Re: S-Log2/S-gamut 10-bit 422 XAVC to Linear ACES RGB EXR

Brendan Bolles <bre...@...>

On Jun 4, 2013, at 9:38 AM, Dithermaster wrote:

It is a small point, but this is incorrect; in Rec. 709 it really is a factor of 4 between them (not 1023/255). According to the specification, 8-bit value 16 is equivalent to 10-bit value 64 and 8-bit 235 is equivalent to 10-bit 940, therefore 8-bit 255 is equivalent to 10-bit 1020 (not 1023).

Interesting! Of course, this only applies if your source is arriving with headroom and footroom, which it often does not. After all, Rec. 709 is really YCrCb, right? So you rarely would access the image without at least an RGB conversion.

If your 10-bit RGB image does have headroom and footroom, you probably want to run tests to make sure you really are getting a 64-940 signal. Of the people who have imported files with headroom in Nuke, I'm sure many have expanded the range by setting white to 0.922 (235/255) where maybe they should have been using 0.919 (940/1023).

The OCIO sample configurations wisely stay out of this and do all their Rec. 709 conversions using the full 0.0-1.0 range.


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