[mapserver-users] Raster data performance

[Tobias Weiß]

Hello list,

this is my first email to this list. I am working with MapServer,
OpenLayers & TileCache for almost 1 year now and the tools are really
working great together, when everything is configured correctly ;-)

At the moment I´m facing a performance problem with raster data that
serves as input data to render my map tiles. The spatial resolution of
this raster data is 720 (rows) x 360 (cols) and it is stored in a simple
.txt file (The size of the file is around 2 MB). I use TileCache to
prerender maptiles for different zoom levels that are later displayed in
a web application using OpenLayers.

I discovered, that the rendering speed of MapServer seems to mainly
depend on how many different classes I define within my mapfile, meaning
how many expressions MapServer has to evaluate during the rendering
process. Without any classes MapServer the rendering speed is really
fast, the more classes I use the slower it gets. With e.g. 10 different
classes defining 10 different styles / pixel colors the rendering time
for one tile can increase up to 60 sec.

CLASS
  EXPRESSION ([pixel] >= -4 AND [pixel] < -3)
    STYLE
	COLOR 69 117 180
   END
END


While working with shape files I haven´t had this problems and
prerendering map tiles was really fast. With raster data the rendering
time for one map and 5 different zoom levels can take up to several
hours and almost overburdens my CPU.

Can anybody give me a hint how I could improve this?

Best, Tobi
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