Perception-based Histogram Equalization for tone mapping applications

Due to the ever increasing commercial availability of High Dynamic Range (HDR) content and displays, backward compatibility of HDR content with Standard Dynamic Range displays is currently a topic of high importance. Over the years, a significant amount of Tone Mapping Operators (TMOs) have been proposed to adapt HDR content to the restricted capabilities of SDR displays. Among them, the Histogram Equalization (HE) is considered to provide good results for a wide set of images. However, the naïve application of HE results either in banding artifacts or noise amplification when the HDR image has large unified areas (i.e. sky). In order to differentiate relevant information from noise in a uniform background, or in dark areas, the authors proposed a ceiling function. Their method results in noise-free but dim images. In this paper we propose a novel ceiling function which is based on the Perceptual Quantizer (PQ) function. Our method uses as threshold the number of code-words that PQ assigns on a luminance range in the original HDR image and the corresponding number of code-words in the resulting SDR image. We limit the number of code-words on SDR to be equal or less than the HDR. The saved code-words during the ceiling operation are redistributed to increase the contrast as well as the brightness of the final image. Results shows that provided SDR images are noise-free and brighter than the one obtained with prior HE operators. Finally since the proposed method is a Global TMO, it is thereby of low complexity and suitable for real time applications.