fix #2519 by checking minimumOverlapRequired on a per-tiled image basis

src/canvasdrawer.js

@@ -143,10 +143,12 @@ class CanvasDrawer extends OpenSeadragon.DrawerBase{
     }
 
     /**
+     * @param {TiledImage} tiledImage the tiled image that is calling the function
      * @returns {Boolean} Whether this drawer requires enforcing minimum tile overlap to avoid showing seams.
+     * @private
      */
-    minimumOverlapRequired() {
+    minimumOverlapRequired(tiledImage) {
-       return true;
+        return true;
     }
 
 

src/drawerbase.js

@@ -141,12 +141,13 @@ OpenSeadragon.DrawerBase = class DrawerBase{
     }
 
     /**
+     * @param {TiledImage} tiledImage the tiled image that is calling the function
      * @returns {Boolean} Whether this drawer requires enforcing minimum tile overlap to avoid showing seams.
      * @private
      */
-    minimumOverlapRequired() {
+    minimumOverlapRequired(tiledImage) {
         return false;
-     }
+    }
 
 
     /**

src/htmldrawer.js

@@ -86,11 +86,13 @@ class HTMLDrawer extends OpenSeadragon.DrawerBase{
     }
 
     /**
+     * @param {TiledImage} tiledImage the tiled image that is calling the function
      * @returns {Boolean} Whether this drawer requires enforcing minimum tile overlap to avoid showing seams.
+     * @private
      */
-    minimumOverlapRequired() {
+    minimumOverlapRequired(tiledImage) {
         return true;
-     }
+    }
 
     /**
      * create the HTML element (e.g. canvas, div) that the image will be drawn into

src/tiledimage.js

@@ -1706,7 +1706,7 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
             tileCenter = positionT.plus( sizeT.divide( 2 ) ),
             tileSquaredDistance = viewportCenter.squaredDistanceTo( tileCenter );
 
-        if(this.viewer.drawer.minimumOverlapRequired()){
+        if(this.viewer.drawer.minimumOverlapRequired(this)){
             if ( !overlap ) {
                 sizeC = sizeC.plus( new $.Point(1, 1));
             }

src/webgldrawer.js

@@ -210,6 +210,16 @@
             return 'webgl';
         }
 
+        /**
+         * @param {TiledImage} tiledImage the tiled image that is calling the function
+         * @returns {Boolean} Whether this drawer requires enforcing minimum tile overlap to avoid showing seams.
+         * @private
+         */
+        minimumOverlapRequired(tiledImage) {
+            // return true if the tiled image is tainted, since the backup canvas drawer will be used.
+            return tiledImage.isTainted();
+        }
+
         /**
         * create the HTML element (canvas in this case) that the image will be drawn into
         * @private