Parameter derivation in cross component mode
A method for visual media processing, including performing a conversion between a current chroma video block of visual media data and a bitstream representation of the current chroma video block, wherein, during the conversion, a chroma residual of the current chroma video block is scaled based on a scaling coefficient, wherein the scaling coefficient is derived at least based on luma samples located in predefined positions.
1. A method of processing video data, comprising:
determining, during a conversion between a current chroma video block of a video and a bitstream of the video, that a scaling process is applied on chroma residual samples of the current chroma video block; and
performing the conversion by applying the scaling process on the chroma residual samples,
wherein in the scaling process, the chroma residual samples are scaled based on at least one scaling factor before being used to reconstruct the current chroma video block, and
wherein the at least one scaling factor is derived based on an averaged luma variable computed based on neighboring luma samples of a video unit of the video which is determined based on a luma sample corresponding to a top-left sample of the current chroma video block, and
wherein the at least one scaling factor is derived by:
checking an availability of each of one or more neighboring luma blocks of the video unit, wherein the each of the one or more neighboring luma blocks comprise at least one sample of the neighboring luma samples,
determining, based on the availability of the each of the one or more neighboring luma blocks, whether to retrieve the neighboring luma samples of the video unit,
deriving the at least one scaling factor based on the averaged luma variable computed using the neighboring luma samples by a rounding-based average operation in response to the neighboring luma samples being available, and
deriving the at least one scaling factor as 1<<(bitDepth−1) in response to the neighboring luma samples being unavailable, where bitDepth is a bit depth of the video, and
wherein a size information of a virtual pipeline data unit is further used to derive the at least one scaling factor.
2. The method of claim 1 , wherein the neighboring luma samples are located in predefined positions neighboring to the video unit.
3. The method of claim 2 , wherein the neighboring luma samples located in the predefined positions neighboring to the video unit include reconstructed luma samples external to the video unit.
4. The method of claim 2 , wherein the neighboring luma samples located in the predefined positions neighboring to the video unit include reconstructed luma samples adjacent to the video unit.
5. The method of claim 4 , wherein the reconstructed luma samples adjacent to the video unit include at least one of one or more left neighboring luma sample columns or one or more above neighboring luma sample rows of the video unit.
6. The method of claim 1 , wherein a total number of the neighboring luma samples of the video unit is N, where N is an integer greater than 1, and a range of N depends on a size information of the video unit.
7. The method of claim 1 , wherein the checking the availability of each of the one or more neighboring luma blocks comprises:
checking the availability of each of the one or more neighboring luma blocks based on at least one of a width and a height of a video region of the video.
8. The method of claim 7 , wherein in response to a top-left sample of the one neighboring luma block being located outside the video region, the neighboring luma block is treated as unavailable.
9. The method of claim 1 , wherein, for a luma video block of a video region of the video, at least one of the following is performed:
1) a forward mapping process for the luma video block, in which prediction samples of the luma video block are converted from an original domain to a reshaped domain; or
2) an inverse mapping process, which is an inverse operation of the forward mapping process, in which reconstructed samples of the luma video block in the reshaped domain are converted to the original domain, and
wherein the neighboring luma samples include reconstructed samples in the reshaped domain.
10. The method of claim 7 , wherein the video region is a picture.
11. The method of claim 1 , wherein the scaling process is based on a piecewise linear model, and wherein an index identifying a piece to which the averaged luma variable belongs, and the at least one scaling factor is derived based on the index.
12. The method of claim 1 , wherein the conversion includes encoding the current chroma video block into the bitstream.
13. The method of claim 1 , wherein the conversion includes decoding the current chroma video block from the bitstream.
14. An apparatus for processing video data comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to:
determine, during a conversion between a current chroma video block of a video and a bitstream of the video, that a scaling process is applied on chroma residual samples of the current chroma video block; and
perform the conversion by applying the scaling process on the chroma residual samples,
wherein in the scaling process, the chroma residual samples are scaled based on at least one scaling factor before being used to reconstruct the current chroma video block, and
wherein the at least one scaling factor is derived based on an averaged luma variable computed based on neighboring luma samples of a video unit of the video which is determined based on a luma sample corresponding to a top-left sample of the current chroma video block, and
wherein the at least one scaling factor is derived by:
checking an availability of each of one or more neighboring luma blocks of the video unit, wherein the each of the one or more neighboring luma blocks comprise at least one sample of the neighboring luma samples,
determining, based on the availability of the each of the one or more neighboring luma blocks, whether to retrieve the neighboring luma samples of the video unit,
deriving the at least one scaling factor based on the averaged luma variable computed using the neighboring luma samples by a rounding-based average operation in response to the neighboring luma samples being available, and
deriving the at least one scaling factor as 1<<(bitDepth−1) in response to the neighboring luma samples being unavailable, where bitDepth is a bit depth of the video, and
wherein a size information of a virtual pipeline data unit is further used to derive the at least one scaling factor.
15. A non-transitory computer-readable storage medium storing instructions that cause a processor to:
determine, during a conversion between a current chroma video block of a video and a bitstream of the video, that a scaling process is applied on chroma residual samples of the current chroma video block; and
perform the conversion by applying the scaling process on the chroma residual samples,
wherein in the scaling process, the chroma residual samples are scaled based on at least one scaling factor before being used to reconstruct the current chroma video block, and
wherein the at least one scaling factor is derived based on an averaged luma variable computed based on neighboring luma samples of a video unit of the video which is determined based on a luma sample corresponding to a top-left sample of the current chroma video block, and
wherein the at least one scaling factor is derived by:
checking an availability of each of one or more neighboring luma blocks of the video unit, wherein the each of the one or more neighboring luma blocks comprise at least one sample of the neighboring luma samples,
determining, based on the availability of the each of the one or more neighboring luma blocks, whether to retrieve the neighboring luma samples of the video unit, and
deriving the at least one scaling factor based on the averaged luma variable computed using the neighboring luma samples by a rounding-based average operation in response to the neighboring luma samples being available, and
deriving the at least one scaling factor as 1<<(bitDepth−1) in response to the neighboring luma samples being unavailable, where bitDepth is a bit depth of the video, and
wherein a size information of a virtual pipeline data unit is further used to derive the at least one scaling factor.
16. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by a video processing apparatus, wherein the method comprises:
determining that a scaling process is applied on chroma residual samples of the current chroma video block; and
generating the bitstream by applying the scaling process on the chroma residual samples,
wherein in the scaling process, the chroma residual samples are scaled based on at least one scaling factor before being used to reconstruct the current chroma video block, and
wherein the at least one scaling factor is derived based on an averaged luma variable computed based on neighboring luma samples of a video unit of the video which covers a luma sample corresponding to a top-left sample of the current chroma video block, and
wherein the at least one scaling factor is derived by:
checking an availability of each of one or more neighboring luma blocks of the video unit, wherein the each of the one or more neighboring luma blocks comprise at least one sample of the neighboring luma samples,
determining, based on the availability of the each of the one or more neighboring luma blocks, whether to retrieve the neighboring luma samples of the video unit, and
deriving the at least one scaling factor based on the averaged luma variable computed using the neighboring luma samples by a rounding-based average operation in response to the neighboring luma samples are available, and
deriving the at least one scaling factor as 1<<(bitDepth−1) in response to the neighboring luma samples being unavailable, where bitDepth is a bit depth of the video, and
wherein a size information of a virtual pipeline data unit is further used to derive the at least one scaling factor.
17. The apparatus of claim 14 , wherein the neighboring luma samples are located in predefined positions neighboring to the video unit,
wherein the neighboring luma samples located in the predefined positions neighboring to the video unit include reconstructed luma samples adjacent to the video unit,
wherein the reconstructed luma samples adjacent to the video unit include at least one of one or more left neighboring luma sample columns or one or more above neighboring luma sample rows of the video unit.
18. The apparatus of claim 14 , wherein a total number of the neighboring luma samples of the video unit is N, where N is an integer greater than 1, and a range of N depends on a size information of the video unit.
19. The apparatus of claim 14 , wherein the checking the availability of each of one or more neighboring luma blocks comprises:
checking the availability of each of one or more neighboring luma blocks based on at least one of a width and a height of a video region of the video.
20. The apparatus of claim 19 , wherein, in response to a top-left sample of the one neighboring luma block being located outside the video region, the neighboring luma block is treated as unavailable.