Hasta La Motion!

I am doing some roto in Nuke and I’m not too sure if these edges look like they’re supposed to. When I look at the edges after the premult, they stand out like they’re including too much background. Is this because the object being roto’d is quite dark and the background is a lot lighter, therefore the blurred edges will naturally be slightly lighter than the core of the object, making them stand out in the premult?
Then when I look at the premult over grey, it looks quite normal.

Should the edges in a premult over black, look the same (or almost) as a premult over gray, or is it inevitable that the edges over black will have more contrast to the black background colour, which makes them stand out more? :confused:

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