… for Codecs & Media
Tip #746: What is HDR Rec. 2020 PQ?
PQ provides for the brightest images, even though technology today can’t fully support it.
High-dynamic-range video (HDR video) describes video having a dynamic range greater than that of standard-dynamic-range video (SDR video). HDR capture and displays are capable of brighter whites and deeper blacks. To accommodate this, HDR encoding standards allow for a higher maximum luminance and use at least a 10-bit dynamic range in order to maintain precision across this extended range.
While technically “HDR” refers strictly to the ratio between the maximum and minimum luminance, the term “HDR video” is commonly understood to imply wide color gamut as well.
There are two ways we can display HDR material: HLG and PQ. (Tip #745 discusses HLG).
Perceptual Quantizer (PQ), published by SMPTE as SMPTE ST 2084, is a transfer function that allows for the display of high dynamic range (HDR) video with a luminance level of up to 10,000 cd/m2 and can be used with the Rec. 2020 color space.
NOTE: cd/m2 refers to “candela per meter squared.” One cd/m2 equals one IRE.
PQ is a non-linear electro-optical transfer function (EOTF). On April 18, 2016, the Ultra HD Forum announced industry guidelines for UHD Phase A, which uses Hybrid Log-Gamma (HLG) and PQ transfer functions with a bit depth of 10-bits and the Rec. 2020 color space. On July 6, 2016, the ITU announced Rec. 2100, which uses HLG or PQ as transfer functions with a Rec. 2020 color space.
The key takeaway here is that PQ supports extremely bright images, but in a format that is not compatible with anything else.