[Qgis-user] Slow rendering of vrt

[Giuseppe Sucameli]

Hi Etienne,

On Aug 23, 2012, at 5:57 PM, Etienne Tourigny <etourigny.dev at gmail.com> wrote:
> On the other hand, wouldn't a simple "gdalinfo -stats file.vrt"
> achieve the same as the linked python script, but easier to run?
the python script calculates approximated stats, so
it works like gdalinfo -approx_stats file.vrt

The problem was the -approx_stats option is not present
in the documentation (either help online or using --help),
then I wrote that few-lines script to achieve the task.

Only now I'm looking at the gdalinfo.c code on the repo
I know it's there since Jan 2007 (r10658)...

BTW a ticket is needed to update the doc.

On Thu, Aug 23, 2012 at 7:39 PM, Etienne Tourigny
<etourigny.dev at gmail.com> wrote:
> it would be nice if someone (ideally the author) could test
> the same with nightly master (1.9).

+1, but I hope you're talking about the wiki page author :)
I cannot do any test because I have no VRT files so big.

Micha, could you try with QGis master and report here,
please?

Regards.

>
> 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
>>>>> _______________________________________________
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>>>>>
>>>>
>>>>
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-- 
Giuseppe Sucameli - Faunalia