Extreme aliasing in multimatte

[SonyBoy]

I can see the advantage of rendering most utility passes separately, since as you say once they are done they are done and usually very quick to output.

But in my mind the approach of rendering RGB matte passes separately seems to involve a lot of potential problems (this being the very topic of this thread), without offering too many advantages. In particular, I personally prefer to use material IDs rather than object IDs, the thinking being that if you want to adjust material A here, you more than likely want to adjust it there as well. This allows for a pretty straightforward material ID and multimatte setup in the beauty pass, which will have none of the antialiasing problems mentioned. Also, using a separate layer or even scene to get mattes out means replacing all materials (otherwise this is of course redundant), then worrying about quality and matching issues. Transparency and refraction are also not taken into account, which would be the case if output with the beauty?

[pixelapocalypse]

This is an interesting topic, and one that's currently relevant to some practices I've inherited at my place of work. I've generally rendered my matte passes along with my beauty renders in the past, so this issue has never come up. I can to some extent see the advantages of rendering a separate matte layer, especially if working with multichannel EXRs in order to keep file sizes manageable. But, if doing this to "save" render time but you have to crank up your settings super high, is there really going to be any noticeable time saving? Also, even if your quality settings are high, surely if rendering mattes with materials other than what's in the beauty passes, you run the risk of sampling not matching precisely, thus getting a matte that doesn't quite match the beauty?

I guess not, for me it wasn't a matter of saving time just that some multimattes and zDepth were incorrectly setup. In this case I thought I would save time by rendering them seperately instead of rendering the entire shot again with all passes. Not realizing these problems would come up. I probably would have been done already if I had known this and re-rendered the whole shot with all passes correctly setup instead of trying to fix this now.