Wavelet coefficient quantization to produce equivalent visual distortions in complex stimuli

This paper describes the results of a psychophysical experiment to quantify sensitivities to supra-threshold distortion caused by wavelet coefficient quantization. Natural images were selected for their energy distributions, subject matter, and edge/detail content. A single subband was quantized and the quantization step size to produce a minimum noticeable distortion (denoted the MNDSS) was recorded; these step sizes are defined to produce a visual distortion unit. The test results indicate that the MNDSS are strongly correlated with band energy, and that at a single frequency, the VDUs for the horizontal and vertical orientations occur when the squared error in the image is constant. Because the quantizers are not operating in the granular region, however, this does not imply that equal step sizes in the two orientations at the same frequency produce equivalent visual distortion, in contrast with common sub-threshold quantization assumptions. Furthermore, contrast sensitivity curves based on these results indicate an increased sensitivity to the orientation with higher energy at each frequency; neither orientation is favored when there is no inequality in energy. The results provide evidence of both orientation-dependent and content-dependent masking in supra-threshold quantization.(1).