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Strange issue here that I cant seem to find any additional info on. I have a scene setup with a camera and an area light etc and a little robot with some animation and a thruster flame generated procedurally. I am attempting to render in out just the animated robot from the scene in an exr format with a transparent background. The render works perfect and displays exactly what I want to see in the render view and in the blender render window, however when this image is written to file I am loosing the bottom of the thruster flame. I am assuming this is some sort of transparency issue as the flame shows when it is overlapping the robot geometry but not when it is alone in space. Below are some images depicting the issue to describe the issue better.

Solid material view from camera:

Solid Material View

Cycles Render View:

Cycles Render View

Blender Final Render View:

Blender Render View:

Saved Exr Image:

Saved exr Frame

Any help, input or suggestion would be much appreciated. I have ran out of ideas!

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1 Answer 1

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You cannot composite flame with transparency or alpha mask because flame emits light, so it means it needs to be added to an image in mathematical sense, not mixed. The pixel values need to be added to whatever is behind it. Alpha masks are not intended to composite flame or other light sources and will not work. You should render emission pass separately instead and add it in compositing.

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  • $\begingroup$ Was thinking this was the case. Thanks for the info and prompt response, much appreciated! $\endgroup$
    – Sungru
    Commented Apr 25, 2023 at 21:03
  • $\begingroup$ I am not sure - it might be that it used to be possible to render flame with alpha, or that it even is possible now, but you shouldn't aim to do that because that will just lead to other problems. $\endgroup$ Commented Apr 25, 2023 at 21:07
  • $\begingroup$ Turns out it was an import issue. I was using after effects to do a quick slap composite and for some reason that info was being lost when importing. Tried importing the exr sequence into nuke and all the flame was preserved with its appropriate transparency! Thanks for your help and advice :) $\endgroup$
    – Sungru
    Commented Apr 26, 2023 at 23:21
  • $\begingroup$ "flame was preserved with its appropriate transparency" Flame cannot have appropriate transparency. No transparency is appropriate for flame. That will be flame over black background mixed with your image below. If there are any light tones in the background, there might be a grey muddy outline around the flame. If it's only tiny bit that is not well visible, maybe it doesn't matter, but it's all wrong if you use transparency. $\endgroup$ Commented Apr 26, 2023 at 23:42

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