Re: [csswg-drafts] Define rec2020 color space to use 2.4 gamma (#12574)

ITU-R BT.709 didn't define an EOTF as at the time, there was only one display technology in use - CRT.

The BT.1886 EOTF only became necessary as more display technologies came in to use.  As I understand it, the gamma value (2.4) is the average value rounded for a CRT working in the HD production environment listed in BT.2035.  This is defined to give an end to end system gamma of 1.2 (when combined with the curve in a BT.709 camera) which is suitable for viewing CRT luminance levels in a dim environment.

Other systems had different end to end gammas (and hence gamma offsets applied for the display) due to the different viewing environments - cinema, printed images etc.

Apple (and I think Silicon Graphics) had a non-linear LUT applied at the OS level.  Charles Poynton has a section in his book about it - I'll try and find it. 

As Chris L states, the inverse OETF is required mainly when combining cameras of different formats.  This is commonly used in live HDR production as not all specialist cameras (such as pylon cameras, stump cameras) are currently available in HDR.

For most uses, the EOTF would be used - overlaying graphics, rendering multiple images on one display etc.

For conversions, it has been common to convert all formats to a common white level.  For SDR this replicates the action of the initial reference display when adjusted.  For HDR it is harder, but in our conversions we have matched the diffuse white of the HDR to the SDR, whilst applying a gamma offset to maintain the artistic look between the two.

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Received on Thursday, 7 August 2025 12:46:23 UTC