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motion tracking

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Lee Tamermotion tracking
by on Nov 17, 2009 at 2:11:16 am

Im new to the forum and fairly new to after effects. I have a kinda tricky question. How would I go about having someone set on fire? I have a green screened video of a person running around as if he was caught on fire. How would I attach the fire elements from action essentials 2 to my actor running around so that the fire is constantly following the actor on his back?

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Michael SzalapskiRe: motion tracking
by on Nov 17, 2009 at 5:08:35 am

From the incomparable Dave LaRonde
Dave's Stock Answer #2:
When you're out on a shoot, and you say, "we'll fix this in post" without knowing PRECISELY HOW you're going to fix it in post, don't shoot it! You'll only end up shooting it over again.
Since post typically costs three times the cost of production, fixing something in post is not a way to save money, but rather a way to spend more of it.
And, before you say "well fix it in post," always consider who's doing the work, especially if you're the one doing the editing.

There is a reason special effect houses charge hundreds of thousands of dollars. Making realistic special effects takes a lot of time, work and experience.

To get you started, try out some of the tutorials on the COW about motion tracking.

Realize that without the light from the fire on the actor or the fire moving in relation to how the person is moving (wind, etc.) it doesn't matter how good your track is, it's going to look fake.

You can compensate for the light on the person with an adjustment layer between the fire layer and the person. This layer should brighten the person and add a bit of orange color. Use a mask to shape the glow to just the part you want. You could then use the same tracking information you get for the fire to keep that glow right there, or simply parent it to the fire (or the fire's null).

As for the air moving the flames as your actor runs around, this might not be too noticeable, but you can always use AE's distortion filters (or maybe the puppet tool) to reshape the flames.

If your actor turns around or has limbs flailing - as is to be expected from someone set aflame - be prepared to enjoy rotoscoping.

Hope that helps.

- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')

No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.

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