What is Hybrid Log Gamma HLG HDR

Ultra HD tech talk: HDR

In order to be able to call up the larger color volume, HDR content and HDR-capable flat screens have to "communicate" with one another according to a new logic. New rules need to be drawn up as to how the television interprets (electronic) data and converts it back into (optical) light signals that it can display. That is why this function is called “Electro-Optical-Transfer-Function” - EOTF for short. This process is essentially the reverse function for the production of content. Here the camera "converts" light signals into data, which is consequently called "Opto-Electrical-Transfer-Function" (OETF). However, a camera “sees” such light signals differently than our eyes. In addition, the human organ of vision is extremely adaptable to different light situations. EOTF and OETF are therefore not simply linear, but rather “curves”. There are several technical suggestions from the TV and video industry as to what these curves should look like. One of these suggestions is already used for Ultra HD Blu-ray or the transmission of Ultra HD streams: the “Perceptual Quantizer” developed by Dolby Laboratories: PQ for short, the basis for HDR10.

Two standards for static HDR: PQ (HDR10) and Hybrid Log-Gamma (HLG)

With static HDR processes, the key data for contrast, brightness and gamma are set only once for the entire film. HDR 10 is based on the PQ curve and offers good HDR quality, but is not backwards compatible with Ultra HD televisions that are not HDR capable. The content would then be misrepresented, viewers would only see “washed out” images. Due to the large number of Ultra HD sets without HDR that have already been sold, it is important for many providers to broadcast their programs in the best possible quality for these TV households as well. That is why TV broadcasters use a different “curve” instead of PQ: Hybrid Log-Gamma (HLG).

HLG content cannot be distinguished from PQ content when played back on HDR flat screens up to a certain brightness. When playing back on SDR flat screens, however, the display (in the BT.2020 color space) is comparable to a conventional TV signal without HDR. PQ (HDR10) and HLG are specified by the International Telecommunication Union ITU in BT.2100 for program production and exchange and are also planned by the DVB consortium in UHD-1 phase 2 for HDR distribution.