Compositing Fire

# 1 25-05-2012 , 07:47 PM

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Hi I have a bright studio scene lit with HDRi.

I rendered flames on another render layer separetely to get dark lighting.

I now want to composite the two.

But After Effects/Photoshop using lighten overlay seems to be the only option and comes out very light looking.

Is there some way to composite these correctly..

Graham

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# 2 25-05-2012 , 08:08 PM

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Do you want to upload your renders so we can have a look...............dave

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# 3 25-05-2012 , 09:22 PM

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ok, do I need some kind of matte?

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# 4 25-05-2012 , 10:10 PM

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Didn't you get an alpha channel with your flame render? If not, could mask it using it's own luminance. Also, I don't think you'd want to use lighten - just add it over as it is (with any colour corrections or grade you may want too).

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# 5 26-05-2012 , 07:38 AM

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(assuming after effects) If you can't get an alpha pass of it from the render as gubar mentioned, best bet would be to throw a simple combination of a feathered elliptical mask (animated to fill most of the fire) and a luma key (removing darker, tweaked to taste). The edges of the flame would would be unaffected by the add mask, and so would fall off naturally according to the luma, but the center (presumably with black smoke as in the above) would be entirely visible due to your keyframed mask. It may sound complicated but it's really quite simple to set up.

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# 6 26-05-2012 , 10:38 AM

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Your saying key it out, like similar to if it was against a blue screen, ya.

Well Im actually rendering the flames myself from a tutorial, so I have complete control that regard. Its just under the same lighting conditions in the scene the flames are too light. So I wanted to render them separately so as to darken them and then composite.

So as an excercise how is it best to render flames on there own?

Thanks

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# 7 26-05-2012 , 02:23 PM

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It was asked a couple of times already, but do you have an alpha? Good advice so far I'd say.

Last edited by honestdom; 26-05-2012 at 04:36 PM.

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# 8 26-05-2012 , 05:52 PM

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I did render with an alpha, but wasn't sure how to extract it in After Effects.

It comes in I say - straight unmatted. Which gives a black background.

How do I make use of the alpha in AE?

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# 9 26-05-2012 , 06:29 PM

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that's odd. what file format are you saving the image? If you're saving jpgs you won't get alpha. I personally save as .iff, but TIFF, TGA, and a whole bunch of other formats carry alpha into AE. Also, when you render, if it's a still frame, can you click on the show alpha channel button at the top of the renderview (it's the white checker circle next to an RGB circle) and see white/black outlines?

Also, be sure in Maya under "Render Settings>Renderable Cameras> Alpha channel (mask)" there's a checkmark. If that's off, maya won't spit out alpha and AE's interpret footage won't see an alpha channel.

I assume you're using Maya fluids? If the above doesn't get your alpha into AE, it may be an issue with your fluid's transparency settings?

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# 10 26-05-2012 , 07:48 PM

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Thanks for those points, I think Im on the right track. I now have files in AE using Alphas.
Im a bit embarrassed by not getting this to work, but may as well show my workings.

The fluid seems to be rendering fine in maya, but when brought in to AE they are more opaque. I did mess a bit with the transparency of the fluid as you said, but should it not be accurate in the Maya render? Also there is a Linear workflow in place.

cheers.

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Last edited by Sircharles; 26-05-2012 at 07:52 PM.

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# 11 28-05-2012 , 10:01 PM

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is the bottom the unaltered version comped onto the wood doll? Or did you change settings in ae for that image? That would seem like a really weird alpha result. maybe you could upload what the alpha matte looks like so we can see what's going on with your fluid?

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# 12 29-05-2012 , 10:35 PM

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when you import the file, and the window pops up that says, The item has an unlabled alpha channel, just select ignore and that "should" theoretically solve this problem.

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# 13 31-05-2012 , 09:07 PM

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is the bottom the unaltered version comped onto the wood doll? Or did you change settings in ae for that image? That would seem like a really weird alpha result. maybe you could upload what the alpha matte looks like so we can see what's going on with your fluid?

Originally Posted by petersrin

Ya Petersin, straight dropped on. But my thinking now is the fluid render needs attention.

It seems lighting the fluid plays an important role and I found a good post on this over at CGsociety:
https://forums.cgsociety.org/archive/...t-1015559.html

But obviously the actual Fluid needs more adjustment in transparency, opacity and texture, which I was at all day.

But if anyone has experience lighting Fluids in a bright environment please let me know.

Thanks

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# 14 01-06-2012 , 07:00 AM

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Can you post the alpha?

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# 15 01-06-2012 , 12:11 PM

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Thanks Dom you were on to it.

I had been working on it and discovered 'Matte Opacity' controls the alpha.

This can be increased above 1, and in my case 2.

So all in all, just needs playing with the texture.

Thanks all for contributing.

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