Render layers with straight (non-premultiplied) alpha for compositing

Hey! Could someone tell me how I render an object with volume scatter material over transparent background using render passes, so I get a layer suitable for compositing with non-premultiplied alpha? :slight_smile:

You can always unpremultiply a layer in compositor if needed using AlphaConvert node or by dividing the RGB channels with alpha channel.

Why exactly do you need the layer to not be premultiplied? You get aliased edges, light rim or wrong colors on semi-transparet parts if you do not use premultiplied layers in operations that by default need them to be premultiplied.

thank you for the answer! I am trying to make a cloud layer to use in the compositor over different background/sky. I believe, that when I render clouds over transparent backgound (Film->Transparency) I will get darker edges of the clouds. I would like to know, which settings should I tweak before rendering this cloud layer so I will not get problems at the post production.

Did I understand you right and I can go with the default and just use Alpha-converter node in the compositor?))

You get dark edges if premultiplication is done twice. Rendering with transparent sky setting is the only way to get alpha channel. If for some reason some node expects unpremultiplied input, use alphaconvert node to remove premultiplication. Default setting is almost always the premultiplied one because (almost) all operations that inviove more than one layer and transparency need premultiplication and it is more efficient to prepare elements for this beforehand.

allrughty) thank you very much for the advice! I will try what you’ve said, and if it does not work out I will have to ask you again to clarify :))) i hope you do not mind)))