How to comp fire/strange alpha?

[Davinski]

Hello! I just purchased student versions of Vray and Phoenix a few weeks ago, and I used it on our latest project at school where a guy throws a molotov cocktail. Everything worked just fine until I had to composite my molotov unto live action footage. The whole setup isn't anything fancy really, just a bottle with a piece of cloth hanging out of it - where I had the fire emitted from. No liquids or anything.

The problem was the alpha, and I suppose that relates to the opacity of the fire. I might have been going all wrong with this, but I tweaked the settings under the Rendering tab to get the look I was going for. Mainly the transparancy. At the time I didn't think of it affecting the alpha (duh), but at the same time I don't know how to create good-looking fire without the transparancy. It turned out like this, without an alpha channel:

So far so good. But the alpha looks like this:

And then, the RGB with alpha:


I actually did get a good alpha if I used the SimpleSmoke setting for transparancy (in this I used temperature), but then the fire obviously wouldn't look the same at all.

Any idea where I went wrong? The project is over now, it turned out okay, but I'm sure I will stumble upon this problem again if I don't know what I did to cause it. Any help is greatly appreciated!

[LarsSonparsson]

I had the same problem. I found out, that if you have 32bit comp in After Effects, everything works just fine. 8 and 16 comps looks exactly like yours. Maybe someone could tell why this is happening. ?

[ivaylo.ivanov]

The VRay output is already premultiplied with the alpha.

[Davinski]

Oh shoot, I hadn't thought of that at all. Switching to 32-bit in AE makes it work like it should, I wish I would have asked this a few days ago.. Thank you!

[LarsSonparsson]

The VRay output is already premultiplied with the alpha.

Yes it is, but in AE it does work in 32bit only. We have had problems with Phoenix alpha in Nuke also. Usually rendering to full or half float exr. Any reason why?

[Ivaylo Katev]

as i understand you problem is the alpha? if you need just fire, you can use adding instead alpha blending in the compositing, and you have to use the atmosphere render element, to remove the bottle.

[LarsSonparsson]

So removing alpha with atmosphere element would work with fire and smoke comp also?

[Skybar]

as i understand you problem is the alpha? if you need just fire, you can use adding instead alpha blending in the compositing, and you have to use the atmosphere render element, to remove the bottle.

Thats a great tip, thank you!