Video encoding and decoding using adaptive color transform

1 . An apparatus for decoding video data, the apparatus comprising at least one processor configured to:

obtain motion information and first residual data for a current block from a bitstream;

determine whether to apply color space transform to the first residual data based on a high-level color transform control flag and a block-level color transform control flag;

in response to a determination that the color space transform is applied to the first residual data, generate second residual data by performing reverse color transform on the first residual data;

generate a prediction block for the current block by inter-predicting the current block using the motion information; and

generate a reconstructed block for the current block based on the prediction block and the second residual data,

wherein the high-level color transform control flag is decoded from the bitstream depending on a maximum transform size allowed in a sequence of pictures including the current block, and the block-level color transform control flag is decoded from the bitstream depending on the high-level color transform control flag,

wherein, when the high-level color transform control flag and the block-level color transform control flag are not decoded, the high-level color transform control flag and the block-level color transform control flag are inferred to be false, and

wherein the at least one processor is further configured to:

decode a syntax element indicating the maximum transform size allowed for the sequence of the pictures from the bitstream; and

decode the high-level color transform control flag from the bitstream when the syntax element indicates that the maximum transform size is smaller than a predetermined value.

2 . The apparatus of claim 1 , wherein the block-level color transform control flag for the current block is decoded from the bitstream when the high-level color transform control flag indicates that the application of the color space transform is allowed for blocks within the sequence.

3 . The apparatus of claim 1 , wherein at least one processor is configured to, when a luma component and chroma components of the current block are encoded based on a dual partitioning tree, determine that the color space transform is not applied to the current block without decoding the block-level color transform control flag.

4 . The apparatus of claim 1 , wherein at least one processor is configured to, when a chroma sampling format for the video data is a chroma sampling format other than 4:4:4, determine that the application of the color space transform is not allowed for the blocks in the sequence without decoding the high-level color transform control flag.

5 . The apparatus of claim 1 , wherein the prediction block is generated for each of color components of the current block,

wherein, when the block-level color transform control flag indicates that the color space transform is applied to the current block, a cross-component linear model (CCLM) is not applied to chroma components of the current block.

6 . The apparatus of claim 1 , wherein the at least one processor is configured to predict at least one of color components of the current block from other color component.

7 . The apparatus of claim 1 , wherein the first residual data includes residual data for each of color components of the current block.

8 . An apparatus for encoding video data, the apparatus comprising at least one processor configured to:

encode a syntax element indicating a maximum transform size allowed for a sequence of pictures of the video data into a bitstream;

when the syntax element indicates that the maximum transform size is smaller than a predetermined value, encode a first control flag indicating whether application of color space transform is allowed for blocks within the sequence into the bitstream;

when the first control flag indicates that the application of the color space transform is allowed for the blocks within the sequence, encode a second control flag indicating whether the color space transform is applied to a current block in the sequence into the bitstream;

determine motion information of the current block and generate a prediction block for the current block using the motion information;

generate first residual data for the current block based on the prediction block;

when the second control flag indicates that the color space transform is applied to the current block, perform color transform on the first residual data to generate second residual data; and encode the second residual data for the current block into the bitstream; and

when the second control flag indicates that the color space transform is not applied to the current block, encode the first residual data for the current block into the bitstream.

9 . An apparatus for providing a video decoding device with video data, the apparatus comprising at least one processor configured to:

encode the video data into a bitstream; and

transmit the bitstream to the video decoding device,

wherein the encoding of the video data includes:

encoding a syntax element indicating a maximum transform size allowed for a sequence of pictures of the video data into the bitstream;

when the syntax element indicates that the maximum transform size is smaller than 64, encoding a first control flag indicating whether application of color space transform is allowed for blocks within the sequence into the bitstream;

when the first control flag indicates that the application of the color space transform is allowed for the blocks within the sequence, encoding a second control flag indicating whether the color space transform is applied to a current block in the sequence into the bitstream;

determining motion information of the current block and generating a prediction block for the current block using the motion information;

generating first residual data for the current block based on the prediction block;

when the second control flag indicates that the color space transform is applied to the current block, performing color transform on the first residual data to generate second residual data and encoding the second residual data for the current block into the bitstream; and

when the second control flag indicates that the color space transform is not applied to the current block, encoding the first residual data for the current block into the bitstream.