Particular Halo

Adobe After Effects Expressions

Particular Halo

Posted by Mark Poole on January 13, 2009 at 5:35 pm

Particular Halo
Here’s the situation:

I am creating an effect for a glowing halo to circle an actor’s head (shot against greenscreen). She moves about a bit.

The concept is a particle effect (Trapcode Particular) with the particle at birth being at it’s largest size, then a rapid falloff to pencil thin. The particles should form a circular trail in the form of a halo. It should look like an electron circling an atom and leaving a glowing tail. The particles will have a slight glow applied. I’ll kill the particles off after one rotation of the emitter. It’s ok if the head moves and the particles move away from the head (slight inherit motion)

I will manually track the halo to the head (the shot is only 2 seconds long). I am thinking that I can use the keyed plate as a 3D layer and have the 3d particles on the backside of her head masked out.

The math and mechanics is what is dogging me. I’d like a mathematical solution, since I will be using the effect on other shots, as well.

My idea is to link the Particular effect to a null, rotate the null, and translate the null about to move the entire setup about.

However, when I pickwhip the location and rotation to the null, it attaches to the nulls rotation point. So when I rotate the null, Particular rotates in place. I then tried a lame x+100, which did move it out, but it still rotates in place.

Any hints?

Thanks in advance!

Mark E. Poole
Director
Anubis Digital Studios

Trent Armstrong replied 17 years, 3 months ago 4 Members · 5 Replies
5 Replies

Victor Nash
January 13, 2009 at 11:12 pm
```
r = 50; //radius (in pixels)
f = 1; //orbits per second

angle = time*Math.PI*2*f;
value + [Math.sin(angle)*r, -Math.cos(angle)*r]
```
I haven’t played with using Particular yet, however you could use 3d Stroke.

Just enable the taper and adjust the Start Thickness to 0 and End to 100. Perhaps adjust the end to a bit less then 100 so it doesn’t ‘connect’ as much. Check Loop, then animate the offset. You could also enable the Comp Camera and parent the camera to the Null.. Then you have complete control.

I’ll play with Particular tonight and see. This is me off the top of my head..

Also, here is nifty Orbiting Expression
Brendan Mccullough
January 13, 2009 at 11:19 pm

I was playing with making the emitter do circles this morning, so maybe this will help you. I may be misunderstanding your problem.

I applied the following to the X/Y position property of particular:

width = someValue; speed = someOtherValue;
value + Math.sin(time*speed)*width;

and then to the Z position property:

width = someValue; speed = someOtherValue;
value + -Math.cos(time*speed)*width;

this is off the top of my head, so please forgive any errors.

Anyway, this will create a halo ring, then animate your null with the camera parented to it.

Victor Nash

January 14, 2009 at 1:20 am

Radius = 100;
Speed = 200;
RotationOffset = 0;

center = [this_comp.width/2,this_comp.height/2,0];
radius = 200;
angle = time * Speed + RotationOffset;
x = Radius * Math.cos(degreesToRadians(angle));
z = Radius * Math.sin(degreesToRadians(angle));
add(center, [x,0,z]);

Here you go.. Try this. Make sure to pick whip the XYZ rotations to the Nulls, as well as the XY Position, and Z Position from Particular. Then Apply this expression the the Null’s Position

Mark Poole
January 14, 2009 at 1:39 am

Thanks, both of you. I’ll give it a try and let you know.
Trent Armstrong
January 14, 2009 at 6:26 pm

Instead of parenting the particular layer to the Null or person, you should use the pick whip. Pick whip the Y rotation of the Null into the Y rotation of the particle emitter.

I also think you should might be better off using 3D Stroke. You’ll get better trails that way. Parent the 3D stroke layer to the girl, set the stroke up with the taper. Then adjust the rotation of stroke using one of the expressions provided by the other contributors in this thread.

Then duplicate the 3D Stroke layer and bring the end point close to the beginning point to get only one dot.

Trent

Trent Armstrong – Creative Cow Leader
https://www.dallasaeug.com

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