Hybrid Log-Gamma (HLG) is a high dynamic range (HDR) standard that was jointly developed by the BBC and NHK. The HLG standard is royalty-free and was approved as ARIB STD-B67 by the Association of Radio Industries and Businesses (ARIB). HLG is compatible with standard dynamic range (SDR) displays. HLG is defined in ATSC 3.0, Digital Video Broadcasting (DVB) UHD-1 Phase 2, and International Telecommunication Union (ITU) Rec. 2100. HLG is supported by HDMI 2.0b, HEVC, VP9, and H.264/MPEG-4 AVC. HLG is supported by video services such as BBC iPlayer, DirecTV, Freeview Play, and YouTube.
- where E is the signal normalized by the reference white level and E' is the resulting nonlinear signal
- where r is the reference white level and has a signal value of 0.5
- where a = 0.17883277, b = 0.28466892, and c = 0.55991073
The signal value is 0.5 for the reference white level while the signal value for 1 has a relative luminance that is 12 times higher than the reference white level. ARIB STD-B67 has a nominal range of 0 to 12. HLG uses a logarithmic curve for the upper half of the signal values due to Weber's law.
HLG does not need to use metadata since it is compatible with both SDR displays and HDR displays. HLG can be used with displays of different brightness in a wide range of viewing environments.
The dynamic range that can be perceived by the human eye in a single image is around 14 stops. An SDR video display with a 2.4 gamma curve and a bit depth of 8-bits per sample can display a range of about 6 stops without visible banding. Professional SDR video displays with a bit depth of 10-bits per sample extend that range to about 10 stops. When HLG is displayed on a 2,000 cd/m2 display with a bit depth of 10-bits per sample it can display a range of 200,000:1 or 17.6 stops without visible banding.
HLG increases the dynamic range of the video compared to a conventional gamma curve by using a logarithmic curve for the upper half of the signal values. HLG also increases the dynamic range by not including the linear part of the conventional gamma curve used by Rec. 601 and Rec. 709. The linear part of the conventional gamma curve was used to limit camera noise in low light video but is no longer needed with HDR cameras.
HLG is supported in HEVC with a formula that is mathematically equivalent to ARIB STD-B67 but has a nominal range of 0 to 1 instead of 0 to 12:
- where Lc has a nominal range of 0 to 1 and V is the resulting nonlinear signal
- where a = 0.17883277, b = 0.28466892, and c = 0.55991073
On October 27, 2015, Colorfront announced that their Transkoder 2016 software would support HDR output using HLG.
On December 3, 2015, Harmonic Inc. and NASA announced the HDR capture of a Atlas V launch which was broadcast the next day on NASA TV using HLG. UHDTV HDR video clips will be published early next year in both HDR10 and HLG.
On April 15, 2016, Dome Productions announced that they will begin trials of HLG to deliver HDR content.
On April 18, 2016, the Ultra HD Forum announced their guidelines for UHD Phase A which includes support for HLG. The Ultra HD Forum also defined HLG10 as HLG, a bit depth of 10-bits, and the Rec. 2020 color space.
On August 3, 2016, Sony announced that in October they will release a firmware update to add HLG to their BVM-X300 OLED monitor.
On September 14, 2016, Sony announced that their VPL-VW675ES projector will support HLG.
On November 17, 2016, the Digital Video Broadcasting (DVB) Steering Board approved UHD-1 Phase 2 with a HDR solution which supports Hybrid Log-Gamma (HLG) and Perceptual Quantizer (PQ). The specification has been published as DVB Bluebook A157 and will be published by the ETSI as TS 101 154 v2.3.1.
On November 18, 2016, Mediapro/Overon announced that they will transmit the Spanish Football League (LFP) worldwide using 4K HDR broadcasts based in HLG
On December 25, 2016, Atomos updated their Shogun Inferno product to include HLG input and output for recording, monitoring editing and layout from Cameras and computers as well as to HLG compatible TV's.
On January 10, 2017, LG Corporation announced that they will add support for HLG to their 2016 OLED TVs and their 2016 Super UHD TVs with a firmware update.
On January 16, 2017, Sony announced that they will add support for HLG to their 2017 4K TVs with a firmware update.
On February 17, 2017, Panasonic announced that they will add support for HLG to several models of their 2016 4K TVs with a firmware update.
On May 31, 2017, Sony began releasing firmware updates for several of their 2016 and 2017 Android TV models which adds support for HLG.
On November 27, 2017, Panasonic began releasing firmware updates for several of their 2016 TV models which adds support for HLG.
On December 5, 2017, the BBC announced that Blue Planet II would be available in 4K HDR on the BBC iPlayer using HLG. Blue Planet II will be available on the BBC iPlayer service from December 10, 2017 to January 16, 2018. The BBC states that almost 400 TV models have support for HLG which includes TV models from Finlux, Hisense, Hitachi, LG, Panasonic, Philips, Samsung, Sony, and Toshiba.
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