diff --git a/src/viewport.js b/src/viewport.js
index 6f5ee841..ddc26f0e 100644
--- a/src/viewport.js
+++ b/src/viewport.js
@@ -46,8 +46,7 @@ $.Viewport = function( options ) {
 
     //backward compatibility for positional args while prefering more
     //idiomatic javascript options object as the only argument
-    var args = arguments,
-    that = this;
+    var args = arguments;
     if(  args.length && args[ 0 ] instanceof $.Point ){
         options = {
             containerSize:  args[ 0 ],
@@ -88,244 +87,7 @@ $.Viewport = function( options ) {
         degrees:            $.DEFAULT_SETTINGS.degrees
 
     }, options );
-    
-    /**
-     * @function
-     * @param {OpenSeadragon.Rect} bounds
-     * @param {Object} options (immediately=null, constraints=false)
-     * @return {OpenSeadragon.Viewport} Chainable.
-     */
-    function fitBounds( bounds, options ) {
-        var newOptions = options || {};
-        var immediately = newOptions.immediately || null;
-        var constraints = newOptions.constraints || false;
-    
-        var aspect = that.getAspectRatio(),
-            center = bounds.getCenter(),
-            newBounds = new $.Rect(
-                bounds.x,
-                bounds.y,
-                bounds.width,
-                bounds.height
-            ),
-            oldBounds,
-            oldZoom,
-            newZoom,
-            referencePoint,
-            newBoundsAspectRatio,
-            newConstrainedZoom;
-        
-        if ( newBounds.getAspectRatio() >= aspect ) {
-            newBounds.height = bounds.width / aspect;
-            newBounds.y      = center.y - newBounds.height / 2;
-        } else {
-            newBounds.width = bounds.height * aspect;
-            newBounds.x     = center.x - newBounds.width / 2;
-        }
-        
-        if ( constraints ) {
-            newBoundsAspectRatio = newBounds.getAspectRatio();
-        }
-        
-        that.panTo( that.getCenter( true ), true );
-        that.zoomTo( that.getZoom( true ), null, true );
-        
-        oldBounds = that.getBounds();
-        oldZoom   = that.getZoom();
-        newZoom   = 1.0 / newBounds.width;
-        
-        if ( constraints ) {
-            newConstrainedZoom = Math.max(
-                Math.min(newZoom, that.getMaxZoom() ),
-                that.getMinZoom()
-            );
-            
-            if (newZoom !== newConstrainedZoom) {
-                newZoom = newConstrainedZoom;
-                newBounds.width = 1.0 / newZoom;
-                newBounds.x = center.x - newBounds.width / 2;
-                newBounds.height = newBounds.width / newBoundsAspectRatio;
-                newBounds.y = center.y - newBounds.height / 2;
-            }
-            
-            newBounds = applyBoundaryConstraints( newBounds, immediately );
-            
-            if( that.viewer ){
-                /**
-                 * Raised when the viewport constraints are applied (see {@link OpenSeadragon.Viewport#applyConstraints}).
-                 *
-                 * @event constrain
-                 * @memberof OpenSeadragon.Viewer
-                 * @type {object}
-                 * @property {OpenSeadragon.Viewer} eventSource - A reference to the Viewer which raised this event.
-                 * @property {Boolean} immediately
-                 * @property {?Object} userData - Arbitrary subscriber-defined object.
-                 */
-                that.viewer.raiseEvent( 'constrain', {
-                    immediately: immediately
-                });
-            }
-        }
-        
-        if ( newZoom == oldZoom || newBounds.width == oldBounds.width ) {
-            return that.panTo( constraints ? newBounds.getCenter() : center, immediately );
-        }
-        
-        referencePoint = oldBounds.getTopLeft().times(
-            that.containerSize.x / oldBounds.width
-        ).minus(
-            newBounds.getTopLeft().times(
-                that.containerSize.x / newBounds.width
-            )
-        ).divide(
-            that.containerSize.x / oldBounds.width -
-            that.containerSize.x / newBounds.width
-        );
-        
-        return that.zoomTo( newZoom, referencePoint, immediately );
-    }
-    
-    /**
-     * @function
-     * @param {OpenSeadragon.Rect} bounds
-     * @param {Boolean} immediately
-     * @return {OpenSeadragon.Viewport} Chainable.
-     */
-    this.fitBounds = function( bounds, immediately ) {
-        return fitBounds( bounds, {
-            immediately: immediately,
-            constraints: false
-        } );
-    };
-    
-    /**
-     * @function
-     * @param {OpenSeadragon.Rect} bounds
-     * @param {Boolean} immediately
-     * @return {OpenSeadragon.Viewport} Chainable.
-     */
-    this.fitBoundsWithConstraints = function( bounds, immediately ) {
-        return fitBounds( bounds, {
-            immediately: immediately,
-            constraints: true
-        } );
-    };
 
-    /**
-     * @function
-     * @param {OpenSeadragon.Rect} bounds
-     * @param {Boolean} immediately
-     * @return {OpenSeadragon.Rect} constrained bounds.
-     */    
-    function applyBoundaryConstraints( bounds, immediately ) {
-        var horizontalThreshold,
-            verticalThreshold,
-            left,
-            right,
-            top,
-            bottom,
-            dx = 0,
-            dy = 0,
-            newBounds = new $.Rect(
-                bounds.x,
-                bounds.y,
-                bounds.width,
-                bounds.height
-            );
-    
-        horizontalThreshold = that.visibilityRatio * newBounds.width;
-        verticalThreshold   = that.visibilityRatio * newBounds.height;
-
-        left   = newBounds.x + newBounds.width;
-        right  = 1 - newBounds.x;
-        top    = newBounds.y + newBounds.height;
-        bottom = that.contentAspectY - newBounds.y;
-
-        if ( that.wrapHorizontal ) {
-            //do nothing
-        } else {
-            if ( left < horizontalThreshold ) {
-                dx = horizontalThreshold - left;
-            }
-            if ( right < horizontalThreshold ) {
-                dx = dx ?
-                    ( dx + right - horizontalThreshold ) / 2 :
-                    ( right - horizontalThreshold );
-            }
-        }
-
-        if ( that.wrapVertical ) {
-            //do nothing
-        } else {
-            if ( top < verticalThreshold ) {
-                dy = ( verticalThreshold - top );
-            }
-            if ( bottom < verticalThreshold ) {
-                dy =  dy ?
-                    ( dy + bottom - verticalThreshold ) / 2 :
-                    ( bottom - verticalThreshold );
-            }
-        }
-
-        if ( dx || dy || immediately ) {
-            newBounds.x += dx;
-            newBounds.y += dy;
-            if( newBounds.width > 1  ){
-                newBounds.x = 0.5 - newBounds.width/2;
-            }
-            if( newBounds.height > that.contentAspectY ){
-                newBounds.y = that.contentAspectY/2 - newBounds.height/2;
-            }
-        }
-        
-        return newBounds;
-    }
-    
-    /**
-     * @function
-     * @return {OpenSeadragon.Viewport} Chainable.
-     * @fires OpenSeadragon.Viewer.event:constrain
-     */
-    this.applyConstraints = function( immediately ) {
-        var actualZoom = this.getZoom(),
-            constrainedZoom = Math.max(
-                Math.min( actualZoom, this.getMaxZoom() ),
-                this.getMinZoom()
-            ),
-            bounds,
-            constrainedBounds;
-        
-        if ( actualZoom != constrainedZoom ) {
-            this.zoomTo( constrainedZoom, this.zoomPoint, immediately );
-        }
-
-        bounds = this.getBounds();
-
-        constrainedBounds = applyBoundaryConstraints( bounds, immediately );
-        
-        if ( bounds.x !== constrainedBounds.x || bounds.y !== constrainedBounds.y || immediately ){
-            this.fitBounds( constrainedBounds, immediately );
-        }
-
-        if( this.viewer ){
-            /**
-             * Raised when the viewport constraints are applied (see {@link OpenSeadragon.Viewport#applyConstraints}).
-             *
-             * @event constrain
-             * @memberof OpenSeadragon.Viewer
-             * @type {object}
-             * @property {OpenSeadragon.Viewer} eventSource - A reference to the Viewer which raised this event.
-             * @property {Boolean} immediately
-             * @property {?Object} userData - Arbitrary subscriber-defined object.
-             */
-            this.viewer.raiseEvent( 'constrain', {
-                immediately: immediately
-            });
-        }
-
-        return this;
-    };    
-    
     this.centerSpringX = new $.Spring({
         initial: 0,
         springStiffness: this.springStiffness,
@@ -557,6 +319,121 @@ $.Viewport.prototype = /** @lends OpenSeadragon.Viewport.prototype */{
             return this.zoomSpring.target.value;
         }
     },
+    
+    /**
+     * @function
+     * @param {OpenSeadragon.Rect} bounds
+     * @param {Boolean} immediately
+     * @return {OpenSeadragon.Rect} constrained bounds.
+     */    
+    _applyBoundaryConstraints: function( bounds, immediately ) {
+        var horizontalThreshold,
+            verticalThreshold,
+            left,
+            right,
+            top,
+            bottom,
+            dx = 0,
+            dy = 0,
+            newBounds = new $.Rect(
+                bounds.x,
+                bounds.y,
+                bounds.width,
+                bounds.height
+            );
+    
+        horizontalThreshold = this.visibilityRatio * newBounds.width;
+        verticalThreshold   = this.visibilityRatio * newBounds.height;
+
+        left   = newBounds.x + newBounds.width;
+        right  = 1 - newBounds.x;
+        top    = newBounds.y + newBounds.height;
+        bottom = this.contentAspectY - newBounds.y;
+
+        if ( this.wrapHorizontal ) {
+            //do nothing
+        } else {
+            if ( left < horizontalThreshold ) {
+                dx = horizontalThreshold - left;
+            }
+            if ( right < horizontalThreshold ) {
+                dx = dx ?
+                    ( dx + right - horizontalThreshold ) / 2 :
+                    ( right - horizontalThreshold );
+            }
+        }
+
+        if ( this.wrapVertical ) {
+            //do nothing
+        } else {
+            if ( top < verticalThreshold ) {
+                dy = ( verticalThreshold - top );
+            }
+            if ( bottom < verticalThreshold ) {
+                dy =  dy ?
+                    ( dy + bottom - verticalThreshold ) / 2 :
+                    ( bottom - verticalThreshold );
+            }
+        }
+
+        if ( dx || dy || immediately ) {
+            newBounds.x += dx;
+            newBounds.y += dy;
+            if( newBounds.width > 1  ){
+                newBounds.x = 0.5 - newBounds.width/2;
+            }
+            if( newBounds.height > this.contentAspectY ){
+                newBounds.y = this.contentAspectY/2 - newBounds.height/2;
+            }
+        }
+        
+        return newBounds;    
+    },
+    
+    /**
+     * @function
+     * @return {OpenSeadragon.Viewport} Chainable.
+     * @fires OpenSeadragon.Viewer.event:constrain
+     */
+    applyConstraints: function( immediately ) {
+        var actualZoom = this.getZoom(),
+            constrainedZoom = Math.max(
+                Math.min( actualZoom, this.getMaxZoom() ),
+                this.getMinZoom()
+            ),
+            bounds,
+            constrainedBounds;
+        
+        if ( actualZoom != constrainedZoom ) {
+            this.zoomTo( constrainedZoom, this.zoomPoint, immediately );
+        }
+
+        bounds = this.getBounds();
+
+        constrainedBounds = this._applyBoundaryConstraints( bounds, immediately );
+        
+        if ( bounds.x !== constrainedBounds.x || bounds.y !== constrainedBounds.y || immediately ){
+            this.fitBounds( constrainedBounds, immediately );
+        }
+
+        if( this.viewer ){
+            /**
+             * Raised when the viewport constraints are applied (see {@link OpenSeadragon.Viewport#applyConstraints}).
+             *
+             * @event constrain
+             * @memberof OpenSeadragon.Viewer
+             * @type {object}
+             * @property {OpenSeadragon.Viewer} eventSource - A reference to the Viewer which raised this event.
+             * @property {Boolean} immediately
+             * @property {?Object} userData - Arbitrary subscriber-defined object.
+             */
+            this.viewer.raiseEvent( 'constrain', {
+                immediately: immediately
+            });
+        }
+
+        return this;
+    },
 
     /**
      * @function
@@ -566,6 +443,128 @@ $.Viewport.prototype = /** @lends OpenSeadragon.Viewport.prototype */{
         return this.applyConstraints( immediately );
     },
     
+    /**
+     * @function
+     * @param {OpenSeadragon.Rect} bounds
+     * @param {Object} options (immediately=null, constraints=false)
+     * @return {OpenSeadragon.Viewport} Chainable.
+     */
+    _fitBounds: function( bounds, options ) {
+        var newOptions = options || {};
+        var immediately = newOptions.immediately || null;
+        var constraints = newOptions.constraints || false;
+    
+        var aspect = this.getAspectRatio(),
+            center = bounds.getCenter(),
+            newBounds = new $.Rect(
+                bounds.x,
+                bounds.y,
+                bounds.width,
+                bounds.height
+            ),
+            oldBounds,
+            oldZoom,
+            newZoom,
+            referencePoint,
+            newBoundsAspectRatio,
+            newConstrainedZoom;
+        
+        if ( newBounds.getAspectRatio() >= aspect ) {
+            newBounds.height = bounds.width / aspect;
+            newBounds.y      = center.y - newBounds.height / 2;
+        } else {
+            newBounds.width = bounds.height * aspect;
+            newBounds.x     = center.x - newBounds.width / 2;
+        }
+        
+        if ( constraints ) {
+            newBoundsAspectRatio = newBounds.getAspectRatio();
+        }
+        
+        this.panTo( this.getCenter( true ), true );
+        this.zoomTo( this.getZoom( true ), null, true );
+        
+        oldBounds = this.getBounds();
+        oldZoom   = this.getZoom();
+        newZoom   = 1.0 / newBounds.width;
+        
+        if ( constraints ) {
+            newConstrainedZoom = Math.max(
+                Math.min(newZoom, this.getMaxZoom() ),
+                this.getMinZoom()
+            );
+            
+            if (newZoom !== newConstrainedZoom) {
+                newZoom = newConstrainedZoom;
+                newBounds.width = 1.0 / newZoom;
+                newBounds.x = center.x - newBounds.width / 2;
+                newBounds.height = newBounds.width / newBoundsAspectRatio;
+                newBounds.y = center.y - newBounds.height / 2;
+            }
+            
+            newBounds = this._applyBoundaryConstraints( newBounds, immediately );
+            
+            if( this.viewer ){
+                /**
+                 * Raised when the viewport constraints are applied (see {@link OpenSeadragon.Viewport#applyConstraints}).
+                 *
+                 * @event constrain
+                 * @memberof OpenSeadragon.Viewer
+                 * @type {object}
+                 * @property {OpenSeadragon.Viewer} eventSource - A reference to the Viewer which raised this event.
+                 * @property {Boolean} immediately
+                 * @property {?Object} userData - Arbitrary subscriber-defined object.
+                 */
+                this.viewer.raiseEvent( 'constrain', {
+                    immediately: immediately
+                });
+            }
+        }
+        
+        if ( newZoom == oldZoom || newBounds.width == oldBounds.width ) {
+            return this.panTo( constraints ? newBounds.getCenter() : center, immediately );
+        }
+        
+        referencePoint = oldBounds.getTopLeft().times(
+            this.containerSize.x / oldBounds.width
+        ).minus(
+            newBounds.getTopLeft().times(
+                this.containerSize.x / newBounds.width
+            )
+        ).divide(
+            this.containerSize.x / oldBounds.width -
+            this.containerSize.x / newBounds.width
+        );
+        
+        return this.zoomTo( newZoom, referencePoint, immediately );
+    },    
+    
+    /**
+     * @function
+     * @param {OpenSeadragon.Rect} bounds
+     * @param {Boolean} immediately
+     * @return {OpenSeadragon.Viewport} Chainable.
+     */
+    fitBounds: function( bounds, immediately ) {
+        return this._fitBounds( bounds, {
+            immediately: immediately,
+            constraints: false
+        } );
+    },
+    
+    /**
+     * @function
+     * @param {OpenSeadragon.Rect} bounds
+     * @param {Boolean} immediately
+     * @return {OpenSeadragon.Viewport} Chainable.
+     */
+    fitBoundsWithConstraints: function( bounds, immediately ) {
+        return this._fitBounds( bounds, {
+            immediately: immediately,
+            constraints: true
+        } );
+    },
+    
     /**
      * @function
      * @param {Boolean} immediately