CCLM-BASED INTRA-PREDICTION METHOD AND APPARATUS THEREFOR
A picture decoding method, performed by a decoding apparatus according to the present dislosure, comprises the steps of: when an intra-prediction mode of a chroma block is a cross-component linear model (CCLM) mode, deriving neighboring chroma reference samples with respect to the chroma block; deriving neighboring luma reference samples of a luma block corresponding to the chroma block and luma samples in the luma block; deriving down-sampled neighboring luma reference samples and downsampled luma samples by down sampling the neighboring luma reference samples and the luma samples, respectively; deriving a linear model parameter on the basis of the down-sampled neighboring luma reference samples and the neighboring chroma reference samples; generating prediction samples with respect to the chroma block on the basis of the linear model parameter and the down-sampled luma samples of the luma block; and reconstructing the chroma block on the basis of the prediction samples with respect to the chroma block.
1 . A decoding apparatus for an image decoding, the decoding apparatus comprising:
a memory; and
at least one processor connected to the memory, the at least one processor configured to:
obtain residual information for a chroma block;
derive neighboring chroma reference samples for the chroma block based on an intra prediction mode of the chroma block being a cross-component linear model (CCLM) mode;
derive neighboring luma reference samples of a luma block corresponding to the chroma block and luma samples in the luma block;
derive downsampled neighboring luma reference samples and downsampled luma samples by downsampling the neighboring luma reference samples and the luma samples;
derive a linear model parameter based on the downsampled neighboring luma reference samples and the neighboring chroma reference samples;
generate prediction samples for the chroma block based on the linear model parameter and the downsampled luma samples of the luma block;
generate residual samples for the chroma block based on the residual information for the chroma block;
reconstruct the chroma block based on the prediction samples for the chroma block and the residual samples for the chroma block; and
perform a deblocking filtering on a reconstructed picture including the chroma block,
wherein the neighboring luma reference samples include top neighboring luma reference samples located above a top boundary of the luma block,
wherein the downsampled neighboring luma reference samples include a downsampled top neighboring luma reference sample, and
wherein based on the top boundary of the luma block being overlapped with a boundary of a coding tree unit (CTU), the downsampled top neighboring luma reference sample is derived based on an equation below,
pTopDsY
[
x
]
=
(
pY
[
2
*
x
-
1
]
[
-
1
]
+
2
*
pY
[
2
*
x
]
[
-
1
]
+
pY
[
2
*
x
+
1
]
[
-
1
]
+
2
)
>>
2
,
where pTopDsY[x] represents the downsampled top neighboring luma reference sample, x represents an x-axis direction position of the downsampled top neighboring luma reference sample, pY[2*x−1][−1], pY[2*x][−1] and pY[2*x+1][−1] represent sample values of the top neighboring luma reference samples, respectively, and a y-axis direction position of the top neighboring luma reference samples is −1.
2 . An encoding apparatus for an image encoding, the encoding apparatus comprising:
a memory; and
at least one processor connected to the memory, the at least one processor configured to:
derive neighboring chroma reference samples for a chroma block based on an intra prediction mode of the chroma block being a cross-component linear model (CCLM) mode;
derive neighboring luma reference samples of a luma block corresponding to the chroma block and luma samples in the luma block;
derive downsampled neighboring luma reference samples and downsampled luma samples by downsampling the neighboring luma reference samples and the luma samples;
derive a linear model parameter based on the downsampled neighboring luma reference samples and the neighboring chroma reference samples;
generate prediction samples for the chroma block based on the linear model parameter and the downsampled luma samples of the luma block;
derive residual samples for the chroma block based on the prediction samples for the chroma block; and
encode image information including residual information related to the residual samples for the chroma block,
wherein the neighboring luma reference samples include top neighboring luma reference samples located above a top boundary of the luma block,
wherein the downsampled neighboring luma reference samples include a downsampled top neighboring luma reference sample, and
wherein based on the top boundary of the luma block being overlapped with a boundary of a coding tree unit (CTU), the downsampled top neighboring luma reference sample is derived based on an equation below,
pTopDsY
[
x
]
=
(
pY
[
2
*
x
-
1
]
[
-
1
]
+
2
*
pY
[
2
*
x
]
[
-
1
]
+
pY
[
2
*
x
+
1
]
[
-
1
]
+
2
)
>>
2
,
where pTopDsY[x] represents the downsampled top neighboring luma reference sample, x represents an x-axis direction position of the downsampled top neighboring luma reference sample, pY[2*x−1][−1], pY[2*x][−1] and pY[2*x+1][−1] represent sample values of the top neighboring luma reference samples, respectively, and a y-axis direction position of the top neighboring luma reference samples is −1.
3 . An apparatus for transmitting data for an image, the apparatus comprising:
at least one processor configured to obtain a bitstream for the image, wherein the bitstream is generated based on deriving neighboring chroma reference samples for a chroma block based on an intra prediction mode of the chroma block being a cross-component linear model (CCLM) mode, deriving neighboring luma reference samples of a luma block corresponding to the chroma block and luma samples in the luma block, deriving downsampled neighboring luma reference samples and downsampled luma samples by downsampling the neighboring luma reference samples and the luma samples, deriving a linear model parameter based on the downsampled neighboring luma reference samples and the neighboring chroma reference samples, generating prediction samples for the chroma block based on the linear model parameter and the downsampled luma samples of the luma block, deriving residual samples for the chroma block based on the prediction samples for the chroma block, and encoding image information including residual information related to the residual samples for the chroma block to output the bitstream; and
a transmitter configured to transmit the data comprising the bitstream,
wherein the neighboring luma reference samples include top neighboring luma reference samples located above a top boundary of the luma block,
wherein the downsampled neighboring luma reference samples include a downsampled top neighboring luma reference sample, and
wherein based on the top boundary of the luma block being overlapped with a boundary of a coding tree unit (CTU), the downsampled top neighboring luma reference sample is derived based on an equation below,
pTopDsY
[
x
]
=
(
pY
[
2
*
x
-
1
]
[
-
1
]
+
2
*
pY
[
2
*
x
]
[
-
1
]
+
pY
[
2
*
x
+
1
]
[
-
1
]
+
2
)
>>
2
,
where pTopDsY[x] represents the downsampled top neighboring luma reference sample, x represents an x-axis direction position of the downsampled top neighboring luma reference sample, pY[2*x−1][−1], pY[2*x][1] and pY[2*x+1][−1] represent sample values of the top neighboring luma reference samples, respectively, and a y-axis direction position of the top neighboring luma reference samples is −1.