[Qgis-user] QGIS Georeferencer and Transparency

[Jukka Rahkonen]

Andreas Neumann <a.neumann at ...> writes:

> 
> Hi,
> 
> Some of my maps I georeference are rotated approx. 45 degrees because
> they did not point to the north before georeferencing. This is desired -
> however - the resulting file has a huge black background as a result of
> the rotation.
> 
> I tried with the option "use 0 for transparency if necessary" - which
> helped - but the map already contains black pixels and the result is
> that I have a lot of tiny transparent pixels in the map, next to the
> transparent background I wanted. Is there a way to set this transparency
> to a custom RGB value that I know it is not yet used within the map?

Hi,

Don't let QGIS to do the warping but just push the button and create the
GDAL commands. Then you can edit the gdal_translate command and have total
control over all the features it supports http://www.gdal.org/gdalwarp.html.
You may be able to set -dstnodata to some value that does not exist in your
data, for example "0 0 255" for total blue. Or then you can add alpha
channel with -dstalpha.

Unfortunately you can't edit the command and run it as edited directly from
the georeferencer but you must run it in a separete GDAL command window.

-Jukka Rahkonen-