Coding apparatus and method, and storage medium

An coding method that divides an input image into blocks and performs coding processing on each block of the input image, comprising:

• a step of executing coding processing on each block using one of prediction modes of
• an inter-prediction mode that generates an inter-predicted image by performing inter-prediction on a block and encodes a difference between the block and the inter-predicted image,
• an intra-prediction mode that generates an intra-predicted image by performing intra-prediction on a block and encodes a difference between the block and the intra-predicted image, and
• a multi-reference prediction mode that generates a multi-predicted image by synthesizing an inter-predicted image and an intra-predicted image generated from a block, and encodes a difference between the block and the multi-predicted image;
• a step of outputting a result of the coding processing as coded data;
• and
• a step of calculating a first coding cost in a case where the block is coded in the inter-prediction mode and a second coding cost in a case where the block is coded in the intra-prediction mode using a predetermined function,
• wherein the coding processing is executed on the block using the multi-reference prediction mode in a case where a difference between the first coding cost and the second coding cost is equal to or less than a threshold, and the coding processing is executed on the block using the prediction mode with a smaller one of the first coding cost and the second coding cost in a case where the difference between the first coding cost and the second coding cost is larger than the threshold.

A non-transitory computer-readable storage medium, the storage medium storing a program that is executable by the computer, wherein the program includes program code for causing the computer to function as an coding apparatus that divides an input image into blocks and performs coding processing on each block of the input image, comprising:

• a coding unit that executes coding processing on each block using one of prediction modes of
• an inter-prediction mode that generates an-inter predicted image by performing inter-prediction on a block and encodes a difference between the block and the inter-predicted image,
• an intra-prediction mode that generates an intra-predicted image by performing intra-prediction on a block and encodes a difference between the block and the intra-predicted image, and
• a multi-reference prediction mode that generates a multi-predicted image by combining an inter-predicted image and an intra-predicted image generated from a block, and encodes a difference between the block and the multi-predicted image;
• an output unit that outputs a result of the coding processing as coded data;
• and
• a calculation unit that calculates a first coding cost in a case where the block is coded in the inter-prediction mode and a second coding cost in a case where the block is coded in the intra-prediction mode using a predetermined function,
• wherein the coding unit executes the coding processing on the block using the multi-reference prediction mode in a case where a difference between the first coding cost and the second coding cost is equal to or less than a threshold, and executes the coding processing on the block using the prediction mode with a smaller one of the first coding cost and the second coding cost in a case where the difference between the first coding cost and the second coding cost is larger than the threshold.