Le 20/02/2024 à 11:15, Elena Ruiz via gdal-dev a écrit :
Hello, I have version 3.6.2 of GDAL and I need to crop an image using
a contour from a GML file, so far there is no problem.
The problem begins when the original image is a GEOTIFF with
transparency and it generates a cropped image, the information of both
images is a little lower.
The color table changes from having 2 values to having 256, this
causes it to lose transparency although it has the label "Mask Flags:
PER_DATASET ALPHA" which indicates that it has an alpha channel.
My question is, why does the color table change?, and is there any
parameter that can be used with gdalwarp or gdal_translate to avoid
this and keep the image transparent?
Strictly speaking your input image has no transparency, just black &
white colors. Not sure which one you interpret as transparent?
You could force a 1-bit output image (ie with a color palette of 2
colors) by adding -co NBITS=1 . But that will likely won't play well
with -dstalpha. You'd then want to use instead -dstnodata 0 if black is
the transparent color or -dstnodata 1 if white is the transparent color
Even
--
http://www.spatialys.com
My software is free, but my time generally not.
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