[gdal-dev] gdalwarp alpha band resampling bug

[Patrick Young]

Hi Even,

Thanks for checking!  I tried this in a docker container (osgeo/gdal) and
sure enough, the output looks fine there.

It reproduces for me with my brew installed gdal (3.3.2) and I was careful
to do everything in a clean directory.  I tried a few other outputs as well
(png, jpeg) and the toucan remains surrounded by white.  I also had a
colleague check (he is also on a mac, using a brew installed GDAL 3.3.0)
and he sees the same artifact that I see.

I have no idea what could be causing it!  Interestingly, if the background
raster is (255, 0, 0), you don't get the white everywhere, but instead you
get solid red wherever the alpha is in [1,254].  I wonder what it could be!

Best,
Patrick



On Fri, Sep 17, 2021 at 5:18 PM Even Rouault <even.rouault at spatialys.com>
wrote:

> 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
>
> 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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>
>
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