Encoder which generates prediction image to be used to encode current block

A non-transitory computer readable medium storing a bitstream, the bitstream comprising a prediction parameter which indicates a prediction process among prediction process candidates and which is read by a decoder to decode a current block provided in the bitstream, the prediction process candidates including a bi-directional optical flow (BDOF) process in which a first parameter, a second parameter, a third parameter, a fourth parameter, and a fifth parameter are used to generate a prediction image of the current block, wherein the first parameter is derived based on the following expression:

• Σ[i,j]∈Ωabs(Ix1+Ix0) [Math. 1];
• the second parameter is derived based on the following expression:
• Σ[i,j]∈Ωabs(Iy1+Iy0) [Math. 2];
• the third parameter is derived based on the following expression:
• Σ[i,j]∈Ω(−sign(Ix1+Ix0)×(I0−I1)) [Math. 3];
• the fourth parameter is derived based on the following expression:
• Σ[i,j]∈Ω(−sign(Iy1+Iy0)×(I0−I1)) [Math. 4];
• and
• the fifth parameter is derived based on the following expression,
• Σ[i,j]∈Ω(sign(Iy1+Iy0)×(Ix1+Ix0)) [Math. 5],
• where:
• Ω indicates a set of relative pixel positions;
• [i, j] indicates a relative pixel position in the set Ω defined by a horizontal position i and a vertical position j;
• Ix0 indicates a horizontal gradient value at a first pixel position in a first gradient image, and Ix1 indicates a horizontal gradient value at the first pixel position in a second gradient image, the first pixel position being determined based on a relative pixel position [i, j], the first gradient image and the second gradient image corresponding to the current block;
• Iy0 indicates a vertical gradient value at the first pixel position in the first gradient image, and Iy0 indicates a vertical gradient value at the first pixel position in the second gradient image;
• I0 indicates a pixel value at the first pixel position in a first interpolated image corresponding to the current block;
• I1 indicates a pixel value at the first pixel position in a second interpolated image corresponding to the current block;
• the abs function outputs an absolute value of an argument;
• and
• the sign function outputs a sign of an argument which is −1, 0, or 1.