Compositing fire.

farss wrote on 6/21/2007, 6:00 AM
Got many 'burning' things from Artbeats and DJ. The basic compositing is fine, alpha or mattes works as they should. The quandry is it never quite looks right. I know this isn't a trivial issue as I see exactly the same wrong look in older or cheap productions but on the other hand you see plenty these days that looks totally photorealistic.
Composited against black all they look just fine, so it's not the source that's wrong, it seems to be in the translucent part of the flame / fire that things go wrong especially against sky or water. I can't really put my finger on exactly what it is that wrong either. I suspect it's got something to do with these things being a source of light as well as transparent.
The bits I do already understand and have dealt with at times are that hot air bends light, displacement mapping takes care of that. I also get the problem of reflections of these luminous events although that's a bit outside my competancy and current toolset to emulate. What I'm really left puzzled about is just the basic addition of one thing onto another.

Bob.

Comments

richard-courtney wrote on 6/21/2007, 7:02 AM
The problem with fire is as you hinted it changes physical state from being
transparent to a solid capable of emitting light.

You can create a mask based on luminosity and come close (blue part
mostly transparent yellow not) The yellow part must then emit light. A pinpoint
spot light will light the objects around it, but that point must move about.

Try generating a luminosity mask and save it as a track. Then use the spotlight
fx passing light through this mask layer. It should give you shadows and
light for the background objects. Then place the flame track over all.
richard-courtney wrote on 6/21/2007, 7:18 AM
Slightly off your subject.....

I did low angle shot of a building with the clouds moving overhead and reflections
in the glass windows. Could have spent days doing a time lapse.
The sky was clear so with a polarizer recorded a beautifull blue chromakey.

The clouds in the sky was no big deal but the glass?

The glass is really a rectangle hanging in space so took a still and made
a mask using a paint program. Then made a "reflection" of the same clouds
using the 3D pan crop in Vegas.

Added this as a new track and reduced the level from 100% to 30%.

Hope that gives you some inspiration.
Tech Diver wrote on 6/21/2007, 9:57 AM
Bob, You could try setting the track composite mode of the fire clip to "Screen" instead of "Source Alpha". I have found this useful for certain effects that emit light and need to be somewhat translucent.
farss wrote on 6/21/2007, 3:20 PM
Thanks for the input.
I think you're close to what the problem is. The alpha channel in the clips causes the forground and background colors to add, so the translucent part of the flame changes color depending on the background and thats the part that looks unnatural. I suspect that in the real world the flame itself is also a color filter, so for example the blue of the sky gets filtered and the result of that is what gets added to the red or yellow of the flame.
From memory when you look through flame at something the image of the background is almost black and white, you see no color in it at all, the only color is from the light generated by the flame.
Need to play around with this some more, more out of interest than any pressing need.

Bob.
GlennChan wrote on 6/21/2007, 11:16 PM
Not sure if this helps... what if you go into the event's properties, into the second tab, and pick straight alpha instead of premultiplied?
farss wrote on 6/21/2007, 11:46 PM
Thanks Glenn but I've tried all alpha types and only straight works at all, it least with the MOVs out of DJ's Juicer.

Bob.
apit34356 wrote on 6/22/2007, 3:59 AM
Farss, one problem with fire, is that it generates light that is reflected back by close objects. So, generating this effect of reflected fire "light" requires some work, masking individual subjects.