Tag Archive: Animation

HTML5 Canvas Fountain

May 22, 2010

A bit more arsing about with HTML5. Purely decorative, completely non-interactive this time. An animated fountain based on one I wrote in AS3 which in turn was based closely on one from the absolutely brilliant Foundation ActionScript 3 Animation: Making Things Move! (seriously, if you need a good basis in programmed animation/interaction this book is a godsend, explained in simple terms and with a brilliant separation of concept from code so the information and tutorials can be easily applied to languages other than Actionscript 3).

View Demo.

Converting to Javascript was pretty straight forward, after all the syntax has the same origins. It’s slightly easier to achieve in Actionscript as Flash has built in the concepts of independent visual objects, wheras the canvas tag does not and so you need to roll your own, which for a simple example as this is not a problem, and I’m guessing there will be some nice JS frameworks around fairly soon which will make the job easier. Strongly tempted to convert a load of the other tutorials from Foundation AS3 to Javascript as well but not a great use of time and the author may (understandably) hunt me down and beat me to death.

The app is pretty simple, it generates a load of circles, each with it’s own direction, velocity, colour and size which move away from the center, when they are off the screen they are reset with new velocity, colour, size, etc and put back to the center again. That’s it. Here’s the code, it’s not very polished but it works…

(download the js source)

The HTML

Just the required bits…

<canvas width="500" height="400" id="MyCanvas">I AM A CANVAS!</canvas>

That was easy. You just need a canvas tag. I’ve given it the original id of ‘MyCanvas’. Next up…

The Javascript

Once again, I’ve used jQuery to get things moving quickly, you don’t need to but it makes life easier.

var tId;
var cHeight = 0;
var cWidth = 0;
var numBalls = 100;
var balls = new Array();

First we set some global variables as follows:

tId: A variable to hold the interval id. We don’t actually need this but it’s handy if we wanted to add more functionality later, and good practice (IMHO).
cHeight, cWidth: The canvas dimensions. Will be set shortly.
numBalls: The number of balls you wish to display. Change this if you fancy.
balls: An array of ball objects.

function Ball(){
        this.xPos = 0;
        this.yPos = 0;
        this.vx = 0;
        this.vy = 0;
        this.radius = 5;
        this.colour;
        this.wind;
        this.gravity;
        this.sizeWobble;
 
        this.reset = function() {
                this.xPos = cWidth / 2;
                this.yPos = cHeight / 2;
                this.vx = Math.random() * 10 – 5;
                this.vy = Math.random() * 10 – 5;
                this.colour = randomColour();
                this.wind = Math.random() * 2 – 1;
                this.gravity = Math.random() * 2 – 1;
                this.radius = Math.random() * 20 + 5;
                this.sizeWobble = Math.random() * 2 – 1;
        }
}

Define a ‘Ball’ class. The xPos/yPos determine the ball’s current position in the canvas. vx and vy are the Ball’s x and y velocity. Wind and Gravity and horizontal and vertical force. The strangely named ‘sizeWobble’ is the amount the ball radius changes each frame. The reset function sets up the ball, most parameters are randomly generated, xPos and yPos are set to the center of the canvas.

jQuery(document).ready(function($){
 
        // Set canvas values
        cHeight = parseInt($(‘#MyCanvas’).height());
        cWidth = parseInt($(‘#MyCanvas’).width());
 
        // Generate balls
        for(var i = 0;i < numBalls;i++){
                balls.push(new Ball());
                balls[i].reset();
        }
 
        tId = setInterval(‘drawBalls()’, 50);
 
});

The core of the script. Wrapped up in a nice jQuery document ready handler. First we set the globals for the canvas dimensions (as used by the reset method in the Ball class). Then we loop as many times as we want balls, create a new ball each loop instance, add it to the balls array and reset it.

Once our balls are created we use setInterval to call the ‘drawBalls’ function every 50 microseconds.

function drawBalls(){
        clearCanvas();
        var context = document.getElementById(‘MyCanvas’).getContext(‘2d’);
        for(var i = 0;i < numBalls;i++){
                drawCircle(balls[i].xPos, balls[i].yPos, balls[i].radius, balls[i].colour);
                balls[i].vy += balls[i].gravity;
                balls[i].vx += balls[i].wind;
                balls[i].xPos += balls[i].vx;
                balls[i].yPos += balls[i].vy;
                if(balls[i].radius > 2){
                        balls[i].radius += balls[i].sizeWobble;
                }
 
                if(
                        balls[i].xPos – balls[i].radius > cWidth||
                        balls[i].xPos + balls[i].radius < 0||
                        balls[i].yPos – balls[i].radius > cHeight||
                        balls[i].yPos + balls[i].radius < 0
                        ){
                                balls[i].reset();
                        }
 
        }
}

This is the function that’s called each ‘frame’ by the setInterval command. First we clear the canvas. Next we get a context for the canvas. Then we loop through the array of balls. For each one we:

Draw it using drawCircle, passing it’s current position, radius and colour
Adjust it’s x and v velocity modifiers using the gravity/wind values
Update it’s stored position using it’s velocity modifiers
If it’s radius is greater than 2 adjust it by it’s sizeWobble! (the min size check ensures it doesn’t completely disappear)
If it is no longer on the canvas, reset it.

That’s pretty much it, the remaining three functions are helpers. clearCanvas clears the canvas by setting it’s width (must find a more logical method of doing this). randomColour returns a random colour in the appropriate format and drawCircle draws a circle to the canvas…

function clearCanvas(){
        $(‘#MyCanvas’).attr(‘width’,$(‘#MyCanvas’).attr(‘width’));
}
 
function randomColour(){
        var red = Math.round(Math.random() * 255);
        var green = Math.round(Math.random() * 255);
        var blue = Math.round(Math.random() * 255);
        return "rgb(" + red + ", " + green + ", " + blue + ")";
}
 
function drawCircle(cx, cy, radius, colour){
    var ctx = document.getElementById(‘MyCanvas’).getContext(‘2d’);
    ctx.fillStyle = colour;
    ctx.strokeStyle = "rgb(60, 80, 50)";
    ctx.beginPath();
    ctx.arc(cx,cy,radius,0,Math.PI*2,true); // Outer circle
    ctx.fill();
    ctx.stroke();
}