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View Full Version : Flame Elements shot on Black.


Daniel-B
02-15-2005, 08:45 PM
Hey guys, how are fire elements shot on a black background composited at a professional effects house. I'm having trouble getting a convincing look to some fire I shot on a black background. How would you go about doing this?

joconnell
02-15-2005, 09:29 PM
There's a good few ways you can use them depending on your personal preference and what looks best in the shot. The quickest way would be to use a blending / layering mode in your compositor - Place the footage over the background using screen or add mode depending on how intense you want it to look.

Alternatively if you want to make the flame look more solid (blending modes will have a lot of transparent areas) you could use a luma keyer to seperate the flame from it's background or alternatively use the brightness of the flame footage as a matte - This method will give you more solid areas of flame where the background doesn't come through quite as much which might be more appropriate.

The last thing to remember is that if a fire was in a background, it'd be chaning the lighting and colouring of everything around it - The background that you are comping the fire over should be colour corrected to take into account the new yellow and red light coming off the fire - you can do some soft masks in your compositor to make the fake lighting fairly intense in the area directly around the fire and make it graduall fall off - As far as I know fire doesn't really provide that much illumination. Another that might be an idea is to animate the intensity of your colour corrections - fire will tend to flicker a little bit and so you could look at animating the colour corrections on your background to flicker subtlely too.

Also remember that the fire should be reflected in shiny objects nearby :)

Best of luck - hope it helps.

Daniel-B
02-17-2005, 04:57 PM
Thanks for the help. Is this how big boy effects stuidios like ILM composite their fire?

curious_69_george
02-19-2005, 08:43 PM
They use a plugin called "Unmult". Which removes the black background, or whatever colour your have, from the footage.

It is not to be confused with a keyer.

Then you can use a filter to soften the edges. The "unmult" will also take care of removing transperency in and amongst the flames.

Hugh
02-20-2005, 11:01 AM
Can you explain more about "unmult"? Sounds to me like a different way of saying MDiv.... (Divide by the matte)

Personally, I've usually used luma-keys - luma-key the flame, put it over the background, and then blur is and screen it over again (this gives a glow around any occluding edges in the scene....)

curious_69_george
02-20-2005, 04:24 PM
It is a form of luma-keyer.

Just type unmult in google. You will get all the info you need.

jussing
02-20-2005, 07:20 PM
Unmult is - as far as I know - developed to un-premultiply image data that is premultiplied with the alpha.

And yes, that works by dividing image data with alpha data.

AFAIK it is developed by John Knoll... or his brother?

- Jonas

beaker
02-21-2005, 01:50 AM
Unmult is not a keyer, it is exactly what Hugh and jussing said it is.

jussing: it was made by john knoll. The math is simple, probably took 30 minutes to make the whole plugin. It's amazing that he sold that thing for $99 all those years.

Daniel-B
02-21-2005, 03:06 AM
Can Unmult be done in After Effects or Combustion?

Ian Jones
02-21-2005, 07:19 AM
yep certainly can, if you check the offical site, do a search for unmult and its the first google result. A demo for AFX 6 can be dloaded.

Hugh
02-21-2005, 09:07 AM
Unmult is not a keyer, it is exactly what Hugh and jussing said it is.

jussing: it was made by john knoll. The math is simple, probably took 30 minutes to make the whole plugin. It's amazing that he sold that thing for $99 all those years.


Beaker: thanks for that... I had a look at the website, and I couldn't quite see how it differed from the way I was doing it manually.... It looked to me just like a macro that I reckon I could put together in 10 minutes....

jussing
02-21-2005, 09:21 AM
The bizarre thing is that After Effects, for the love of god, STILL doesn't have that simple division implemented, so a plug-in like Unmult is needed. Sheesh. :rolleyes: :D

Cheers,
- Jonas

joconnell
02-21-2005, 03:02 PM
Yeah it comes with ultimatte and knoll lens flare to generate alphas for their flares too - You could also use the footage as its own luma matte to do something similar.

P.s. Hugh - hows life in mpc? I nearly ended up working there last year on troy in the 3d dept. Will probably be looking at moving to london again soon - what are those guys like to work for?

Hugh
02-21-2005, 03:19 PM
Ahh - okay - if it comes with another (useful) tool, then I can understand a lot of people using it.... I just can't see how a company with it's own technical people would pay money for something that would take one of their tech people (or a slightly technical shake person) 10 minutes to do....


MPC is good these days - lots of work on over here at the moment - we're just finishing up on Kingdom of Heaven, and Charlie and the Chocolate Factory is in its last few months too....

We've got a few big projects coming up - I'm not sure if we've got anything that involves big armies or cities again, though (I think there are a few people here who are absolutely sick of soldiers by now, after Troy, Alexander and Kingdom of Heaven....)

Blur1
03-05-2005, 05:42 AM
I read an email post a long time ago from a guy working in the Rebel Mac crew and he bought up this exact issue, but also it refers to lens flares, car headlights, any bright light source being comped. Basically yes he described unmulting, but the way they did it manually was to take the flame layer and use its luminance as a matte for itself, then add or screen it on top of the background. This solves a common compositing snafu where bright light sources placed on top of darker backgrounds with an Add or Screen mode also brighten the scene behind them which is physically inaccurate. I have seen lots of flame* operators make this kind of mistake ;)

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