AE fire and physics....Wow.

Andrew Brinkhaus

Andrew Brinkhaus

Veteran
AE fire mixed with live action!

Ok, so I have a scene coming up to shoot and do VFX for, where two characters are in a beautiful forest, then suddenly, Character "A"' brings fire from his shoulder armor, these are two warriors fighting. As he angers, the fire ignites his sword, engulfing it in flames, as he fights with the other character. The flames on his shoulders die down.

WHEW.

So, the shoulder stuff I can do, easy. I am using Trapcodes Particular, and just customizing a fire particle. The reason I can do this, is because the emitter is the characters shoulder, and that remains somewhat static, and upright, so the gravity of the fire is upward, congruent to his body. Simple enough. When the fire engulfs the sword, how can I make the angle of fire change as he swings around, etc.? I will be roto'ing the movement, frame by frame matching to the swords movement, like I would if it were a lightsaber sequence...But how do I get the flames to "whip" around like the wind is affecting the flames?! For a little smoke to excrete from the flame also, how do I get it to rest in the shot like it is dissipating off from the flame?

I have plenty of time to do these, but I am just so blown away by all Particulars physics and Aux settings, I don't know where to start. I searched all over Google and Trapcodes site, for any firm tutorial info, but nothing came up in terms of fire, and physics. I also PM'd Andrew Kramer, so hopefully he can help me out, as well as all of you. Thanks!

AE 7 Pro
Trapcode Particular
 
It will be a lot of manual keyframing trying to do this with Particular. It has a small physics system built in, but it isn't procedural like a 3D package would be. If you motion track (or roto) the sword you might be able to do something with expressions. You could add an expression that tells the wind direction of Particular to change with the position of the sword.

The smoke you can do with the Aux Particles in Particular and changing their life settings to die out slow.
 
Yeah, that is what I was thinking, I just don't know a lot about expressions in AE.

I was planning on using the Aux system to do the smoke, but the thing is, if you can imagine the speed that someone would be swinging a sword around, how can I get that smoke to "hang" in the air, THEN dissolve, instead of being dragged around by the emitter, then dissolving?

Lets say for a second, that I decide I just want to use fire, with no smoke...I somehow need to get the angle of the flame to have a dedicated position that it is trying to go, like the wind is pushing it, so if I motion track the flame to the sword, or roto it, when I rotate the fire layer, that changes particulars reference on the direction the flame was going, so now the wind and physics is unrealistic, because it looks like the wind is changing directions every few seconds...
 
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I would imagine to accomplish this via keyframing would be terribly hard. Expressions would be pretty complex if/else statements using arrays, etc.

How thick is the sword? What is footage shot on? What kind of realism do you want?

I can't access youtube here at work...I might test something out tonight...seems like an interesting challenge.
 
The motion blur helps alot, but the direction that the flames whip when the sword is moved is whats challenging me.

The sword has about a 2 inch surface width, and this is shot in HD 1080i. Realism isn't as much of a concern, because I tested flames out already, that I did with Particular, and I can get the look of the flame that I want, just not the most realistic motion.
 
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One suggestion, if you have access to it, is to motion track the sword with a 3d tracking program and do a 3d solve on the scene. You then export an AE comp with a 3d camera and tracking points for the sword. You can parent the particular emitter to the sword tracking nulls, and through expression you link the direction of the wind to be the inverse of the direction of the null. The trick is the 3d tracker will export movement on all 3 axes, which you can use that to drive the respective axis of the emitter and wind (providing that both provide X, Y, & Z movement, can't remember if wind does) then you have movement in all three dimensions. The wind will be the inverse of the emitter movement causing it to "blow" away from the emitter origin.

Turn on some moblur and you're good to go.
 
That sounds like it would definitely work... I don't have access to 3D tracking util's though... Any way you think I can manipulate this process inside AE? Couldn't I just use expressions in AE, inversing the wind direction of the null, to "blow" away from the emitter origin? I feel like I could track this in AE pretty well, however I would only have 2 dimensions, I think that would be enough.
 
why dont you turn the layer first into a 3d layer, then put in a approximate geometery something like a 3d box (but with the dimensions of the sword) then hand track it in 3d to macth the real sword, then link or attach the particle system to the 3d box.

I'm just learning AE myself so thats just my random crack at it.

Of course for this a object tracking (using a 3d tracker) would be the best way to go, then of course you should be able to bring that data back into AE.
 
Well, its not so much the tracking of the emitter to the sword that i'm having trouble with, its the "flame physics" ex. the way the flame whips around when the sword is swung, etc. Making it follow its own wind path, instead of just drag along with the emitter origin.
 
I'll see if I can whip up an expression for you tonight. My company just picked up SynthEyes and I would offer to help out with a 3d object track, but we're heavy in a project and I'm not sure if I would have time.
 
The simplest and best way to pull off this shot wold be to use Particle Illusion. They even have a preset called Flaming Sword, though that particular emitter is not that great. Some of their simpler fire emitters put along the length and then angeled as the sword swings give suprsingly good results.

It won't look like the Youtube example, but it is FAST, and allows for the use of (albiet limited) particle dynamics.

I have done the flaming sword thing, and it works wonderfully. Mostly, it's about timing and changing the emitter angle accordingly to get the best results.

Particle Illusion is a hell of a cheat, but it does have some simply kick ass results, but limited to 2D. Limited, when you don't know how to map an animation to a 3D object, that is.
 
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Thanks Matt, i'll give it a try, see what kind of results I can get. Still though, I would love to be able to do this with Particular, because I am more pleased with the fire I am able to achieve, as well as AE's interface.
 
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Ohh, you want the flames to wip round and move realistically, that can be done pretty easly in most 3d apps using particles, wind forces and gravity forces.

I think its a case where you will need to look in a 3d app,

object tracking + CG flames + Compositing.
 
Hmm...I wish I knew more 3D. I don't have much experience in that realm. I am starting to get some good results with Particular, I figured out the whooshing effect, using physics time factor, and motion velocity. I should have a test up soon.
 
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I messed around with Particular a little bit trying to figure it out with expressions with no luck. Plus, Particular in AE on a MBPro is no fun :(
 
can you send me a jpeg sequence of that shot, maybe a photo of the prop (so I can make a proxy model, dimensions would be nice)

I'll see if I can track the object in 3d and then attach fire particle and some nice physics with it,

should be a laugh...
 
Test is almost done, should be online tonight. I just shot some test footage, as this is just a test sequence to make sure this effect is going to be achievable.
 
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