Hybrid Log-Gamma

混合对数伽马 (Hybrid Log-Gamma, HLG) is an HDR signal system jointly developed by the BBC and NHK, which was later incorporated into ITU-R BT.2100. Its photoelectric conversion function uses a square-root form similar to that of traditional television in the lower signal range and transitions to a logarithmic curve in the higher range, hence the name “混合对数伽马.”

HLG represents programs in terms of relative scene brightness, unlike PQ, which directly associates each code value with a fixed absolute display brightness. The display applies a system gamma based on its own peak brightness and viewing environment, allowing the same signal to be adapted to screens with different capabilities. This design is well-suited for live production and broadcast links, and it also reduces the need for each program to rely on static master display metadata.

Compatibility is one of the key design priorities of HLG. If a traditional SDR display directly processes HLG as a standard gamma signal, it can usually still produce a recognizable image, but the highlights, contrast, and colors will not be identical to those of a specifically converted SDR version; wide-color-gamut signals may also display incorrectly if color markers are not recognized. Therefore, “viewable” does not imply full backward compatibility.

HLG is generally combined with BT.2020 primary colors and 10-bit or higher bit depth, and can be used for 1080p, 4K, and 8K. It specifies the method of dynamic range transmission but does not specify a video codec; HEVC, broadcast transmission, or other systems are responsible for compression and encapsulation. HLG and HDR10 cannot be compared solely based on the presence or absence of dynamic metadata. The fundamental difference between the two lies in relative scene representation versus absolute display representation, as well as the corresponding production, monitoring, and tone mapping methods. Converting between the two requires handling reference white and the display environment; simply rewriting the metadata will not result in a correct conversion.