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<p>Tim,</p>
<p>Reviewing the warping logic, I see we have support for a *input*
mask, but not creating/updating an *output* mask. AFAICS, there
isn't any particular strong reason not to have support for that.
Likely just a few hours of coding away. Your best workaround is to
generate an output with an alpha band (possibly as a VRT) and use
gdal.Translate(dst, src, options="-b 1 -b 2 -b 3 -mask 4") for
example to transform a RGBA into a RGB+mask product</p>
<p>Even<br>
</p>
<div class="moz-cite-prefix">Le 12/02/2025 à 23:47, Tim Harris via
gdal-dev a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:CAK3fJgUSvG5qdTZQFDoS==CMKpkp3C8sF81KKKFod=wMiBuWZQ@mail.gmail.com">
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<div>Not sure if this is a bug, or expected behavior, or user
error. If I warp one TIF into another, and both have nodata
masks, it seems that gdalwarp isn't updating the destination
TIF's nodata mask to unmask the new pixels.</div>
<div><br>
</div>
<div>Here's a short script to generate two example TIFs. One has
a red box in its upper left corner, the other has a green box
in its upper right corner. Both have nodata masks so that only
the colored boxes are visible.</div>
<div><br>
</div>
<div><span style="font-family:monospace">---</span></div>
<div><span style="font-family:monospace">import numpy as np<br>
from osgeo import gdal<br>
<br>
gdal.UseExceptions()<br>
gdal.SetConfigOption("GDAL_TIFF_INTERNAL_MASK", "YES")<br>
drv = gdal.GetDriverByName("GTiff")<br>
<br>
zeros = np.zeros((3, 1024, 1024))<br>
<br>
red = np.copy(zeros)<br>
red[0, 100:200, 100:200] = 255<br>
<br>
green = np.copy(zeros)<br>
green[1, 100:200, 800:900] = 255<br>
<br>
ds = drv.Create("red.tif", 1024, 1024, 3, gdal.GDT_Byte)<br>
ds.SetGeoTransform((0, 1, 0, 0, 0, -1))<br>
ds.WriteArray(red)<br>
ds.CreateMaskBand(gdal.GMF_PER_DATASET)<br>
ds.GetRasterBand(1).GetMaskBand().WriteArray(red[0, :, :])<br>
ds = None<br>
<br>
ds = drv.Create("green.tif", 1024, 1024, 3, gdal.GDT_Byte)<br>
ds.SetGeoTransform((0, 1, 0, 0, 0, -1))<br>
ds.WriteArray(green)<br>
ds.CreateMaskBand(gdal.GMF_PER_DATASET)<br>
ds.GetRasterBand(1).GetMaskBand().WriteArray(green[1, :, :])<br>
ds = None</span></div>
<div><span style="font-family:monospace">---</span></div>
<div><span style="font-family:monospace"><br>
</span></div>
<div>Then if you copy red.tif to warp.tif, then warp green.tif
into warp.tif:</div>
<div><span style="font-family:monospace">cp red.tif warp.tif</span></div>
<div><span style="font-family:monospace">gdalwarp green.tif
warp.tif</span></div>
<div><br>
</div>
<div>I would expect the result to be a combined image with both
the red and green boxes. But when I load this result into
QGIS, I only see the red box. If I change the layer in QGIS to
ignore the mask band, I do see both boxes (and the remaining
image is black, as expected). So the pixel data is there, but
the mask didn't get updated.</div>
<div><br>
</div>
<div>Is there a way to get gdalwarp to update the nodata mask
automatically? Maybe some hidden setting I'm missing? I can
work around this by separately warping the masks, then copying
the result into the final TIF's mask band. But it would be
nice if the warp took care of this for me.</div>
<div><br>
</div>
<div>Thanks</div>
</div>
<br>
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</blockquote>
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