Hybrid Log-Gamma


Hybrid Log-Gamma is a backwards-compatible high dynamic range standard that was jointly developed by the BBC and NHK. It provides the ability to encode a wide dynamic range, while still being compatible with the existing transmission standards in the standard dynamic range region. This makes HLG compatible with standard dynamic range displays, reducing complexity and cost for both equipment manufacturers and content distributors.
The HLG standard is royalty-free and was approved as ARIB STD-B67 by the Association of Radio Industries and Businesses. HLG is defined in ATSC 3.0, Digital Video Broadcasting UHD-1 Phase 2, and International Telecommunication Union Rec. 2100. HLG is supported by HDMI 2.0b, HEVC, VP9, and H.264/MPEG-4 AVC, and is used by video services such as BBC iPlayer, DirecTV, Freeview Play, and YouTube.

Technical details

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
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 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

Inception

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. On June 9, 2015, HLG was proposed to the JCT-VC for High Efficiency Video Coding and added to the June 2015 draft of the screen content coding extensions.
Later that year, Sony showed HLG video on a modified HDR display at the SMPTE 2015 conference. Colorfront announced that their Transkoder 2016 software would support HDR output using HLG. LG announced that their 2015 4K OLED TVs would support HDR from HLG and Perceptual Quantizer. Blackmagic Design released an update for DaVinci Resolve that added support for HLG.
SKY PerfecTV! announced that they will use HLG to transmit 4K UHDTV HDR programming to their satellite subscribers in Japan. Harmonic Inc. and NASA announced the HDR capture of an Atlas V launch which was broadcast the next day on NASA TV using HLG. Vatican Television Center broadcast the ceremony of the Holy Door using HLG and the Rec. 2020 color space.

2016

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