Patent ID: 8224105

Claim:
An image coding method using an image coding device, in which one of a plurality of orthogonal transform sizes is selected per block, and a coding target block image is coded by an orthogonal transform based on the selected orthogonal transform size, the image coding method including: an edge detection step which detects an edge of the coding target block image and outputs a detection result thereby obtained as edge information; an edge decision step which compares the edge information to an edge decision threshold value to decide whether there is the edge in the coding target block image; a flat portion detection step which detects a flat portion in the coding target block image after the edge decision step decides that the coding target block image includes the edge and outputs a detection result thereby obtained as flat portion information; a flat portion decision step which compares the flat portion information to a flat portion decision threshold value to decide whether there is the flat portion in the coding target block image; a first orthogonal transform size selection step which divides the plurality of orthogonal transform sizes into a first group of orthogonal transform sizes including one or a plurality of orthogonal transform sizes and a second group of orthogonal transform sizes including one or a plurality of orthogonal transform sizes greater than the first group of orthogonal transform sizes, the first orthogonal transform size selection step further selecting an orthogonal transform based on one of the first group of orthogonal transform sizes for the coding target block image after the flat portion decision step decides that the coding target block image includes the flat portion; and a second orthogonal transform size selection step which selects an orthogonal transform based on one of the second group of orthogonal transform sizes for the coding target block image after the edge decision step decides that the coding target block image does not include the edge or after the flat portion decision step decides that the coding target block image does not include the flat portion.