Canvas Bitmap Operations - bitblt in JavaScript
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Canvas Bitmap Operations - bitblt in JavaScript
ImageData object


The complete program is:

function augmentImageData(o) {
 o.getPixel = function (x, y) {
               var i = (x + y * this.width) * 4;
               return {R:[i],
                       G:[i + 1],
                       B:[i + 2],
                       A:[i + 3]

 o.setPixel = function (x, y, c) {
                var i = (x + y * this.width) * 4;
      [i] = c.R;
      [i + 1] = c.G;
      [i + 2] = c.B;
      [i + 3] = c.A;

function createCanvas(h, w) {
      var c = document.createElement("canvas");
      c.width = w;
      c.height = h;
      return c;

function draw() {
      var ctx = document.body.appendChild(
      createCanvas(400, 400)).getContext("2d");
      var img = new Image();
      img.onload = function () {
       ctx.drawImage(img, 0, 0, 400, 300);
       var ImDat = ctx.getImageData(0, 0, 400, 300);
       for (var x = 0; x < 400; x++) {
        for (var y = 0; y < 300; y++) {
          var c1 = ImDat.getPixel(x, y);
          var c2 = ImDat.getPixel(x, y + 3);
          var r = Math.abs(c1.R - c2.R) + 128;
          var g = Math.abs(c1.G - c2.G) + 128;
          var b = Math.abs(c1.B - c2.B) + 128;
          var grey = (r + g + b) / 3;
          ImDat.setPixel(x, y,
            {R: grey, G: grey, B: grey, A: c1.A});
        ctx.putImageData(ImDat, 0, 0);

     img.src = "test.jpg";

A security problem

If you try this program out you will probably find that it doesn't work. The reason is that we are loading an image from the local file system and this is a security risk. You cannot access the pixels of an image that has been loaded from a different URL and accessing pixels of a local file is particularly forbidden. If you download the file from a server with the same URL as the script then there's no problem.

To allow for testing, Chrome has a command line switch that turns off the security check - no doubt other browsers have similar features. All you have to do is locate the shortcut that you use to launch Chrome and change the target to read:

"C:\Program Files\Google\Chrome\Application\

Alternatively simply use the command from the command prompt or add:


to whatever command gets Chrome.

With this command line switch it all works and you can see the result of the effect. It is also worth saying that it runs remarkably fast for a JavaScript-based image processing routine.




If you would like the code for this project then you can find it in the CodeBin.

Now available as a paperback or ebook from Amazon.

JavaScript Bitmap Graphics
With Canvas




  1. JavaScript Graphics
  2. Getting Started With Canvas
  3. Drawing Paths
      Extract: Basic Paths 
      Extract: Bezier Curves 
  4. Stroke and Fill
      Extract: Stroke Properties 
      Extract: Fill and Holes
      Extract: Gradient & Pattern Fills 
  5. Transformations
      Extract: Transformations
      Extract: Custom Coordinates 
      Extract  Graphics State
  6. Text
      Extract: Text, Typography & SVG 
      Extract: Unicode 
  7. Clipping, Compositing and Effects
      Extract: Clipping & Basic Compositing 
  8. Generating Bitmaps
      Extract:  Introduction To Bitmaps
      Extract :  Animation 
  9. WebWorkers & OffscreenCanvas
      Extract: Web Workers **NEW!
      Extract: OffscreenCanvas
  10. Bit Manipulation In JavaScript
      Extract: Bit Manipulation  
  11. Typed Arrays
  12. Files, blobs, URLs & Fetch
      Extract: Blobs & Files
      Extract: Read/Writing Local Files 
  13. Image Processing
      Extract: ImageData
      Extract: The Filter API
  14. 3D WebGL
      Extract: WebGL 3D 
  15. 2D WebGL
    Extract: WebGL Convolutions 







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Last Updated ( Thursday, 06 September 2018 )