[OpenLayers-Dev] WMS-Performance

[Slawomir Messner]

Hi,
I thought about optimizing the WMS Layers and the simplest thing would 
be if the extents are known(which is the most of the cases) just to get 
the tiles which are needed and for the others only one static file so 
the browser has to cache only one file with no opacity. It should reduce 
memory usage and number of request in cost of some calculations. So I 
made the Extents WMS Layer(I know not a good name, I'm looking for a 
good one) which I paste at the bottom. Than I thought I could save some 
calculations by saving a tile's "empty"-state in it, by a class derived 
from Image-class (just override the renderTile-funciton and add an 
isEmpty-attribute in the class), but tiles are reused so this leads to 
some ugly empty tiles in some cases(especially panning, zoom is no 
problem). So I want to share the class with interested and hope for some 
feedback and optimization suggestions. This solution is better then the 
normal WMS especially in cases where a small WMS picture lies on an 
bigger one and even better if someone has some small WMS layers in 
"space", it could be interesting for mobiles too and it is interesting 
if your caching your tiles like us so there are no empty tiles in cache 
any more there is only one image for all. It should be inferior to the 
normal one if your view is filled with the not empty WMS tiles because 
intersection is calculated every time, but I don't know if the 
calculations impact that much.
Regards
Slawomir

OpenLayers.Layer.WMS.Extents = OpenLayers.Class(OpenLayers.Layer.WMS, {

*extents:null,*
*emptyTileImageUrl:null,*

     getURL: function (bounds) {
         bounds = this.adjustBounds(bounds);
*if(this.extents == null || this.emptyTileImageUrl == null || 
this.extents.intersectsBounds(bounds)) {*
             var imageSize = this.getImageSize();
             var newParams = {};
             // WMS 1.3 introduced axis order
             var reverseAxisOrder = this.reverseAxisOrder();
             newParams.BBOX = this.encodeBBOX ?
                 bounds.toBBOX(null, reverseAxisOrder) :
                 bounds.toArray(reverseAxisOrder);

             newParams.WIDTH = imageSize.w;
             newParams.HEIGHT = imageSize.h;
             var requestString = this.getFullRequestString(newParams);
             return requestString;
*} else {
             return this.emptyTileImageUrl;
         }*
     },

     CLASS_NAME: "OpenLayers.Layer.WMS.Extents"
});
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