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

An image encoder comprising:

• memory;
• and
• circuitry coupled to the memory, wherein
• in operation, the circuitry:
• generates a prediction block by performing an interpolation process using a value of a sample included in a reference picture;
• generates a gradient block using the prediction block, the gradient block having a size that is same as a size of the prediction block;
• generates a predicted image using the gradient block;
• and
• encodes a current block based on the predicted image, wherein
• the generation of the gradient block includes a process of calculating a gradient value indicating a difference between a value of a lower sample and a value of an upper sample, the lower sample being adjacent to a bottom of a target sample, the upper sample being adjacent to a top of the target sample, the target sample being included in the prediction block,
• a first gradient value located on an upper edge of the gradient block is calculated using, as the value of the upper sample, a value of a first sample which is co-located with the first gradient value in the prediction block, and
• a second gradient value that is adjacent to a bottom of the first gradient value is calculated using the value of the first sample as the value of the upper sample.