Hybrid Log Gamma (HLG), like Perceptual Quantization(PQ), is a correction curve used in the transfer functions for electro-optical and optical-to-electrical conversion, EOTF and OETF, in optical transmission systems and displays.
HLG correction is similar to gamma correction in traditional television. With Hybrid Log Gamma (HLG) and also with Perceptual Quantization (PQ), - both processes are standardized by the International Telecommunication Union ITU in BT.2100 -, the aim is to adapt the dynamic range of the display on the UHD displays to the dynamic range of the recording devices. Only then does the display correspond to the viewer's perception of brightness.
High Dynamic Range( HDR) displays follow a logarithmic function. The brightness range corresponds to a luminance between 100 nits and 5,000 nits. The HLG correction curve requires a resolution of at least 10 bits and is used with High Dynamic Range HDR10 and Dolby Vision in transmission and mastering. When simultaneously transmitting video in SDR technology and HDR technology in one data stream, the image content is encoded with the HLG curve so that it can be reproduced on both SDR displays and HDR displays.