Image encoding/decoding method and device

1. An image decoding method, comprising:

determining an intra prediction mode of a chroma block;

determining a neighboring region for inter-component reference of the chroma block based on the intra prediction mode, the neighboring region including at least one of a top neighboring region or a left neighboring region adjacent to a luma block corresponding to the chroma block;

deriving down-sampled neighboring luma samples by down-sampling neighboring luma samples in the neighboring region,

deriving a parameter for the inter-component reference of the chroma block based on the down-sampled neighboring luma samples; and

predicting the chroma block based on the parameter for the inter-component reference,

wherein deriving the parameter for the inter-component reference comprises:

classifying the down-sampled neighboring luma samples into a first group and a second group, the first group including down-sampled neighboring luma samples having relatively large values and the second group including down-sampled neighboring luma samples having relatively small values; and

calculating a maximum value by averaging the down-sampled neighboring luma samples included in the first group and a minimum value by averaging the down-sampled neighboring luma samples included in the second group,

wherein the parameter for the inter-component reference is derived based on a difference between the maximum value and the minimum value,

wherein when the top neighboring region belongs to a different coding tree unit from the luma block, only three neighboring luma samples are used to derive one down-sampled neighboring luma sample, and

wherein when the top neighboring region belongs to a same coding tree unit as the luma block, five or six neighboring luma samples are used to derive the one down-sampled neighboring luma sample.

2. The method of claim 1 , wherein the intra prediction mode of the chroma block is derived based on an index and a predetermined group configured with inter-component reference-based prediction modes, and

wherein the inter-component reference-based prediction modes includes a first mode which refers to both left and top neighboring regions adjacent to the chroma block, a second mode which refers to only the left neighboring region adjacent to the chroma block and a third mode which refers to only the top neighboring region adjacent to the chroma block.

3. The method of claim 1 ,

wherein the top neighboring region includes N sample lines and the left neighboring luma region includes M sample lines, and

wherein N is set to a different value than M.

4. The method of claim 1 , wherein the down-sampling is performed by using only a neighboring luma sample at a specific position and a-neighboring samples adjacent to the neighboring luma sample.

5. The method of claim 4 , wherein the specific position is determined based on a position of one or more pixels selected among a plurality of samples belonging to a neighboring region of the chroma block.

6. The method of claim 5 , wherein the selected one or more pixels are one or more pixels positioned at every predetermined interval in the neighboring region of the chroma block.

7. The method of claim 4 , wherein the neighboring samples include a sample positioned in at least one of a left, right, top, bottom, top-left, bottom-left, top-right or bottom-right direction of the neighboring luma sample.

8. An image encoding method, comprising:

determining an intra prediction mode of a chroma block;

determining a neighboring region for inter-component reference of the chroma block, the neighboring region including at least one of a top neighboring region or a left neighboring region adjacent to a luma block corresponding to the chroma block;

deriving down-sampled neighboring luma samples by down-sampling neighboring luma samples in the neighboring region,

deriving a parameter for the inter-component reference of the chroma block based on the down-sampled neighboring luma samples; and

predicting the chroma block based on the parameter for the inter-component reference,

wherein deriving the parameter for the inter-component reference comprises:

classifying the down-sampled neighboring luma samples into a first group and a second group, the first group including down-sampled neighboring luma samples having relatively large values and the second group including down-sampled neighboring luma samples having relatively small values; and

calculating a maximum value by averaging the down-sampled neighboring luma samples included in the first group and a minimum value by averaging the down-sampled neighboring luma samples included in the second group,

wherein the parameter for the inter-component reference is derived based on a difference between the maximum value and the minimum value,

wherein when the top neighboring region belongs to a different coding tree unit from the luma block, only three neighboring luma samples are used to derive one down-sample neighboring luma sample, and

wherein when the top neighboring region belongs to a same coding tree unit as the luma block, five or six neighboring luma samples are used to derive the one down-sample neighboring luma sample.

9. A non-transitory computer-readable medium storing a bitstream generated by an image encoding method, the image encoding method comprising:

determining an intra prediction mode of a chroma block based on the intra prediction mode;

determining a neighboring region for inter-component reference of the chroma block, the neighboring region including at least one of a top neighboring region or a left neighboring region adjacent to a luma block corresponding to the chroma block;

deriving down-sampled neighboring luma samples by down-sampling neighboring luma samples in the neighboring region;

deriving a parameter for the inter-component reference of the chroma block based on the down-sampled neighboring luma samples; and

predicting the chroma block based on the parameter for the inter-component reference,

wherein deriving the parameter for the inter-component reference comprises:

classifying the down-sampled neighboring luma samples into a first group and a second group, the first group including down-sampled neighboring luma samples having relatively large values and the second group including down-sampled neighboring luma samples having relatively small values; and

calculating a maximum value by averaging the down-sampled neighboring luma samples included in the first group and a minimum value by averaging the down-sampled neighboring luma samples included in the second group,

wherein the parameter for the inter-component reference is derived based on a difference between the maximum value and the minimum value,

wherein when the top neighboring region belongs to a different coding tree unit from the luma block, only three neighboring luma samples are used to derive one down-sample neighboring luma sample, and

wherein when the top neighboring region belongs to a same coding tree unit as the luma block, five or six neighboring luma samples are used to derive the one down-sample neighboring luma sample.