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HTML5 Canvas Twirl Sphere

05.04.2012
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Today, I would like to show you how to create an animated Twirl Sphere (I modified our previous 3D sphere, and I used same way of accessing pixels on the canvas). Our sphere goes around the canvas continuously. This example is a cross browser solution (because it uses html5).

In the end, you should get something like this:

HTML5 Canvas Twirl Sphere

Here are our demo and downloadable package:

Live Demo

download in package

Ok, please download our source files and let’s start coding !

Step 1. HTML

This is the markup of our page:

index.html
<div class="container">
    <canvas id="slideshow" width="1024" height="630"></canvas>
    <canvas id="obj" width="256" height="256"></canvas>
</div>

I prepared two canvas objects: the first for the resource image, and the second – for our Twirl sphere.

Step 2. CSS

css/main.css
.container {
    height: 630px;
    margin: 50px auto;
    position: relative;
    width: 1024px;
    z-index: 1;
}
#obj {
    position: absolute;
    z-index: 2;
}

We should keep our Sphere object above our main canvas.

Step 3. JS

And now, our main js script file:

js/script.js
var canvas, ctx;
var canvasObj, ctxObj;
var iDstW = 256;
var iDstH = 256;
var iXSpeed = 4;
var iYSpeed = 3;
var iLastX = iDstW / 2;
var iLastY = iDstH / 2;
var oImage;
var aMap = [];
var aMapT = [];
var aBitmap;

var mathTwirl = function(px,py) {
    var x = px - iDstW / 2;
    var y = py - iDstH / 2;
    var r = Math.sqrt(x * x + y * y);
    var maxR = iDstW / 2;
    if (r > maxR) return {'x':px, 'y':py, 't': 1};

    var a = Math.atan2(y, x);
    a -= 1 - r / maxR;

    var dx = Math.cos(a) * r;
    var dy = Math.sin(a) * r;

    return {'x': dx+iDstW/2, 'y': dy+iDstH/2, 't': 1.5}
}

window.onload = function(){

    // load background
    oImage = new Image();
    oImage.src="images/bg.jpg";
    oImage.onload = function () {

        // creating canvas and context objects
        canvas = document.getElementById('slideshow');
        ctx = canvas.getContext('2d');
        canvasObj = document.getElementById('obj');
        ctxObj = canvasObj.getContext('2d');

        // clear context
        ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);

        // and draw source image
        ctx.drawImage(oImage, 0, 0);

        aBitmap = ctx.getImageData(0, 0, iDstW, iDstH);
        for (var y = 0; y < iDstH; y++) {
            for (var x = 0; x < iDstW; x++) {
                var t = mathTwirl(x, y);
                aMap[(x + y * iDstH) * 2 + 0] = Math.max(Math.min(t.x, iDstW - 1), 0);
                aMap[(x + y * iDstH) * 2 + 1] = Math.max(Math.min(t.y, iDstH - 1), 0);
                aMapT[(x + y * iDstH)] = t.t;
            }
        }

        // begin updating scene
        updateScene();
    };

    function updateScene() {

        // update last coordinates
        iLastX = iLastX + iXSpeed;
        iLastY = iLastY + iYSpeed;

        // reverse speed
        if (iLastX + 1 > ctx.canvas.width - iDstW/2) {
            iXSpeed = -4;
        }
        if (iLastX - 1 < iDstW/2) {
            iXSpeed = 4;
        }
        if (iLastY + 1 > ctx.canvas.height - iDstH/2) {
            iYSpeed = -3;
        }
        if (iLastY - 1 < iDstH/2) {
            iYSpeed = 3;
        }

        // shifting of the second object
        canvasObj.style.left = iLastX - Math.floor(iDstW / 2) + 'px';
        canvasObj.style.top = iLastY - (Math.floor(iDstH / 2)) + 'px';

        // draw result Twirl sphere
        var aData = ctx.getImageData(iLastX - Math.ceil(iDstW / 2), iLastY - Math.ceil(iDstH / 2), iDstW, iDstH + 1);
        for (var j = 0; j < iDstH; j++) {
            for (var i = 0; i < iDstW; i++) {
                var u = aMap[(i + j * iDstH) * 2];
                var v = aMap[(i + j * iDstH) * 2 + 1];
                var t = aMapT[(i + j * iDstH)];
                var x = Math.floor(u);
                var y = Math.floor(v);
                var kx = u - x;
                var ky = v - y;
                for (var c = 0; c < 3; c++) {
                    aBitmap.data[(i + j * iDstH) * 4 + c] =
                      (aData.data[(x + y * iDstH) * 4 + c] * (1 - kx) + aData.data[((x + 1) + y * iDstH) * 4 + c] * kx) * (1-ky) * t +
                      (aData.data[(x + (y + 1) * iDstH) * 4 + c] * (1 - kx) + aData.data[((x + 1) + (y + 1) * iDstH) * 4 + c] * kx) * (ky) * t;
                }
            }
        }
        ctxObj.putImageData(aBitmap,0,0);

        // update timer
        setTimeout(updateScene, 16);
    }
};

During initialization, the script prepares two canvas objects and two contexts. After initialization, it loads the background image, and drasw it at our first context. Then it prepares a hash table for sphere transformations: aMap (using the new mathTwirl function). And, finally – it starts the timer that updates the main scene. This function (updateScene) updates the coordinates of our Sphere object, and draws the updated sphere at our second context.

Live Demo

download in package

Conclusion

I hope that today’s Twirl HTML5 Sphere lesson has been interesting for you. We have finished another nice html5 example. I will be glad to see your thanks and comments. Good luck!

Published at DZone with permission of Andrey Prikaznov, author and DZone MVB. (source)

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