Convert SVG to PNG with applied images as background to svg elements

Question

I have an external SVG file which contains some embedded image tags in pattern. Whenever I convert this SVG into PNG using toDataURL(), the generated PNG images does not contain the image I have applied as pattern to some SVG paths. Is there any way to solve this problem?

Answer 1

Yes there are : append the svg into your document and encode all the included images to dataURIs.

I am writing a script that does this and also some other stuff like including external style-sheets and some other fix of where toDataURL will fail (e.g external elements referenced through xlink:href attribute or <funciri>).

Here is the function I wrote for parsing the images content :

function parseImages(){
    var xlinkNS = "http://www.w3.org/1999/xlink";
    var total, encoded;
    // convert an external bitmap image to a dataURL
    var toDataURL = function (image) {

        var img = new Image();
        // CORS workaround, this won't work in IE<11
        // If you are sure you don't need it, remove the next line and the double onerror handler
        // First try with crossorigin set, it should fire an error if not needed
        img.crossOrigin = 'Anonymous';

        img.onload = function () {
            // we should now be able to draw it without tainting the canvas
            var canvas = document.createElement('canvas');
            canvas.width = this.width;
            canvas.height = this.height;
            // draw the loaded image
            canvas.getContext('2d').drawImage(this, 0, 0);
            // set our <image>'s href attribute to the dataURL of our canvas
            image.setAttributeNS(xlinkNS, 'href', canvas.toDataURL());
            // that was the last one
            if (++encoded === total) exportDoc();
        };

        // No CORS set in the response      
        img.onerror = function () {
            // save the src
            var oldSrc = this.src;
            // there is an other problem
            this.onerror = function () {
                console.warn('failed to load an image at : ', this.src);
                if (--total === encoded && encoded > 0) exportDoc();
            };
            // remove the crossorigin attribute
            this.removeAttribute('crossorigin');
            // retry
            this.src = '';
            this.src = oldSrc;
        };
        // load our external image into our img
        img.src = image.getAttributeNS(xlinkNS, 'href');
    };

    // get an external svg doc to data String
    var parseFromUrl = function(url, element){
        var xhr = new XMLHttpRequest();
        xhr.onload = function(){
            if(this.status === 200){
                var response = this.responseText || this.response;
                var dataUrl = 'data:image/svg+xml; charset=utf8, ' + encodeURIComponent(response);
                element.setAttributeNS(xlinkNS, 'href', dataUrl);
                if(++encoded === total) exportDoc();
                }
            // request failed with xhr, try as an <img>
            else{
                toDataURL(element);
                }
            };
        xhr.onerror = function(){toDataURL(element);};
        xhr.open('GET', url);
        xhr.send();
        };

    var images = svg.querySelectorAll('image');
    total = images.length;
    encoded = 0;

    // loop through all our <images> elements
    for (var i = 0; i < images.length; i++) {
        var href = images[i].getAttributeNS(xlinkNS, 'href');
        // check if the image is external
        if (href.indexOf('data:image') < 0){
            // if it points to another svg element
            if(href.indexOf('.svg') > 0){
                parseFromUrl(href, images[i]);
                }
            else // a pixel image
                toDataURL(images[i]);
            }
        // else increment our counter
        else if (++encoded === total) exportDoc();
    }
    // if there were no <image> element
    if (total === 0) exportDoc();
}

Here the svgDoc is called svg,
and the exportDoc() function could just be written as :

var exportDoc = function() {
    // check if our svgNode has width and height properties set to absolute values
    // otherwise, canvas won't be able to draw it
    var bbox = svg.getBoundingClientRect();

    if (svg.width.baseVal.unitType !== 1) svg.setAttribute('width', bbox.width);
    if (svg.height.baseVal.unitType !== 1) svg.setAttribute('height', bbox.height);

    // serialize our node
    var svgData = (new XMLSerializer()).serializeToString(svg);
    // remember to encode special chars
    var svgURL = 'data:image/svg+xml; charset=utf8, ' + encodeURIComponent(svgData);

    var svgImg = new Image();

    svgImg.onload = function () {
        var canvas =  document.createElement('canvas');
        // IE11 doesn't set a width on svg images...
        canvas.width = this.width || bbox.width;
        canvas.height = this.height || bbox.height;

        canvas.getContext('2d').drawImage(svgImg, 0, 0, canvas.width, canvas.height);
        doSomethingWith(canvas)
    };

    svgImg.src = svgURL;
};

But once again, you will have to append your svg into the document first (either through xhr or into an <iframe> or an <object> element, and you will have to be sure all your resources are CORS compliant (or from same domain) in order to get these rendered.

var svg = document.querySelector('svg');
var doSomethingWith = function(canvas) {
  document.body.appendChild(canvas)
};

function parseImages() {
  var xlinkNS = "http://www.w3.org/1999/xlink";
  var total, encoded;
  // convert an external bitmap image to a dataURL
  var toDataURL = function(image) {

    var img = new Image();
    // CORS workaround, this won't work in IE<11
    // If you are sure you don't need it, remove the next line and the double onerror handler
    // First try with crossorigin set, it should fire an error if not needed
    img.crossOrigin = 'anonymous';

    img.onload = function() {
      // we should now be able to draw it without tainting the canvas
      var canvas = document.createElement('canvas');
      canvas.width = this.width;
      canvas.height = this.height;
      // draw the loaded image
      canvas.getContext('2d').drawImage(this, 0, 0);
      // set our <image>'s href attribute to the dataURL of our canvas
      image.setAttributeNS(xlinkNS, 'href', canvas.toDataURL());
      // that was the last one
      if (++encoded === total) exportDoc();
    };

    // No CORS set in the response  
    img.onerror = function() {
      // save the src
      var oldSrc = this.src;
      // there is an other problem
      this.onerror = function() {
        console.warn('failed to load an image at : ', this.src);
        if (--total === encoded && encoded > 0) exportDoc();
      };
      // remove the crossorigin attribute
      this.removeAttribute('crossorigin');
      // retry
      this.src = '';
      this.src = oldSrc;
    };
    // load our external image into our img
    var href = image.getAttributeNS(xlinkNS, 'href');
    // really weird bug that appeared since this answer was first posted
    // we need to force a no-cached request for the crossOrigin be applied
    img.src = href + (href.indexOf('?') > -1 ? + '&1': '?1');
  };

  // get an external svg doc to data String
  var parseFromUrl = function(url, element) {
    var xhr = new XMLHttpRequest();
    xhr.onload = function() {
      if (this.status === 200) {
        var response = this.responseText || this.response;
        var dataUrl = 'data:image/svg+xml; charset=utf8, ' + encodeURIComponent(response);
        element.setAttributeNS(xlinkNS, 'href', dataUrl);
        if (++encoded === total) exportDoc();
      }
      // request failed with xhr, try as an <img>
      else {
        toDataURL(element);
      }
    };
    xhr.onerror = function() {
      toDataURL(element);
    };
    xhr.open('GET', url);
    xhr.send();
  };

  var images = svg.querySelectorAll('image');
  total = images.length;
  encoded = 0;

  // loop through all our <images> elements
  for (var i = 0; i < images.length; i++) {
    var href = images[i].getAttributeNS(xlinkNS, 'href');
    // check if the image is external
    if (href.indexOf('data:image') < 0) {
      // if it points to another svg element
      if (href.indexOf('.svg') > 0) {
        parseFromUrl(href, images[i]);
      } else // a pixel image
        toDataURL(images[i]);
    }
    // else increment our counter
    else if (++encoded === total) exportDoc();
  }
  // if there were no <image> element
  if (total === 0) exportDoc();
}

var exportDoc = function() {
  // check if our svgNode has width and height properties set to absolute values
  // otherwise, canvas won't be able to draw it
  var bbox = svg.getBoundingClientRect();

  if (svg.width.baseVal.unitType !== 1) svg.setAttribute('width', bbox.width);
  if (svg.height.baseVal.unitType !== 1) svg.setAttribute('height', bbox.height);

  // serialize our node
  var svgData = (new XMLSerializer()).serializeToString(svg);
  // remember to encode special chars
  var svgURL = 'data:image/svg+xml; charset=utf8, ' + encodeURIComponent(svgData);

  var svgImg = new Image();

  svgImg.onload = function() {
    var canvas = document.createElement('canvas');
    // IE11 doesn't set a width on svg images...
    canvas.width = this.width || bbox.width;
    canvas.height = this.height || bbox.height;

    canvas.getContext('2d').drawImage(svgImg, 0, 0, canvas.width, canvas.height);
    doSomethingWith(canvas)
  };

  svgImg.src = svgURL;
};
window.onload = parseImages;

canvas {
  border: 1px solid green !important;
}

<svg width="200" height="200" xmlns="http://www.w3.org/2000/svg" version="1.1">
  <defs>

    <pattern id="Pattern" x="0" y="0" width=".25" height=".25">
      <image xlink:href="https://dl.dropboxusercontent.com/s/1alt1303g9zpemd/UFBxY.png" width="100" height="100"/>
    </pattern>

  </defs>

  <rect fill="url(#Pattern)" x="0" y="0" width="200" height="200"/>
</svg>