[OpenLayers-Users] Performance?

[Jeff Dege]

________________________________

	From: Erik Uzureau [mailto:euzuro at gmail.com] 
	Sent: Tuesday, August 15, 2006 8:59 PM
	To: Jeff Dege
	Cc: users at openlayers.org
	Subject: Re: [OpenLayers-Users] Performance?
	
	
	Hi Jeff, 
	
	Are you running on 1.0 version or on the svn HEAD? 

I'm running svn HEAD, as of a week or so ago.

	 
	We just put a bunch of new funky gridding stuff into the trunk,
like
	reusing divs, adjustable tile-buffer, and spiral gridding.  

I just did an svn update, and I can see the changes you're talking
about.

	 To modify the off-current-view tiles, try fiddling with the
'buffer'
	property in Grid.js -- ex:
	
	        function init(){
	            map = new OpenLayers.Map( $('map') );
	            layer = new OpenLayers.Layer.WMS( "OpenLayers WMS", 
	                    "http://labs.metacarta.com/wms/vmap0",
{layers: 'basic'}, 
	                    { tileSize: new OpenLayers.Size(800,475),
	                      buffer: 1 });
	            map.addLayer(layer);
	
	            map.setCenter(new OpenLayers.LonLat(lon, lat),
zoom);
	            map.addControl( new
OpenLayers.Control.LayerSwitcher() );
	        }
	
	
	The default is set to 2 but you can take it down to 1 (as above)
and that will probably
	speed things up for you, though UI-wise it might be less sweet.

I think it's an inherent trade-off.  Faster initial load vs. faster
panning after the initial load.  Though if you're loading tiles in a
spiral from the center, and the UI remains responsive while the
non-visible tiles are still loading, you might be achieving the best of
both worlds.
 
Are you spiraling within each layer?  Or for all layers?
 
That is, is it:
 
  for each layer
      for each tile, starting at the center and spiraling out
        draw the layer's tile
 
or is it:
 
   for each tile, starting at the center and spiraling out
      for each layer
        draw the layer's tile
 
???
 
 Note that setting the buffer to 0 will not work. 


So yeah, the reason you are seeing so many tiles loaded is because it is
loading a buffer
of two tiles around the viewport. Also, the reason you dont see simple
numbers like 9
and 25 is because of the way we have implemented gridding -- with
cacheing in mind, OL
uses a sort of world-grid at each zoom level, which means that
specifying a tile size of the 
same size as the viewport won't necessarily (in fact quite unlikely)
mean that it will load one
tile smack-dab in the center of the viewport. Does that make sense?

--Erik 
 
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