html5-canvas Compositing Invert or Negate image with "difference"
Example
Render a white rectangle over an image with the composite operation
ctx.globalCompositeOperation = 'difference';
The amount of the effect can be controled with the alpha setting
// Render the image
ctx.globalCompositeOperation='source-atop';
ctx.drawImage(image, 0, 0);
// set the composite operation
ctx.globalCompositeOperation='difference';
ctx.fillStyle = "white";
ctx.globalAlpha = alpha; // alpha 0 = no effect 1 = full effect
ctx.fillRect(0, 0, image.width, image.height);
