Fawcett, David (MPCA) wrote:
I don’t know anything about your work flow, but if possible, you could
add a pre-processing step where you use a script to read your text file
and pre-classify the data. You would end up with integers representing
each class as ‘cell’ values.
Instead of :
EXPRESSION([pixel] >= -4 AND [pixel] < -3)
You could have: EXPRESSION “2”
This is the fastest one.
Or
EXPRESSION /2/
Be careful with this because the // mean you are doing a regular
expression evaluation which is slower. and you need to be more precise
in the expression:
/2/ match a "2" anywhere in the string so would match any of the follow
where "?" is a wildcard:
"2"
"2?"
"?2"
"2??"
"?2?"
"??2"
If you want to match exactly "2" with a regex then you would need /^2$/
Complex expressions are compiled into structures that have to be
evaluated for each [pixel] so the more pixels you have the more
computation you have and the more complex the expression the more
computation that is involved.
-Steve W
Matching a single string should be the fastest expression to evaluate.
David.
*From:* [email protected]
[mailto:[email protected]] *On Behalf Of *Tobias Weiß
*Sent:* Wednesday, July 28, 2010 8:25 AM
*To:* [email protected]
*Subject:* Re: [mapserver-users] Raster data performance
Hi Alex,
I tried some of the performance tips you mentioned, e.g. converted the
.txt to a tiled TIFF to optimize performance. I can not see significant
changes in rendering speed.
gdal_translate -co TILED=YES input.txt output.tif
The major brake still is the expression stuff. I really wonder what
makes this task so time-consuming, it should not be more than a simple
comparison.
Do you have experiences with this issue? Or do I have to live with this
constraint?
Best, Tobi
===
Am 28.07.10 14:27, schrieb Alexandre Dube:
Hi Tobi,
The following documentation topics about raster might help :
http://mapserver.org/input/raster.html#rasters-and-tile-indexing
http://mapserver.org/input/raster.html#raster-display-performance-tips
and about expressions :
http://mapserver.org/mapfile/expressions.html
The string comparison type is the fastest, so you could try define
more classes but using string comparison to see if it gets faster.
Also, a good performance tip is to put your most frequent classes types
on top first. For example, if 80% of your data has "pixel = -3" then
put it on top.
Kind regards,
Alexandre
On 10-07-28 04:54 AM, Tobias Weiß wrote:
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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Alexandre Dubé
Mapgears
www.mapgears.com <http://www.mapgears.com>
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