transfer function

A mathematical formula that converts the pixel values of images to corresponding light values on the screen. These functions are computed within the display device. There are several common transfer functions:

• Rec. 709 Gamma: An image-referenced (or scene-referenced) transfer function with a luminance range of 100 nits. Rec. 709 is used by SDR monitors, such as regular HDTV displays and non-HDR computer monitors.

• Hybrid Log-Gamma (HLG): An image-referenced (or scene-referenced) transfer function with a luminance range of 1,000 nits. HLG is used by HDR monitors, such as 4K Ultra HD televisions. HLG-enabled displays automatically adapt the luminance levels based on the content and display capabilities. HLG is compatible with the gamma curve used in SDR (defined in ITU-R Recommendation BT.2100).

• Perceptual Quantizer (PQ): A display-referenced transfer function that relates image luminosity values to absolute light levels. PQ has a luminance range of 10,000 nits. PQ-enabled HDR monitors use content metadata to display PQ-coded images and implement tone mapping to adapt luminance levels. PQ replaces the gamma curve used in SDR (defined in both SMPTE ST 2084 and Rec. 2100).

HLG is based on the older SDR (Rec. 709) standard, but displays broader luminance ranges and offers backward compatibility and simplicity in the production process. PQ doesn’t offer backward compatibility, but does provide a well-defined dynamic range.