Animate a Fill Circle using Canvas - html

Question

Basically I want to be able to Fill a Circle using canvas, but it animate to a certain percentage.
I.e only have the circle fill up 80% of the way.
My canvas knowledge isn't amazing, Here is an image i made in photoshop to display what i want.
I want the circle to start empty and then Fill up to say 70% of the circle.
Is this possible with Canvas, if so? can anyone shed some light on how to do it?
Here is a fiddle of what I've managed
http://jsfiddle.net/6Vm67/
var canvas = document.getElementById('Circle');
var context = canvas.getContext('2d');
var centerX = canvas.width / 2;
var centerY = canvas.height / 2;
var radius = 80;
context.beginPath();
context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
context.fillStyle = '#13a8a4';
context.fill();
context.lineWidth = 10;
context.strokeStyle = '#ffffff';
context.stroke();
Any help would be massively appreciated

Answer 1

Clipping regions make this very easy. All you have to do is make a circular clipping region and then fill a rectangle of some size to get a "partial circle" worth of fill. Here's an example:
var canvas = document.getElementById('Circle');
var context = canvas.getContext('2d');
var centerX = canvas.width / 2;
var centerY = canvas.height / 2;
var radius = 80;
var full = radius*2;
var amount = 0;
var amountToIncrease = 10;
function draw() {
context.save();
context.beginPath();
context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
context.clip(); // Make a clipping region out of this path
// instead of filling the arc, we fill a variable-sized rectangle
// that is clipped to the arc
context.fillStyle = '#13a8a4';
// We want the rectangle to get progressively taller starting from the bottom
// There are two ways to do this:
// 1. Change the Y value and height every time
// 2. Using a negative height
// I'm lazy, so we're going with 2
context.fillRect(centerX - radius, centerY + radius, radius * 2, -amount);
context.restore(); // reset clipping region
context.beginPath();
context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
context.lineWidth = 10;
context.strokeStyle = '#000000';
context.stroke();
// Every time, raise amount by some value:
amount += amountToIncrease;
if (amount > full) amount = 0; // restart
}
draw();
// Every second we'll fill more;
setInterval(draw, 1000);
http://jsfiddle.net/simonsarris/pby9r/

Answer 2

This is a little more dynamic, object-oriented version, so you can configure the options as the circle radius, border width, colors, duration and step of animation, you can also animate the circle to a certain percentage. It was quite fun to write this.
<canvas id="Circle" width="300" height="300"></canvas>
<script>
function Animation( opt ) {
var context = opt.canvas.getContext("2d");
var handle = 0;
var current = 0;
var percent = 0;
this.start = function( percentage ) {
percent = percentage;
// start the interval
handle = setInterval( draw, opt.interval );
}
// fill the background color
context.fillStyle = opt.backcolor;
context.fillRect( 0, 0, opt.width, opt.height );
// draw a circle
context.arc( opt.width / 2, opt.height / 2, opt.radius, 0, 2 * Math.PI, false );
context.lineWidth = opt.linewidth;
context.strokeStyle = opt.circlecolor;
context.stroke();
function draw() {
// make a circular clipping region
context.beginPath();
context.arc( opt.width / 2, opt.height / 2, opt.radius-(opt.linewidth/2), 0, 2 * Math.PI, false );
context.clip();
// draw the current rectangle
var height = ((100-current)*opt.radius*2)/100 + (opt.height-(opt.radius*2))/2;
context.fillStyle = opt.fillcolor;
context.fillRect( 0, height, opt.width, opt.radius*2 );
// clear the interval when the animation is over
if ( current < percent ) current+=opt.step;
else clearInterval(handle);
}
}
// create the new object, add options, and start the animation with desired percentage
var canvas = document.getElementById("Circle");
new Animation({
'canvas': canvas,
'width': canvas.width,
'height': canvas.height,
'radius': 100,
'linewidth': 10,
'interval': 20,
'step': 1,
'backcolor': '#666',
'circlecolor': '#fff',
'fillcolor': '#339999'
}).start( 70 );
</script>