//invert.js

  $scope.addInvertFilter = function(){
     var image = new Image();
     image.src = $scope.userPost.imageURI;

     var canvas = document.createElement('canvas');
     canvas.height = canvas.width;
     document.body.appendChild(canvas);

     var ctx = canvas.getContext('2d');
     ctx.drawImage(image, 0, 0);
     image.style.display = 'none';
     var imageData = ctx.getImageData(0,0, canvas.width, canvas.height);
     var data = imageData.data;

     for (var i = 0; i < data.length; i += 4) {
       data[i]     = 255 - data[i];     // red
       data[i + 1] = 255 - data[i + 1]; // green
       data[i + 2] = 255 - data[i + 2]; // blue
     }

     ctx.putImageData(imageData, 0, 0);
     $scope.userPost.imageURI = canvas.toDataURL();
   };