Calculating maxResolution instead of having it in defaults.

lib/OpenLayers/Layer.js

@@ -255,8 +255,8 @@ OpenLayers.Layer = OpenLayers.Class({
      * APIProperty: maxResolution
      * {Float} Default max is 360 deg / 256 px, which corresponds to
      *     zoom level 0 on gmaps.  Specify a different value in the layer 
-     *     options if you are not using a geographic projection and 
-     *     displaying the whole world.
+     *     options if you are not using the default <OpenLayers.Map.tileSize>
+     *     and displaying the whole world.
      */
     maxResolution: null,
 
@@ -638,12 +638,6 @@ OpenLayers.Layer = OpenLayers.Class({
                          this.units || this.map.units;
             
             this.initResolutions();
-            if (!this.resolutions) {
-                throw(
-                    "Could not calculate resolutions for layer " + this.name +
-                    ". Configure maxResolution or resolutions or scales."
-                );
-            }
             
             if (!this.isBaseLayer) {
                 this.inRange = this.calculateInRange();
@@ -896,6 +890,16 @@ OpenLayers.Layer = OpenLayers.Class({
                 props.resolutions = this.resolutionsFromScales(props.scales);
             }
             if(props.resolutions == null) {
+                var maxExtent = this.maxExtent;
+                if (!props.maxResolution && maxExtent) {
+                    // maxResolution for default grid sets assumes that at zoom
+                    // level zero, the whole world fits on one tile.
+                    var tileSize = this.tileSize || this.map.getTileSize();
+                    props.maxResolution = Math.max(
+                        maxExtent.getWidth() / tileSize.w,
+                        maxExtent.getHeight() / tileSize.h
+                    );
+                }
                 props.resolutions = this.calculateResolutions(props);
             }
         }