer a value of the bin using the selected partial interval; and
perform a renormalization of the updated current probability interval width value and the offset state value comprising a continuation of reading bits from the data stream.
8 . The decoder according to claim 1 , wherein the entropy decoder is configured to initialize state and value variables of each of the plurality of contexts at a beginning of each slice of the video.
9 . The decoder according to claim 1 , wherein the slope value and the offset value are determined based on first and second sets of 4 bits of the 8-bit initialization value, respectively, using a look-up table.
10 . The decoder according to claim 1 , wherein the data stream comprises at least a portion associated with color samples related to the video.
11 . The decoder according to claim 1 , wherein the data stream comprises at least a portion associated with depth values related to a depth map related to the video.
12 . An encoder for encoding a video into a data stream, comprising:
an entropy encoder configured to encode syntax elements related to the video into the data stream based on context-adaptive binary arithmetic coding (CABAC), wherein the entropy encoder is configured to:
initialize at least one context of a plurality of contexts based on a slope value of an initialization equation and an offset value of the initialization equation, wherein:
the slope value is computed by applying a first arithmetic operation on an 8-bit initialization value associated with the at least one context, and
the offset value is computed by applying a second arithmetic operation on the 8-bit initialization value.
13 . The encoder according to claim 12 , further comprising a symbolizer configured to binarize the syntax elements to obtain binarizations of the syntax elements, wherein the entropy encoder is configured to encode the binarizations based on the CABAC.
14 . The encoder according to claim 12 , wherein the entropy encoder is configured to select a context from the plurality of contexts to encode a syntax element based on one or more of:
a bin position of a bin of the syntax element,
a syntax element type of the syntax element, and
a value associated with a syntax element previously encoded.
15 . The encoder according to claim 12 , wherein the entropy encoder is configured to update probability states associated with each of the plurality of contexts by transitioning from a current probability state associated with the context to a new probability state among 126 probability sates based on a portion of a syntax element being currently encoded.
16 . The encoder according to claim 12 , wherein the data stream comprises at least a portion associated with color samples related to the video.
17 . The encoder according to claim 12 , wherein the data stream comprises at least a portion associated with depth values related to a depth map related to the video.
18 . A method for encoding a video into a data stream, comprising:
initializing at least one context of a plurality of contexts based on a slope value of an initialization equation and an offset value of the initialization equation, wherein:
the slope value is computed by applying a first arithmetic operation on an 8-bit initialization value associated with the at least one context, and
the offset value is computed by applying a second arithmetic operation on the 8-bit initialization value; and
encoding syntax elements related to the video into the data stream based on context-adaptive binary arithmetic coding (CABAC).
19 . A method for storing a data stream into which a video is encoded according to the method of claim 18 .