Modifying a scan order to limit scan distance
View Patent ↗A method for coding a transform block of coefficients includes generating a new scan order from the original scan order such that a maximum scan distance of the new scan order is smaller than or equal to a predetermined distance, and coding the coefficients based on the new scan order. An apparatus for decoding a transform block of coefficients. The apparatus includes a memory and a processor. The memory includes instructions executable by the processor to identify an original scan order for encoding the coefficients, generate a new scan order from the original scan order such that a maximum scan distance of the new scan order is less than or equal to a predetermined distance, and decode, from an encoded bitstream, the coefficients based on the new scan order.
1. A method for coding a transform block of coefficients, comprising:
identifying an original scan order for coding the coefficients;
generating a new scan order from the original scan order such that a maximum scan distance of the new scan order is less than or equal to a predetermined distance, wherein generating the new scan order includes:
assigning a new scan index to a current coefficient,
the current coefficient being immediately followed by a next coefficient in the original scan order, and
the current coefficient being a context coefficient for entropy coding another coefficient; and
on condition that a scan distance of the other coefficient is greater than or equal to a conservative maximum distance, assigning a next scan index to the other coefficient before assigning the next scan index to the next coefficient;
and
coding the coefficients based on the new scan order.
2. The method of claim 1 , wherein the predetermined distance is a size of a line buffer used for caching coded transform coefficients.
3. The method of claim 1 , wherein generating the new scan order from the original scan order comprises:
assigning, to at least some of the coefficients, respective scan indexes in the new scan order.
4. The method of claim 1 , wherein the coefficient is immediately below or to an immediate right of the current coefficient in the transform block.
5. The method of claim 1 , wherein
the maximum scan distance is a maximum of respective scan distances of at least some of the coefficients, and
a respective scan distance of a coefficient is a maximum of respective scan distances between the coefficient and each of context coefficients of the coefficient.
6. An apparatus for encoding a transform block of coefficients, comprising:
a memory;
a processor, wherein the memory includes instructions executable by the processor to:
identify an original scan order for encoding the coefficients;
generate a new scan order from the original scan order such that a maximum scan distance of the new scan order is less than or equal to a predetermined distance, wherein to generate the new scan order comprises to:
assign a new scan index to a current coefficient, wherein the current coefficient is preceded by a previous coefficient in the original scan order;
identify a coefficient that uses the previous coefficient as context coefficient; and
on condition that a scan distance of the coefficient is greater than or equal to a conservative maximum distance, assign a next scan index to the coefficient before assigning the next scan index to another coefficient subsequent to the current coefficient;
and
encode, in an encoded bitstream, the coefficients based on the new scan order.
7. The apparatus of claim 6 , wherein the predetermined distance is a size of a line buffer used for caching coded transform coefficients.
8. The apparatus of claim 6 , wherein to generate the new scan order from the original scan order comprises to:
assign, to at least some of the coefficients, respective scan indexes in the new scan order.
9. The apparatus of claim 6 , wherein the coefficient is immediately below or to an immediate right of the current coefficient in the transform block.
10. The apparatus of claim 6 , wherein
the maximum scan distance is a maximum of respective scan distances of at least some of the coefficients, and
a respective scan distance of a coefficient is a maximum of respective scan distances between the coefficient and each of context coefficients of the coefficient.
11. An apparatus for decoding a transform block of coefficients, comprising:
a memory;
a processor, wherein the memory includes instructions executable by the processor to:
identify an original scan order for decoding the coefficients;
generate a new scan order from the original scan order such that a maximum scan distance of the new scan order is less than or equal to a predetermined distance, wherein to generate the new scan order includes to:
assign a new scan index to a current coefficient, wherein the current coefficient is preceded by a previous coefficient in the original scan order;
identify a coefficient that uses the previous coefficient as context coefficient; and
on condition that a scan distance of the coefficient is greater than or equal to a conservative maximum distance, assign a next scan index to the coefficient before assigning the next scan index to another coefficient subsequent to the current coefficient;
and
decode, from an encoded bitstream, the coefficients based on the new scan order.
12. The apparatus of claim 11 , wherein the predetermined distance is a size of a line buffer used for caching coded transform coefficients.
13. The apparatus of claim 12 , wherein to generate new scan order from the original scan order comprises to:
assign, to at least some of the coefficients, respective scan indexes in the new scan order.
14. The apparatus of claim 12 , wherein
the maximum scan distance is a maximum of respective scan distances of at least some of the coefficients, and
a respective scan distance of a coefficient is a maximum of respective scan distances between the coefficient and each of context coefficients of the coefficient.