[Mapserver-users] Gdaladdo

[Ed McNierney]

Omry -

You should do some standalone ECW tests (and talk to other ECW users) to see if this isn't an artifact of ECW.  UNLIKE raster data formats (i.e. where the data is stored as a raster), it has been my experience (VERY unscientific) that wavelet compression formats require MORE effort to generate a small (zoomed-in) area than they do to create a low-resolution overview.  Raster formats perform best when the output image is close to the resolution of the source data (or one of the resolutions, if you're using gdaladdo or other pyramiding schemes).

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242 

-----Original Message-----
From: Omry Yadan [mailto:omry at telmap.com]
Sent: Tuesday, October 21, 2003 11:43 AM
To: Frank Warmerdam
Cc: mapserver-users at lists.gis.umn.edu
Subject: RE: [Mapserver-users] Gdaladdo


Frank,
By high zoom I meant zoomed in.
For zoom out, the performance is relatively alright, but some times I get
very poor time for zoom in.
I think it's a specific zoom that cause me the problems...
Maybe the zoom where I get the image around the real maximum resolution,
with little scaling.
Does it make sense?


-----Original Message-----
From: Frank Warmerdam [mailto:warmerdam at pobox.com] 
Sent: Tuesday, October 21, 2003 5:08 PM
To: Omry Yadan
Cc: mapserver-users at lists.gis.umn.edu
Subject: Re: [Mapserver-users] Gdaladdo

Omry Yadan wrote:
> As I suspected...
> I did not properly benchmark MapServer yet, but I noticed that sometimes
> image generation takes much more time for images in high 'zoom'.
> As long as 8-9 seconds some times, compared to 1-1.5 second for other
maps..
> Why do you think that is? Any suggestions how to improve it?

Omry,

There are many factors that could account for that.  I assume by "high zoom"
you mean "zoomed out", is that right?  Is this primarily working with ECW
rasters?

Even though the ECW technology supports efficient extraction of reduced
resolution images, I don't think that the GDAL ECW driver takes proper
advantage of this capability.  I imagine that is why overview generation
is relatively slow.

If server disk space isn't a big issues I would suggest converting the
ecw files to GeoTIFF and building overviews.  Alternatively we could look
at improving the ECW driver in GDAL to be optimized for MapServer use, but
I don't have the time or unfunded inclination to work on that at this time.

Best regards,

-- 
---------------------------------------+------------------------------------
--
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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