[Mapserver-users] ECW not showing below certain scale

[Frank Warmerdam]

Delfos, Jacob wrote:
> Hi All,
> 
> I've been running into a little problem with mapserver 4.2, which I was 
> also experiencing in Chameleon 1.99; when I zoom in quite far on the map 
> (holding an ecw image), the map doesn't show up. It keeps trying to 
> load, but nothing comes, suggesting it got stuck somewhere. CPU activity 
> is initially at maximum for a few seconds (as normal when loading the 
> map) but then drops while the map doesn't show (I'm loading the map in 
> the browser on the server itself). The mapserv process won't allow 
> itself to get killed either.
> 
> If I simply replace my mapserv.exe file with one of version 4.0, the 
> problem is gone. Of course the map takes quite a few seconds to show, 
> which is normal when zooming close on an ECW, but it does show.
> 
> I've tried this quite a few times in different browsers, but the map 
> never shows at that scale when the ECW is in there. I have tried 
> different mapfiles with different ECW's as well. Has the mechanism to 
> display the ECW changed? It seems that as soon as I reach a resolution 
> where I go beyond the ECW resolution (1pixel:1pixel) it dies.

Jacob,

I was able to reproduce a problem with ECW supersampling when assertions
are turned on in GDAL 1.2.0.  I have just issued a GDAL 1.2.1 with this issue
fixed, and suggested to Assefa that he upgrade to that as time permits.

I am not absolutely positive if that is your problem or not.

I suspect you are seeing differences between MapServer 4.0 and 4.2 because
MapServer 4.2 tries to use "dataset" level io calls when available.  This
allows reading red, green and blue in one pass against ECW for instance,
resulting in better performance.  However, it uses different code paths
which may have different issues.

So ... if and when new binaries are available with GDAL 1.2.1 I would
suggest you try MapServer 4.2 again.  In the meantime you could just set an
appropriate MINSCALE so the ECW would stop rendering when supersampled,
or switch back to the build that works.

Finally, I will once again strongly suggest that folks running into problems
like this try and reproduce it at the commandline with shp2img and appropriate
extent and debug options.  This often results in much more meaningful error
messages than you get on in a web application.

Best regards,
-- 
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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