Methods of coding images/videos with alpha channels

1 . A method of video processing, comprising:

determining, for a conversion between a video comprising a plurality of pictures and a bitstream of the video, first layer information which is utilized in a first process performed on a first layer of each picture;

determining, second layer information which is utilized in a second process performed on a second layer of each picture; and

performing the conversion for the second layer based on the first layer information and the second layer information,

wherein the first layer is a base layer corresponding to a color channel of the plurality of pictures, the second layer is an alpha layer corresponding to a degree of transparency of the plurality of pictures,

wherein the first layer information includes first block information of a first block in the first layer, and the method further comprises:

determining a predictive context based on the first layer information by performing a context modeling for context-based adaptive binary arithmetic coding (CABAC), the context modeling being determined based on the first block information; and

determining the second layer information based on a second block in the second layer and the predictive context,

wherein the conversion is performed in a single-pass conversion between the first layer and the second layer, the bitstream is generated by interleaving the first layer information and the second layer information in a coding tree unit basis and signaling one or more syntaxes for an individual frame to separate an alpha bitstream from a color bitstream,

wherein performing the conversion for the second layer comprises:

predicting a current block in the alpha layer by determining a weighted average of a plurality of reference blocks associated with the base layer, wherein the plurality of reference blocks are from a plurality of previously-coded pictures, and

wherein a syntax element used for coding the alpha layer by an alpha layer coding component is different from a syntax element used for coding the base layer by a base layer coding component, the alpha layer coding component disables a coding tool employed by the base layer coding component to determine the first layer information, the coding tool is a loop filtering process, and a conformance bitstream satisfies that the coding tool is disabled for coding the alpha layer of the plurality of pictures.

2 . The method of claim 1 , wherein the first block information is used as a predictor for coding second block information of a second block in the second layer.

3 . The method of claim 1 , wherein the first block information is used for coding a second block in the second layer, and wherein the first block has a same top-left position and a same size with those of the second block.

4 . The method of claim 1 , further comprising determining prediction information based on a portion of the first layer information associated with a luma component of the first layer; and wherein the second layer information is determined based on the prediction information.

5 . The method of claim 1 , further comprising:

performing a bit depth alignment on a plurality of samples of the first layer to determine bit aligned sample information as the first layer information; and

determining prediction information based on a portion of the first layer information associated with a luma component of the first layer; and wherein the second layer information is determined based on the prediction information.

6 . The method of claim 1 , wherein the first layer information includes at least one of the following: intra-prediction mode information, motion information, a motion candidate index, or partition information for block structures of the first process and the second process.

7 . The method of claim 1 , further comprising:

a second block in the second layer is reconstructed based on samples of at least a first block in the first layer.

8 . The method of claim 7 , wherein the first block has a same top-left position and a same size with those of the second block or neighboring blocks around the first block.

9 . The method of claim 7 , wherein the second block is reconstructed by performing a bitwise operation on the first block.

10 . The method of claim 9 , wherein the bitwise operation is denoted by C=R*a+b; where C is a sample of the second block, and R is a sample of the first block, a and b are constant.

11 . The method of claim 9 , wherein the bitwise operation is denoted by C=(R<<a)+b or by C=(R>>a)+b; where C is a sample of the second block, and R is a sample of the first block, a and b are constant.

12 . The method of claim 7 , wherein the first block is denoted by the samples before performing a loop filtering process or by the samples after performing a loop filtering process.

13 . The method of claim 12 , wherein the loop filtering process includes at least one of sample adaptive offset (SAO), adaptive loop filter (ALF) or deblocking filters.

14 . The method of claim 1 , wherein the conversion includes encoding the video into the bitstream.

15 . The method of claim 1 , wherein the conversion includes decoding the video from the bitstream.

16 . 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, for a conversion between a video comprising a plurality of pictures and a bitstream of the video, first layer information which is utilized in a first process performed on a first layer of each picture;

determine, second layer information which is utilized in a second process performed on a second layer of each picture; and

perform the conversion for the second layer based on the first layer information and the second layer information,

wherein the first layer is a base layer corresponding to a color channel of the plurality of pictures, the second layer is an alpha layer corresponding to a degree of transparency of the plurality of pictures,

wherein the first layer information includes first block information of a first block in the first layer, and the instructions further cause the processor to:

determine a predictive context based on the first layer information by performing a context modeling for context-based adaptive binary arithmetic coding (CABAC), the context modeling being determined based on the first block information; and

determine the second layer information based on a second block in the second layer and the predictive context,

wherein the conversion is performed in a single-pass conversion between the first layer and the second layer, the bitstream is generated by interleaving the first layer information and the second layer information in a coding tree unit basis and signaling one or more syntaxes for an individual frame to separate an alpha bitstream from a color bitstream,

wherein the instructions causing the processor to perform the conversion for the second layer further cause the processor to:

predict a current block in the alpha layer by determining a weighted average of a plurality of reference blocks associated with the base layer, wherein the plurality of reference blocks are from a plurality of previously-coded pictures, and

wherein a syntax element used for coding the alpha layer by an alpha layer coding component is different from a syntax element used for coding the base layer by a base layer coding component, the alpha layer coding component disables a coding tool employed by the base layer coding component to determine the first layer information, the coding tool is a loop filtering process, and a conformance bitstream satisfies that the coding tool is disabled for coding the alpha layer of the plurality of pictures.

17 . A non-transitory computer-readable recording medium, wherein the computer-readable recording medium stores instructions, and the instructions, when executed by a processor, cause the processor to:

determine, for a conversion between a video comprising a plurality of pictures and a bitstream of the video, first layer information which is utilized in a first process performed on a first layer of each picture;

determine, second layer information which is utilized in a second process performed on a second layer of each picture; and

generate generating a bitstream of the video from the video based on the first layer information and the second layer information,

wherein the first layer is a base layer corresponding to a color channel of the plurality of pictures, the second layer is an alpha layer corresponding to a degree of transparency of the plurality of pictures,

wherein the first layer information includes first block information of a first block in the first layer, and the instructions further cause the processor to:

determine a predictive context based on the first layer information by performing a context modeling for context-based adaptive binary arithmetic coding (CABAC), the context modeling being determined based on the first block information; and

determine the second layer information based on a second block in the second layer and the predictive context,

wherein the conversion is performed in a single-pass conversion between the first layer and the second layer, the bitstream is generated by interleaving the first layer information and the second layer information in a coding tree unit basis and signaling one or more syntaxes for an individual frame to separate an alpha bitstream from a color bitstream,

wherein the instructions causing the processor to perform the conversion for the second layer further cause the processor to:

predict a current block in the alpha layer by determining a weighted average of a plurality of reference blocks associated with the base layer, wherein the plurality of reference blocks are from a plurality of previously-coded pictures, and

wherein a syntax element used for coding the alpha layer by an alpha layer coding component is different from a syntax element used for coding the base layer by a base layer coding component, the alpha layer coding component disables a coding tool employed by the base layer coding component to determine the first layer information, the coding tool is a loop filtering process, and a conformance bitstream satisfies that the coding tool is disabled for coding the alpha layer of the plurality of pictures.

18 . A non-transitory computer-readable storage medium storing instructions that cause a processor to:

determine, for a conversion between a video comprising a plurality of pictures and a bitstream of the video, first layer information which is utilized in a first process performed on a first layer of each picture;

determine, second layer information which is utilized in a second process performed on a second layer of each picture; and

perform the conversion for the second layer based on the first layer information and the second layer information,

wherein the first layer is a base layer corresponding to a color channel of the plurality of pictures, the second layer is an alpha layer corresponding to a degree of transparency of the plurality of pictures,

wherein the first layer information includes first block information of a first block in the first layer, and the instructions further cause the processor to:

determine a predictive context based on the first layer information by performing a context modeling for context-based adaptive binary arithmetic coding (CABAC), the context modeling being determined based on the first block information; and

determine the second layer information based on a second block in the second layer and the predictive context,

wherein the conversion is performed in a single-pass conversion between the first layer and the second layer, the bitstream is generated by interleaving the first layer information and the second layer information in a coding tree unit basis and signaling one or more syntaxes for an individual frame to separate an alpha bitstream from a color bitstream,

wherein the instructions causing the processor to perform the conversion for the second layer further cause the processor to:

predict a current block in the alpha layer by determining a weighted average of a plurality of reference blocks associated with the base layer, wherein the plurality of reference blocks are from a plurality of previously-coded pictures, and

wherein a syntax element used for coding the alpha layer by an alpha layer coding component is different from a syntax element used for coding the base layer by a base layer coding component, the alpha layer coding component disables a coding tool employed by the base layer coding component to determine the first layer information, the coding tool is a loop filtering process, and a conformance bitstream satisfies that the coding tool is disabled for coding the alpha layer of the plurality of pictures.