Update to allow setting of globalCompositeOperation when

html <canvas> is used to process multiple tiledImages to blend
is a specific way.
(special handling,
   when compositeOperation is 'source-over' and opacity is 1,
   useSketch is false, otherwise useSketch is true
)
Valid values are 'source-atop', 'source-in', 'source-out',
  'destination-over', 'destination-atop', 'destination-in',
  'destination-out', 'lighter', 'copy' or 'xor'
http://www.w3schools.com/tags/canvas_globalcompositeoperation.asp

src/drawer.js

@@ -373,13 +373,14 @@ $.Drawer.prototype = /** @lends OpenSeadragon.Drawer.prototype */{
      * @param {Float} opacity The opacity of the blending.
      * @returns {undefined}
      */
-    blendSketch: function(opacity) {
+    blendSketch: function(opacity, compositeOperation) {
         if (!this.useCanvas || !this.sketchCanvas) {
             return;
         }
 
         this.context.save();
         this.context.globalAlpha = opacity;
+        this.context.globalCompositeOperation = compositeOperation;
         this.context.drawImage(this.sketchCanvas, 0, 0);
         this.context.restore();
     },

src/openseadragon.js

@@ -206,6 +206,11 @@
   * @property {Number} [opacity=1]
   *     Default opacity of the tiled images (1=opaque, 0=transparent)
   *
+  * @property {String} [compositeOperation='source-over']
+  *     Valid values are 'source-atop', 'source-in', 'source-out',
+  *     'destination-over', 'destination-atop', 'destination-in',
+  *     'destination-out', 'lighter', 'copy' or 'xor'.<br>
+  *
   * @property {String|CanvasGradient|CanvasPattern|Function} [placeholderFillStyle=null]
   *     Draws a colored rectangle behind the tile if it is not loaded yet.
   *     You can pass a CSS color value like "#FF8800".
@@ -1044,6 +1049,7 @@ if (typeof define === 'function' && define.amd) {
 
             // APPEARANCE
             opacity:                    1,
+            compositeOperation:         'source-over',
             placeholderFillStyle:       null,
 
             //REFERENCE STRIP SETTINGS

src/tile.js

@@ -268,6 +268,7 @@ $.Tile.prototype = /** @lends OpenSeadragon.Tile.prototype */{
         context.save();
 
         context.globalAlpha = this.opacity;
+        context.globalCompositeOperation = this.compositeOperation;
 
         //if we are supposed to be rendering fully opaque rectangle,
         //ie its done fading or fading is turned off, and if we are drawing

src/tiledimage.js

@@ -65,6 +65,7 @@
  * @param {Boolean} [options.alwaysBlend] - See {@link OpenSeadragon.Options}.
  * @param {Number} [options.minPixelRatio] - See {@link OpenSeadragon.Options}.
  * @param {Number} [options.opacity=1] - Opacity the tiled image should be drawn at.
+ * @param {String} [options.compositeOperation='source-over'] - How a tiled source image are drawn onto an existing image.
  * @param {Boolean} [options.debugMode] - See {@link OpenSeadragon.Options}.
  * @param {String|CanvasGradient|CanvasPattern|Function} [options.placeholderFillStyle] - See {@link OpenSeadragon.Options}.
  * @param {String|Boolean} [options.crossOriginPolicy] - See {@link OpenSeadragon.Options}.
@@ -131,7 +132,6 @@ $.TiledImage = function( options ) {
         _midDraw:       false, // Is the tiledImage currently updating the viewport?
         _needsDraw:     true,  // Does the tiledImage need to update the viewport again?
         _hasOpaqueTile: false,  // Do we have even one fully opaque tile?
-
         //configurable settings
         springStiffness:      $.DEFAULT_SETTINGS.springStiffness,
         animationTime:        $.DEFAULT_SETTINGS.animationTime,
@@ -145,7 +145,8 @@ $.TiledImage = function( options ) {
         debugMode:            $.DEFAULT_SETTINGS.debugMode,
         crossOriginPolicy:    $.DEFAULT_SETTINGS.crossOriginPolicy,
         placeholderFillStyle: $.DEFAULT_SETTINGS.placeholderFillStyle,
-        opacity:              $.DEFAULT_SETTINGS.opacity
+        opacity:              $.DEFAULT_SETTINGS.opacity,
+        compositeOperation:   $.DEFAULT_SETTINGS.compositeOperation
 
     }, options );
 
@@ -583,6 +584,21 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
         this._needsDraw = true;
     },
 
+    /**
+     * @returns {String} The TiledImage's current compositeOperation.
+     */
+    getCompositeOperation: function() {
+        return this.compositeOperation;
+    },
+
+    /**
+     * @param {String} compositeOperation the tiled image should be drawn with this globalCompositeOperation.
+     */
+    setCompositeOperation: function(compositeOperation) {
+        this.compositeOperation = compositeOperation;
+        this._needsDraw = true;
+    },
+
     // private
     _setScale: function(scale, immediately) {
         var sameTarget = (this._scaleSpring.target.value === scale);
@@ -1301,7 +1317,8 @@ function drawTiles( tiledImage, lastDrawn ) {
         drawDebugInfo( tiledImage, lastDrawn );
         return;
     }
-    var useSketch = tiledImage.opacity < 1;
+    var useSketch = (tiledImage.compositeOperation == 'source-over') ? (tiledImage.opacity < 1):true;
+
     if ( useSketch ) {
         tiledImage._drawer._clear( true );
     }
@@ -1360,7 +1377,7 @@ function drawTiles( tiledImage, lastDrawn ) {
     }
 
     if ( useSketch ) {
-        tiledImage._drawer.blendSketch( tiledImage.opacity );
+        tiledImage._drawer.blendSketch( tiledImage.opacity, tiledImage.compositeOperation );
     }
     drawDebugInfo( tiledImage, lastDrawn );
 }

src/viewer.js

@@ -1207,6 +1207,7 @@ $.extend( $.Viewer.prototype, $.EventSource.prototype, $.ControlDock.prototype,
      * (portions of the image outside of this area will not be visible). Only works on
      * browsers that support the HTML5 canvas.
      * @param {Number} [options.opacity] Opacity the tiled image should be drawn at by default.
+     * @param {String} [options.compositeOperation] How a tiled source image are drawn onto an existing image.
      * @param {Function} [options.success] A function that gets called when the image is
      * successfully added. It's passed the event object which contains a single property:
      * "item", the resulting TiledImage.
@@ -1239,6 +1240,9 @@ $.extend( $.Viewer.prototype, $.EventSource.prototype, $.ControlDock.prototype,
         if (options.opacity === undefined) {
             options.opacity = this.opacity;
         }
+        if (options.compositeOperation === undefined) {
+            options.compositeOperation = this.compositeOperation;
+        }
 
         var myQueueItem = {
             options: options
@@ -1335,6 +1339,7 @@ $.extend( $.Viewer.prototype, $.EventSource.prototype, $.ControlDock.prototype,
                     clip: queueItem.options.clip,
                     placeholderFillStyle: queueItem.options.placeholderFillStyle,
                     opacity: queueItem.options.opacity,
+                    compositeOperation: queueItem.options.compositeOperation,
                     springStiffness: _this.springStiffness,
                     animationTime: _this.animationTime,
                     minZoomImageRatio: _this.minZoomImageRatio,