Image encoding device, image decoding device, and image processing method

1. An image encoding device that performs, in an image, an intra-prediction on a block obtained by dividing the image so as to encode the block, the image encoding device comprising:

a memory; and

a processor coupled to the memory and the processor configured to:

determine whether the block is a rectangular block; and

select a certain prediction direction as a selection target intra-prediction direction when the block is the rectangular block, wherein the certain prediction direction is one of prediction directions in which a pixel that is adjacent to a short side of the rectangular block is referred to, and an adjacent pixel in a left or upper block that is adjacent to a long side of the rectangular block is referred to in another prediction direction that is 180 degrees opposite to the certain prediction direction.

2. The image encoding device according to claim 1 , wherein

when the rectangular block has a vertically long shape, the adjacent pixel is included in the left block that is adjacent to the long side of the rectangular block or to an extension obtained by extending the long side in a vertically downward direction, and when the rectangular block has a horizontally long shape, the adjacent pixel is included in a upper block that is adjacent to the long side of the rectangular block or to an extension obtained by extending the long side in a horizontally right direction.

3. The image encoding device according to claim 1 , wherein

with respect to the rectangular block, the processor determines an intra-prediction direction from lower left to upper right or an intra-prediction direction from upper right to lower left to be the selection target intra-prediction direction.

4. The image encoding device according to claim 3 , wherein

when a prediction target pixel in the rectangular block is used as a reference, the intra-prediction direction from lower left to upper right is a direction of a counterclockwise angle between 180 degrees and 225 degrees, with the horizontally right direction being 0 degrees, and the intra-prediction direction from upper right to lower left is a direction of a counterclockwise angle between 45 degrees and 90 degrees, with the horizontally right direction being 0 degrees.

5. The image encoding device according to claim 2 , wherein

with respect to at least the certain intra-prediction direction that has been added as a selection target intra-prediction direction and the another intra-prediction direction that is 180 degrees opposite to the added certain prediction direction, the processor calculates a sum of distances between all of the prediction target pixels in the rectangular block and respective adjacent pixels in the intra-prediction direction from lower left to upper right, and a sum of distances between all of the prediction target pixels in the rectangular block and respective adjacent pixels in the intra-prediction direction from upper right to lower left, and determines, as a result of selecting the intra-prediction direction, an intra-prediction direction that corresponds to a smaller sum from among the calculated sums of distances.

6. The image encoding device according to claim 2 , wherein

with respect to at least the certain intra-prediction direction that has been added as a selection target intra-prediction direction and the another intra-prediction direction that is 180 degrees opposite to the added certain prediction direction, the processor calculates a sum of distances between prediction target pixels on a upper left corner and on a lower right corner in the rectangular block and respective adjacent pixels in the intra-prediction direction from lower left to upper right, and a sum of distances between the prediction target pixels on the upper left corner and on the lower right corner in the rectangular block and respective adjacent pixels in the intra-prediction direction from upper right to lower left, and determines, as a result of selecting the intra-prediction direction, an intra-prediction direction that corresponds to a smaller sum from among the calculated sums of distances.

7. The image encoding device according to claim 2 , wherein

with respect to at least the certain intra-prediction direction that has been added as a selection target intra-prediction direction and the another intra-prediction direction that is 180 degrees opposite to the added certain prediction direction, the processor calculates a distance between a prediction target pixel on a lower right corner in the rectangular block and a respective adjacent pixel in the intra-prediction direction from lower left to upper right, and a distance between the prediction target pixel on the lower right corner in the rectangular block and a respective adjacent pixel in the intra-prediction direction from upper right to lower left, and determines, as a result of selecting the intra-prediction direction, an intra-prediction direction that corresponds to a smaller distance from among the calculated distances.

8. The image encoding device according to claim 1 , wherein

in a process of selecting the intra-prediction direction when the rectangular block is selected, the processor does not add a certain prediction direction as the selection target intra-prediction direction, wherein the certain prediction direction is one of prediction directions in which a pixel adjacent to the short side of the rectangular block is referred to, an adjacent pixel in a left or upper block that is adjacent to the long side of the rectangular block is referred to in another prediction direction that is 180 degrees opposite to the certain prediction direction, and a pixel in a left or upper block that is adjacent to the short side of the rectangular block is referred to in the another prediction direction.

9. An image decoding device that performs, in an image, an intra-prediction on a block obtained by dividing the image so as to decode the block, the image decoding device comprising:

a memory; and

a processor coupled to the memory and the processor configured to:

determine whether the block is a rectangular block; and

select a certain prediction direction as a selection target intra-prediction direction when an intra-prediction direction used when the block the rectangular block,

wherein the certain prediction direction is one of prediction directions in which a pixel that is adjacent to a short side of the rectangular block is referred to, and an adjacent pixel in a left or upper block that is adjacent to a long side of the rectangular block is referred to in another prediction direction that is 180 degrees opposite to the certain prediction direction.