[gdal-dev] Black borders after reprojection GDAL_WARP

[Lorenzo Di Giacomo]

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 <cgodkin at gmail.com> 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
> .)
>
> 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 <loridigia at gmail.com>
> 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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