1. A circuit implemented method for encoding an input signal representing at least a portion of an image, the method comprising: (a) filtering the input signal with a circuit implemented filter to obtain a first set of spectral functions; and (b) based on a set of at least one perceptual threshold signal, generating a quantized output signal with a quantizer, the quantized output signal being representative of at least one of the first set of spectral functions, each said perceptual threshold signal representing a measure of human visual sensitivity to distortion in the first set of spectral functions, the perceptual threshold signal being generated in accordance with a perceptual model, the perceptual model being responsive to: (i) at least one of the first set of spectral functions; and (ii) a second set of spectral functions representing a plurality of other images.