Absolute calculation of tile bounds

This avoids cumulated tile bounds errors for layer types that do not use a
tile row/column index in requests (e.g. WMS).

lib/OpenLayers/Layer/Grid.js

@@ -867,7 +867,8 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
      *
      * Returns:
      * {Object} containing properties tilelon, tilelat, tileoffsetlat,
-     * tileoffsetlat, tileoffsetx, tileoffsety
+     * tileoffsetlat, tileoffsetx, tileoffsety and optional startrow, startcol
+     * for grid layouts where absolute tile bounds calculation is possible.
      */
     calculateGridLayout: function(bounds, origin, resolution) {
         var tilelon = resolution * this.tileSize.w;
@@ -894,7 +895,8 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
         return { 
           tilelon: tilelon, tilelat: tilelat,
           tileoffsetlon: tileoffsetlon, tileoffsetlat: tileoffsetlat,
-          tileoffsetx: tileoffsetx, tileoffsety: tileoffsety
+          tileoffsetx: tileoffsetx, tileoffsety: tileoffsety,
+          startcol: tilecol, startrow: tilerow
         };
 
     },
@@ -1012,12 +1014,16 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
                         Math.pow(tileCenter.lat - center.lat, 2)
                 });
      
-                tileoffsetlon += tilelon;       
+                tileoffsetlon = 'startcol' in tileLayout ?
+                    origin.lon + (tileLayout.startcol + colidx) * tilelon :
+                    tileoffsetlon + tilelon;       
                 tileoffsetx += Math.round(tileSize.w);
             } while ((tileoffsetlon <= bounds.right + tilelon * this.buffer)
                      || colidx < minCols);
              
-            tileoffsetlat -= tilelat;
+            tileoffsetlat = 'startrow' in tileLayout ?
+                origin.lat + (tileLayout.startrow - rowidx) * tilelat :
+                tileoffsetlat - tilelat;
             tileoffsety += Math.round(tileSize.h);
         } while((tileoffsetlat >= bounds.bottom - tilelat * this.buffer)
                 || rowidx < minRows);

tests/Layer/WMS.html

@@ -533,6 +533,18 @@
         
         map.destroy();
     }
+    
+    function test_tileBounds(t) {
+        t.plan(1);
+        var map = new OpenLayers.Map("map", {projection: "EPSG:3857"});
+        var layer = new OpenLayers.Layer.WMS("wms", "../../img/blank.gif");
+        map.addLayer(layer);
+        map.setCenter([0, 0], 1);
+        map.pan(2, -100);
+        map.zoomIn();
+        t.eq(layer.grid[1][0].bounds, new OpenLayers.Bounds(-10018754.17, 0, 0, 10018754.17), "no floating point errors");
+        map.destroy();
+    }
 
 
   </script>