With a single band raster map (eg. topographical black/white map) if
there is a collar (eg. white border) around the raster, using
gdalbuilvrt (and BuildVRT plugin) the result is a vrt file where the map
have white strips in overlap area [1].
In this case making little modification in the vrt text file you can
obtain a nice result [2]. What is important is to set the NODATA value
in vrt for each raster and replace SimpleSource with ComplexSource.
I don't remember where I find this trick (mailing list?) and I'm not a
gdal guru (vrt specification in gdal web site [3]).
When created the vrt file (via command-line or the very useful new
plugin) I run:
sed -e 's/<SimpleSource>/<ComplexSource>/g' -e 's/<\/SimpleSource>/
<NODATA>0<\/NODATA>\n <\/ComplexSource>/g' file_created.vrt
and now all is ok.
Note that the trasparency of the final map must be set in qgis in the
usual manner.
Maybe this can be add to the plugin (an otpion to select NODATA value)?
Hope this can help (and sorry for the bad english...).
Regards,
giovanni
[1] http://www.iouppo.com/lite/pics/de0b44236bd2b2398de0e3172269f81b.png
[2] http://www.iouppo.com/lite/pics/5d9e1e816caea9c33f1f62595bffaa59.png
[3] http://www.gdal.org/gdal_vrttut.html
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