1. An 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 processor configured to: allocate 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 generate the output pixels, the processor 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, by applying a first spatial filter for generating an output pixel value at a required pixel position to a sub-pixel accuracy; in which: in the first mode, the first spatial filter is applied to a group of pixel values derived from the input image(s) such that the group does not extend into an image area having different motion characteristics to those of the output pixel by re-using pixel values from an image area having the same motion characteristics as those of the output pixel, so that at least one such pixel value is applied to more than one non-zero filter tap of the first spatial filter; and in the second mode, the first spatial filter is applied to a group of pixel values derived from the input image(s), there being one such pixel value for each non-zero filter tap.