[mapserver-users] Display multiple classes using WMS

[geographika]

Hi list,

I'm trying to do something I thought would be quire simple, but seems 
may not be possible in MapServer.

I have a client JavaScript application that uses MapServer as a WMS/WFS 
backend. The UI has a checkbox that allows the user to see the data with 
or without labels.
With the deprecation of the ANNOTATION layer I started to update my 
MAPFILE LAYER definitions. Some of the DATA clauses are very complicated 
so I moved my LABEL class into the same layer as the line features which 
are also colour coded.

I can switch between by LABEL class and the other classes using 
CLASSGROUP and the WMS STYLES parameter.

E.g. in the MapFile:

     CLASSGROUP "default"

     CLASS
         NAME 'Complete'
         GROUP "default"
         ...
     END

     CLASS
         NAME 'Incomplete'
         GROUP "default"
         ...
     END

    # a single class containing labels for all features
     CLASS
         GROUP "labels" # hidden by default, but can be activated with STYLE
     ...
     END


I can then use the STYLES property of the WMS server to hide/show labels 
by requesting the same layer twice with two different styles e.g.

No labels: http://localhost/mapserver/?LAYERS=Mylayer......STYLES=
Labels: 
http://localhost/mapserver/?LAYERS=Mylayer,MyLayer......STYLES=default,labels

This works well, but has a major drawback that the database is queried 
twice for the same data, which in my case causes a performance issue. Is 
there anyway to do this in a single-pass query?
It seems MapServer will only ever apply one class per feature. I could 
add a LABEL class to each default CLASS using includes, however I see no 
way of then turning these off, except maybe using MapScript?
Any workarounds / pointers helpful.

Regards,

Seth

--
web:http://geographika.co.uk
twitter: @geographika


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