Black borders are completely expected: reprojection changes the geometry
of the image.
That said, if the area of interest is sufficiently small and the
geometry change being approximatively an affine transformation, you can
try https://gdal.org/programs/gdalmove.html instead of gdalwarp to only
alter the georeferencing information and not touch pixel values at all.
Even
Le 18/11/2021 à 15:34, Lorenzo Di Giacomo a écrit :
Hi Carl, thanks for your reply, i noticed that it happens even if i
just reproject the image, without cutting.
Of course the dstalpha works, but it increases the size of the image
and it changes its nature (adding another band) since this operation
is an intermediate operation the resulting image can't be different
from the original, just reporojected.
Il giorno gio 18 nov 2021 alle ore 14:11 Carl Godkin
<[email protected] <mailto:[email protected]>> ha scritto:
Hi Lorenzo,
I have faced this and there are two things to consider.
First, you can specify the georeferenced extents _and_ the SRS of
the extents using something like this:
gdalwarp -t_srs EPSG:3857 \
-te_srs EPSG:4326 -te -109 32 -102 36 \
input.tif output.tif
Note that I'm warping to one coordinate system but trimming based
on another one. Is it possible that your black boundary is due to
trimming in the wrong coordinate system? For instance, if your
input map is projected but the boundary of the map consists of
parallels of latitude or meridians of longitude, then you could
use something like the above. (You can actually get even fancier
by trimming with polygons too; see this example for inspiration:
https://gis.stackexchange.com/questions/45053/gdalwarp-cutline-along-with-shapefile
<https://gis.stackexchange.com/questions/45053/gdalwarp-cutline-along-with-shapefile>
.)
Second, recognize that the output will always be a rectangle in
the output SRS. If the black border is due to "no data" areas
outside the input map's extent appearing in the output map, then
you can use -dstalpha ("Create an output alpha band to identify
nodata (unset/transparent) pixels.") to mark the output pixels
that shouldn't be part of the map. Basically your black pixels
become transparent in this case.
I hope that helps,
carl
On Wed, Nov 17, 2021 at 8:11 AM Lorenzo Di Giacomo
<[email protected] <mailto:[email protected]>> wrote:
Hi all, i have a question, how is it possible to avoid black
borders after reprojection with "gdal_warp" ?
Basically i have an image that is little rotated, when i
reprojected from 32632 to 4326 the resulting image has black
borders more or less tight depending on the rotation.
I saw i can change the colors of those no_data (using
-dstnodata) but how can i do if i dont want it at all? Adding
another band result in a size increase, that i don't want either.
Thanks !!
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