[webmap-discuss] OGC and google style tiling

[Steven M. Ottens]

Hi Dave,

In my opinion the edge problem is a bug in mapserver and should be
solved in mapserver, not with a workaround. But i'm not too knowledgable
on mapserver so I could be mistaken. Anyhow I don't think we should need
any post-rendering processing of the mapserver-images. The whole point
of tiling is a performance boost. So if every tile needs to be recut
after being rendered it'll generate more overhead. I'm believing in a
WMS-server solution which creates and cuts tiles like tile.php does now.
This should solve the edge *and* labeling issues.

Steven


On Thu, 2006-04-27 at 15:28 -0400, dblasby at openplans.org wrote:
> I've been thinking of tiling rendering for WMS in general, and geoserver
> in particular (especially with OpenLayers apparently coming out soon
> and mapbuilder also adding similar functionality).
> 
> Paul gave a good account of the problems that I've been thinking about:
> a) Edge Rendering Effects
> b) cache HTTP headers
> c) Get capabilities (ie. tell clients about how to make requests --
> hopefully OGC will define this)
> d) Labeling
> 
> I think a-c are actually fairly easy to handle with a "dumb" proxy WMS.
> Take a quick look at the two images I attached.
> 
> a) Edge Rendering Effects
>      This can be handled simply by intercepting an incoming WMS
> request, expanding the request bounding box and image size and sending
> the request down to the actual WMS server.  The image returned is then
> "cookie cut" so its the proper size/extent of the original request.
> 
>      You'd have to tune the expansion size, but generally you'll want it
> to be a few pixels wider than your "widest style."  For example, if
> your roads are 12 pixels wide you'd want to expand the image by about
> 14 pixels in all directions (extra pixels are to account for
> anti-aliasing).


> 
> b) cache HTTP headers
>       Geoserver allows you to set the cache expire headers, but you can
> also have the WMS proxy mangle header for servers that dont allow this.
> 
> c) Get capabilities
>       At some time in the future, the WMS 'get capabilities' document
> will contain the grid information the server wants the client to use
> for tiling.  Once this is defined, the proxy WMS can augment the real
> server's get capabilities so that clients make "proper" requests.
> 
> 
> Unfortunately, I dont see a good solution to labeling issues (except
> making requests for very large images) - anyone have any ideas?
> 
> dave
> 
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