Study how to insert easing functions

build.js

@@ -66,6 +66,7 @@ var filesToInclude = [
   'src/util/dom_style.js',
   'src/util/dom_misc.js',
   'src/util/dom_request.js',
+  'src/util/anim_ease.js',
   
   ifSpecifiedInclude('parser', 'src/parser.js'),
   

src/object.class.js

@@ -1313,6 +1313,7 @@
       fabric.util.animate({
         startValue: options.from,
         endValue: to,
+        easing: options.easing,
         duration: options.duration,
         onChange: function(value) {
           obj.set(property, value);

src/util/anim_ease.js

@@ -0,0 +1,12 @@
+(function(){
+
+function sine(pos){
+	return (-Math.cos(pos * Math.PI) / 2) + 0.5; 
+}
+
+/** @namespace */
+fabric.util.ease = {
+  sine: sine
+};
+
+}());