HTML5 canvas rounded corners

How to make accurate rounded corners using HTML5 canvas.

by jwerre

HTML

<canvas id="myCanvas" width="500" height="400">
    <!-- Insert fallback content here -->
</canvas>

JavaScript

var canvas = $("#myCanvas");
var context = canvas.get(0).getContext("2d");

// Set rectangle and corner values
var rectX = 50;
var rectY = 50;
var rectWidth = 100;
var rectHeight = 100;
var cornerRadius = 20;

// Reference rectangle without rounding, for size comparison
/* context.fillRect(200, 50, rectWidth, rectHeight); */

// Set faux rounded corners
context.lineJoin = "round";
context.lineWidth = cornerRadius;

// Change origin and dimensions to match true size (a stroke makes the shape a bit larger)
context.strokeRect(rectX+(cornerRadius/2), rectY+(cornerRadius/2), rectWidth-cornerRadius, rectHeight-cornerRadius);
context.fillRect(rectX+(cornerRadius/2), rectY+(cornerRadius/2), rectWidth-cornerRadius, rectHeight-cornerRadius);

// You can do the same thing with paths, like this triangle
// Remember that a stroke will make the shape a bit larger so you'll need to fiddle with the
// coordinates to get the correct dimensions.
/* context.beginPath();
context.moveTo(400, 60);
context.lineTo(440, 140);
context.lineTo(360, 140);
context.closePath();
context.stroke();
context.fill(); */