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copied!<p>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:</p> <pre><code>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); </code></pre> <p><a href="http://jsfiddle.net/simonsarris/pby9r/">http://jsfiddle.net/simonsarris/pby9r/</a></p>

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