1. A signal-processing system comprising: input means for receiving an input image signal S in ; output means for outputting an enhance image signal; and a processing component that computes upper-envelope and lower-envelope functions U and L with respect to the input image signal S in ; modifies the upper-envelope and lower-envelope functions U and L to produce a modified upper-envelope function U′ and a modified lower-envelope function L′; and employs the modified upper-envelope function U′ and the modified lower-envelope function L′ to generate an enhanced output image.