A special effects system has an image processing system (50) to receive an input image signal representing an image to be folded and to provide a folded output image signal. A key processing system (80) receives an input key signal to define a portion of the image selected for viewing. It operates in parallel with the image processing system to generate an output key signal which has undergone the same selected spatial transformation relative to the input key signal as did the image. A spatial transformation system (26,800,900) operates on the input image signal so as to be able to spatially transform the input image signal on a folded curved surface. The spatial transformation system can be arranged as a separable, two pass transformation process and functions either with h-pass first operations, followed by v-pass operations or vice versa.