[Qgis-user] Slow rendering of vrt

[Radim Blazek]

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