Building Your Own 3D Particle Generator
Dan Ebberts July 18, 2023
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In this tutorial, Dan Ebberts explores how you can use expressions in After Effects 5.5 to build a fairly sophisticated 3D particle generator. Our particles will respond to initial velocity, gravity, wind, drag, and much more. Using this particle generator, we'll look at how you might create explosions, smoke, a fire blast, falling snow, and even a liquid-like flow. We'll explore how to make each particle re-usable, which will allow us to simulate fountain-like particle streams. This one is quite advanced.
Explosion MP4
3D Particle Generator .zip
the basics The basic idea is that we will import a graphic representation of a single particle. We'll drag it into our comp and make it 3D. We'll apply our physical simulation expressions to our particle and then duplicate that layer enough times to get a convincing result. We'll set the expressions up so that each particle (layer) will behave differently. We'll be applying expressions to many of the particle's properties, including position, scale, opacity, and rotation.
We'll start off with a simple explosion and develop more complex expressions from there. It will eventually involve some pretty hairy math and some physics (sorry!) but the good news is that you don't necessarily have to understand exactly how the expressions work - you just need to know how to modify them to get them to do what you want. In the first part of the tutorial, what I'll try to do, as much as possible, is to cover the basics of how the expressions work and how you would modify them. Then, towards the end, the stouthearted can follow me through the hairy math and physics.
the strategy We want each of our particles to have a defined life span that will be random within a certain range. Since we'll be applying expressions to multiple properties of the particle, we need some way to communicate information about the particle's life span to all the expressions that need it. We'll do this by performing the life span calculations in an expression control (if you want to learn more about expression controls, see my tutorial Building the World's Greatest Cameraman (COW Admin: coming soon)). We'll then use the expression control as a read-only global variable that can be accessed by all the other expressions applied to our particle.
let's begin! Go ahead and open the project file and then open the explosion basic comp. You'll notice a graphic layer called star flare and a null layer, which both have 3D turned on. We'll be using the position of the null to set the center of the explosion. We'll refer to the null as the particle emitter and later we'll move the null around to impart emitter velocity to the particles. Select the star flare layer and type u to display all the properties that have expressions applied to them. Also type Ctrl Shift t (PC) or Cmd Shift t (Mac) to display the effects applied the layer. You'll see that a point control has been applied and renamed life . You may be wondering why we'd use a point control instead of a slider control to hold the life value. The reason is that later on (when we get to re-usable particles) we're going to need to pass both life and birth times so we need a control that can pass two values. Couldn't we just use two slider controls? No, because the calculations for birth and life have to be done within the same expression to make it work. In the timeline, click the twirly next to Point to reveal the expression applied to the life point control:
lmin = 1.5; //minimum particle life
lmax = 2.5; //maximum particle life
seed_random(1,true);
life=random(lmin,lmax);
birth=0;
[birth,life]
This expression will generate a random life value for our particle that is between 1.5 and 2.5 seconds. This value will be plugged into the second element of our point control variable. The first element ( birth ) is just set to zero since for this first explosion comp all our particles will be born at time zero. The seed_random call just tells After Effects that we want to get the same random value for life every time we run the expression (otherwise the life value would hop around on every frame - which doesn't help us here). If you want to learn more about seed_random , see my tutorial Generating Random Motion (COW Admin: coming soon). If you wanted to change the life span of the particles, you would just change the lmin and lmax values.
Click the twirly next to Position to reveal this expression:
vmin=500; //minimum initial velocity
vmax=700; //maximum initial velocity
birth=effect( life ).param( Point )[0];
life=effect( life ).param( Point )[1];
origin=this_comp.layer( Null 1 ).position.value_at_time(birth);
age=time-birth;
seed_random(1,true);
s=random(vmin,vmax);//initial speed
a=degrees_to_radians(random(180)); //angle from vertical
r=degrees_to_radians(random(360)) //rotation around y axis
x=s*Math.sin(a)*Math.cos(r);
y=-s*Math.cos(a);
z=s*Math.sin(a)*Math.sin(r);
v=[x,y,z];
origin v*time
This is the basic position expression for a zero g explosion. Our particle will launch at a random direction in 3D space from the origin (which is just the position of the null layer) with an initial speed between 500 and 700 units (which roughly equates to pixels per second). The particle will continue at that speed and direction until it dies out. We haven't yet factored in wind, gravity, and drag. I'm sure you noticed the three lines that use the JavaScript trig functions to calculate the x, y, and z components of the particle's velocity vector. This vector is generated from the random values of speed ( s ), launch angle from vertical ( a ) and the rotation relative to the comp's y
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Building Your Own 3D Particle Generator 
