An encoding method, a decoding method, a bitstream, an encoder, a decoder, and a storage medium are disclosed in this disclosure. At the encoding/decoding end, multiple candidate template regions used for an illuminance compensation mode for a current block are determined. A target template region corresponding to a minimum cost value is determined by performing cost calculation on the multiple candidate template regions based on template matching. A target illuminance compensation model parameter is determined according to the target template region. A second prediction block of a current block is determined by performing illuminance compensation on a first prediction block of the current block according to the target illuminance compensation model parameter.