How to composite exr without alpha?

[verty (Wolfgang)]

I have a set of exr render passes (e.g. diffuse, volume, glossy).
Those exrs don’t have a transparent background - instead where there is no data they are just black.

All the tutorials I found use the alpha over node to composite. But how to do it when there is no alpha?

In a 32bit capable image editing software I can use the “add blend mode” to recreate the beauty pass, but how to do it in Blender?

[sozap]

the passes should follow a strict pattern to recreate the beauty pass.
I think it’s (diffuse direct + indirect) * color
And you add diffuse , gloss and volume like that.

Then for the transparency, you need to export that as well and add alpha as a last step.
You can use the set alpha node for that.

So for mixing passes you don’t need alpha, but you still need to have one alpha pass to finish the comp if that’s needed.

[verty (Wolfgang)]

Thanks for the answer.
I just found out that Color Mix set to ADD works like the add blend mode in the image editing software. My LPEs are set in a way that all light passes appear in only one pass and include their respective color information. So a simple add works here without alpha.