Preservation of Perceived Colorfulness of Down-Mapped, Hybrid Log-Gamma (HLG) to Standard Dynamic Range Content Using a Color Appearance Model (CAM)

Abstract

Most of today’s high dynamic range (HDR) hybrid log-gamma (HLG) to standard dynamic range (SDR) conversions rely on a mathematical approach for color volume reduction from an International Telecommunication Union-Radioommunication (ITU-R) Recommendation BT.2100 container into an ITU-R BT.709 one, typically through “tone-mapping” applied to a luminance signal. However, this might not be the only plausible option for HLG to SDR conversion. A color appearance model (CAM) has been extended in this work to compute HLG to SDR down-mapping. This alternative down-mapping approach can be carried out through a CAM combined with a chromatic adaptation transform (CAT). The CAT transforms the image into a uniform color space (UCS), and the CAM calculates values of perceived quantities of the viewed media, such as lightness and colorfulness given specific display parameters. Perceptual quantities in the model are, for the most part, conserved to compute the down-mapping. This method has shown great promise in several natural images and colors, while having limitations with engineering test signals. SDR footage derived from an HDR capture using a CAM look-up table (LUT) results in natural pictures that are more visually accurate than those from conventional conversion technologies or SDR capture since many colors in nature exist outside of a BT.709 container. Other examples of down-mapping can be found in the ITU-R report BT.2446. Such down-mapping conversions attempt to maintain color fidelity between the two visual representations and are usually implemented in a 3D LUT. Such approaches, however, cannot take full account of psychovisual factors and may struggle to differentiate between colors such as yellow and brown that are similar in hue but vary in luminance. A CAM-based approach may not deliver mathematically ideal output values for artificial test patterns, so the article will also discuss what changes may need to be made to TV production workflows if the improvements offered by CAM-based down-mapping are to be fully realized.