Suprathreshold wavelet coefficient quantization in complex stimuli: psychophysical evaluation and analysis

A psychophysical experiment is described that quantifies human sensitivities to suprathreshold distortions caused by wavelet coefficient quantization in natural images, and the resulting analysis is explained. The quantizer step sizes that cause the first three visible degradations relative to the original image are well predicted by exponential functions of subband standard deviation. The resulting root-mean-square (RMS) error in the image is constant for a spatial frequency and is independent of orientation. Contrast sensitivity calculations suggest a higher sensitivity to bands with higher energy, and threshold elevations for the second and third visible degradations are predicted well by the constant-RMS model. A quantization strategy based on the results is proposed for low-bit-rate applications.