Hybrid Log Gamma (HLG)

Hybrid Log Gamma (HLG) is a standard developed by the BBC and NHK.

Today's HDR productions (HDR = High Dynamic Range vs. SDR = Standard Dynamic Range) still pose the challenge of having to prepare HDR content for specific terminal devices in the final production process. Since it depends on preset profiles and the respective terminal device (display) how the HDR signal is distributed in the available dynamic range, separate gradings are currently still required for the respective distribution channels. It does not matter whether you use HDR-10 or Dolby Vision.
If you want to broadcast HDR on television, different streams would have to be offered for HDR in addition to the existing SDR streams. But this is not practical.
This challenge can be met with a transmission standard that can send HDR and SDR signals over the same channel and the terminal devices can display the picture correctly within the scope of their design possibilities, regardless of whether they are SDR or HDR displays.
Hybrid Log Gamma has been developed for this purpose.  For displaying HDR content on both SDR and HDR displays there is no need for metadata.
The HLG standard is a license-free development and has been standardized by the Association of Radio Industries and Businesses (ARIB) as ARIB STD-B67. The ITU defines HLG in the Rec.2100. The HDMI 2.0b interface as well as the HEVC and VP9 codec are also compatible with HLG.
To achieve this functionality, the Hybrid Log Gamma uses a Rec.709-typical gamma curve in the lower part, while a log function is appended for the upper part. Put simply, an SDR display will only "see" the Rec.709 typical part, while an HLG compatible HDR display can evaluate the entire curve for display.
Both camera and display manufacturers are now concentrating on the implementation of HLG in their new developments. Compatibility is also "retrofitted" with firmware updates. Hyper Log Gamma is a standardized curve.