Encoder, decoder, encoding method, and decoding method

A non-transitory medium storing a bitstream and readable by a computer,

• the bitstream including at least a bit depth of an image to perform a process of determining a transform curve, wherein
• the computer:
• performs a mapping process of Luma Mapping with Chroma Scaling (LMCS), using line segments forming the transform curve, the mapping process being for transforming a first pixel value space applied to a luma display image signal into a second pixel value space applied to a luma encoding process signal, the line segments each corresponding to a different one of sections obtained by partitioning the first pixel value space;
• and
• decodes the image, and
• in the performing of the LMCS, the computer determines the transform curve so that among boundary values in the second pixel value space which are located at boundaries between the line segments, a first value obtained by dividing a boundary value by a base width is not equal to a second value obtained by dividing another boundary value by the base width, the base width being changed according to the bit depth of the image.