Intra prediction-based video encoding/decoding method and device

1. A method of decoding an image based on intra prediction with a decoding apparatus, comprising:

determining a reference region for the intra prediction of a current block in the image;

selecting, based on prediction mode candidate group information signaled from a bitstream, one of a first prediction mode candidate group and a second prediction mode candidate group, wherein a number of intra prediction modes in the first prediction mode candidate group is different from a number of intra prediction modes in the second prediction mode candidate group;

selecting, based on MPM information, one of a first candidate group, a second candidate group and a third candidate group that are divided from the selected prediction mode candidate group;

determining an intra prediction mode of the current block from the selected candidate group; and

predicting the current block based on the determined intra prediction mode,

wherein the reference region is spatially adjacent to the current block and includes at least one of a top reference region, a left reference region, a top-left reference region, a bottom-left reference region, or a top-right reference region,

wherein determining the reference region comprises searching for whether an unavailable pixel exists in the reference region,

wherein the search on the left reference region is performed from top to bottom,

wherein when the unavailable pixel is found during the search on the left reference region, the unavailable pixel is replaced with a pixel above the unavailable pixel, and

wherein the first candidate group includes an intra prediction mode of a neighboring block adjacent to the current block, the second candidate group includes an intra prediction mode with a diagonal direction and an intra prediction mode derived by adding or subtracting a predetermined constant to an index of the intra prediction mode of the neighboring block, and the third candidate group includes remaining modes except for intra prediction modes in the first candidate group and the second candidate group among intra prediction modes belonging to the selected prediction mode candidate group.

2. The method of claim 1 , wherein the current block is obtained through a block division based on at least one of a binary tree or a ternary tree, and

wherein the binary tree is representative of a division type of dividing one coding block into two coding blocks, and the ternary tree is representative of a division type of dividing one coding block into three coding blocks.

3. The method of claim 1 , wherein the reference region further includes at least one of a right reference region or a bottom-right reference region according to a predetermined coding order.

4. The method of claim 1 , wherein the predetermined constant is a natural number greater than or equal to 5.

5. A method of encoding an image based on intra prediction with an encoding apparatus, comprising:

determining a reference region for the intra prediction of a current block in the image;

selecting one of a first prediction mode candidate group and a second prediction mode candidate group and encoding prediction mode candidate group information indicating the selected prediction mode candidate group, wherein a number of intra prediction modes in the first prediction mode candidate group is different from a number of intra prediction modes in the second prediction mode candidate group; and

selecting one of a first candidate group, a second candidate group and a third candidate group that are divided from the selected prediction mode candidate group and encoding MPM information indicating the selected candidate group;

predicting the current block based on the reference region and an intra prediction mode from the selected candidate group,

wherein the reference region is spatially adjacent to the current block and includes at least one of a top reference region, a left reference region, a top-left reference region, a bottom-left reference region, or a top-right reference region,

wherein determining the reference region comprises searching for whether an unavailable pixel exists in the reference region,

wherein the search on the left reference region is performed from top to bottom,

wherein when the unavailable pixel is found during the search on the left reference region, the unavailable pixel is replaced with a pixel above the unavailable pixel,

wherein the first candidate group includes an intra prediction mode of a neighboring bock adjacent to the current block, the second candidate group includes an intra prediction mode with a diagonal direction and an intra prediction mode derived by adding or subtracting a predetermined constant to an index of the intra prediction mode of the neighboring block, and the third candidate group includes remaining modes except for intra prediction modes in the first candidate group and the second candidate group among intra prediction modes belonging to the selected prediction mode candidate group.

6. A non-transitory computer-readable storage medium having a bitstream that is decoded by a method of decoding an image, wherein the bitstream comprises:

prediction mode candidate group information indicating a prediction mode candidate group for decoding a block;

MPM information indicating a candidate group for decoding the block;

wherein the method comprises:

determining a reference region for the intra prediction of a current block in the image;

selecting, based on the prediction mode candidate group information, one of a first prediction mode candidate group and a second prediction mode candidate group, wherein a number of intra prediction modes in the first prediction m candidate group is different from a number of intra prediction modes in the second prediction mode candidate group;

selecting, based on the MPM information, one of a first candidate group a second candidate group and a third candidate group that are divided from the selected prediction mode candidate group;

determining an intra prediction mode of the current block from the selected candidate group; and

predicting the current block based on the determined intra prediction mode,

wherein the reference region is spatially adjacent to the current block and includes at least one of a top reference region, a left reference region, a top-left reference region, a bottom-left reference region, or a top-right reference region,

wherein determining the reference region comprises searching for whether an unavailable pixel exists in the reference region,

wherein the search on the left reference region is performed from top to bottom,

wherein when the unavailable pixel is found during the search on the left reference region, the unavailable pixel is replaced with a pixel above the unavailable pixel, and

wherein the first candidate group includes an intra prediction mode of a neighboring block adjacent to the current block, the second candidate group includes an intra prediction mode with a diagonal direction and an intra prediction node derived by adding or subtracting a predetermined constant to an index of the infra prediction mode of the neighboring block, and the third candidate group includes remaining modes except for intra prediction modes in the first candidate group and the second candidate group among infra prediction modes belonging to the selected prediction mode candidate group.