[Qgis-developer] Raster resampling

[Marco Hugentobler]

Hi Radim

>It seems that sometimes it is slower reprojection and sometimes
>resampling, depending on which resampling algorithm is used,
>complexity of projection and if approx repro is used. We need some
>benchmarks on this. Would it be possible to have an optional order of
>projection and resampling and choose the faster one on runtime?

Not sure I understand exactly what you mean.

Is it to have resampling on layer (and color) level and additionaly 
resampling on provider (and raster band) level in the method 
QgsRasterDataProvider::readBlock? Then if otf-reprojection is on and 
raster type is not paletted, use the one inside the provider level, else 
the one on layer level?
This sounds like a good solution to me.

Regards,
Marco




On 19.01.2012 18:07, Radim Blazek wrote:
> On Tue, Jan 17, 2012 at 1:32 PM, Marco Hugentobler
>> The problem here is that resampling sometimes can only be done once you know
>> the pixel color. E.g. in case of paletted raster, two adjacent pixel values
>> can have totally different color (therefore resampling needs to be done on
>> color components, not on raster value).
> OK.
>
>> How is it with GRASS? it probably has the resampling on a lower level?
> GRASS supports  only nearest neighbour.
>
>>> I am especially concerned about reprojection, because reprojection of
>>> images oversampling^2 times bigger may be significantly slower.
>>   True, I need to do some more tests here. At the moment, the oversampling
>> rate is kept at a maximum of 4 (that should probably be a user option
>> later). Ok, maximum 16 times more data to reproject (hopefully no problem
>> with your approximative reprojection :-) ).
> It seems that sometimes it is slower reprojection and sometimes
> resampling, depending on which resampling algorithm is used,
> complexity of projection and if approx repro is used. We need some
> benchmarks on this. Would it be possible to have an optional order of
> projection and resampling and choose the faster one on runtime?
>
> Radim


-- 
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