[Qgis-developer] Provider native raster band stats

[Etienne]

Tim, my impression is that it speeds things up, but I have a suggestion to make it faster.

From what I have seen in your code, the  QgsGdalProvider::bandStatistics function calls GDALComputeRasterStatistics (), which computes the statistics every time - it doesn't fetch pre-calculated data ( which is stored in gtiff metadata, or aux.xml file).  

A better way would be to use the GDALGetRasterStatistics() which calls GDALComputeRasterStatistics() when needed (when bForce = TRUE)
double myerval =  GDALGetRasterStatistics ( myGdalBand, bApproxOK, TRUE, &pdfMin, &pdfMax, &pdfMean, &pdfStdDev);



I have tested it on a few rasters, it gets the same stats.

The downside is that GDALGetRasterStatistics() does not offer a progress callback, rendering the progressbar useless.  
If it"s really needed, I could re-implement the function in  QgsGdalProvider (about 50 lines of code), or try to have GDALGetRasterStatistics support a progress callback.

regards, Etienne

----- Original Message -----
From: Tim Sutton <lists at linfiniti.com>
To: Marco Hugentobler <marco.hugentobler at sourcepole.ch>
Cc: qgis-developer at lists.osgeo.org
Sent: Monday, July 11, 2011 7:04:08 PM
Subject: Re: [Qgis-developer] Provider native raster band stats

Hi


On Mon, Jul 11, 2011 at 5:29 PM, Marco Hugentobler
<marco.hugentobler at sourcepole.ch> wrote:
> Hi Tim
>
> I'm not a raster guru, but the changes look good to me.
> +1 for merging.

Thanks for your feedback Marco. I guess I'll write some unit tests
(wrong way around I know :-) ) and then merge it if they all pass and
there are no other objections. :-)

Regards

Tim


>
> Regards,
> Marco
>
> Am Donnerstag, 7. Juli 2011, 00.47:25 schrieb Tim Sutton:
>> Hi
>>
>> During the Lisbon hackfest (between all those meetings!) I worked on
>> updating the raster providers to be able to generate stats themselves.
>> The raster provider base class has a default (and historically
>> inefficient as we all know) implementation for collecting stats by
>> walking over every cell in the raster twice. This method remains, but
>> the providers can now supply stats themselves (heopfully using more
>> efficient mechanisms). I have implemented gdal native stats already.
>> Over the last few weeks I did some further testing and cleanups to
>> this work. From my testing some of my worst case files opened in
>> ~1minute rather than ~8+ minutes. It would be great if interested
>> parties could test (details below) before I put it into master.
>>
>> git remote add timlinux git://github.com/timlinux/Quantum-GIS.git
>> git fetch timlinux
>> git branch --track timlinux raster-stats
>> git checkout raster-stats
>>
>> I look forward to your feedback.
>>
>> Regards
>
>
> --
> Dr. Marco Hugentobler
> Sourcepole -  Linux & Open Source Solutions
> Churerstrasse 22, CH-8808 Pfäffikon SZ, Switzerland
> marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
> Technical Advisor QGIS Project Steering Committee
>



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