[Qgis-user] Slow rendering of vrt

[Etienne Tourigny]

On Thu, Aug 23, 2012 at 11:00 AM, Even Rouault
<even.rouault at mines-paris.org> wrote:
> Selon Etienne Tourigny <etourigny.dev at gmail.com>:
>
>> It would be nice to have this bultin to gdalbuildvrt (as an optin of
>> course) - could the authors make a patch?
>
> For a *byte* data type, which must be the common case, why wouldn't QGIS just
> use min=0 and max=255 when statistics are not computed ?

good question.

Unless I am mistaken, min/max are set to 0/255 by default, unless
QgsRasterDataProvider::bandStatistics() or
QgsRasterLayer::bandStatistics()  is called - which is probably what
happens.

I don't understand what is the problem, does the VRT load slowly
initially, or is getting statistics rather slow?

On the other hand, wouldn't a simple "gdalinfo -stats file.vrt"
achieve the same as the linked python script, but easier to run?

Etienne

>
>>
>> Regards,
>> Etienne
>>
>> On Thu, Aug 23, 2012 at 7:34 AM, Micha Silver <micha at arava.co.il> wrote:
>> > On 23/08/2012 11:33, Giovanni Manghi wrote:
>> >>
>> >> http://trac.osgeo.org/gdal/wiki/CatalogueForQIS
>> >
>> >
>> > Magic! Many thanks, and also to Andrea Peri for posting the python code.
>> > In windows I had to run it as: " python computestats.py -approx
>> > <filename.vrt> "
>> >>
>> >> it would be very nice to have this added as tool directly in QGIS...
>> >>
>> >> cheers
>> >>
>> >>
>> >> -- Giovanni --
>> >>
>> >>
>> >> On Thu, 2012-08-23 at 11:27 +0300, Micha Silver wrote:
>> >>>
>> >>> Hello all:
>> >>>
>> >>> I have a batch of over 100 raster tiles in ecw format. Each is 50 -
>> >>> 100 MB in size. When I choose 20 or so files to load they appear
>> >>> (render) very quickly - in a matter of a few seconds or less. If I
>> >>> create a vrt of that same batch of tiles, it takes a long time to
>> >>> first render - upwards of 15 - 30 seconds.  Once the vrt is loaded
>> >>> response is excellent (zooming, etc). But that initial delay is a bit
>> >>> annoying. Anything I can do to improve it? (QGIS 1.8, Win 7 64 bit)
>> >>>
>> >>> Many thanks,
>> >>> Micha
>> >>>
>> >>> --
>> >>> Micha Silver
>> >>> 052-3665918
>> >>> _______________________________________________
>> >>> Qgis-user mailing list
>> >>> Qgis-user at lists.osgeo.org
>> >>> http://lists.osgeo.org/mailman/listinfo/qgis-user
>> >>
>> >>
>> >>
>> >> This mail was received via Mail-SeCure System.
>> >>
>> >>
>> >
>> >
>> > --
>> > Micha Silver
>> > 052-3665918
>> >
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