Patrick Galbraith

Web developer - Adelaide, Australia

Drawing Animated Curves 9

Using Javascript and HTML5 canvas

After digging through some older projects today I came across this code from used to animate the flight path from the Radiohead tour map. It was originally written in AS3 but I ported it to JS for fun.

Basically drawBezierSplit() allow you to draw a section of a quadratic bezier curve. It uses a quadratic bezier curve (quadraticCurveTo) because this is the default curve type built into the AS3 drawing API. For reference the HTML5 canvas API uses cubic curves by default.

 * Animates bezier-curve
 * @param ctx       The canvas context to draw to
 * @param x0        The x-coord of the start point
 * @param y0        The y-coord of the start point
 * @param x1        The x-coord of the control point
 * @param y1        The y-coord of the control point
 * @param x2        The x-coord of the end point
 * @param y2        The y-coord of the end point
 * @param duration  The duration in milliseconds
function animatePathDrawing(ctx, x0, y0, x1, y1, x2, y2, duration) {
    var start = null;
    var step = function animatePathDrawingStep(timestamp) {
        if (start === null)
            start = timestamp;
        var delta = timestamp - start,
            progress = Math.min(delta / duration, 1);
        // Clear canvas
        ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
        // Draw curve
        drawBezierSplit(ctx, x0, y0, x1, y1, x2, y2, 0, progress);
        if (progress < 1) {

 * Draws a splitted bezier-curve
 * @param ctx       The canvas context to draw to
 * @param x0        The x-coord of the start point
 * @param y0        The y-coord of the start point
 * @param x1        The x-coord of the control point
 * @param y1        The y-coord of the control point
 * @param x2        The x-coord of the end point
 * @param y2        The y-coord of the end point
 * @param t0        The start ratio of the splitted bezier from 0.0 to 1.0
 * @param t1        The start ratio of the splitted bezier from 0.0 to 1.0
function drawBezierSplit(ctx, x0, y0, x1, y1, x2, y2, t0, t1) {
    if( 0.0 == t0 && t1 == 1.0 ) {
        ctx.moveTo( x0, y0 );
        ctx.quadraticCurveTo( x1, y1, x2, y2 );
    } else if( t0 != t1 ) {
        var t00 = t0 * t0,
            t01 = 1.0 - t0,
            t02 = t01 * t01,
            t03 = 2.0 * t0 * t01;
        var nx0 = t02 * x0 + t03 * x1 + t00 * x2,
            ny0 = t02 * y0 + t03 * y1 + t00 * y2;
        t00 = t1 * t1;
        t01 = 1.0 - t1;
        t02 = t01 * t01;
        t03 = 2.0 * t1 * t01;
        var nx2 = t02 * x0 + t03 * x1 + t00 * x2,
            ny2 = t02 * y0 + t03 * y1 + t00 * y2;
        var nx1 = lerp ( lerp ( x0 , x1 , t0 ) , lerp ( x1 , x2 , t0 ) , t1 ),
            ny1 = lerp ( lerp ( y0 , y1 , t0 ) , lerp ( y1 , y2 , t0 ) , t1 );
        ctx.moveTo( nx0, ny0 );
        ctx.quadraticCurveTo( nx1, ny1, nx2, ny2 );

 * Linearly interpolate between two numbers v0, v1 by t
function lerp(v0, v1, t) {
    return ( 1.0 - t ) * v0 + t * v1;

    var docCanvas = document.getElementById('canvas'),
        ctx = docCanvas.getContext('2d');

    animatePathDrawing(ctx, 0, 100, 150, -50, 300, 100, 5000);

Note: Click on the result tab to restart the animation.

Credit to Andre Michelle ( for the original inspiration.


Leave a reply


    Nice Castle defense game

  2. Hello colleagues, good article and pleasant urging commented at this place, I am truly enjoying by these.

  3. Nice bro

  4. Arshaan said

    Great work!
    I was wondering, does it have some sort of cubic Bezier? Because in the result it looks like it has.

  5. It’s remarkable designed for me to have a website, which is good designed for my know-how.

    thanks admin

  6. Doris V Walker said

    This was very interesting young man keep up the good work, I’m a fan of atari space fighter games.

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