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, and VP9. HLG is supported by video services such as Freeview Play and YouTube.
Chart showing a conventional SDR gamma curve and Hybrid Log-Gamma (HLG). HLG uses a logarithmic curve for the upper half of the signal values which allows for a larger dynamic range.
HLG defines a nonlinear transfer function in which the lower half of the signal values use a gamma curve and the upper half of the signal values use a logarithmic curve.
- 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. SDR video with a 2.4 gamma curve and a bit depth of 8-bits per sample has a dynamic range of about 6 stops. Professional SDR video with a bit depth of 10-bits per sample has a dynamic range of about 10 stops. When HLG is displayed on a 2,000 cd/m2 display with a bit depth of 10-bits per sample it has a dynamic range of 200,000:1 or 17.6 stops.
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 Rec. 2100 with a nominal peak luminance of 1,000 cd/m2 and a system gamma value that can be adjusted depending on background luminance.
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 May 15, 2015, the BBC announced that they had begun work with the NHK to develop a joint HDR proposal that would be proposed to the International Telecommunication Union (ITU).
On June 9, 2015, HLG was proposed to the JCT-VC for High Efficiency Video Coding (HEVC) and added to the June 2015 draft of the screen content coding extensions.
On October 26, 2015, Sony showed HLG video on a modified HDR display at the SMPTE 2015 conference.
On October 27, 2015, Colorfront announced that their Transkoder 2016 software would support HDR output using HLG.
On November 4, 2015, SKY PerfecTV! announced that they will use HLG to transmit 4K UHDTV HDR programming to their satellite subscribers in Japan.
On November 6, 2015, LG announced that their 2015 4K OLED TVs would support HDR from HLG and Perceptual Quantizer (PQ).
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 December 14, 2015, Vatican Television Center broadcast the ceremony of the Holy Door using HLG and the Rec. 2020 color space.
On December 23, 2015, Blackmagic Design released an update for DaVinci Resolve that added support for HLG.
On January 12, 2016, Leader Electronics Corporation announced their 12G-SDI waveform monitors with support for HLG.
On January 29, 2016, Avid Technology released an update for Media Composer that added support for HLG.
On March 29, 2016, Harmonic Inc. released an update for the ViBE 4K UHD encoder that added support for HLG.
On April 13, 2016, Canon Inc. announced that they will release firmware updates for the DP-V2410 and DP-V3010 reference displays to add support for HLG.
On April 15, 2016, Dome Productions announced that they will begin trials of HLG to deliver HDR content.
On April 18, 2016, Sony announced the PVM-X550 OLED monitor with support for HLG. Sony also announced a firmware update for the BVM-X300 OLED monitor to add support for HLG.
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 July 6, 2016, the ITU announced Rec. 2100 which defines two HDR transfer functions which are HLG and PQ.
On July 29, 2016, SKY Perfect JSAT Group announced that on October 4 they will start the world's first 4K HDR broadcasts using HLG.
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 7, 2016, Eutelsat announced that it will launch a new channel using HLG.
On September 9, 2016, Google announced Android TV 7.0 which supports HLG.
On September 14, 2016, Sony announced that their VPL-VW675ES projector will support HLG.
On September 23, 2016, Digital UK published their 2017 specification for Freeview Play which includes support for HDR using HLG.
On November 4, 2016, Samsung announced that all of their 2016 HDR TVs could support HLG with a firmware update.
On November 7, 2016, Google announced that YouTube will start streaming HDR videos which can be encoded with HLG or PQ.
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 8, 2016, the BBC announced that it was adding a 4-minute HLG edit from their Planet Earth II series to its iplayer IPTV platform for public UHD testing.
On December 12, 2016, HDMI announced that HLG support had been added to the HDMI 2.0b standard.
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 3, 2017, LG Corporation announced that their 2017 Super UHD TVs will support HLG.
On January 4, 2017, Panasonic announced that their 2017 OLED TV will support HLG.
On January 4, 2017, Sony announced that their 2017 OLED TVs will support HLG.
On January 4, 2017, Eutelsat announced that their Hot Bird video service would include the Travelxp 4K channel which uses HLG.
On January 5, 2017, JVC announced that their 2017 4K projectors will support HLG.
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 March 13, 2017, Philips announced that their 2017 4K TVs will support HLG and that they will add support for HLG to several models of their 2016 4K TVs with a firmware update.
On April 19, 2017, Adobe Systems announced updates to Adobe Creative Cloud which includes support for HLG.
On May 19, 2017, ATSC released the video standard for ATSC 3.0 which includes support for HLG.
On May 31, 2017, Sony began releasing firmware updates for several of their 2016 and 2017 Android TVs which adds support for HLG.
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