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

Discussions - Using ACES VFX
2

Welcome Dorian,

Long time no see! :slight_smile:

So what is happening here is that:

  1. The First OCIOColorSpace node converts from Non-Linear sRGB to Linear ACES-2065-1 by applying the sRGB Electro-Optical Transfer Function (EOTF) on your image via a 1D LUT and then applying the sRGB to ACES-2065-1 via two concatenated transformation matrices.
  2. The Second OCIOColorspace node then applies the Reference Rendering Transform (RRT) plus the sRGB Output Device Transform (ODT) via a shaper LUT + a 3D LUT. That transformation is entirely different than the Inverse sRGB EOTF that you would need to use here to get back to your original image without a View Transform.

So basically, your second OCIOColorspace needs to re-encode your image to Non-Linear sRGB by applying the exact inverse transformation that the first OCIOColorSpace node applied, i.e swap the in and out. Note that in this particular case if you keep everything Raw, you will see the exact same image as no encoding or decoding will occur.

Hope that helps,

Cheers,

Thomas