Brownian Movement in Actionscript | Random Motion Tutorial


Having things drift around or move randomly has always interested me. Having an animation that is never going to be the exact same thing is very exciting. The focus turns from key-ing exact animations to programming a feel and letting the animations take car of themselves! One type of seemingly random motion is Brownian motion. This gives the movement a random walk wandering look, it will just drift around with no real direction.


Step by step this process is very simple. In every random motion you create the random number, and apply it to the property. If you want constant random action (motion) rather than just random placement, you repeat that over and over.

  1. Make a random number (random velocity)
  2. Apply the random number (apply velocity to property)
  3. Repeat (if needed)

To create a random number in actionscript, use Math.random(), which creates a random number between 0 and 1. Usually you’ll want to scale it to a range you want to use. If you want a number between 50 and 100, you’d do Math.random() * 50 + 50. *50 to scale it to 0-50, and + 50 to bring it up to 50 – 100. Also if we want to get a 100 range around 0 (-50 – 50) we would do Math.random() * 100 – 50. In the code below I’ve abstracted this to Math.random() * this.randomRange – this.randomRange/2.


[kml_flashembed fversion=”9.0.0″ movie=”” targetclass=”flashmovie” publishmethod=”dynamic” width=”500″ height=”500″]

Get Adobe Flash player


Here I’ve got dots created and placed randomly, with randomly set scale and alpha. On every frame each dot has a random velocity applied to it’s x and y coordinates.
The yellow dot is the simple example (code below) and the rest are included in the complex example below.


Simple Example:
[cc lang=”actionscript”]
dotOne.onEnterFrame = function() {
//create a random velocity for x and y direction
vx = Math.random() * 4 – 2;
vy = Math.random() * 4 – 2;
//apply velocity to coordinates
this._x += vx;
this._y += vy;

Complex example:
[cc lang=”actionscript”]
var numDots = 25;
var randomRange = 1;

for(var i=1; i<=numDots; i++) {
//create a new dot
duplicateMovieClip(, “dot”+i, i);
//save it’s ref path for use
theDot = _root[“dot”+i];
//give it random coordinates
theDot._y = Math.random() * Stage.height;
theDot._x = Math.random() * Stage.width;
//give each dot a distinct random range
theDot.randomRange = i/numDots;
//give each dot a random size and transparency
theDot._xscale =
theDot._yscale =
theDot._alpha = i*4;

//apply this code on the dot every frame
theDot.onEnterFrame = function() {
//create a random velocity for x and y direction within the specifically created random range for each dot
vx = Math.random() * this.randomRange – this.randomRange/2;
vy = Math.random() * this.randomRange – this.randomRange/2;
//apply velocity to coordinates
this._x += vx;
this._y += vy;



2 thoughts on “Brownian Movement in Actionscript | Random Motion Tutorial

  1. Hi, thenks for the file, Ive just applied to my animation, but the thing is that all the random elements are in the first layer, no matter in what layer I put the random element.

    So I cant use it, I really like the visual effect, but how can I do to this stays at the background?

    (Sorry for my english Im still learning

  2. @andres – Not sure how you’ve set up your file, but you could try initially putting all the random elements onto their own layer and extending them throughout your entire animation. Sounds like you’re doing some timeline animation? If you have the keyframe on frame 1 and another on your last frame, the script should continue to run throughout your animation.

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