Multimate element leaves 1 pixel edge in objects

[thanulee]

Hi, not sure if this is the place to ask, but what i see in After Effects sometimes, is that my selections from multimatte element are not clean. I use the "set matte" effect of AE, where u pick a pure RGB value to make an alpha channel out of it. It always leaves this annoying fringe on objects though which makes me use extra effects to clean it up and after stacking 2-3 selections the whole compositing becomes a bit problematic.
So, i am wondering, is this a multimatte problem or AE problem? Would a possible element of pure white on black work better? I always forget to ask this, cause i find ways of getting rid of this fringe but it would be nice to know the solution on the first place. Thanks

[LudvikKoutny]

That would be probably wrong gamma

[thanulee]

That would be probably wrong gamma

Nope its not that

[LudvikKoutny]

Ah, okay. It was my first guess because I had fringe problems with some mattes in past too, and it was because of the wrong gamma. Gamma affects gradient falloff at the antialiased edges of multimattes, and you said that stacking 3 mattes on top does the trick, which to me sounds very similar to adding gamma correction. (*2.2)

[thanulee]

Also another thing which needs adjustment. When i render my beauty with final quality is all is good in multimatte pass (except the fringe issue). When i wanna make some changes though in the selections (and this happens very often after render is done and while in post), i re adjust the colors of the matte element and i turn off everything like lights/gi/reflection etc, so i can render very fast the matte element, but seems like its working with samples. Therefore it renders very jagged and broken. Which shouldnt be the case cause this element should be calculated like alpha channel (idk the algorithm behind it but u get the idea), only geometries. Maybe this needs another approach? I cannot re render my beauty everytime i wanna change a selection. Ofc i have solutions for that, but are far from handy when i make constant changes among files and project versions/revisions. Any word for this as well? thanks

[Neilg]

When i wanna make some changes though in the selections (and this happens very often after render is done and while in post),

how are you in a situation where you need new multimatte colors but the geometry hasn't changed needing the base re-rendering?

you can render more than one at a time.

[thanulee]

I know that u can render as much as u want, but here is a practical example.

I have 3 selections in total. So ids 1,2,3 for rgb. Then in post i decide that 1 and 2 fit better my compositing purposes. So i need to go and make these both the same number. And thats just a tiny example of how things can get escalated in post.

And ofc i do not beforehand export dozens of selections that i dont know if i need. I just need the flexibility to re render very fast my mattes thats it.

[Neilg]

I think it would be easier to add those 2 mattes together in comp before using them rather than re render anything anyway.

if it's a single mask you're after then use something like the quickmask script - ( http://www.scriptspot.com/3ds-max/scripts/quick-mask )
multimattes are designed to render out masks of near everything in the scene at the same time as the base.

[thanulee]

I think it would be easier to add those 2 mattes together in comp before using them rather than re render anything anyway.

Thanks for the tip, but thats not always possible, in fact is very rare to happen in animations, especially ones with mblur/dof from 3d. As i already mentioned, i know how to deal with all those issues in post, im just pointing out a small problem in the pipeline.

Element should work as alpha does and shouldnt need samples.

[Neilg]

i'm pretty sure the alpha needs samples too.

you can also raise the minimum rate of your AA to what your old max was to do a fixed rate AA and get the mattes to have the same clean edge.

[squintnic]

You can just add more elements or render select ones with an extratex or quickmask

[thanulee]

Thanks guys, but thats not my point here, either how can i have other ways of doing it. I know how to solve this in post, is just the need to re render multimatte very fast and not need samples calculation for it. Alpha doesnt need samples, u can render anything without lights or color information and alpha is perfect.

Multimattes needs fixed sampling if u dont wanna re render the beauty and thats not optimal. It shouldnt work like this.

[svetlozar.draganov]

This issue is widely discussed on the forums not only for MultiMatte render elements but for all render elements.
The problem comes from the fact that sampling is based mostly on RGB pass, if it is black or near black V-Ray won't waste time to fire samples in other render elements since those won't make it to the RGB anyway.
Of course compositors would like to have more freedom and break the rules by making some invisible render element visible for the RGB pass and then the bad sampling pops up immediately.

A lot of people complains about that but I'm not sure when and how this is going to be solved since if sampling evaluates all elements rendering would become considerably slower than currently is.
Maybe we'll have to fix it for the MM elements at least - can you send us two example scenes by the way for demonstration? One where RGB pass is as it should where the MM elements get the correct sampling and another one which is optimized for MM-rendering only and where the sampling is incorrect. I tried to create one by myself but I failed, MM-element were identical.

You may also try Vlado's suggestion here:
http://forums.chaosgroup.com/showthr...610#post643610

[thanulee]

I will pm u the scene cause i cant share it, its an ongoing project but look at these 2 images. The one is rgb with element, the other is just the matte with no light/gi/maps etc. Ya maybe making multimatte only behave different is the solution. How can i send u the scene in private or doesnt needed it? thanks

[Raven]

If I have to recreate zdepth or multimattes for whatever reason, I use material override with grey material, disabled gi and a default light. It needs the contrast to get the samples.
If you are using material ids for your multi matte, then don't use material override, but disable reflection, refraction and glossy.