[osgeo4w-dev] QGIS and ECW: slow

[Frank Warmerdam]

On Tue, Mar 10, 2009 at 6:23 AM, Luca Manganelli
<luca_manganelli at comune.trento.it> wrote:
> Hi,
>
> I've installed QGIS 1.0.0 and used the ECW with these istructions:
>
> http://wiki.qgis.org/qgiswiki/OsGeo4wSetup
>
> then I updated to QGIS 1.0.1 and all libraries.
>
> Before I can import an ECW inside QGIS in about 3-4 seconds.
>
> Now it requires from 30-40 seconds, that's 10 times slower...
>
> The ECW is a local file and it's about 153 MB.

Luca,

I'm not sure when "before" was.  The ECW driver does not
report having any specific overview levels and this defeats
certain optimizations in the statistics computation.  I believe
this has a bad effect when an ecw layer is first added to qgis
but browsing after that should be fast.

The fix is to make the histogram computing logic smarter
(I'm not positive if QGIS is using it's own or GDAL's logic)
so that for formats like ECW with "arbitrary overviews" a
small overview sample of data will be fetched for statistics.

I believe this issue is the same in all modern QGIS+GDAL
builds, not just specifically in OSGeo4W or windows.

If you file a QGIS ticket, and ensure it is assigned to me
("warmerdam") I'll try to resolve it.

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