1. A method for applying a motion effect using two input images, comprising: generating a single channel image for each of two input images according to a function that measures, for each pixel, occurrence of a desired characteristic, other than luminance alone, in the input images at each pixel location to provide a single value for each output pixel in the single channel image from a range of values that represent a likelihood of the occurrence of the desired characteristic; computing an estimate of motion of the desired characteristic between the two input images based on the single channel images generated for the two input images; and processing at least one of the two input images to generate an output image that includes a motion-based effect, wherein the processing uses the computed estimate of motion of the desired characteristic.