Rendering MultiMatte elements only

[tricky]

This must have been covered before, but I can't find a post.

I need to render out a whole sequence of multimatte elements (5 of them) but I don't want to re-render the actual rgb frame - just the multimatte elements.

Is this possible as presumably it would be much quicker than re-rendering everything?

[joconnell]

Not as such but if you turn off lights and shadows and put on a flat black material in the override slot it'll whip through quite quickly. Unfortunately the render elements are a consequence of the full render so you can't render just one aspect of the final image.

[Mats_O]

Check the RenderMask script. Has saved me days of rendertime when I just need to make masks. If you uncheck the "Use VRay Render" it uses the scanline instead and it is just lightning fast.

[joconnell]

Absolutely - only thing is it won't support vray dof or motion blur so your mattes won't line up.

[Mats_O]

No, not with the scanline but with the VRay it does. Tyhe nice thing is it doesn´t render the whole image, just the mask, which is a great timesaver in some cases.

[tricky]

Yes, I've used RenderMask on many occasions and it is great. I just had all my 5 3-channel MultiMattes set up, so thought it would be nice just to re-render from there rather than using RenderMask.

Also, scanline AA won't match up correctly, so although lightening fast (I sometimes miss scanline!), it isn't ideal.

I know that render elements are a consequence of the rgb beauty pass, but I had hoped MultiMattes were different as they are referring to the IDs of the objects only, and so they *could* be rendered independantly. Oh well.

I am resorting to an overide black material, but it does have its drawbacks: materials etc are still all loaded at rendertime, even though there is an overide on everything - this takes time. Also, I need to save each black-overide rgb-beauty image somewhere in order for it to save the MultiMatte images. This is a pain and can easily lead to mistakes with files being overwritten.

[instinct]

Mind you that you might get non-matching AA/Moblur when not rendering the actual RGB as the sampling might be a lot different. Esp with black there wont be any oversampling i'd guess.

Regards,
Thorsten