Encoder, decoder, encoding method, decoding method, and recording medium

A decoder that decodes a current block in a current picture, the decoder comprising:

• a processor;
• and
• memory, wherein
• using the memory, the processor:
• derives two motion vectors indicating two prediction blocks included in two reference pictures referred to for the current block;
• obtains two prediction images from two reference pictures by performing motion compensation using the two motion vectors;
• obtains a gradient value of each of pixels included in the two prediction images, based on a pixel value of each of the pixels;
• derives a local motion estimation value for each of sub-blocks based on the pixel value and the gradient value of each of the pixels, the sub-blocks being obtained by partitioning the current block;
• and
• generates a final prediction image for the current block using the pixel value and the gradient value of each of the pixels, and the local motion estimation value derived for each of the sub-blocks, wherein
• each of the pixels in the two prediction images is interpolated with sub-pixel accuracy, and
• a reference range for the interpolation is included in a normal reference range that is referred to for motion compensation for the current block in normal inter prediction performed without using the local motion estimation value.

An encoding method for encoding a current block in a current picture, the encoding method comprising:

• deriving two motion vectors indicating two prediction blocks included in two reference pictures referred to for the current block;
• obtaining two prediction images from the two reference pictures by performing motion compensation using the two motion vectors;
• deriving a local motion estimation value for each of sub-blocks based on the pixel value and the gradient value of each of the pixels, the sub-blocks being obtained by partitioning the current block;
• and
• generating a final prediction image for the current block using the pixel value and the gradient value of each of the pixels, and the local motion estimation value derived for each of the sub-blocks, wherein
• each of the pixels in the two prediction images is interpolated with sub-pixel accuracy, and
• a reference range for the interpolation is included in a normal reference range that is referred to for motion compensation for the current block in normal inter prediction performed without using the local motion estimation value.

A decoding method for decoding a current block in a current picture, the decoding method comprising:

• deriving two motion vectors indicating two prediction blocks included in two reference pictures referred to for the current block;
• obtaining two prediction images from the two reference pictures by performing motion compensation using the two motion vectors;
• deriving a local motion estimation value for each of sub-blocks based on the pixel value and the gradient value of each of the pixels, the sub-blocks being obtained by partitioning the current block;
• and
• generating a final prediction image for the current block using the pixel value and the gradient value of each of the pixels, and the local motion estimation value derived for each of the sub-blocks, wherein
• each of the pixels in the two prediction images is interpolated with sub-pixel accuracy, and
• a reference range for the interpolation is included in a normal reference range that is referred to for motion compensation for the current block in normal inter prediction performed without using the local motion estimation value.

A non-transitory computer-readable recording medium for use in a computer, the recording medium having a computer program recorded thereon for causing the computer to execute:

• deriving two motion vectors indicating two prediction blocks included in two reference pictures referred to for the current block;
• obtaining two prediction images from the two reference pictures by performing motion compensation using the two motion vectors;
• deriving a local motion estimation value for each of sub-blocks based on the pixel value and the gradient value of each of the pixels, the sub-blocks being obtained by partitioning the current block;
• and
• generating a final prediction image for the current block using the pixel value and the gradient value of each of the pixels, and the local motion estimation value derived for each of the sub-blocks, wherein
• each of the pixels in the two prediction images is interpolated with sub-pixel accuracy, and
• a reference range for the interpolation is included in a normal reference range that is referred to for motion compensation for the current block in normal inter prediction performed without using the local motion estimation value.