Signaling scaling matrices in video coding

A device for decoding video data, the device comprising:

• a memory configured to store video data;
• one or more processors implemented in circuitry and configured to:
• receive, from the bitstream of encoded video data, a first syntax element indicating that the encoded video data is encoded using a 4:4:4 chroma subsampling format;
• in response to the first syntax element indicating that the encoded video data is encoded using the 4:4:4 chroma subsampling format, receive, from the bitstream of encoded video data, a second syntax element indicating whether chroma scaling matrices are signaled separately for chroma components of the encoded video data, wherein the first syntax element is different than the second syntax element, wherein the second syntax element has one of at least two different values and wherein a first value of the at least two different values for the second syntax element indicates the chroma scaling matrices are signaled separately for the chroma components of the encoded video data and a second value of the at least two different values for the second syntax element indicates the chroma scaling matrices are not signaled separately for the chroma components of the encoded video data;
• in response to the second syntax element being equal to the first value indicating that chroma scaling matrices are signaled separately for the chroma components of the encoded video data, determine a chroma scaling matrix for a block of the encoded video data;
• determine a block of chroma transform coefficients for the block, wherein the block of chroma transform coefficients comprises at least a first chroma transform coefficient and a second chroma transform coefficient;
• determine a quantization parameter (QP) value for the block of chroma transform coefficients;
• dequantize the first chroma transform coefficient of the block of chroma transform coefficients using a first scaling value from the chroma scaling matrix and based on the QP value;
• dequantize the second chroma transform coefficient of the block of chroma transform coefficients using a second scaling value from the chroma scaling matrix and based on the QP value, wherein the second scaling value is different than the first scaling value;
• determine a chroma residual block for the block based on the first dequantized chroma transform coefficients and the second dequantized chroma transform coefficient;
• add the chroma residual block to a chroma prediction block to determine a decoded chroma block;
• and
• output decoded video data that includes the decoded chroma block.

A non-transitory computer-readable storage medium storing instructions that when executed by one or more processors cause the one or more processors to:

• receive, from the bitstream of encoded video data, a first syntax element indicating that the encoded video data is encoded using a 4:4:4 chroma subsampling format;
• in response to the first syntax element indicating that the encoded video data is encoded using the 4:4:4 chroma subsampling format, receive, from the bitstream of encoded video data, a second syntax element indicating whether chroma scaling matrices are signaled separately for chroma components of the encoded video data, wherein the first syntax element is different than the second syntax element, wherein the second syntax element has one of at least two different values and wherein a first value of the at least two different values for the second syntax element indicates the chroma scaling matrices are signaled separately for the chroma components of the encoded video data and a second value of the at least two different values for the second syntax element indicates the chroma scaling matrices are not signaled separately for the chroma components of the encoded video data;
• in response to the second syntax element being equal to the first value indicating that chroma scaling matrices are signaled separately for the chroma components of the encoded video data, determine a chroma scaling matrix for a block of the encoded video data;
• determine a block of chroma transform coefficients for the block, wherein the block of chroma transform coefficients comprises at least a first chroma transform coefficient and a second chroma transform coefficient;
• determine a quantization parameter (QP) value for the block of chroma transform coefficients;
• dequantize the first chroma transform coefficient of the block of chroma transform coefficients using a first scaling value from the chroma scaling matrix and based on the QP value;
• dequantize the second chroma transform coefficient of the block of chroma transform coefficients using a second scaling value from the chroma scaling matrix and based on the QP value, wherein the second scaling value is different than the first scaling value;
• determine a chroma residual block for the block based on the first dequantized chroma transform coefficients and the second dequantized chroma transform coefficient;
• add the chroma residual block to a chroma prediction block to determine a decoded chroma block;
• and
• output decoded video data that includes the decoded chroma block.