The visual masking effects make the signal distortion(noise) at the pixels (picture elements) with luminance gradient be less perceptible to human eyes. The effect is investigated with the gradient being extended to temporal direction in addition to spatial gradients. It is measured in the form of visibility function of additive distortion(noise) for pixels of level of gradients. The characteristics of the spatio-temporal visibility function are compared with the spatial visibility function for two test image sequences.
The visibility function are applied in the design of as adaptive quantization of interframe DPCM(differential PCM) for image sequence. The mean square subjective distortion based on the spatio-temporal visibility function is found to be an effective measure of subjective test, but the adavantage over spatial visibility function is not remarkable.
