Intra prediction method of chrominance block using luminance sample, and apparatus using same
Disclosed are an intra prediction method of a chrominance block using a luminance sample and an apparatus using the same. An image decoding method comprises the steps of: calculating an intra prediction mode of a chrominance block on the basis of an LM mapping table when the chrominance block uses an LM; and generating a prediction block for the chrominance block on the basis of the calculated intra prediction mode of the chrominance block. When intra prediction mode information of chrominance blocks are decoded, mutually different tables are used depending on whether or not an LM is used, so that encoding and decoding can be performed without an unnecessary waste of bits.
1. A picture decoding method performed by a decoding apparatus, the method comprising:
receiving a bitstream including index information on an intra chroma prediction mode for a current chroma block;
determining an intra luma prediction mode for a current luma block which is related to the current chroma block;
deriving the intra chroma prediction mode based on the intra luma prediction mode and the index information;
deriving a prediction sample for the current chroma block based on the derived intra chroma prediction mode; and
generating a reconstructed picture for the current chroma block based on the prediction sample,
wherein the index information is obtained based on a binary codeword for the index information,
wherein when a LM mode is applied to the current chroma block, a predicted sample among the predicted samples of the current chroma block is derived based on at least one of linear model parameters and a linearly interpolated reconstructed sample of the current luma block, and
wherein the LM mode is an intra prediction method of deriving the predicted samples of the current chroma block based on linearly interpolated reconstructed samples of the current luma block comprising the linearly interpolated reconstructed sample of the current luma block which is derived by linearly interpolating reconstructed samples of the current luma block.
2. The method of claim 1 , wherein the at least one of linear model parameters comprise a first linear model parameter and a second linear model parameter,
wherein the first linear model parameter is determined based on the linearly interpolated reconstructed samples of the current luma block, the reconstructed samples of the current luma block, and reconstructed samples of neighboring chroma block which is adjacent to the current chroma block, and
wherein the second linear model parameter is determined based on the reconstructed samples of the neighboring chroma block, the first linear model parameter and the linearly interpolated reconstructed samples of the current luma block.
3. The method of claim 1 , wherein the reconstructed samples of the current luma block comprise a first reconstructed sample of the current luma block which is located in a coordinate of (2x, 2y) according to a coordinate system and a second reconstructed sample of the current luma block which is located in a coordinate of (2x, 2y+1) according to the coordinate system,
wherein a top-left sample of the current luma block and a top-left sample of the current chroma block are located in a coordinate of (0, 0) and the predicted sample of the current chroma block is located in a coordinate of (x, y) according to the coordinate system,
wherein the linearly interpolated reconstructed sample of the current luma block is derived by linearly interpolating the first reconstructed sample of the current luma block and the second reconstructed sample of the current luma block.
4. The method of claim 1 , wherein when the LM mode is available for the current chroma block, the index information represents one of a first mode index which is related to a planar mode, a second mode index which is related to a vertical mode, a third mode index which is related to a horizontal mode, a fourth mode index which is related to a DC mode, a fifth mode index which is related to a DM mode, and a sixth mode index which is related to the LM mode, and
wherein the DM mode represents that an intra prediction mode of the current chroma block is same as the intra prediction mode of the current luma block.
5. The method of claim 4 , wherein the binary codeword related to the sixth mode index represents ‘10’.
6. The method of claim 4 , wherein first two bits of the binary codeword are decoded based on context-based decoding when the LM mode is available for the current chroma block.
7. The method of claim 4 , wherein the binary codeword is one of a binary codeword for the first mode index which represents ‘1100’, a binary codeword for the second mode index which represents ‘1101’, a binary codeword for the third mode index which represents ‘1110’, a binary codeword for the fourth mode index which represents ‘1111’, and a binary codeword for the fifth mode index which represents ‘0’.
8. The method of claim 1 , wherein when the LM mode is not available for the current chroma block, the index information represents one of a first mode index which is related to a planar mode, a second mode index which is related to a vertical mode, a third mode index which is related to a horizontal mode, a fourth mode index which is related to a DC mode, and a fifth mode index which is related to a DM mode, and
wherein the DM mode represents that an intra prediction mode of the current chroma block is same as the intra prediction mode of the current luma block.
9. The method of claim 8 , wherein first one bit of the binary codeword is decoded based on context-based decoding when the LM mode is not available for the current chroma block.
10. The method of claim 8 , wherein the binary codeword is one of a binary codeword for the first mode index which represents ‘100’, a binary codeword for the second mode index which represents ‘101’, a binary codeword for the third mode index which represents ‘110’, a binary codeword for the fourth mode index which represents ‘111’, and a binary codeword for the fifth mode index which represents ‘0’.
11. The method of claim 8 , wherein when the index information represents the first mode index and the intra prediction mode of the current luma block is the planar mode, the intra prediction mode of the current chroma block is a right-up diagonal intra mode,
wherein when the index information represents the first mode index and the intra prediction mode of the current luma block is an intra prediction mode other than the planar mode, the intra prediction mode of the current chroma block is the planar mode,
wherein when the index information represents the second mode index and the intra prediction mode of the current luma block is the vertical mode, the intra prediction mode of the current chroma block is the right-up diagonal intra mode,
wherein when the index information represents the second mode index and the intra prediction mode of the current luma block is an intra prediction mode other than the vertical mode, the intra prediction mode of the current chroma block is the vertical mode,
wherein when the intra chroma prediction mode information represents the third index and the intra prediction mode of the current luma block is the horizontal mode, the intra prediction mode of the current chroma block is the right-up diagonal intra mode,
wherein when the index information represents the third mode index and the intra prediction mode of the current luma block is an intra prediction mode other than the horizontal mode, the intra prediction mode of the current chroma block is the horizontal mode,
wherein when the index information represents the fourth mode index and the intra prediction mode of the current luma block is the DC mode, the intra prediction mode of the current chroma block is the right-up diagonal intra mode, and
wherein when the index information represents the fourth mode index and the intra prediction mode of the current luma block is an intra prediction mode other than the DC mode, the intra prediction mode of the current chroma block is the DC mode.
12. The method of claim 1 , further comprising receiving flag information indicating whether the LM mode is available for the current chroma block.
13. A picture encoding method performed by an encoding apparatus, the method comprising:
determining an intra luma prediction mode for a current luma block;
determining an intra chroma prediction mode for the current chroma block which is related to the current luma block;
generating index information on the intra chroma prediction mode based on the intra luma prediction mode and the intra chroma prediction mode; and
encoding image information including the index information on the intra chroma prediction mode to generate a bitstream,
wherein the index information is encoded based on a binary codeword,
wherein when a LM mode is available for the current chroma block, the index information represents one of a first mode index which is related to a planar mode, a second mode index which is related to a vertical mode, a third mode index which is related to a horizontal mode, a fourth mode index which is related to a DC mode, a fifth mode index which represents that an intra prediction mode of the current chroma block is same as the intra prediction mode of the current luma block, and a sixth mode index which is related to the LM mode, and
wherein the LM mode is an intra prediction method of deriving the predicted samples of the current chroma block based on linearly interpolated reconstructed samples of the current luma block which is derived by linearly interpolating reconstructed samples of the current luma block.
14. The method of claim 13 , wherein the at least one of linear model parameters comprise a first linear model parameter and a second linear model parameter,
wherein the first linear model parameter is determined based on the linearly interpolated reconstructed samples of the current luma block, the reconstructed samples of the current luma block, and reconstructed samples of neighboring chroma block which is adjacent to the current chroma block, and
wherein the second linear model parameter is determined based on the reconstructed samples of the neighboring chroma block, the first linear model parameter and the linearly interpolated reconstructed samples of the current luma block.
15. A non-transitory decoder-readable storage medium storing a bitstream generated by performing determining an intra luma prediction mode for a current luma block, determining an intra chroma prediction mode for the current chroma block which is related to the current luma block, generating index information on the intra chroma prediction mode based on the intra luma prediction mode and the intra chroma prediction mode and encoding image information including the index information on the intra chroma prediction mode to generate a bitstream, wherein the index information is encoded based on a binary codeword, wherein when a LM mode is available for the current chroma block, the index information represents one of a first mode index which is related to a planar mode, a second mode index which is related to a vertical mode, a third mode index which is related to a horizontal mode, a fourth mode index which is related to a DC mode, a fifth mode index which represents that an intra prediction mode of the current chroma block is same as the intra prediction mode of the current luma block, and a sixth mode index which is related to the LM mode, and wherein the LM mode is an intra prediction method of deriving the predicted samples of the current chroma block based on linearly interpolated reconstructed samples of the current luma block which is derived by linearly interpolating reconstructed samples of the current luma block.