[gdal-dev] Black borders after reprojection GDAL_WARP

[Lorenzo Di Giacomo]

Thanks Even, didn't know about gdalmove. I'll try !!


Il giorno gio 18 nov 2021 alle ore 15:47 Even Rouault <
even.rouault at spatialys.com> ha scritto:

> 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 <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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>>> gdal-dev at lists.osgeo.org
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>
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