Generating Random Motion Using AE 5.5's Expressions
Dan Ebberts July 26, 2023
0 Comments
Originally published in 2002
In this tutorial, Dan Ebberts demonstrates a method of generating motion that is random in both time and space and allows you to quickly assemble a composition with a pleasantly fluid, chaotic movement.
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Project File: .sit (currently unavailable)
The Problem:
The concept behind this tutorial came out of an online discussion. A fellow After Effects enthusiast was trying to come up with an expression to generate motion that was random in time and space. That is, target positions would be random as would the time it takes to get there. The problem that you encounter when trying to develop such an expression is that there just doesn't seem to be a good way to carry the value of variables forward in time from one frame to the next. Any variables that you define will be gone at the next frame. Consider the following example expression for opacity:
if (time = 0){
x = 10
}
else{
x = x 1
}
This works fine at the first frame. At the second frame you get an error because x is undefined. The expression doesn't remember that x was 10 on the previous frame. This becomes a serious limitation when you are trying to do something over a random amount of time because once you decide how long it should take, you need to check at every frame to see if you're done yet. Hard to do if you can't pass that value from one frame to the next.
Seed_random To The Rescue:
It turns out that you can get around this problem with the enhanced (for AE 5.5) seed_random function. This function allows you to establish a repeatable sequence of random numbers and regenerate them at will, just by restoring the seed to a previous value and then calling the random() function. This gives us a solution to the problem because even though we can't pass our random values from one frame to the next, we can recreate them using seed_random. Let's look at how seed_random works.
Open the project file and then open the basic comp . This is just a 640 480 comp with a 50 50 solid layer added. Select Solid 1 and reveal the position property. Alt-click (Option-click) on the stopwatch to activate the expression for this property. Enter the following expression:
[random(0,this_comp.width),random(0,this_comp.height)];
This will cause the square to jump to a random position in the comp. Preview the comp. You'll notice that the square jumps to a new position on every frame. Not very useful. Add a new line to the expression so that it now reads:
seed_random(1,true);
[random(0,this_comp.width),random(0,this_comp.height)];
Notice that again the square jumps to a random position but when you preview it stays there. This new behavior is due to a change that Adobe made to the seed_random function for AE 5.5. They've added a second parameter, that when set to true sets the random function so that random numbers generated don't depend on time. The random numbers depend on the layer number, the property number, the value of the first parameter in the seed_random call (the seed ), and the number of times the random function has been called. That means that the same expression will generate different random numbers in a different layer or property (later we will take advantage of this to quickly add complexity to our comp). Try changing the seed parameter to some other number. You'll notice that the square moves to a different location and stays there. Change the second parameter to false and notice that when you preview, the square jumps all over the place again.
Add a little more code to your expression so that it looks like this:
if (time < 1){
seed_random(1,true)
}
else{
seed_random(2,true)
}
[random(0,this_comp.width),random(0,this_comp.height)];
Now you'll notice that when you preview, the square stays at one location for the first second and then jumps to another location for the remainder of the comp.
So how do we use this new, more powerful seed_random function? Here's the key concept: With the second parameter set to true , for a given layer, property, and seed value, the random number you get when calling the random() function only depends on how many times you have called the function since setting the seed. That is, the first time you call random() after setting the seed you will get a particlar random number (let's call it x). The second call will generate a second random number (let's call this one y) and so on. But if you call seed_random again with the original seed, the sequence will start over. That is, the next random() call will regenerate the original random number (x in this case). Using a different seed value will cause a completely different sequence to be generated. Consider the following example:
seed_random(1,true); //set the seed value
A=random();
B=random();
C=random();
seed_random(1,true); //reset the seed to the same value
I=random();
J=random();
K=random();
seed_random(2,true) //new seed
Q=random();
R=random();
S=random();
seed_random(1,true); // reestablish original seed
X=random();
Y=random();
Z=random();
In this case, A,B, and C will all be different random numbers. However, A and I will be the same, B and J will be the same, and C and K will be the same because the seed has been reset (to 1 in this case) and the random sequence starts over. Q,R and S will be a new sequence of random numbers because the seed has been changed to 2. The sequence X,Y,Z will be the same as A,B,C and I,J,K because the seed has been set to 1 again. So we can return to a sequence of random numbers by reestablishing the seed value that generated those numbers.
How does this help us? To generate our random motion, at any given frame we need to know several things about the current segment of motio
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Generating Random Motion Using AE 5.5's Expressions 
