Cryptomatte + roto = edging problem

[JulianD]

Hey guys!

I come into a problem when it comes to use cryptomatte as a mask of a roto node. It's not really about vray for nuke but as I didn't find any other place to ask, so I'm here
I want to mask a part of a cryptomate and it gives me a 1 pixel wide white trim on the border of the crytomatted object, everytime.

Here's a quick setup to show you the issue:



I tried to use a OCIO converter from ACESCG to Raw for the cryptomatte, I also tried to unpremult the exr, those didn't change anything.

Vray Next, 3ds max 2020, Nuke v12, cryptomatte (version downloaded a week ago), Encapsulated 16bit EXR (I also tried 32bit, same result)

Thank you

[frueter]

I think this is likely due to your premult setup. I can't quite make sense of your node arrangement, but try unpremultiplying the cg, then creating a new alpha with Cryptomatte, copy the result into the cg and premult. Or you could create the cryptomatte of the bit you want to remove, then subtract that from the alpha in unpremultiplied state, then re-premultiply. There are other ways as well (as always), but one of those should do the trick.

[JulianD]

I can't quite make sense of your node arrangement

Well I have my CG with a roto on it that cut a piece of the image. As I don't want my character to be affected by this cut I use a mask (cryptomatte) on my roto that only selects the "mesh" I want to cut, and then I premult. The result works but there's a one pixel line that remains on the edge of the cryptomatted object.

but try unpremultiplying the cg, then creating a new alpha with Cryptomatte, copy the result into the cg and premult.

That won't give me the result I'm looking for: I want to cut a piece on a specified mesh not on the whole image.

Or you could create the cryptomatte of the bit you want to remove, then subtract that from the alpha in unpremultiplied state, then re-premultiply.

Yes but how could I create the cryptomatte of the bit I want to remove?

Here's my mask from my cryptomatte and you can see the roto on it.