Create a Customizable Snow Effect with ActionScript 3.0

It's been a cold winter all over the place, so let's recognize that fact with a nice wintery scene. In this tutorial we will create a falling snow effect using Flash and ActionScript 3.0.

Final Result

Let's take a quick look at the final effect:

Step 1: Brief Overview

Using an image as a background we will create a falling snow effect. The snow will be a user defined MovieClip that will be exported to be used as a class. We will duplicate and animate this clip with AS3.

Step 2: Set Up

Open Flash and create a new Flash File (ActionScript 3.0).

Set the stage size to 500 x 375 px and set the frame rate to 24fps.

Step 3: Getting the Background

A snowy scene will be required for this effect.

You can create your own or adapt a custom background. In this example I used a photograph from Andy Armstrong, which has a Creative Commons License.

Step 4: Importing the Image

When you're done choosing the background go back to Flash, click File > Import > Import to Stage and select the image you downloaded.

Place the image in the center of the stage and go to the next step.

Step 5: Create your Snowflake

A MovieClip will be used as a Snowflake, I used this basic Snowflake bitmap:

Convert the bitmap to a MovieClip, mark the Export for ActionScript checkbox, and name it Snowflake.

Alternatively, you could create a new MovieClip called Snowflake and just draw a circle inside it, with a gradient fill to make it look soft and round.

Step 6: Create your Document Class

Create a new ActionScript Document and save it as Snow.as.

Step 7: Start your Code

The package keyword allows you to organize your code into groups that can be imported by other scripts; it's recommended to name them starting with a lowercase letter and use intercaps for subsequent words -- for example: myClasses.

If you don't want to group your files in a package or you have only one class you can use it right from your source folder, but the idea here is to be organized.

1
2	package
3	{

Step 8: Import Required Classes

These are the required classes, for a more detailed description about each class, please refer to the Flash Help (press F1 within Flash).


import flash.display.MovieClip;
import flash.events.Event;
import flash.utils.Timer;
import flash.events.TimerEvent;

Step 9: Declare the Class

The extends keyword defines a class that is a subclass of another class. The subclass inherits all the methods, properties and functions; that way we can use them in our class.

Our document class is going to extend the MovieClip class.

1
2	public class Snow extends MovieClip
3	{

Step 10: Set Up Variables

Let's take a look at the variables we'll use.


//A Vector object that will store the snowflakes movieclips
private var flakesVector:Vector.<MovieClip> = new Vector.<MovieClip>();
//Timer object that controls the horizontal movement
private var timer:Timer = new Timer(2000);

(If you're using Flash CS3, you can use an Array instead of a Vector.)

Step 11: Write the Constructor

The constructor is a function that runs when an object is created from a class; this code is the first to execute when you make an instance of an object or run using the Document Class. In this case, it's the first code to run when our SWF is started.

Here we've specified two parameters that have values by default: the speed parameter, which stores the pixels each snowflake moves down by every frame, and flakesNumber, which is the number of snowflakes presented on screen.

1
2	public function Snow(speed:int = 3, flakesNumber = 150):void
3	{

Step 12: Duplicate the Snowflakes

A for statement handles this action.


	for(var i:int = 0; i < flakesNumber; i++)
	{
		//Instantiates the snowflake clip
		var flake:Snowflake = new Snowflake(); 
					
		//Gives a unique and random falling speed to each snowflake
		flake.vel = (Math.random() * speed) + 0.5; 
		//Picks a random value between 0.5 and -0.5, this is the horizontal movement speed
		flake.xSpeed = Math.floor(Math.random() * (0.5 - -0.5 + 1)) + -0.5; 				
		flake.scaleX = (Math.random() * 1) + 0.3; 	//Scales the snowflake
		flake.scaleY = flake.scaleX;
		flake.x = Math.random() * stage.stageWidth; 	//Random Position
		flake.y = Math.random() * stage.stageHeight;
					
		addChild(flake); //Adds the clip to the stage
	
		//Adds the clip to the Vector object so it can be accessed by other functions
		flakesVector.push(flake); 
	}

	//Adds a listener to the stage to execute the fall function every frame
	addEventListener(Event.ENTER_FRAME, fall); 

	timer.addEventListener(TimerEvent.TIMER, 	changeMovement);
	timer.start(); //Starts the timer
}

Step 13: Add the Fall Function

The falling function.

This will be triggered every frame, due to the ENTER_FRAME event listener. It will move the snowflakes horizontally and down each frame according to the speed calculated in xSpeed and vel. It will also re-position the snowflakes at the top when they pass the bottom of the stage.


private function fall(e:Event):void
{
	//This for loops through the snowflakes to apply the code to every clip
	for(var i:int = 0; i < flakesVector.length; i++) 	{

Step 14: Horizontal Movement

Move each snowflake horizontally. At first each will only move to the right, but the timer event handler will change that.

1	flakesVector[i].x += flakesVector[i].xSpeed;

Step 15: Vertical Movement

The snowflake will be moved down every frame by its vel (velocity) variable, which has a different value for each flake.

1
2	flakesVector[i].y += flakesVector[i].vel;

Step 16: Reset Positions

If the snowflake clip gets below the stage, it will be moved back to the top and assigned with a new random x-postition.


if(flakesVector[i].y > stage.stageHeight)
{
	flakesVector[i].x = Math.random() * stage.stageWidth;
	flakesVector[i].y = -flakesVector[i].height;
}

Step 17: Change Movement

A Timer object will change the horizontal movement of the flakes by multiplying the xSpeed of each flake by -1 every couple of seconds. This function is triggered by the TIMER event listener we added earlier.


		private function changeMovement(e:TimerEvent):void
		{
			for(var i:int = 0; i < flakesVector.length; i++)
			{
				flakesVector[i].xSpeed *= -1;
			}
		}
	}
}