canvasFingerprintObstructer.js

// ==UserScript==
// @name        Canvas Tracking Blocker
// @namespace   ctb
// @description Monkey patches toDataURL for canvas to make it non-deterministic
// @include     *
// @version     1
// @grant       none
// @run-at document-start
// ==/UserScript==
var toDataURL = HTMLCanvasElement.prototype.toDataURL;
HTMLCanvasElement.prototype.toDataURL = function () {
  if ( !this.__obstructed ) {
    var ctx = this.getContext("2d");
    ctx.fillStyle = "rgba(255, 255, 255, 0.1)";
    ctx.fillRect(Math.random(1), Math.random(1), Math.random(0.2), Math.random(0.2));
    this.__obstructed = true;
  }
  return toDataURL.apply(this, arguments);
}