Problem Rendering Mattes with MR

[origin]

Just did test rendering, I got the same image pixel by pixel (constant white sphere on constant black grid VS constant white sphere on 0-alpha background) alpha=pixel luminance/r/g/b

[Lord Futzi Voldemort]

Yes. it´s true. With linear exr there is no difference. This is definitely the way to go then.

Thanks again

Achim

PS: only for the record, and to avoid misunderstandings in the future: premultiplication is the process of multiplying an images colorvalues with it´s own alpha, while multiplying a foreground or a background with it´s respective matte, although technically the same, is some thing different workflowwise, and does not neccessarily need an alpha channel.

[Lord Futzi Voldemort]

Now here comes the fun part. Below is the exact same thing done in Blender. I wonder why I didn´t do that earlier....
I´m sorry to say, this proves you all wrong. Simple 8 bit png, saved from blender, no alphas, no premultiplication, no complicated setups in nuke to resemble a merge node, no linear gamma.
It works EXACTLY as it is supposed to work, and how it works for 15 years for me. There IS a mentalray issue.
(And yes, I´ve tried png with mr too)

Achim