High Dynamic Range increases the difference between black and white beyond the contrast range of conventional television. That means water can glisten, stars can twinkle, and sunlight can be golden, all whilst maintaining detail in the shadows. Hybrid Log-Gamma (HLG) is an HDR system that was specifically developed for television by the BBC and Japanese broadcaster NHK.
Project from - present
What we're doing
Consumer television displays are getting brighter and capable of presenting stunning UHD images which are almost like looking through a window on to a scene. Having seen some fantastic HDR demonstrations we wanted to understand how our viewers could benefit from HDR in the future.
Broadcast television, and particularly live television, has a very complex content workflow. Our production and broadcast activities are spread over a number of sites, each at different points in their equipment replacement cycle. We started to investigate how HDR could fit into a typical broadcast infrastructure such as ours. We concluded that any approach couldn't rely on end-to-end metadata as metadata often gets lost or becomes out-of-sync with the content as it passes through the production chain and that standard presentation techniques such as mixing video sources become overly complex with metadata. We also wanted the captured signal to be display-independent because a wide range of different displays and lighting conditions are used both in production environments and in the home. Finally we wanted an approach that was compatible with our current 10 bit infrastructure and only needed changes to the cameras and critical monitoring displays. This led us, and NHK who shared many of our concerns, to invent the Hybrid Log-Gamma (HLG) system for HDR.
In order to meet all of our requirements, HLG was designed to be a scene-referred system, just like conventional television. The signal represents relative light levels in the original scene, which allows pictures from a single mastering process or live production to be adapted to give the same artistic effect on brighter or darker screens at home. Only the display itself needs information about its own capabilities and environment to faithfully render the scene-referred signal, so metadata that describes the mastering display is not required. HLG also has native compatibility with standard dynamic range (SDR) television within the same colour format, which can be used for ultra-high definition (UHD) SDR displays.
We are working to develop a complete HDR ecosystem. This starts with fundamental research into the human visual system, which then informs development of technical systems including format conversion techniques, HDR production guidelines and adaptation requirements for displays of different brightness. We then see these systems through to international standards that cover the whole video chain, from the basic video parameters, through professional interfaces, compression, programme delivery and broadcast and IP distribution. We also support consumer electronics manufacturers in verifying their implementations and developing consumer interfaces to the final display. HLG is included in ITU-R BT.2100, the international standard for high dynamic range programme production and exchange, and is royalty-free.
Why it matters
High Dynamic Range will offer a step-change in quality to viewers, making pictures more realistic and more immersive. In order for HDR programmes to be enjoyed in the home, a complete broadcast infrastructure must be in place. HLG's native compatibility allows much of the existing SDR infrastructure to be re-used for HDR.
How it works
In the coming months we will add a series of guides below which will describe the technical details of HDR and HLG as they become available.
- Frequently Asked Questions on High Dynamic Range and Hybrid Log-Gamma
- List of Standards and Publications
- Perceptual Quantiser (PQ) to Hybrid Log-Gamma (HLG) Transcoding
- Use of Look-Up Tables (LUTs) in FFmpeg
- HLG in Production and Playout
- HLG in Receivers
- Production Guidelines
- Creative Intent
- Colour Volumes
- HLG, SDR and PQ Format Conversion LUTs Available Under Licence
- How to Measure Displays
- QuickTime file parameter editor for modifying transfer function, colour primary, matrix characteristics - Github
Quite simply, our audiences will be able to enjoy a brilliant home TV experience.
UHD Production Training and Skills from the BBC Academy including:
This project is part of the Broadcast and Connected Systems section
This project is part of the Immersive and Interactive Content section