A Mental Breakdown of Alphabet Soup: HDR, 4K, WCG

The below is being posted in a relatively raw format.

One of the clearest things that I’ve heard about “HDR” is if anyone claims to an expert take their input with a grain of salt. So allow me to begin this by claiming the opposite, this is intended to be a regurgitation of information for my own purposes as much as it is a public document.

First, to address a presence of and general ambiguity of terms, HDR from a consumer standpoint is often presumed to be inclusive of 4K60 content and includes both increased resolution, gamma, and color space. However, these terms are not mutually exclusive in many workflows and the presences of another should not be presumed.

  • Color Gamut, inclusive of many existing standards.
    • REC.601
    • REC.709
    • REC.2020 — this stupid thing doesn’t just define color gamut but also seems to include some gamma correction
      • The gamma correction uses the same functions as 709, but with more resolution… apparently, see wiki.
      • How are we supposed to reconcile the handling of gamma by the HDR standard from the influences of the color standard? Is it even fair to make the delineation?
      • Show this on a graph sort of like sample rate, perhaps increase or double the frequency of a dashed line to illustrate?
  • WCG – Wide Color Gaumet, see also REC/BT.2020
  • HDR – High Dynamic Range, Refers to the gamma curve applied to a particular source material.
  • OTF – Optical Transfer Function
  • Electro-Optical Transfer Function (EOTF) which maps the non-linear signal value into display light. [Rec.2100 Wikipedia]
  • Opto-Optical Transfer Function (OOTF) which maps relative scene linear light to display linear light. [Rec.2100 Wikipedia]
  • Opto-Electronic Transfer Function (OETF) which maps relative scene linear light into the non-linear signal value. [Rec.2100 Wikipedia]
  • HLG – Hybrid Log-Gamma (AKA HLG10)
    • BBC + NHK joint venture.
    • Approved as ARIB STD-B67
    • Included in REC.2100
    • Supported in H264/265, VP9, and HDMI 2.0
    • ** My own interpretation, it uses the same amplitude as SDR but is interpreted sort of like a knee, compressing the highlights into the top end of the signal – by understanding this predefined curve it can be stretched back out and displayed based on the parameters of the available display device.
  • PQ – Perceptual Quantization (AKA HDR10)
    • Included in REC.2100
    • ** No really sure on the exact curve, but an image equivalent to 100 nits is expected to be displayed at 100 nits on the receiving display regardless — should a display not be able to display all of the data in the signal, too bad… it gets clipped. Miss you maybe.


What is Rec. 2100? [Spec]

Table 1 – Image spatial and temporal characteristics
Parameter Values
Image Container Shape 16:9
Container Pixel count Horizontal × Vertical 7680 × 4320
3840 × 2160
1920 × 1080
Pixel aspect ratio 1:1 (square pixels)
Frame frequency (Hz) 120, 120/1.001,100, 60, 60/1.001, 50, 30, 30/1.001, 25, 24, 24/1.001
Image Format Progressive [no support for interlaced, huzzah!]

Source: ITU-R BT.2100-2 (07/2018)
Note: A few lines of the table above have been removed to simplify. Pithy comments are free, I’ll leave you to reconcile those.

REC.2100 also includes guidance on the handling of OTF, the short version – you can use HLG or PQ.

Leave a Reply

Your email address will not be published. Required fields are marked *