Effective gamma, global brightness contrast and perceptual image quality

The perceptual image quality of different scenes as a function of the gamma parameter of the imaging chain, determined by the slope in the inflection point of the luminance reproduction function, has a definite optimum. Although the optimal gamma values were found to be subject-independent, they did vary from scene to scene. If gamma is varied, global brightness contrast is clearly the changing perceptual attribute. Scaled global brightness contrast as a function of gamma also appears different for different scenes, while subjects are consistent. If scaled perceptual quality is plotted against scaled global brightness contrast, however, the curves of the different scenes tend to coincide. This confirms earlier results indicating that global brightness contrast is a dominant sensorial dimension of perceptual image quality. The scene dependence of the optimal gamma values was the immediate motive of the present study. An effective gamma can be defined taking into account the distribution of the luminance levels of the scene elements with respect to the luminance-reproduction curve. If this gamma is used, both perceptual quality versus gamma and global brightness contrast versus gamma for the different scenes merge. The optimal value of gamma is found to be 1.2-1.5.