Threshold image colors – Base64

I would like to use threshold filter on base64 string (data:image/png;base64,iVBOR...) using javaScript like this:

function threshold(base64) {
    some action whith base64 string...

    return base64; //base64 is updated by threshold filter    
}

Is it possible and if it is, how can I do this?

Answer

var base64string = "data:image/png;base64,iVBOR..........",
    threshold = 180, // 0..255
    ctx = document.createElement("canvas").getContext("2d"),
    image = new Image();

image.onload = function() {

  var w = ctx.canvas.width  = image.width,
      h = ctx.canvas.height = image.height;

  ctx.drawImage(image, 0, 0, w, h);      // Set image to Canvas context
  var d = ctx.getImageData(0, 0, w, h);  // Get image Data from Canvas context


  for (var i=0; i<d.data.length; i+=4) { // 4 is for RGBA channels
    // R=G=B=R>T?255:0
    d.data[i] = d.data[i+1] = d.data[i+2] = d.data[i+1] > threshold ? 255 : 0;
  }

  ctx.putImageData(d, 0, 0);             // Apply threshold conversion
  document.body.appendChild(ctx.canvas); // Show result

};
image.src = base64string;

MDN – putImageData
MDN – getImageData