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ABSTRACT

The migration from High Definition (HD) TV to Ultra High Definition (UHD) is already underway. In addition to an increase of picture spatial resolution, UHD potentially provides more colour by introducing a wider colour gamut (WCG), and better contrast by moving from Standard Dynamic Range (SDR) to High Dynamic Range (HDR).

The transition from SDR to HDR will require distribution solutions supporting some level of SDR backward compatibility. This paper presents the HDR content distribution scheme jointly developed by Technicolor and Philips. The solution is based on a single layer codec design and provides SDR compatibility, thanks to a pre- processing step applied prior to the encoding.

The resulting SDR video can be compressed and distributed then decoded using standard-compliant decoders (e.g. HEVC Main 10 compliant) and directly rendered on SDR displays. Dynamic metadata of limited size are used to reconstruct the HDR signal from the decoded SDR video, using a post-processing that is the functional inverse of the pre-processing. Both HDR quality and artistic intent are preserved.

Pre- and post-processing are applied independently per picture, do not involve any inter-pixel dependency, and are codec agnostic.

INTRODUCTION

The arrival of the High Efficiency Video Coding (HEVC) standard enables the deployment of new video services with enhanced viewing experience, such as Ultra HD broadcast services. In addition to an increased spatial resolution, Ultra HD can bring a wider colour gamut (WCG) and a higher dynamic range (HDR) than the Standard dynamic range (SDR) HD-TV currently deployed.

Different solutions for the representation and coding of HDR/WCG video have been proposed [1, 2, 3, 4]. As stated in [5, 6, 7, 8], SDR backward compatibility with decoding and rendering devices is an important feature in some video distribution systems, such as broadcasting or multicasting systems.

Dual-layer coding is one solution to support this feature. However, due to its multi-layer design, this solution is not adapted to all distribution workflows. An alternative is to transmit HDR content and to apply at the receiving device an HDR-to-SDR adaptation process (tone mapping).

One issue in this scenario is that the tone mapped content may be out of control of the content provider or creator. Another issue is that a new HDR-capable receiving device is needed to apply this tone mapping for existing SDR displays. Alternatively, the Hybrid Log Gamma (HLG) transfer function [2] has been designed as a straightforward solution to address the SDR backward compatibility, that is, an HDR video graded on a display using the HLG transfer function can be in principle directly displayed on an SDR display (using the BT.1886 transfer function [9]) without any adaptation.

However, this solution may result in colour shifting when the HLG-graded video is displayed on an SDR rendering device, especially when dealing with content with high dynamic range and peak luminance [10, 11, 12]. Also, there is no way to optimize the brightness and contrast of the SDR image.

Technicolor and Philips have jointly developed a new Single Layer HDR distribution solution, SL-HDR1 [14], aiming at addressing these issues. The solution is SDR compatible and leverages SDR distribution networks and services already in place. It enables both high quality HDR rendering on HDR-enabled CE devices, while also offering high quality SDR rendering on SDR CE devices.

The main features of the HDR distribution system are as follows:

Single layer with metadata: the HDR system is based on a single layer coding process, with side metadata (of a few bytes per video frame or scene) that can be used as a post-decoding processing stage to reconstruct the HDR signal.
Distribution codec agnostic: the HDR system is codec independent (a 10 bits codec is recommended).
Direct SDR compatibility: a decoded bitstream can be directly displayed on an SDR display. A post-processing is applied to convert the decoded SDR picture to HDR, thanks to the metadata, with preservation of the artistic intent.
Preserved quality of HDR content: there is no visible impairment due to the SDR compatibility feature in comparison with coding of the HDR10 signal.
Limited complexity: the post-processing of limited complexity can be implemented in a low-cost CE device. The involved operations are strictly pixel-based.
Independent from the HDR transfer function: the HDR system is independent from the HDR video signal transfer function that is input at the pre-processing stage.
No dedicated production metadata required: in case of live broadcasting, the SDR signal can be automatically derived by the pre-processing of the distribution encoder.

The remaining of the paper is structured as follows. The solution overview is presented in the next section. Then the HDR decomposition and reconstruction processes are detailed in the two following sections. The next section relates to the metadata signalling. Finally, complexity and performance are commented, and closing remarks are made.