Coding position data for the last non-zero transform coefficient in a coefficient group
Methods and devices for video coding are described. When encoding a non-zero coefficient group (CG) prior to the last such group in a group scan ordering within a transform block, data representative of the true (x,y)-position of the last non-zero transform coefficient in a scan ordering within that CG is modified, to improve coding efficiency, based upon at least one of (a) an intra-prediction mode used to encode the block and (b) at least one coefficient group neighboring that CG. Upon decoding a CG thus encoded, the modification is un-done to retrieve the true (x,y)-position.
1. A method for reconstructing a current block of residual data, using a video decoder, from an encoded block of residual data in a bitstream of encoded video, the current block partitioned into coefficient groups, each coefficient group comprising transform coefficients, the method comprising:
for a current coefficient group, which is a non-zero coefficient group prior to the last non-zero coefficient group in a group scan ordering of the coefficient groups within the current block:
decoding coordinates (X, Y) representative of the true (x,y)-position, in the current coefficient group, of the last non-zero transform coefficient in a scan ordering of the transform coefficients in the current coefficient group; and
applying a non-identity modification to the coordinates (X, Y) to compute the true (x,y)-position of the last non-zero transform coefficient, wherein the modification is based upon at least one of: an intra-prediction mode used in generating the encoded block; and at least one coefficient group neighbouring the current coefficient group.
2. The method of claim 1 , wherein the current coefficient group has size 4×4, and wherein the modification includes one or both of: converting the coordinate X to 3-X; and converting the coordinate Y to 3-Y.
3. The method of claim 1 , wherein the modification is based upon a classification of the mode.
4. The method of claim 3 , further comprising:
decoding data indicating the mode; and
determining the classification of the mode.
5. The method of claim 3 , wherein the current coefficient group has size 4×4, and wherein the modification comprises:
if the mode is in a diagonal class, converting the coordinates (X, Y) to (3-X, 3-Y);
if the mode is in a vertical class, converting the coordinates (X, Y) to (3-X, Y); and
if the mode is a horizontal class, converting the coordinates (X, Y) to (X, 3-Y).
6. The method of claim 1 , wherein at least one of the neighbouring coefficient group right of the current coefficient group and the neighbouring coefficient group below the current coefficient group comprises a non-zero coefficient group, and wherein the modification is based upon at least one of said neighbouring coefficient groups.
7. The method of claim 6 , wherein the current coefficient group has size 4×4, and wherein the modification comprises:
if the neighbouring coefficient group right of the current coefficient group is a zero coefficient group and the neighbouring coefficient group below the current coefficient group is a non-zero coefficient group, converting the coordinates (X, Y) to (X, 3-Y);
if the neighbouring coefficient group right of the current coefficient group is a non-zero coefficient group and the neighbouring coefficient group below the current coefficient group is a zero coefficient group, converting the coordinates (X, Y) to (3-X, Y); and
if the neighbouring coefficient group right of the current coefficient group and the neighbouring coefficient group below the current coefficient group are both non-zero coefficient groups, converting the coordinates (X, Y) to (3-X, 3-Y).
8. The method of claim 1 , wherein the current coefficient group has size 4×4, wherein the neighbouring coefficient group right of the current coefficient group and the neighbouring coefficient group below the current coefficient group are both zero coefficient groups, wherein the mode is in one of a horizontal class and a vertical class; and wherein the modification comprises:
if the mode is in a vertical class, converting the coordinates (X, Y) to (3-X, Y); and
if the mode is in a horizontal class, converting the coordinates (X, Y) to (X, 3-Y).
9. The method of claim 1 , wherein the group scan ordering comprises an ordering of the coefficient groups in the current block, starting from the upper-left-most coefficient group; and wherein the coefficient groups of the block are decoded by the decoder in the reverse of the group scan ordering.
10. The method of claim 9 , wherein the group scan ordering comprises a zig-zag ordering.
11. The method of claim 1 , wherein the scan ordering comprises an ordering of transform coefficients in each coefficient group, starting from the upper-left-most transform coefficient in the group, and wherein level, run, and sign data for the transform coefficients in each coefficient group are decoded by the decoder in the reverse of the scan ordering.
12. The method of claim 11 , wherein the scan ordering comprises a zig-zag ordering.
13. The method of claim 1 , wherein decoding coordinates (X, Y) is based upon a context derived at least in part from to which of three regions, into which the coefficient groups of the current block have been partitioned, the current coefficient group belongs.
14. The method of claim 13 , wherein the three regions are each transpose-invariant.
15. The method of claim 14 ,
wherein the coefficient groups are arranged in rows and columns within the current block;
wherein one of the three regions consists of the coefficient groups in both the top row and the left column;
wherein another of the three regions consists of the coefficient groups in neither the top row nor the left column; and
wherein the remaining one of the three regions consists of the coefficient groups in either the top row or the left column, but not both.
16. A decoder for reconstructing a current block of residual data, from an encoded block of residual data in a bitstream of encoded video, the decoder comprising:
one or more processors;
a memory; and
a decoding application stored in memory and containing instructions for configuring the one or more processors to perform the method claimed in claim 1 .
17. A non-transitory processor-readable medium storing processor-executable instructions which, when executed, configures one or more processors to perform the method claimed in claim 1 .
18. A method for encoding video using a video encoder, the video including a picture partitioned into blocks, residual data for the blocks being spectrally transformed into transform blocks, each transform block being further partitioned into coefficient groups, the method comprising:
for a current coefficient group, which is a non-zero coefficient group prior to the last non-zero coefficient group in a group scan ordering of the coefficient groups within a transform block:
obtaining the (x,y)-position (X, Y), in the current coefficient group, of the last non-zero transform coefficient in a scan ordering of the transform coefficients in said coefficient group;
converting the (x,y)-position (X, Y) to a modified (x,y)-position (X′, Y′) using a non-identity modification based upon at least one of: an intra-prediction mode used in generating the transform block; and at least one coefficient group neighbouring the current coefficient group; and
binarizing and entropy encoding the modified (x,y)-position (X′, Y′), using a context model.
19. An encoder for encoding video, the encoder comprising:
one or more processors;
a memory; and
an encoding application stored in memory and containing instructions for configuring the one or more processors to perform the method claimed in claim 18 .
20. A non-transitory, processor-readable medium storing processor-executable instructions which, when executed, configures one or more processors to perform the method claimed in claim 18 .