1. Image processing apparatus in which output pixels of an output image are generated from one or more input images using a set of motion vectors having a sub-pixel accuracy, the apparatus comprising: a motion vector allocator for allocating motion vectors in the set to pixels of the output image, the motion vector allocator being arranged to compare a current output pixel with test image areas pointed to by motion vectors in the set to detect a most suitable motion vector for the current output pixel; and a pixel generator for generating the output pixels, the pixel generator operating in a first mode if the output pixel is within a threshold displacement from an image region having substantially different motion characteristics to those of that output pixel, and in a second mode otherwise, the pixel generator comprising a spatial filter for generating an output pixel value at a required pixel position to a sub-pixel accuracy; in which: the pixel generator is operable, in the first mode, to apply its spatial filter to a group of pixel values derived from the input image(s) such that the group does not extend into an image area having substantially different motion characteristics, at least one such pixel value being applied to more than one non-zero filter tap; and the pixel generator is operable, in the second mode, to apply its spatial filter to a group of pixel values derived from the input image(s), there being one such pixel value for each non-zero filter tap.