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| HLG (Hybrid Log‑Gamma) | |
|---|---|
| Name | HLG (Hybrid Log‑Gamma) |
| Introduced | 2014 |
| Standardised | 2015 |
| Developer | BBC and NHK |
| Application | High dynamic range video |
| Bit depth | 10‑bit typical |
| Color space | BT.2020, BT.709 |
HLG (Hybrid Log‑Gamma) is a high dynamic range (HDR) electro‑optical transfer function developed jointly by the BBC and NHK to enable HDR video distribution compatible with existing standard dynamic range (SDR) infrastructures. It was standardized by international bodies and adopted in broadcast, streaming, and production workflows alongside formats from Dolby Laboratories, Technicolor, and ITU‑R. HLG is designed for backward compatibility with legacy displays and for flexible use in live events such as FIFA World Cup, Olympic Games, and network broadcasts by organizations like the American Broadcasting Company and the British Broadcasting Corporation.
HLG originated from research at the BBC Research and Development and NHK Science & Technology Research Laboratories to address HDR without metadata dependencies used by Dolby Vision and HDR10. It was included in recommendations by ITU‑R and later referenced in standards from the European Broadcasting Union and the Advanced Television Systems Committee. HLG enables broadcasters such as NHK and BBC and services like Sky UK, DAZN, and YouTube to deliver HDR signals over infrastructures developed for HDTV, SDI links, and satellite transmissions like those of Eutelsat or SES S.A..
The HLG transfer function combines a gamma curve for darker values with a logarithmic function for highlights; the specification was published with contributions from engineers associated with SMPTE, ITU‑R BT.2100, and the European Broadcasting Union. Typical implementations use 10‑bit precision and colorimetry defined by BT.2020 or BT.709 primaries with reference white at 6500 K as in SMPTE RP 177. HLG does not require scene‑referred metadata such as that in SMPTE ST 2084 and CTA‑861.3; instead, it relies on a function that maps scene or display luminance to signal values enabling compatibility with legacy signal chains like HD‑SDI. Key parameters reference mastering levels seen in productions for BBC Sport broadcasts and NHK test transmissions.
HLG works with color primaries defined in Rec. 2020 and chromatic adaptation practices used in standards from SMPTE and ITU‑R. Color encoding uses Y′CbCr with transfer characteristics specified alongside BT.2100 for wide color gamut support in workflows used by studios like Warner Bros. and post houses such as Deluxe Entertainment Services Group Inc.. Conversions between color spaces involve matrices similar to those adopted by Technicolor and color grading systems in products from Blackmagic Design and Avid Technology, while mastering and monitoring frequently leverages displays certified by Dolby Laboratories, VESA, and vendors like Sony and Samsung Electronics.
Broadcasters, streaming platforms, and camera manufacturers from Canon Inc., Sony Corporation, Panasonic Corporation, and RED Digital Cinema have implemented HLG options in cameras, encoders, and playout servers. Major streaming providers such as Netflix and Amazon Prime Video have historically evaluated HDR options including HLG; live sports platforms like Sky Sports and public broadcasters including NHK and the BBC use HLG for over‑the‑air HDR. Equipment certification and interoperability testing have been coordinated through bodies like EBU and ATSC test suites, while consumer TVs from LG Electronics, Samsung Electronics, and Sony often include HLG compatibility alongside support for HDR10 and Dolby Vision.
HLG’s hybrid transfer allows compatible SDR displays to render images without metadata up‑conversion tools provided by vendors such as Apple Inc. or Google LLC. In live workflows used by Sky UK and international broadcasters at events like the UEFA European Championship, HLG reduces latency compared with dynamic metadata systems. Compatibility challenges arise when interfacing with legacy color grading systems from companies like Avid Technology or when converting between scene‑referenced pipelines used in productions for Walt Disney Studios and display‑referred pipelines used in broadcast. Monitoring accuracy often requires reference displays calibrated using standards from SMPTE and colorimeters by X‑Rite.
HLG is widely used for live sports broadcasts by organizations such as BBC Sport, Sky Sports, and Eurosport, live events coverage at the Olympic Games and FIFA World Cup, satellite distribution by operators like SES S.A. and Intelsat, and streaming services supporting live OTT feeds like YouTube and DAZN. It is also used in news production chains at broadcasters including NHK and CNN for HDR newsgathering, and in camera production settings at manufacturers like Panasonic Corporation and Sony Corporation which provide HLG capture modes for ENG workflows.
Compared with HDR10 and HDR10+, HLG does not rely on static or dynamic metadata, unlike Dolby Vision which uses scene‑by‑scene metadata from Dolby Laboratories. HLG’s scene‑oriented hybrid transfer contrasts with the perceptual quantizer in SMPTE ST 2084, used by HDR10 and Dolby Vision, and this yields different tradeoffs in highlight roll‑off and display mapping observed in tests by ITU‑R, EBU, and manufacturers such as Sony and Samsung Electronics. While Dolby Vision targets premium mastered content distributed by studios like Warner Bros. and Netflix, HLG targets live broadcast and backward compatibility in deployments by broadcasters including BBC and NHK.
Category:Video encoding