Hybrid log–gamma

[1] It is backward compatible with the transfer function of SDR (the gamma curve).

[1][3][4] It is also defined in ATSC 3.0, Digital Video Broadcasting (DVB) UHD-1 Phase 2, and International Telecommunication Union (ITU) Rec.

However, HLG is not intended to be fully backward compatible with traditional SDR displays that cannot interpret BT.2020 colorimetry.

[1][10][9] They are supported by HDMI 2.0b, HEVC, VP9, and H.264/MPEG-4 AVC,[11][12][13][14] and are used by video services such as BBC iPlayer, DirecTV, Freeview Play, and YouTube.

[15][16][17][18][19] HLG is designed to be better-suited for television broadcasting, where the metadata required for other HDR formats is not backward compatible with non-HDR displays, consumes additional bandwidth, and may also become out-of-sync or damaged in transmission.

HLG defines a non-linear optical-electro 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.

[25] The HLG transfer function is backward compatible with SDR's gamma curve.

2020 color primaries which produce a de-saturated image with visible hue shifts on non-compatible devices.

[26] HLG is therefore backward compatible with SDR-UHDTV and will show color distortion on common SDR devices that only support Rec.

[26] 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.

[28] HLG uses a logarithmic curve for the upper half of the signal values due to Weber's law.

[1][3] HLG can be used with displays of different brightness in a wide range of viewing environments.

[3] The dynamic range that can be perceived by the human eye in a single image is around 14 stops.

[27] Professional SDR video displays with a bit depth of 10-bits per sample extend that range to about 10 stops.

[27] HLG also increases the dynamic range by not including the linear part of the conventional gamma curve used by Rec.

[5][30] 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:[28]

[31] 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.

[32][12] Later that year, Sony showed HLG video on a modified HDR display at the SMPTE 2015 conference.

[33][34] Colorfront announced that their Transkoder 2016 software would support HDR output using HLG.

[35] LG announced that their 2015 4K OLED TVs would support HDR from HLG and perceptual quantizer (PQ).

[36] Blackmagic Design released an update for DaVinci Resolve that added support for HLG.

announced that they will use HLG to transmit 4K UHDTV HDR programming to their satellite subscribers in Japan.

[39] Vatican Television Center broadcast the ceremony of the Holy Door using HLG and the Rec.