How do you convert a color image to black/white using Javascript?

Question

How do you convert a color image to black/white using only Javascript?

AND, also make it cross compatible in most browsers because I hear Internet E

Answer 1

Using HTML5 Canvas and JavaScript

 ctx.drawImage(img, 0, 0, w, h);

    var imgPixels = ctx.getImageData(0, 0, w, h);
    for(var y = 0; y < imgPixels.height; y++){
     for(var x = 0; x < imgPixels.width; x++){
          var i = (y * 4) * imgPixels.width + x * 4;
          var avg = (imgPixels.data[i] + imgPixels.data[i + 1] + imgPixels.data[i + 2]) / 3;
          imgPixels.data[i] = avg;
          imgPixels.data[i + 1] = avg;
          imgPixels.data[i + 2] = avg;
     }
}
ctx.putImageData(imgPixels, 0, 0, 0, 0, imgPixels.width, imgPixels.height);