aerogel..... key and premul differences...

hi guys, this seems like a bug to me… but i’ll show you anyway.

i am rendering some aerogel, but when the images are saved as Key rather than premul, they are completely useless when re-composited back to having a black background… in theory with a black background put back on the two images should be identical

the image on the left is the Key rendering, the image ont he right is the Premul rendering…

this is very concerning to me since i need Key to recomposite.

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I don’t know how advanced your skills are but a premultiplied image should work fine unless you are using Blender for professional work or if you are renndering close ups of transparent materials such as glass where a the edges are rendering black due to the render being premultiplied with black along the antialiased edges in order to knock out transparent image areas.
It looks to me as though you may have only rendered an alpha channel for the keyed image and left out the image information because I can only see the luminance values that you would expect to see in the alpha and all detail is suspiciously absent.
Just out of curiosity, why are you wanting to use a straight alpha rather than a premult? Is that some sort of product promotion for DVD, television, or kiosk?