Fix multiple applications of blend image filter with transparent images

src/filters/blendimage_filter.class.js

@@ -148,7 +148,7 @@
           canvas1, context, image = this.image, blendData;
 
       if (!resources.blendImage) {
-        resources.blendImage = document.createElement('canvas');
+        resources.blendImage = fabric.util.createCanvasElement();
       }
       canvas1 = resources.blendImage;
       if (canvas1.width !== width || canvas1.height !== height) {
@@ -159,6 +159,7 @@
       context.setTransform(image.scaleX, 0, 0, image.scaleY, image.left, image.top);
       context.drawImage(image._element, 0, 0, width, height);
       blendData = context.getImageData(0, 0, width, height).data;
+      context.clearRect(0, 0, width, height);
       for (var i = 0; i < iLen; i += 4) {
 
         r = data[i];

test/visual/blendimage_filter.js

@@ -0,0 +1,94 @@
+(function() {
+  fabric.enableGLFiltering = false;
+  var _pixelMatch = pixelmatch;
+  if (fabric.isLikelyNode) {
+    var fs = global.fs;
+    _pixelMatch = global.pixelmatch;
+  }
+  var fabricCanvas = this.canvas = new fabric.Canvas(null, {enableRetinaScaling: false, renderOnAddRemove: false});
+  var pixelmatchOptions = {
+    includeAA: false,
+    threshold: 0.01
+  };
+  fabric.Object.prototype.objectCaching = false;
+
+  function getAbsolutePath(path) {
+    var isAbsolute = /^https?:/.test(path);
+    if (isAbsolute) { return path; };
+    var imgEl = fabric.document.createElement('img');
+    imgEl.src = path;
+    var src = imgEl.src;
+    imgEl = null;
+    return src;
+  }
+
+  function getFixtureName(filename) {
+    var finalName = '/../fixtures/' + filename;
+    return fabric.isLikelyNode ? (__dirname + finalName) : getAbsolutePath('test/fixtures/' + filename);
+  }
+
+  function getImage(filename, original, callback) {
+    if (fabric.isLikelyNode && original) {
+      try {
+        fs.statSync(filename);
+      }
+      catch (err) {
+        var dataUrl = original.toDataURL().split(',')[1];
+        console.log('creating original for ', filename);
+        fs.writeFileSync(filename, dataUrl, { encoding: 'base64' });
+      }
+    }
+    var img = fabric.document.createElement('img');
+    img.onload = function() {
+      callback(img);
+    };
+    img.onerror = function(err) {
+      console.log('Image loading errored', err);
+    };
+    img.src = filename;
+  }
+
+  QUnit.module('Blend image filter test', {
+    afterEach: function() {
+      fabricCanvas.clear();
+      fabricCanvas.renderAll();
+    }
+  });
+
+  var testName = 'Multiple applications of filter';
+  QUnit.test(testName, function(assert) {
+    var done = assert.async();
+    getImage(getFixtureName('greyfloral.png'), false, function(img) {
+      getImage(getFixtureName('greyfloral_partial_transparent.png'), false, function(backdrop) {
+        var image = new fabric.Image(img);
+        var backdropImage = new fabric.Image(backdrop);
+        image.filters.push(new fabric.Image.filters.BlendImage({image: backdropImage}));
+        image.applyFilters();
+
+        fabricCanvas.add(image);
+        fabricCanvas.renderAll();
+
+        var renderedCanvas = fabricCanvas.lowerCanvasEl;
+
+        var width = renderedCanvas.width;
+        var height = renderedCanvas.height;
+        var totalPixels = width * height;
+        var imageDataCanvas = renderedCanvas.getContext('2d').getImageData(0, 0, width, height).data;
+
+        image.applyFilters();
+        fabricCanvas.renderAll();
+        var imageDataCanvas2 = renderedCanvas.getContext('2d').getImageData(0, 0, width, height).data;
+
+        var differentPixels = _pixelMatch(imageDataCanvas, imageDataCanvas2, null, width, height, pixelmatchOptions);
+        var percDiff = differentPixels / totalPixels * 100;
+        var okDiff = totalPixels * 0.01;
+        assert.ok(
+          differentPixels < okDiff,
+          testName + ' has too many different pixels ' + differentPixels + '(' + okDiff + ') representing ' + percDiff + '%'
+        );
+        console.log('Different pixels:', differentPixels, '/', totalPixels, ' diff:', percDiff.toFixed(3), '%');
+        done();
+      });      
+    });
+  });
+})();