[gdal-dev] "wrong" overview selected when warping from ortho to webmercator

[Even Rouault]

Hi Thomas,

> * I added the -to AREA_OF_INTEREST= options while trying to debug the issue
> but it makes no difference with or without it

Yes, -to AREA_OF_INTEREST is only taken into account for informing PROJ of the area of 
interest in its choice of candidate transformations when datum transformations are involved. 
So completely useless here (and likely in most other cases as well, unless to cut a bit 
computation time in PROJ when researching candidate coordinate transformation pipelines)

> Is there an easy way to fix this? Or could there be an initial first step
> that programatically sets the -ovr AUTO-N parameter depending on the size
> of the source window that is going to be used?

The issue you observe comes from using GDALSuggestedWarpOutput2()
https://github.com/OSGeo/gdal/blob/master/gdal/apps/gdalwarp_lib.cpp#L2308
that looks at reprojecting the whole source dataset, without taking into account the 
potential restriction of the target extent set by -te. I guess using 
GDALWarpOperation::ComputeSourceWindow() instead would be more appropriate. You 
could file a ticket about that.

A workaround (untested) in the mean time could be that you restrict your input dataset as a 
VRT that includes "just" the needed source pixels. You could probably try do that 
automatically with gdal_translate -projwin with bounds in the the target SRS and -projwin_srs 
being that target SRS. However I'm not sure how well it will guess the source pixels (-
projwin_srs effect is quite naive: it just reprojects the upper-left and lower-right corners of -
projwin. No fancy sampling along the edges of the bounding box). So you might need to code 
something smarter to create that source VRT. Or add sufficient margins to coordinates 
passed to projwin so that the VRT includes the needed source pixels.
I'd expect access to overviews of the source file should still work through VRT.

Even


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