BBC R&D

Abstract

Modern video displays provide improved performance compared to, now obsolete, cathode ray tube (CRT) displays. In particular they are now starting to provide brighter images and deeper blacks. In other words they are starting to provide significantly higher dynamic range than CRT displays. A television system that can take advantage of this would be able to offer improved pictures that are more realistic and have greater impact. However, this cannot easily be achieved whilst retaining the conventional non-linear transfer functions, known as gamma curves, which were designed for CRT displays. Instead enhanced “gamma” curves are needed in both cameras and displays to support improved picture quality. Ideally an enhanced gamma curve should be simple, broadly compatible with the existing conventional gamma curves, and based on relative, not absolute, brightness. Such enhanced gamma curves would minimise the change to both existing equipment and television production process, thereby simplifying and minimising the cost of a transition to high dynamic range video. This paper discusses the background to the conventional non-linear gamma curves used for video. Based on this discussion it suggests a new, simple, and compatible, gamma curve to support the production, exchange and end user distribution of high dynamic range video. For end user distribution this proposal would increase the dynamic range by about a factor of 50. This increase would likely be sufficient for domestic use for the foreseeable future. This suggestion was proposed to ITU (International Telecommunication Union) in March 2014 . The proposal to the ITU highlighted the inconsistencies between existing standards for gamma curves in cameras and displays. So this paper goes on to reformulate the ITU proposal in a simpler, but essentially equivalent, way that removes these inconsistencies.

Note (February 2017): The pre-standard HLG (Hybrid Log-Gamma) transfer characteristic described in this paper has been superseded by ITU-R BT.2100: https://www.itu.int/rec/R-REC-BT.2100.

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