Code and Stuff

Nov 29, 2011

HTML5 Canvas Image composition

A view on the composition of two images at changing position can result in quite interesting effects. Especially if we use the correct images and composition function.

The Code

The implementation is quite simple. In the initialization javascript function we will load the two images and trigger the drawing.
var images = [];
function init() {
   images[0] = new Image();
   images[0].src = "URL to first image";
   images[1] = new Image();
   images[1].src = "URL to second image";
   window.setTimeout("draw()", 50);

The draw function will simply place the two images at different positions. The globalCompositeOperation attribute sets the composition type.
var t = 0;
function draw() {
   var canvas = document.getElementById("ID of canvas");  
   var ctx = canvas.getContext("2d");

   ctx.globalCompositeOperation = 'copy'; // forget the past
   // draw first image
   ctx.drawImage(images[0], 0, 0);

   var px = -150.0 + 150.0 * Math.sin(0.86 * t + 2.3) * Math.sin(13 + t * 0.52);
   var py = -200.0 + 200.0 * Math.sin(t * 1.3)*  Math.sin(13 + t * 0.32);
   ctx.globalCompositeOperation = 'lighter'; 
   ctx.drawImage(images[1], px, py);

   // increase the iteration
   t = t + 0.06;

   // reschedule
   window.setTimeout("draw()", 50);
Note: the position of the first image can also be changed to alter the behaviour (as in the examples below).

On the HTML part we simply have to place the canvas and have someone call the initialization function.

<canvas width="300" height="200" id="canvas_ID"></canvas>
// For simplicity we us a link to start the animation
<a onclick="init()" href="#">Start</a>

Other examples

The choice of input images, how we move them and the composition function will greatly affect the result.

The effect works also when the two images are the same. Still adding composition with an image of circles.

Concentric circles and using xor composition.

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