[gdal-dev] gdalwarp alpha band resampling bug

[Even Rouault]

I can't reproduce the issue you describe. There's not a single white 
pixel in the output image (the max of the green and blue bands is < 
255). But I suspect this could be related to a pre-existing 
composite-bl.tif file you might have. gdalwarp doesn't overwrite by 
default the output image. It blends into it. If you add -overwrite, you 
should get a reasonable output.

Le 18/09/2021 à 01:06, Patrick Young a écrit :
> Hello,
>
> I've found behavior in gdalwarp related to compositing an image with a 
> variable alpha band.  Under bilinear resampling, the alpha compositing 
> in areas where the alpha band is between 0 and 255 results in 
> completely white pixels.
>
> To reproduce, I've first grabbed a rgba image from the test suite, e.g.:
>
> wget 
> https://github.com/OSGeo/gdal/raw/a2db646d1c34905e47eb21657284b529b7478d66/autotest/gcore/data/stefan_full_rgba.tif 
> <https://github.com/OSGeo/gdal/raw/a2db646d1c34905e47eb21657284b529b7478d66/autotest/gcore/data/stefan_full_rgba.tif>
>
> Then,
>
> gdal_create -ot Byte -bands 3 -burn 71 147 240 -outsize 162 150 -a_srs 
> EPSG:3857 -a_ullr 0 162 150 0 background.tif
> gdal_translate -a_srs EPSG:3857 -a_ullr 0 162 150 0 
> stefan_full_rgba.tif foreground.tif
> gdalwarp -r bilinear background.tif foreground.tif composite-bl.tif
>
> In this case, composite-bl.tif is white wherever the alpha is in [1, 
> 254].  Trying
>
> gdalwarp background.tif foreground.tif composite-nn.tif
>
> produces a reasonable image, although it is blocker than when I 
> overlay background.tif and foreground.tif in qgis.
>
> Thanks for any tips about what I'm seeing, happy to file a bug if it 
> is one!  I've tested this on gdal 3.2.2 on macos (from brew).
> Patrick
>
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