A method for video processing includes performing a conversion between a current video block and a bitstream representation of the current video block using an inter-prediction coding scheme based on an n-order polynomial model using a first variable associated with the current video block, where n is an integer greater than or equal to zero, wherein the conversion includes generating the current video block from the bitstream representation or generating the bitstream representation from the current video block; and performing illumination compensation for the current video block using the n-order polynomial model during the conversion.