Use old school data URIs for generating image if it helps working aro…

…und tainted canvas issues. Closes #88

src/svg2image.js

@@ -3,6 +3,51 @@ var svg2image = (function (ayepromise, window) {
 
     var module = {};
 
+    var urlForSvg = function (svg, useBlobs) {
+        if (useBlobs) {
+            return URL.createObjectURL(new Blob([svg], {"type": "image/svg+xml"}));
+        } else {
+            return "data:image/svg+xml;charset=utf-8," + encodeURIComponent(svg);
+        }
+    };
+
+    var simpleForeignObjectSvg = '<svg xmlns="http://www.w3.org/2000/svg" width="1" height="1"><foreignObject></foreignObject></svg>';
+
+    var supportsReadingObjectFromCanvas = function (url) {
+        var canvas = document.createElement("canvas"),
+            image = new Image(),
+            defer = ayepromise.defer();
+
+        image.onload = function () {
+            var context = canvas.getContext("2d");
+            try {
+                context.drawImage(image, 0, 0);
+                // This will fail in Chrome & Safari
+                canvas.toDataURL("image/png");
+                defer.resolve(true);
+            } catch (e) {
+                defer.resolve(false);
+            }
+        };
+        image.src = url;
+
+        return defer.promise;
+    };
+
+    var readingBackFromCanvasBenefitsFromOldSchoolDataUris = function () {
+        // Check for work around for https://code.google.com/p/chromium/issues/detail?id=294129
+        return supportsReadingObjectFromCanvas(urlForSvg(simpleForeignObjectSvg, true))
+            .then(function (supportsReadingFromBlobs) {
+                if (supportsReadingFromBlobs) {
+                    return false;
+                }
+                return supportsReadingObjectFromCanvas(urlForSvg(simpleForeignObjectSvg, false))
+                    .then(function (s) {
+                        return s;
+                    });
+            });
+    };
+
     var supportsBlobBuilding = function () {
         if (window.Blob) {
             // Available as constructor only in newer builds for all browsers
@@ -14,18 +59,39 @@ var svg2image = (function (ayepromise, window) {
         return false;
     };
 
-    var useBlobs = supportsBlobBuilding() && window.URL;
+    var checkBlobSupport = function () {
+        var defer = ayepromise.defer();
 
-    var buildImageUrl = function (svg) {
-        if (useBlobs) {
-            return URL.createObjectURL(new Blob([svg], {"type": "image/svg+xml"}));
+        if (supportsBlobBuilding && window.URL) {
+            readingBackFromCanvasBenefitsFromOldSchoolDataUris()
+                .then(function (doesBenefit) {
+                    defer.resolve(! doesBenefit);
+                });
         } else {
-            return "data:image/svg+xml;charset=utf-8," + encodeURIComponent(svg);
+            defer.resolve(false);
+        }
+
+        return defer.promise;
+    };
+
+    var checkForBlobsResult;
+
+    var checkForBlobs = function () {
+        if (checkForBlobsResult === undefined) {
+            checkForBlobsResult = checkBlobSupport();
         }
+
+        return checkForBlobsResult;
+    };
+
+    var buildImageUrl = function (svg) {
+        return checkForBlobs().then(function (useBlobs) {
+            return urlForSvg(svg, useBlobs);
+        });
     };
 
     var cleanUpUrl = function (url) {
-        if (useBlobs) {
+        if (url instanceof Blob) {
             URL.revokeObjectURL(url);
         }
     };
@@ -43,8 +109,6 @@ var svg2image = (function (ayepromise, window) {
                 }
             };
 
-        url = buildImageUrl(svg);
-
         image = new window.Image();
         image.onload = function() {
             resetEventHandlers();
@@ -58,7 +122,10 @@ var svg2image = (function (ayepromise, window) {
             // Webkit calls the onerror handler if the SVG is faulty
             defer.reject();
         };
-        image.src = url;
+        buildImageUrl(svg).then(function (imageUrl) {
+            url = imageUrl;
+            image.src = url;
+        });
 
         return defer.promise;
     };