Efficient context handling in arithmetic coding

A method for entropy decoding a current entropy coded transform coefficient from a current block, the current block being decoded in a scan order, the scan order being non-contiguous, the method comprising:

• determining, based on the scan order, a first scan order distance between a scan order location corresponding to the current entropy coded transform coefficient and a scan order location corresponding to a first context coefficient;
• identifying a first context coefficient value from a first location in a context coefficient register, the first location corresponding to the first scan order distance;
• determining, based on the scan order, a second scan order distance between the scan order location corresponding to the current entropy coded transform coefficient and a scan order location corresponding to a second context coefficient, wherein the scan order location corresponding to a second context coefficient is spatially proximate to the scan order location corresponding to the current entropy coded transform coefficient;
• identifying a second context coefficient value from a second location in the context coefficient register, the second location corresponding to the second scan order distance;
• and
• entropy decoding the current entropy coded transform coefficient based on the first context coefficient value and the second context coefficient value;
• and
• including the entropy decoded current transform coefficient in an output bitstream for display.

An apparatus for decoding a current entropy coded transform coefficient of a current block, the current block being decoded using a scan order, the apparatus comprising:

• a memory;
• and
• a processor configured to execute instructions stored in the memory to:
• entropy decode the current entropy coded transform coefficient by:
• determining, based on the scan order, a left scan order distance between a scan order location corresponding to the current entropy coded transform coefficient and a scan order location corresponding to a left context coefficient;
• determining, based on the scan order, an above scan order distance between the scan order location corresponding to the current entropy coded transform coefficient and a scan order location corresponding to an above context coefficient;
• identifying a left context coefficient value from a first location in a context coefficient register, the first location corresponding to the left scan order distance;
• identifying an above context coefficient value from a second location in the context coefficient register, the second location corresponding to the above scan order distance;
• and
• entropy decoding the current entropy coded transform coefficient based on the left context coefficient value and the above context coefficient value;
• and
• include the entropy decoded current transform coefficient in an output bitstream.