Perception-based contrast enhancement model for complex images in high dynamic range

Contrast in image processing is typically scaled using a power function (gamma) where its exponent specifies the amount of the physical contrast change. While the exponent is normally constant for the whole image, we observe that such scaling leads to perceptual nonuniformity in the context of high dynamic range (HDR) images. This effect is mostly due to lower contrast sensitivity of the human eyes for the low luminance levels. Such levels can be reproduced by an HDR display while they can not be reproduced by standard display technology. We conduct two perceptual experiments on a complex image: contrast scaling and contrast discrimination threshold, and we derive a model which relates changes of physical and perceived contrasts at different luminance levels. We use the model to adjust the exponent value such that we obtain better perceptual uniformity of global and local contrast scaling in complex images.