Hybrid log gamma is a backward-compatible high dynamic range (HDR) 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 (SDR) region. This makes HLG compatible with SDR 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 (ARIB). 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, and is used by video services such as BBC iPlayer, DirecTV, Freeview Play, and YouTube.
HLG is a royalty-free HDR standard jointly developed by the BBC and NHK. 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. In practice, the signal is interpreted as normal by standard-dynamic-range displays (albeit capable of displaying more detail in highlights), but HLG-compatible displays can correctly interpret the logarithmic portion of the signal curve to provide a wider dynamic range. In contrast the other HDR formats it does not use metadata.
The HLG transfer function is backward compatible with SDR's gamma curve. However, HLG has commonly used with Rec. 2020 color primaries which produce a de-saturated image with visible hue shifts on non-compatible devices. HLG is therefore backward compatible with SDR-UHDTV and will show color distortion on common SDR devices that only support Rec. 709 color primaries.
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 the BBC iPlayer, DirecTV, Freeview Play, and YouTube.