[Qgis-developer] get mean of pixel values within defined area around current mouse position

[Etienne Tourigny]

probably A would be slow because of the external call to gdalwarp

On Wed, Jul 3, 2013 at 8:58 AM, Fred Lehodey <lehodey at gmail.com> wrote:

> Hi,
> I have no idea in terms of performance but I would bet on B option in your
> case.
> Thanks to give feedback.
>
> Regards.
>
>
> On Wed, Jul 3, 2013 at 8:36 AM, Otto Dassau <dassau at gbd-consult.de> wrote:
>
>> Hi Fred,
>>
>> thanks for the hints, A and B sound interesting - PostGIS is not an option
>> here.
>>
>> One more question, because you already used these workarounds. What
>> solution
>> A or B should be more usable (faster) from your opinion, if I need to
>> calculate the mean while moving the mouse over the map canvas?
>>
>> Regards
>> Otto
>>
>> Am Tue, 2 Jul 2013 18:05:10 +0100
>> schrieb Fred Lehodey <lehodey at gmail.com>:
>>
>> > Hi,
>> >
>> > Not sure it's the best way but I did something similar with these
>> > workarounds:
>> >
>> > A - with gdal:
>> > 1 - the user click and you get your buffer. (already done)
>> > 2 - use gdalwarp with "-cutline" and "*-*crop_to_cutline" options to
>> > extract a new raster. (python subprocess)
>> > 3 - read the created raster in a numpy array to do all the statistics
>> you
>> > need.
>> >
>> > B - with matplotlib:
>> > 1 - with the buffer: create an array of the vertices
>> > 2 - read the raster into another numpy array
>> > 3 - use the "points_inside_poly" function of "matplotlib" lib to create
>> a
>> > numpy mask.
>> >
>> > Not tried but postgis raster should do the job.
>> >
>> > Hth,
>> > Fred
>> >
>> >
>> > On Tue, Jul 2, 2013 at 1:12 PM, Otto Dassau <dassau at gbd-consult.de>
>> wrote:
>> >
>> > > Hi,
>> > >
>> > > for a qgis 1.8 plugin I would like to query the mean of pixel values
>> of a
>> > > raster layer within a defined buffer for the current mouse position
>> (not
>> > > for
>> > > an existing point layer).
>> > >
>> > > For the buffer it works:
>> > >
>> > > ...
>> > > feat.geometry().asPoint()
>> > > (3.5176e+06,5.4062e+06)
>> > > buff_geom = feat.geometry().buffer(1000, 5)
>> > > buff_geom.asPolygon() [[(3.5186e+06,5.4062e+06),
>> > > (3.51855e+06,5.4059e+06), (3.51841e+06,5.40562e+06),
>> > > (3.51819e+06,5.4054e+06), (3.51791e+06,5.40525e+06),
>> > > (3.5176e+06,5.4052e+06), (3.51729e+06,5.40525e+06),
>> > > (3.51701e+06,5.4054e+06),...]]
>> > >
>> > > but now I try to find a solution to query the raster pixels within the
>> > > buffer zone and calculate the mean, but I don't know how and if this
>> is
>> > > possible in QGIS 1.8 at all. I found QgsZonalStatistics, but that
>> doesn't
>> > > work for my task.
>> > >
>> > > Does anybody have a solution for this or a better idea, how to solve
>> > > this?
>> > >
>> > > Thanks
>> > >
>> > > Otto
>> > > _______________________________________________
>> > > Qgis-developer mailing list
>> > > Qgis-developer at lists.osgeo.org
>> > > http://lists.osgeo.org/mailman/listinfo/qgis-developer
>> > >
>>
>>
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