[gdal-dev] gdalwarp alpha band resampling bug

[Patrick Young]

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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