reading render ID pass in comp

[sharktacos]

I rendered out a multichannel exr with a render ID pass.

It looks fine in the VFB, but in Nuke it just looks black. As I understand it, the render ID pass is integer numbers rather than colors. Does anyone know how I would be able to extract mattes from this in Nuke, or in any other comp program for that matter?

thanks!

(p.s. yes I realize that Chaos does not make Nuke)

[instinct]

The better way to render is using VRay's Multimatte. You can have up to 3 mattes per element (red green and blue). You get proper antialiasing as a "bonus"

[sharktacos]

The limitation with multi-matte is that it needs to be assigned manually. Render ID is automatic, so in a scene with lots of objects that can be very useful.

Out of curiosity, how are you extracting the mattes from the multimatte? In the test I did using Nuke there were errors (which i think is happening on the Nuke side, rather than on the vray side). For example of I have a matte of a characters shirt and another for the head and then combine them I get a thin black line alone the neck when it should be seamless.

[instinct]

RenderID is never antialiased so that will give you most likely a LOT of issues.
There are various scripts floating around to set objectIDs automatically if i am not mistaken. I am pretty sure that the RenderID (beeing automatic) might change when the scene is changed, no ? So you might
end up with a corrupted comp after a re-rendering. Don't know tho.

When combining mattes you have to use disjoint over.

Regards,
Thorsten

[aweidenhammer]

Totally agree with Thorsten... An ID assignment ideally should never change from one render to another. It will infuriate your compers we use materialIDs with multimatte passes and some simple scripts to assign them. You can still get mistakes in a complicated rushed pipeline, where people are editing like mad... but the mattes are perfectly antialiased. They can also produce mattes through refractive objects, and in reflections. See my workflow video here:
http://www.youtube.com/watch?v=xZNTUPDSPEo

Andrew

[sharktacos]

Thanks for the insights guys. Thorsten that's a great tip with disjoint over. I'm still getting used to Nuke from Shake, so was not familiar with that node, but I'll give it a try.
Andrew those are awesome videos!

So I'm wondering what the practical usefulness of the render ID pass is then? I was thinking that it would be an easy way to automatically isolate all the mattes in a scene for comp, but it sounds like the fact that it is integer based (ie no AA) makes that a no go. So I'm wondering what the render ID pass would be good for at all? Should there be a feature request to change render ID to output consistant AA colors?