[Qgis-user] Slow rendering of vrt

[Etienne Tourigny]

Author was experiencing this in 1.8, it would be nice if someone
(ideally the author) could test the same with nightly master (1.9).

Etienne

On Thu, Aug 23, 2012 at 2:27 PM, Radim Blazek <radim.blazek at gmail.com> wrote:
> On Thu, Aug 23, 2012 at 5:57 PM, Etienne Tourigny
> <etourigny.dev at gmail.com> wrote:
>> On Thu, Aug 23, 2012 at 11:00 AM, Even Rouault
>> <even.rouault at mines-paris.org> wrote:
>>>> 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.
>
> Default contrast enhancement in current master (may be changed in
> Options > Rendering > Rasters):
>   Single band gray: Stretch to min / max
>   Multiband color (byte/band): No stretch
>   Multiband color (>byte/band): Stretch to min / max
>   Limits (min/max): Cumulative count cut.
>
> The min/max are calculated using 250000 pixels sample, which should be
> fast. It would be useful to test the VRT without stats collected with
> current master.
>
> It would be possible to add another default contrast enhancement for
> Single band gray byte, but I believe that if the whole raster can be
> rendered in reasonable time, the min/max calculation must be also fast
> and contrast enhancement may be important even with byte data.
>
> Radim
>
>> 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
>>>> >
>>>> > _______________________________________________
>>>> > Qgis-user mailing list
>>>> > Qgis-user at lists.osgeo.org
>>>> > http://lists.osgeo.org/mailman/listinfo/qgis-user
>>>> _______________________________________________
>>>> Qgis-user mailing list
>>>> Qgis-user at lists.osgeo.org
>>>> http://lists.osgeo.org/mailman/listinfo/qgis-user
>>>>
>>>
>>>
>> _______________________________________________
>> Qgis-user mailing list
>> Qgis-user at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/qgis-user