Image coding apparatus, method for coding image, program therefor, image decoding apparatus, method for decoding image, and program therefor

A method for an image decoding apparatus for decoding quantization parameters for subblocks in a block representing a part of an image, wherein the block is divided into subblocks so as to form a quadtree, the method comprising:

• decoding a first quantization parameter for a first subblock, coded data related to a first difference value between the first quantization parameter for the first subblock and a second quantization parameter for a second subblock, and coded data related to a second difference value between a third quantization parameter for a third subblock and a rounding value of an average of quantization parameters for subblocks in the block, wherein the first subblock, the second subblock, and the third subblock are generated by dividing the block so as to form a quadtree;
• acquiring the second quantization parameter based on the first quantization parameter and the first difference value decoded by the decoding, and acquiring the third quantization parameter based on the second difference value decoded by the decoding and the rounding value of the average of quantization parameters for subblocks decoded before the third subblock, and
• reproducing parts of an image corresponding to the first subblock, the second subblock, and the third subblock, respectively, using the acquired first quantization parameter, the acquired second quantization parameter, and the acquired third quantization parameter.