An example method of video processing includes determining, for a conversion between a current picture of a video and a bitstream of the video, a scaling ratio based on a first window in the current picture and a second window in a reference picture associated with the current picture; resampling the reference picture by using the determined scaling ratio; and performing the conversion based on the result of the resampling, wherein a dimension of the first window is W×H and a dimension of the second window is W′×H′, where W, H, W′ and H′ are integers, and wherein W, H, W′ and H′ satisfy a constraint.