[Tilecache] cascading TileCache / WMS layers cached individually

[Bart van den Eijnden (OSGIS)]

Hi Pierre,

with respect to Mapserver WMS client combining layers from the same server,
there is the following METADATA to prevent this:

wms_force_separate_request

I don't have a solution though for the BBOX problem but have seen this
before. Does your BBOX and WIDTH/HEIGHT have the same ratio in the request?
Can you share the full request?

Best regards,
Bart

--
Bart van den Eijnden
OSGIS, Open Source GIS
http://www.osgis.nl


--------- Oorspronkelijk bericht --------
Van: Pierre GIRAUD <bluecarto at gmail.com>
Naar: tilecache at openlayers.org <tilecache at openlayers.org>
Onderwerp: [Tilecache] cascading TileCache / WMS layers cached individually
Datum: 03/04/08 07:38

> Hello,
> 
> Context :
> 
>   We have a project with one OL WMS layer combining several WMS layers
> (coma separated layers list param in the WMS getMap service).
> By using MapFish layerTree widget (and OL mergeNewParams), the list of
> WMS layers may be changed by the user. This said, it's obvious that we
> can't get this OL layer cached. One can easily imagine that for 3
> different WMS layers, there are 4*3*2*1=24 possible combinations.
> 
>   But as performance is really important, we propose to find an
> alternative to that.
> 
> Proposal :
> 
>   Each WMS layers could be cached individually and then merged by a
> WMS client also being a WMS server for OL.
>   This way, no geospatial data would be processed. Only image
> transformations (blending) would be required.
> 
> Tests :
> 
>   To do this, we had the idea to use UMN Mapserver as WMS Client for
> TileCache. We tried it this way :
>   The mapfile includes a layer pointing to a tileCache layer.
> ---------------------
> LAYER
>     NAME &quot;bar&quot;
>     TYPE RASTER
>     CONNECTION &quot;tilecache/tilecache.py&quot;
>     CONNECTIONTYPE WMS
>     METADATA
>       &quot;wms_srs&quot;             &quot;EPSG:4326&quot;
>       &quot;wms_name&quot;            &quot;bar&quot;
>       &quot;wms_server_version&quot;  &quot;1.1.1&quot;
>       &quot;wms_format&quot;          &quot;image/png&quot;
>     END
> END
> -----------------------
>   And then, in OpenLayers we create a WMS layer pointing to the MapServer
layer.
>   If several layers are requested, MapServer would request all WMS
> services (cached with TileCache) and then would merge (blend) the
> received images.
> 
> Problems :
> 
>   The first issue we encounter is that the bbox sent in the MapServer
> request to TileCache [1] differs slightly from the OL request to
> MapServer [2]. And because it is different, TileCache complains
> because the resolution doesn't match with its configuration.
> 
>   The second problem is that MapServer is able to know that different
> layers come from the same WMS service and sends a single request to
> this service.
> 
> I would be pleased if someone could point me to a solution for that.
> I would also appreciate if you also think that it can enhance
> performances for the described use-case or if it is worth having it.
> If so, is it something TileCache could adress itself ?
> 
> Regards,
> Pierre
> 
> [1] &amp;BBOX=-179.296875,-89.296875,179.296875,269.296875
> [2] &amp;BBOX=-180,-90,180,270
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