Mattes don't seem to be matching up exactly

[evanerichards]

So I have a scene. It has two elements. I added a multimatte material to each object. When the matte renders out, it looks fine. The two objects meet perfectly and there is no space between the two. When I separate the two mattes out into individual alpha channels though and then join the two back together though, there is a gap between them.

I feel like they should be meeting perfectly and forming one solid white object. What's going on? Is there a way around this? Am I making some mistake?

Please find the attachment for reference.


Thanks!

[joconnell]

For any matte things, you need to unpremultiply them first. What program are you using to composite?

[evanerichards]

Nuke. I used a merge node with an "over" operation.

I tried unpremultiplying but it makes everything look blocky and unaliased.
Maybe that's what is supposed to happen though. I'm not much of a compositor myself I'm just trying to prepare some renders for our flame artists.

Edit: the "Over" seems to be the default merge operation but if I change it to a simple plus (A+B) then it seems to work.

[Svetlozar Draganov]

Glad to hear you sorted it out.
In general compositing separate matte-objects involves unpremultiply node before merge node and premultiply node after it.
Unpremultiply will of course make the edges jagged but it will ensure correct merge-process and then premultiply node will fix the edges.

[CerpinTaxt]

You can try "disjoint-over" in the merge node. That seems to be the correct blend mode for this scenario.

[Morbid Angel]

How we generally do this is shuffle out each id and the add them to produce single matte.

[evanerichards]

It ended up being a flame problem. In nuke it was solved with a simple "plus" node.

[Morbid Angel]

It ended up being a flame problem. In nuke it was solved with a simple "plus" node.

its funny I head this a lot. Flame is being used more and more, but yet it has a lot of issues with vray passes. I peronally dealty with that, it was reading all the buffers with [] and couldn't make sense of them.

Whats the benefit of flame anyway?

[evanerichards]

Our artists really like it for working with clients in the room as it can do most things in real time and doesn't have to do a lot of rendering. It's more real time than nuke I think (and after effects certainly).

We have had a lot of problems getting it to read things correctly though. You're right about that. For instance it doesn't do well with multilayer EXR's that have more than 8 or 9 layers. It can't read vrays zdepth pass (we have to break it out separately in nuke). Also any exr's we tweak and nuke and then export again all the layers have the red and blue channels flipped (all mattes that were blue are now red and vice versa for instance). So...yeah, A lot of issues but our artists seem to like it and I'm not the one making the software decisions so I guess they can do whatever they want

[joconnell]

Whats the benefit of flame anyway?

None from a quality point of view. The big sell is client interactivity. Flame comes on a fairly decent pc with a very fast disk array so you can play back footage real time, and when you're adding effects the feedback is very very smooth. If you're tweaking a blur or a grade for example, a client can see it change in real time with no stuttering so it's very handy for doing last minute changes. It can't perform miracles though, if you've got a huge node tree similar to doing bits in nuke then it's still going to be dog slow to render a 100 frame hd shot. Nuke is better for big comps, flame is very good for client attended finishing though.