Evaluation and Enhancement of HDR Image Appearance on Displaysof Varying Dynamic Range

Abstract

The increasingly widespread availability of high dynamic range (HDR) technology has led to active study of the characteristics of the human visual system (HVS) in terms of brightness, lightness, contrast, and color perception and the application of the results of these studies to computer graphics. Because the development of HDR technology gives us display devices with much broader dynamic range for both high and low luminances, it is especially important to revise the models of HVS for the luminance ranges which are not covered by classical psychophysics, but required by the new HDR technology.In this dissertation, we focus on the evaluation and enhancement of the appearance of HDR images as reproduced on low dynamic range (LDR) media. First, we conducted a psychophysical experiment on seven tone mapping operators (TMOs)to assess how tone mapped images are perceived differently by human observers and to find out which attributes of image appearance account for these differences. The results show qualitative differences in TMOs, however, it also turned out that it was hard to choose the consistently best algorithm in terms of the fidelity of tone mapped images to real-world scenes. Based on this result, we conducted another series of psychophysical experiments with a generic easy to understand TMO. This experiment focused on three parameters - brightness, contrast, and color saturation - in order to measure user preference for, and fidelity of, tone mapped HDR images across several types of emulated limited dynamic range displays. The results of this study provide novel guidance for creating more advanced TMO designs.To evaluate the enhancement of image appearance, we conducted two series of experiments on contrast and brightness enhancement. A non-linear change in perceived contrast with respect to given physical contrast and a different adaptation luminance levels were measured by contrast scaling and contrast discrimination threshold experiments on HDR displays. These results lead to a model of just noticeable difference (JND) unit which provides uniformly changing perceived contrast in complex images. Finally, brightness enhancement caused by the glare illusion was investigated. We employed two profiles to evoke the glare illusion: a point spread function (PSF) of the human eye and a Gaussian kernel. The outcome of this study shows that the glare illusion increases the perceived luminance (brightness) by 20 - 35% for both convolution methods of a PSF and a Gaussian kernel. This means that faithful simulation of the human eye optics, which has been proposed before, is not necessary to achieve a strong brightness enhancement of the glare illusion because the Gaussian kernel, which has no theoretical justification in human perception, evokes the brightness enhancement at the same or higher strength than the PSF.