1. A method of processing video signals to achieve a visual effect involving three-dimensional manipulation of an input image includes determining whether in a resulting transformed image the front or the back of the input image should be displayed by the steps of: defining an incremental surface in said transformed image by at least three adjacent non-collinear pixel addresses from said transformed image; determining from said transformed pixel addresses whether a vector normal to said incremental surface points towards or away from a viewing point; and selecting for display at one of said transformed pixel addresses the corresponding portion of the front or the back of said input image in dependence on whether said vector points towards or away from said viewing point.