NoCanvasFingerprinting.js

// ==UserScript==
// @name        NoCanvasFingerprinting
// @namespace   NCF
// @description Inspects every script element before it gets executed to determine if it is bad. If so, replaces it.
// @include     *
// @version     1
// @grant       none
// @run-at      document-start
// ==/UserScript==

document.orig_createelement = document.createElement;
document.createElement = function(x) {
  var ret_canvas = document.orig_createelement(x);
 
  //Here we can override canvas functions
  ret_canvas.orig_todataurl = ret_canvas.toDataURL;
  ret_canvas.toDataURL = function() {
    var randomID = Math.random().toString(10);
    var saved_style = ret_canvas.getContext('2d').fillStyle;
    var r = 255*Math.random()|0,
        g = 255*Math.random()|0,
        b = 255*Math.random()|0
        a = .01;
    ret_canvas.getContext('2d').fillStyle = 'rgba(' + r + ',' + g + ',' + b + ',' + a + ')';
    ret_canvas.getContext('2d').fillRect(0,0,ret_canvas.width,ret_canvas.height);
    ret_canvas.getContext('2d').fillStyle = saved_style;
   
    return ret_canvas.orig_todataurl();
  }
 
  ret_canvas.orig_getcontext = ret_canvas.getContext;
  ret_canvas.getContext = function(x) {
    var ret_ctx = ret_canvas.orig_getcontext(x);
  
    //Here we can override ctx functions
    ret_ctx.orig_getimagedata = ret_ctx.getImageData;
    ret_ctx.getImageData = function(w,x,y,z) {
      var imagedata = ret_ctx.orig_getimagedata(w,x,y,z);
      for (var i = 0; i < imagedata.data.length; i+=4) {
        //alpha blend the pixel to emulate ctx fill
        var r = 255*Math.random()|0,
            g = 255*Math.random()|0,
            b = 255*Math.random()|0
        imagedata[i] = r +  (imagedata[i] * .99);
        imagedata[i+1] = g +  (imagedata[i+1] * .99);
        imagedata[i+2] = b +  (imagedata[i+2] * .99);
        imagedata[i+3] = a +  (imagedata[i+3] * .99);
      }
        
      return imagedata;
    }
       
    return ret_ctx;
  }
 
  return ret_canvas;
}