I am rendering a fire to a multilayer EXR and then composite it using the method described in this answer ontop of my footage. One comment states that it is difficult to work with my EXR because I only have a beauty pass and my emissions and transparency aren't separated. I also get all sorts of wired effects compositing my exr when I try to pre divide/post multiply. But if I do not do that everything tourns with.

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Here it gets blocky

Nothing I was just trying to bring the highlights of the fire a bit down but it messed the image up so I pressed premultiply / post divide and then it messed the rest up. I am wondering how blender outputs the exr is it already premultiplied or is it straight?

Reference files: https://drive.google.com/drive/folders/1HatjWYAmmXHfEiaTNYChJYBK0PZVvMo7?usp=sharing

  • $\begingroup$ Please describe what you are doing with the alpha channel. $\endgroup$
    – susu
    Sep 24, 2020 at 0:57
  • $\begingroup$ Added some info $\endgroup$
    – Phönix 64
    Sep 24, 2020 at 9:33
  • $\begingroup$ Proper alpha encoding, aka associated (premultiplied) is the only alpha encoding that a path tracer can generate, and is the only alpha encoding that works. By default, all path tracers will generate associated alpha encodings. It is literally impossible to model things like fires, flares, reflections, glows, etc. without it, as it is the only encoding that allows encoding of emissive yet unoccluding mixtures such as 0.0 3726.2 0.0 0.0 for example. A generic alpha over is how fire is composited. $\endgroup$
    – troy_s
    Sep 24, 2020 at 13:52

1 Answer 1


As @troy_s stated in this case a normal alpha over should be used. In Resolve, this translates to a merge node with the Alpha Gain turned to 0.


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