HTML5 Canvas JavaScript Code to a Draw Bezier Curve

By Stephen Bucaro

The <canvas> element was added in HTML5. The canvas element allows you, in certain circumstances, to not need to load a graphic element on your page, but instead to use JavaScript to the draw the graphics. It's easy to use the canvas, just define a HTML canvas element similar to how you would define a div element, and then configure the context. The context is like a box of drawing tools.

The Bezier curve is named after the French mathematician Pierre Bezier who developed the formulas for drawing precise curves using the Cartesian coordinate system. In this article I describe how to use JavaScript to draw Bezier curves on the HTML5 canvas element.

The canvas context bezierCurveTo() method is used to draw a curve.

Bezier Curve Syntax

bezierCurveTo(c1x, c1y, c2x, c2y, x, y)

Below is the code for the example shown above.

<canvas id="myCanvas" width="280" height="250" style="border:solid 1px; margin-top:10px;"></canvas>

<script>
var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");

ctx.strokeStyle = "purple";
ctx.lineWidth = 3;
ctx.fillStyle = "yellow";

// draw a Bezier curve (c1x, c1y, c2x, c2y, x, y)
ctx.beginPath();
ctx.moveTo(100,50);
ctx.bezierCurveTo(20,100,250,80,200,200);
ctx.stroke();

ctx.lineWidth = 2;

// draw dots at control point locations
ctx.beginPath();
ctx.arc(20,100,2,0,2*Math.PI);
ctx.stroke();

ctx.beginPath();
ctx.arc(250,80,2,0,2*Math.PI);
ctx.stroke();

</script>

Note that you use the context moveTo() method to define first point of the curve.

The above example demonstrates a cubic Bezier curve. A cubic Bezier curve requires two control points. A quadratic cubic Bezier curve requires only one control point. Shown below is the syntax for a A quadratic cubic Bezier curve.

bezierCurveTo(cx, cy, x, y)

Below is the code for the example shown above.

<canvas id="myCanvas2" width="220" height="130" style="border:solid 1px; margin-top:10px;"></canvas>

<script>
var canvas = document.getElementById("myCanvas2");
var ctx = canvas.getContext("2d");

ctx.strokeStyle = "purple";
ctx.lineWidth = 3;
ctx.fillStyle = "yellow";

// draw a quadratic Bezier curve (cx, cy, x, y)
ctx.beginPath();
ctx.moveTo(20,20);
ctx.quadraticCurveTo(20,100,200,20);
ctx.stroke();

ctx.lineWidth = 2;

// draw dot at control point location
ctx.beginPath();
ctx.arc(20,100,2,0,2*Math.PI);
ctx.stroke();

</script>

How to Determine Control Point Coordinates

There are several ways to determine the coordinates of control points. The first method is just wing it, understanding that the control point attracts the closest area of the line to the control point. Estimate the x,y coordinates of a control point, test it, and then adjust it as necessary. The second method is to use a vector graphics application like the free open-source Inkscape application, which provides rulers that display the x,y coordinates of a control point. Unfortunately Inkscape's y axis in numbered in reverse, so you'll need to convert the y coordinate. The third method is to use an online canvas code generator like BezierCurveTo command generator to design your curve, and then transfer the coordinates to your code.

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