Drawing Bezier Curves

The canvas supports drawing two kinds of Bezier curves: cubic and quadratic.

We pick a start and end point and then add one or more control points that shape the curve.

The curve methods:

Name	Description	Returns
bezierCurveTo(cx1, cy1, cx2, cy2, x, y)	Draws a Bezier curve to the point (x, y) with the control points (cx1, cy1) and (cx2, cy2).	void
quadraticCurveTo(cx, xy, x, y)	Draws a quadratic Bezier curve to (x, y) with the control point (cx, cy).	void

Drawing Cubic Bezier Curves

 
<!DOCTYPE HTML>
<html>
<head>
<title>Example</title>
<style>
canvas {
      border: thin solid black
}

body>* {
      float: left;
}
</style>
</head>
<body>
      <canvas id="canvas" width="500" height="140"> 
      Your browser doesn't support the <code>canvas</code> element 
      </canvas>
      <script>
            var canvasElem = document.getElementById("canvas");
            var ctx = canvasElem.getContext("2d");
            var startPoint = [ 20, 100 ];
            var endPoint = [ 200, 100 ];
            var cp1 = [ 20, 50 ];
            var cp2 = [ 30, 50 ];
            function draw() {
                  ctx.lineWidth = 3;
                  ctx.strokeStyle = "black";
                  ctx.beginPath();
                  ctx.moveTo(startPoint[0], startPoint[1]);
                  ctx.bezierCurveTo(cp1[0], cp1[1], cp2[0], cp2[1], endPoint[0],endPoint[1]);
                  ctx.stroke();
                  ctx.lineWidth = 1;
                  ctx.strokeStyle = "red";
                  var points = [ startPoint, endPoint, cp1, cp2 ];
                  for ( var i = 0; i < points.length; i++) {
                        drawPoint(points[i]);
                  }
                  drawLine(startPoint, cp1);
                  drawLine(endPoint, cp2);
            }
            function drawPoint(point) {
                  ctx.beginPath();
                  ctx.strokeRect(point[0] - 2, point[1] - 2, 4, 4);
            }
            function drawLine(from, to) {
                  ctx.beginPath();
                  ctx.moveTo(from[0], from[1]);
                  ctx.lineTo(to[0], to[1]);
                  ctx.stroke();
            }
            draw();
      </script>
</body>
</html>

Click to view this demo.

Drawing Quadratic Bezier Curves

A quadratic Bezier curve has only one control point.

 
<!DOCTYPE HTML>
<html>
<head>
<title>Example</title>
<style>
canvas {
      border: thin solid black
}

body>* {
      float: left;
}
</style>
</head>
<body>
      <canvas id="canvas" width="500" height="140"> 
      Your browser doesn't support the <code>canvas</code> element 
      </canvas>
      <script>
            var canvasElem = document.getElementById("canvas");
            var ctx = canvasElem.getContext("2d");
            var startPoint = [ 150, 100 ];
            var endPoint = [ 400, 100 ];
            var cp1 = [ 250, 50 ];
            draw();
            function draw() {
                  ctx.lineWidth = 3;
                  ctx.strokeStyle = "red";
                  ctx.beginPath();
                  ctx.moveTo(startPoint[0], startPoint[1]);
                  ctx.quadraticCurveTo(cp1[0], cp1[1], endPoint[0], endPoint[1]);
                  ctx.stroke();
                  ctx.lineWidth = 1;
                  ctx.strokeStyle = "red";
                  var points = [ startPoint, endPoint, cp1 ];
                  for ( var i = 0; i < points.length; i++) {
                        drawPoint(points[i]);
                  }
                  drawLine(startPoint, cp1);
                  drawLine(endPoint, cp1);
            }
            function drawPoint(point) {
                  ctx.beginPath();
                  ctx.strokeRect(point[0] - 2, point[1] - 2, 4, 4);
            }
            function drawLine(from, to) {
                  ctx.beginPath();
                  ctx.moveTo(from[0], from[1]);
                  ctx.lineTo(to[0], to[1]);
                  ctx.stroke();
            }
      </script>
</body>
</html>

Click to view this demo.

Creating a Clipping Region

The clip method:

Name	Description	Returns
clip()	Creates a new clipping region	void

 
<!DOCTYPE HTML>
<html>
<head>
<title>Example</title>
<style>
canvas {
      border: thin solid black
}

</style>
</head>
<body>
      <canvas id="canvas" width="500" height="140"> 
      Your browser doesn't support the <code>canvas</code> element 
      </canvas>
      <script>
            var ctx = document.getElementById("canvas").getContext("2d");
            ctx.fillStyle = "yellow";
            ctx.beginPath();
            ctx.rect(20, 20, 500, 140);
            ctx.fill();
            ctx.beginPath();
            ctx.rect(100, 20, 300, 100);
            ctx.clip();
            ctx.fillStyle = "red";
            ctx.beginPath();
            ctx.rect(0, 0, 500, 140);
            ctx.fill();
      </script>
</body>
</html>

Click to view this demo.