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When I F12-render a scene with a mix of Octane world-light and added lamps (Octane materials), the F12-rendered image is 4 or 5 stops brighter than the Viewport render. Plus, a black and white image gets pink(ish).

What's wrong?

Color Management is set to None.

But the difference between the two renders is - literally - like day and night...

The overall image quality of the Viewport render is also richer; there's more contrast but at the same time a wider dynamic range; more definition in lights and shadows.

Viewport-render and F12-render, same area...

OK, this is really weird:

You know the way you can turn Exposure and/or Gamma up and down in the Viewport render and get logical, dramatic and predictable results?

The F12-render hardly responds at all to Exposure changes in comparison.

Even extreme adjustments from, say, 0.2 to 1.2, only slightly modifies the exposure.

Am I missing something, or does the Viewport render not serve any practical purpose?

OK, now I get it; some of it anyway:

The Viewport render is simply useless; it doesn't compare to F12 in any predictable way. And the Exposure and Gamma setting under Octane Camera don't do anything either.

BUT you can achieve the most gorgeous Exposure and Gamma effects under Color Management! In real time! So that's just an awesome and completely functional replacement for the Viewport render.

I was under the impression that anything under Color Management should be disabled or ignored (as in changing sRGB to None), but that was really, really a mistake.

Seriously, I'm not gonna get any sleep tonight, need to play with Gamma...

If you want to have exact image in vewport and in final render you have to remember of two things:
- Turn on "Octane Imager" check box in camera settings.
- In scene settings -> Color management > set Display Device to "None", Exposure to "0.000" and Gamma to "1.000"

You can still use Blender's Color Management in final render but only with above settings it will match viewport and final render.

J.C wrote:- Turn on "Octane Imager" check box in camera settings.

Where specifically is the check box - I can't find that. It seems to me that Octane Imager is always on and we just choose which presets (or linear) to use?

It's here but I think it was removed from V3.x. I'm still on V2.x.

J.C wrote:If you want to have exact image in vewport and in final render you have to remember of two things:
- Turn on "Octane Imager" check box in camera settings.
- In scene settings -> Color management > set Display Device to "None", Exposure to "0.000" and Gamma to "1.000"

You can still use Blender's Color Management in final render but only with above settings it will match viewport and final render.

Thank you again J.C -- but I did use these settings in my samples (except for the Octane Imager check box, which doesn't exist in my version; please see JimStar's explanatory link in this thread).

But, as you can see in the samples, viewport and final render are like night and day...

So, for now, I stick to my conclusion:

The viewport render is useless, but the Exposure and Gamma settings in the Color Management make up for this since they can be used to modify final renders in real time, so you don't really need a preview render.

@justavisitor why don't you upgrade to latest version? It works as it supposed to work. No differences in renders.