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var imgData = context.getImageData(0,0,canvas.width,canvas.height);
var data = imgData.data;

// enumerate all pixels
// each pixel's r,g,b,a datum are stored in separate sequential array elements

for(var i=0; i<data.length; i+=4) {
  var red = data[i];
  var green = data[i+1];
  var blue = data[i+2];
  var alpha = data[i+3];
}

// save any altered pixel data back to the context
// the image will reflect any changes you made

context.putImageData(imgData, 0, 0);