Post-Filtering for Color Appearance in Synthetic Images Visualization

In the photorealistic image synthesis process, an accurate approximation of the spectral light radiance field of a synthetic scene is carefully reproduced, with the goal of generating a synthetic image that is indistinguishable from the actual one. The paradigm of photorealism requires a comparison of the real scene and its synthetic reproduction, and the two visual representations should be studied under the same conditions to make a correct comparison and evaluate the degree of accuracy. To reproduce the same observing conditions, we need to define a sort of synthetic observer (given the impossibility to enter into a synthetic world) to compensate the deep differences in the viewing conditions, between the real and synthetic images. Various solutions have been proposed to this end; most of them are based more on perceptive measures of the human visual system (HVS) under controlled conditions, rather than on the HVS behavior under real conditions, e.g., observing a natural image and not a controlled black and white or colored pattern. Besides the comparison problem, difficulties can arise from the visualization phase, whose purpose is to display the final results of the simulation model on a monitor screen or printed paper. This is known as the tone reproduction problem, and in most cases, one has to find the best solution to compress an extended dynamic range of the computed light field into the limited range of displayable colors. Several solutions have been proposed to solve this problem. On the contrary, no mapping is usually made in case of low luminance and extremely limited dynamic range images, and consequently photorealism and visual appearance are lost. We propose a working hypothesis to solve the appearance and the tone reproduction problems in the synthetic image generation, integrating the Retinex model into the photorealistic image synthesis context, including in this way a model of the HVS in the image synthesis process