[gdal-dev] gdalwarp different results (vrt vs direct)

[Even Rouault]

Le vendredi 04 septembre 2015 21:29:02, vous avez écrit :
> Hi
> 
> On Thu, Sep 3, 2015 at 10:58 AM, Even Rouault
> 
> <even.rouault at spatialys.com> wrote:
> > This occurs in the middle of a source tile, or at the boundary between 2
> > tiles ?
> 
> In the middle of a source file.
> 
> > Just to be sure: you mean you are using the above -s_srs and -t_srs on
> > one of the tile that makes the ortho2014.vrt ?
> 
> Yes.
> 
> > And what if you make a VRT with just that source image ?
> 
> I think it has nothing to do with the VRT. Using the gdalwarp command
> on a single tif file it has the same breaking edge when using the -te
> option. I did try the -et 0 option. Same error but not at the same
> place (but only few pixels to the left) and much slower.
> 
> gdalwarp -overwrite -s_srs "+proj=somerc +lat_0=46.952405555555555
> +lon_0=7.439583333333333 +ellps=bessel +x_0=600000 +y_0=200000
> +units=m +k_0=1 +nadgrids=./chenyx06a.gsb" -t_srs "+proj=somerc
> +lat_0=46.952405555555555 +lon_0=7.439583333333333 +ellps=bessel
> +x_0=2600000 +y_0=1200000 +units=m +k_0=1 +nadgrids=@null"  -tr 0.125
> 0.125 -te 2611000 1233000 2612000 1234000 -co 'TILED=YES'
> 611233_12_5cm.tif 2611_1233_12_5cm.tif

Stefan,

as you work with sub-meter accuracy, forcing the bounds to nearest meter, 
while using the default resampling (nearest neighbour) is the likely cause. 
The issue seems to be a pixel shift in the vertical direction, likely due to a 
pixel coordinate expressed as a floating point value being rounded suddenly 
from the floor integer to the the ceiling integer. 

I've quickly tried with cubic resampling (-r cubic) and the result seems to be 
much better/smoother. Bilinar could probably work as well.

Even

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