Below is the script I wrote that will take two canvases and find the differences between them – a canvas diff. What it returns is the bounding area – upper left coordinates and lower-right coordinates – with which you can do as you like.

The challenge here is iterating through the pixel data. The data itself is an array of each pixel’s RGBA values in sequence. For example, if we are looking at four pixels then the pixel array representing them would have a length of 16 (RGBA = 4 array elements x the number of pixels = 16). This imaginary array could look like:

  • 0,0,122,5,100,12,123,6,16,100,43,123,55,55,100,50

With a little formatting to help make it make more sense we can easily see the groupings:

  • 0,0,122,5,    100,12,123,6,    16,100,43,123,    55,55,100,50

Looking at the first group of 4 numbers we can see that the pixel they represent has these RGBA values:

  • R: 0
  • G: 0
  • B: 122
  • A: 5

Imagine a 1024×768 image represented by a single data array of RGBA values. Such an image would have a data array length of 3,145,728 (1024 x 768 x 4). In order to manipulate the pixel data you’d have to discover a way of looping through the array keeping in mind that every group of 4 array elements is itself a single pixel. You would also need to realize that any given group of array elements represents a specific pixel somewhere within your image/canvas.

Image Comparison Example

In the example shown here I’m comparing the pixel data between two images, keeping track of the differences, and returning a bounding area that encompasses them.

This example is in an iframe – view source to see the script that loads the images into image objects, then loads those images into two canvas elements, extracts the image data from the canvases, compares them, and then finally draws the red bounding boxes to illustrate where the images diverge from each other.

Diff Function – Comparing Two Canvases

The function I wrote below compares all the pixels and notes the coordinates of the ones that don’t match. I then use the sort array method to sort the resulting “diff” array by either the X or Y of each diff’d coordinate so that I can find the extremes of each one.

While I’m looking for the differences I am also keeping track of the X and Y coordinate representing each RGBA quadruplet. Note the use of the modulus operator as that is what makes the coordinate-tracking work.

To use this function all you have to do is create two image data objects using the canvas getImageData() method then pass them to the canvasDiff function where the first data object is the original image and the second is the one that has changed. Refer to the iframed example above – view source to see how the diff function seen below was used within the example to produce the bounding differential boxes.

When using images as the source data they need to be of identical size and ideally in PNG format. PNG is optimal because JPG is a lossy compression algorithm that will make it hard to do a legit diff – provided you are using images at all – you could easily just have programmatically-generated canvas art – point is that canvasDiff needs two image data objects where the images are the same physical dimensions.

function canvasDiff(imageData1,imageData2){
    // - please leave the attribution!
    var w = imageData1.width;
    var h = imageData1.height;
    var diffs = [];
    var start = {x:null,y:null};
    var end   = {x:null,y:null};
    var pA1_r,pA1_g,pA1_b,pA1_a,
    var y = 0;
    var x = 0;
var len =;
    for (var i=0;i b.x){
	    return 1;
	} else {
	    return 0;
    start.x = diffs[0].x || 0;
    end.x = diffs[diffs.length-1].x || w;
	if (a.y < b.y){
	    return -1;
	} else if (a.y > b.y){
	    return 1;
	} else {
	    return 0;
    start.y = diffs[0].y || 0;
    end.y = diffs[diffs.length-1].y || h;

    // DONE
    // "start" and "end" have the bounding coordinates
    return [start,end];