Spatial prediction method, image decoding method, and image coding method

A non-transitory recording medium having a computer program stored thereon, which codes an image per block included in the image, the computer program causing a processor to execute operations comprising:

• calculating an integer slope indicating an integer value of a slope, based on at least one of a horizontal gradient and a vertical gradient;
• determining, for each of pixel positions within a current block to be coded, a sub-pel position that is an intersection between a line having the integer slope and passing through the pixel position within the current block and a boundary of a block adjacent to the current block;
• generating a predicted block by predicting, for each of the pixel positions within the current block, a pixel value at the pixel position, based on a pixel value interpolated in the sub-pel position determined for the pixel position;
• and
• coding the current block by subtracting the predicted block from the current block,
• wherein in the calculating, a result of a division with reference to a division table stored in a memory is obtained, and the integer slope is calculated using the obtained result of the division, the division using, as a divisor, a value indicating the one of the horizontal gradient and the vertical gradient, the division table indicating, for each predetermined value, a result of a division of a value scaled by a c-th power of 2, using the predetermined value as a divisor, where c is a positive integer, and
• wherein in the generating, the pixel value interpolated in the sub-pel position is calculated by multiplying the integer slope generated by scaling by a coordinate value horizontal or vertical to a pixel position to be predicted within the current block, and shifting the multiplied integer slope right by c bits.