<div dir="ltr">Hi Even,<div><br></div><div>Thanks for checking! I tried this in a docker container (osgeo/gdal) and sure enough, the output looks fine there.</div><div><br></div><div>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.</div><div><br></div><div>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!</div><div><br></div><div>Best,</div><div>Patrick</div><div><br></div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Sep 17, 2021 at 5:18 PM Even Rouault <<a href="mailto:even.rouault@spatialys.com" target="_blank">even.rouault@spatialys.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<p>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.<br>
</p>
<div>Le 18/09/2021 à 01:06, Patrick Young a
écrit :<br>
</div>
<blockquote type="cite">
<div dir="ltr">Hello,
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>To reproduce, I've first grabbed a rgba image from the test
suite, e.g.:<br>
<br>
wget <a href="https://github.com/OSGeo/gdal/raw/a2db646d1c34905e47eb21657284b529b7478d66/autotest/gcore/data/stefan_full_rgba.tif" target="_blank">https://github.com/OSGeo/gdal/raw/a2db646d1c34905e47eb21657284b529b7478d66/autotest/gcore/data/stefan_full_rgba.tif</a><br>
</div>
<div><br>
</div>
<div>Then,</div>
<div><br>
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<br>
gdal_translate -a_srs EPSG:3857 -a_ullr 0 162 150 0
stefan_full_rgba.tif foreground.tif<br>
gdalwarp -r bilinear background.tif foreground.tif
composite-bl.tif<br>
</div>
<div><br>
</div>
<div>In this case, composite-bl.tif is white wherever the alpha
is in [1, 254]. Trying<br>
<br>
gdalwarp background.tif foreground.tif composite-nn.tif<br>
<br>
produces a reasonable image, although it is blocker than when
I overlay background.tif and foreground.tif in qgis.</div>
<div><br>
</div>
<div>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).</div>
<div>Patrick</div>
</div>
<br>
<fieldset></fieldset>
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