Nuke: sRGB -> ACES -> sRGB = Not the same

[narann (Dorian Fevrier)]

Hi Thomas!

Yeah, I’m always lurking into CGI production techs and tried to jump and test ACES by practice.

I’m not under my desk so I can’t test for now.

So, just to be sure I get your (precise) comment, I repeat:

Your point 1: sRGB EOTF (1D LUT) makes values linear, then sRGB to ACES-2065-1 convert primaries. The white point doesn’t change as it’s the same between both standards (D65).

Am I getting it right? So this is not where my image become darker (and it make sense). OK.

Now the reason why I get a darker output is related to RRT+sRGB ODT, especially, RRT.

ACES output apply a filmic look (S-shaped curve) and is not made to display “already OK” images (TV logo incrustation).

So if you need to apply a logo on your image, you need to mess with some inverted “RRT/ODT”.

Am I right?

Understanding the sRGB RRT/ODT

A 1.0 in ACES or ACEScg is not meant to map to a 1.0 in an output encoding such as sRGB. ACEScg [1.0] -> RRT -> sRGB ODT -> sRGB [0.812] The reason ACEScg [1,1,1] becomes sRGB [0.8,0.8,0.8] is because it is being rendered from scene-referred encoding to an output referred encoding. It uses an S-shape curve and so, as in most imaging systems trying to make a reproduction of a scene in a different viewing environment, a 100% reflector (1.0) in the scene does not map to 100% output (1.0) on a di…

I’m still investigating… Thanks for the precious information!