Preload.js

_html2canvas.Preload = function( options ) {

  var images = {
    numLoaded: 0,   // also failed are counted here
    numFailed: 0,
    numTotal: 0,
    cleanupDone: false
  },
  pageOrigin,
  methods,
  i,
  count = 0,
  element = options.elements[0] || document.body,
  doc = element.ownerDocument,
  domImages = element.getElementsByTagName('img'), // Fetch images of the present element only
  imgLen = domImages.length,
  link = doc.createElement("a"),
  supportCORS = (function( img ){
    return (img.crossOrigin !== undefined);
  })(new Image()),
  timeoutTimer;

  link.href = window.location.href;
  pageOrigin  = link.protocol + link.host;

  function isSameOrigin(url){
    link.href = url;
    link.href = link.href; // YES, BELIEVE IT OR NOT, that is required for IE9 - http://jsfiddle.net/niklasvh/2e48b/
    var origin = link.protocol + link.host;
    return (origin === pageOrigin);
  }

  function start(){
    h2clog("html2canvas: start: images: " + images.numLoaded + " / " + images.numTotal + " (failed: " + images.numFailed + ")");
    if (!images.firstRun && images.numLoaded >= images.numTotal){
      h2clog("Finished loading images: # " + images.numTotal + " (failed: " + images.numFailed + ")");

      if (typeof options.complete === "function"){
        options.complete(images);
      }

    }
  }

  // TODO modify proxy to serve images with CORS enabled, where available
  function proxyGetImage(url, img, imageObj){
    var callback_name,
    scriptUrl = options.proxy,
    script;

    link.href = url;
    url = link.href; // work around for pages with base href="" set - WARNING: this may change the url

    callback_name = 'html2canvas_' + (count++);
    imageObj.callbackname = callback_name;

    if (scriptUrl.indexOf("?") > -1) {
      scriptUrl += "&";
    } else {
      scriptUrl += "?";
    }
    scriptUrl += 'url=' + encodeURIComponent(url) + '&callback=' + callback_name;
    script = doc.createElement("script");

    window[callback_name] = function(a){
      if (a.substring(0,6) === "error:"){
        imageObj.succeeded = false;
        images.numLoaded++;
        images.numFailed++;
        start();
      } else {
        setImageLoadHandlers(img, imageObj);
        img.src = a;
      }
      window[callback_name] = undefined; // to work with IE<9  // NOTE: that the undefined callback property-name still exists on the window object (for IE<9)
      try {
        delete window[callback_name];  // for all browser that support this
      } catch(ex) {}
      script.parentNode.removeChild(script);
      script = null;
      delete imageObj.script;
      delete imageObj.callbackname;
    };

    script.setAttribute("type", "text/javascript");
    script.setAttribute("src", scriptUrl);
    imageObj.script = script;
    window.document.body.appendChild(script);

  }

  function loadPseudoElement(element, type) {
    var style = window.getComputedStyle(element, type),
    content = style.content;
    if (content.substr(0, 3) === 'url') {
      methods.loadImage(_html2canvas.Util.parseBackgroundImage(content)[0].args[0]);
    }
    loadBackgroundImages(style.backgroundImage, element);
  }

  function loadPseudoElementImages(element) {
    loadPseudoElement(element, ":before");
    loadPseudoElement(element, ":after");
  }

  function loadGradientImage(backgroundImage, bounds) {
    var img = _html2canvas.Generate.Gradient(backgroundImage, bounds);

    if (img !== undefined){
      images[backgroundImage] = {
        img: img,
        succeeded: true
      };
      images.numTotal++;
      images.numLoaded++;
      start();
    }
  }

  function invalidBackgrounds(background_image) {
    return (background_image && background_image.method && background_image.args && background_image.args.length > 0 );
  }

  function loadBackgroundImages(background_image, el) {
    var bounds;

    _html2canvas.Util.parseBackgroundImage(background_image).filter(invalidBackgrounds).forEach(function(background_image) {
      if (background_image.method === 'url') {
        methods.loadImage(background_image.args[0]);
      } else if(background_image.method.match(/\-?gradient$/)) {
        if(bounds === undefined) {
          bounds = _html2canvas.Util.Bounds(el);
        }
        loadGradientImage(background_image.value, bounds);
      }
    });
  }

  function getImages (el) {
    var elNodeType = false;

    // Firefox fails with permission denied on pages with iframes
    try {
      _html2canvas.Util.Children(el).forEach(function(img) {
        getImages(img);
      });
    }
    catch( e ) {}

    try {
      elNodeType = el.nodeType;
    } catch (ex) {
      elNodeType = false;
      h2clog("html2canvas: failed to access some element's nodeType - Exception: " + ex.message);
    }

    if (elNodeType === 1 || elNodeType === undefined) {
      loadPseudoElementImages(el);
      try {
        loadBackgroundImages(_html2canvas.Util.getCSS(el, 'backgroundImage'), el);
      } catch(e) {
        h2clog("html2canvas: failed to get background-image - Exception: " + e.message);
      }
      loadBackgroundImages(el);
    }
  }

  function setImageLoadHandlers(img, imageObj) {
    img.onload = function() {
      if ( imageObj.timer !== undefined ) {
        // CORS succeeded
        window.clearTimeout( imageObj.timer );
      }

      images.numLoaded++;
      imageObj.succeeded = true;
      img.onerror = img.onload = null;
      start();
    };
    img.onerror = function() {
      if (img.crossOrigin === "anonymous") {
        // CORS failed
        window.clearTimeout( imageObj.timer );

        // let's try with proxy instead
        if ( options.proxy ) {
          var src = img.src;
          img = new Image();
          imageObj.img = img;
          img.src = src;

          proxyGetImage( img.src, img, imageObj );
          return;
        }
      }

      images.numLoaded++;
      images.numFailed++;
      imageObj.succeeded = false;
      img.onerror = img.onload = null;
      start();
    };
  }

  methods = {
    loadImage: function( src ) {
      var img, imageObj;
      if ( src && images[src] === undefined ) {
        img = new Image();
        if ( src.match(/data:image\/.*;base64,/i) ) {
          img.src = src.replace(/url\(['"]{0,}|['"]{0,}\)$/ig, '');
          imageObj = images[src] = {
            img: img
          };
          images.numTotal++;
          setImageLoadHandlers(img, imageObj);
        } else if ( isSameOrigin( src ) || options.allowTaint ===  true ) {
          imageObj = images[src] = {
            img: img
          };
          images.numTotal++;
          setImageLoadHandlers(img, imageObj);
          img.src = src;
        } else if ( supportCORS && !options.allowTaint && options.useCORS ) {
          // attempt to load with CORS

          img.crossOrigin = "anonymous";
          imageObj = images[src] = {
            img: img
          };
          images.numTotal++;
          setImageLoadHandlers(img, imageObj);
          img.src = src;

          // work around for https://bugs.webkit.org/show_bug.cgi?id=80028
          img.customComplete = function () {
            if (!this.img.complete) {
              this.timer = window.setTimeout(this.img.customComplete, 100);
            } else {
              this.img.onerror();
            }
          }.bind(imageObj);
          img.customComplete();

        } else if ( options.proxy ) {
          imageObj = images[src] = {
            img: img
          };
          images.numTotal++;
          proxyGetImage( src, img, imageObj );
        }
      }

    },
    cleanupDOM: function(cause) {
      var img, src;
      if (!images.cleanupDone) {
        if (cause && typeof cause === "string") {
          h2clog("html2canvas: Cleanup because: " + cause);
        } else {
          h2clog("html2canvas: Cleanup after timeout: " + options.timeout + " ms.");
        }

        for (src in images) {
          if (images.hasOwnProperty(src)) {
            img = images[src];
            if (typeof img === "object" && img.callbackname && img.succeeded === undefined) {
              // cancel proxy image request
              window[img.callbackname] = undefined; // to work with IE<9  // NOTE: that the undefined callback property-name still exists on the window object (for IE<9)
              try {
                delete window[img.callbackname];  // for all browser that support this
              } catch(ex) {}
              if (img.script && img.script.parentNode) {
                img.script.setAttribute("src", "about:blank");  // try to cancel running request
                img.script.parentNode.removeChild(img.script);
              }
              images.numLoaded++;
              images.numFailed++;
              h2clog("html2canvas: Cleaned up failed img: '" + src + "' Steps: " + images.numLoaded + " / " + images.numTotal);
            }
          }
        }

        // cancel any pending requests
        if(window.stop !== undefined) {
          window.stop();
        } else if(document.execCommand !== undefined) {
          document.execCommand("Stop", false);
        }
        if (document.close !== undefined) {
          document.close();
        }
        images.cleanupDone = true;
        if (!(cause && typeof cause === "string")) {
          start();
        }
      }
    },

    renderingDone: function() {
      if (timeoutTimer) {
        window.clearTimeout(timeoutTimer);
      }
    }
  };

  if (options.timeout > 0) {
    timeoutTimer = window.setTimeout(methods.cleanupDOM, options.timeout);
  }

  h2clog('html2canvas: Preload starts: finding background-images');
  images.firstRun = true;

  getImages(element);

  h2clog('html2canvas: Preload: Finding images');
  // load <img> images
  for (i = 0; i < imgLen; i+=1){
    methods.loadImage( domImages[i].getAttribute( "src" ) );
  }

  images.firstRun = false;
  h2clog('html2canvas: Preload: Done.');
  if ( images.numTotal === images.numLoaded ) {
    start();
  }

  return methods;

};