Coefficient-based transform and mode signaling
Techniques are described for express and implied signaling of transform mode selections in video coding. Information derived from coefficient samples in a given transform unit (TU) or prediction unit (PU) may constrain or modify signaling of certain syntax elements at the coding block (CB), TU, or PU levels. For instance, based on the spatial locations of decoded coefficients, the spatial patterns of coefficients, or the correlation with the coefficients in neighboring blocks, various syntax elements such as the transform type and related flags/indices or secondary transform modes/flags indices, a residual coding mode, intra and inter prediction modes, and scanning order may be disabled or constrained. In another case, if the coefficient samples match a desired spatial pattern or have other desired properties then a default transform type, a default secondary transform type, a default intra and inter prediction mode or other block level modes may be inferred at the decoder side. Similar ideas may be extended to infer intra-block copy (IBC) flags/modes/indices from coefficient samples, or to infer flags and modes related to predictive coding techniques such as block-wise differential pulse code modulation (BDPCM), i.e., horizontal and vertical BDPCM modes, palette mode syntax and other block level flags/indices.
1. A video decoding method comprising:
receiving coded data of a transform unit from a channel according to a coding protocol in which an identifier of a position of a last significant quantized transform coefficient of the transform unit precedes an identification of transform type in signaling order;
comparing the position of the last significant quantized transform to a threshold;
assigning a transform type to the transform unit based on the comparison, wherein the assigned transform type is inferred from i) multiple reference line data received from the channel or ii) angle delta offset data received from the channel;
dequantizing received transform coefficients received from the channel; and
transforming the dequantized transform coefficients according to a default transform type.
2. The method of claim 1 , wherein the position is identified by its scan position within the transform unit.
3. The method of claim 1 , wherein the position is identified by its x, y coordinate within the transform unit.
4. The method of claim 1 , wherein when the comparison indicates an implied signaling condition is satisfied, the assigned transform type is the default transform type determined by a governing coding protocol.
5. The method of claim 1 , wherein the assigned transform type is identified in a coding unit level of the coding protocol.
6. The method of claim 1 , wherein the assigned transform type is inferred from a prediction mode of the transform unit.
7. The method of claim 1 , further comprising:
determining if a transform type identifier is present in the received data representing the coded transform unit,
if the transform type identifier is not present in the received data of the coded transform unit, determining one of multiple position ranges to which the position identifier belongs, and determining the coded transform unit's transform type from the determined range.
8. The method of claim 1 , wherein the comparison determines that the position is less than the threshold and, if so, assigns the transform type as a two-dimensional transform.
9. The method of claim 1 , wherein the comparison determines that the position is less than the threshold and, if so, assigns the transform type from a two-member set binary symbol received from a channel.
10. The method of claim 1 , wherein the comparison determines that the position is greater than the threshold and, if so, assigns the transform type as an identity transform.
11. The method of claim 1 , wherein the comparison
determines whether the position is between first and second thresholds and,
if so, compares a transform type received in the channel to an array of candidate transform types, the array being smaller than an entirety of transform types supported by a governing coding protocol, and
selects one of the candidate transform types based on the transform type comparison.
12. The video decoding method of claim 1 , wherein the coding protocol supports coding of transform units in a quadtree representation, and assigned transform type is identified in a transform unit level at a higher level of the quadtree representation than the transform unit level in which the position identifier is received.
13. The method of claim 1 , further comprising deriving a secondary transform type from a processing context of the coded transform unit.
14. A video decoder, comprising:
a processing device,
a memory storing program instructions that, when executed by the processing device, cause the processing device to decode coded video by:
receiving coded data of a transform unit from a channel according to a coding protocol in which an identifier of a position of a last significant quantized transform coefficient of the transform unit precedes an identification of transform type in signaling order;
comparing the position of the last significant quantized transform to a threshold;
assigning a transform type to the transform unit based on the comparison, wherein the assigned transform type is inferred from i) multiple reference line data received from the channel or ii) angle delta offset data received from the channel;
dequantizing received transform coefficients received from the channel;
transforming the dequantized transform coefficients according to the default transform type;
determining if a transform type identifier is present in the received data representing the coded transform unit; and
if the transform type identifier is not present in the received data of the coded transform unit, determining one of multiple position ranges to which the position identifier belongs, and determining the coded transform unit's transform type from the determined range.
15. The decoder of claim 14 , wherein the position is identified by its scan position within the transform unit.
16. The decoder of claim 14 , wherein the position is identified by its x, y coordinate within the transform unit.
17. The decoder of claim 14 , wherein when the comparison indicates an implied signaling condition is satisfied, the assigned transform type is the default transform type determined by a governing coding protocol.
18. The decoder of claim 14 , wherein the assigned transform type is identified in a coding unit level of the coding protocol.
19. The decoder of claim 14 , wherein the assigned transform type is inferred from a prediction mode of the transform unit.