1. A computer implemented method of measuring the differences between a first video signal and a second video signal, said method comprising: processing the first and second video signals with a sensory emulation process and a subjective emulation process, the sensory emulation process modeling the physical sensitivity of the human visual system and the subjective emulation process modeling the subjective significance of any errors present in a video signal; analyzing the processed video signals to identify perceptually relevant boundaries and disregarding perceptually irrelevant boundaries of video images depicted therein; comparing only said perceptually relevant boundaries so defined in the first signal with those in the second signal and disregarding said perceptually irrelevant boundaries, the comparison including determination of the extent to which the properties of boundaries defined in the first image are preserved in the second image; and generating an output indicative of perceptual difference between the first and second signals.