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See the attached image: I set up 100% transparency on 0 value, but the white collars are still shown.
Debian, QGIS compiled yesterday from trunk.
Thanks.

Hi,

I tested also a different tiff/vrt on ubuntu 9.04 with qgis 1.3 installed from the ubuntugis repositories. Transparency works for any chosen colour on both formats.

Replying to [comment:2 pcav]:

See the attached image: I set up 100% transparency on 0 value, but the white collars are still shown.

This is not a transparency issue, it seems to be a registration problem. I turned my map canvas green so I can tell if the transparency is working - making white in the virtual raster 100% transparent - which ends up giving me black on green. See the attached screen grab.

An interesting note from Giovanni Pasini:

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 r1.
In this case making little modification in the vrt text file you can obtain a nice result r2. 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 r3).

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>\
<\\/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.

This could be a buildvrt bug... I've opened a ticket on GDAL ticket.
They can add a new param to set which color is blanc (e.g. -nodata 0) or resolve this bug in another way.

Replying to [comment:6 brushtyler]:

This could be a buildvrt bug... I've opened a ticket on GDAL ticket.
They can add a new param to set which color is blanc (e.g. -nodata 0) or resolve this bug in another way.

Hi all,

the GDAL bug was closed as fixed in gdal 1.7.0 (the dev version actually in trunk)

Below the ticket closing comment:

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Enhancement implemented in :

/trunk/gdal/apps/ (gdal_utilities.dox gdalbuildvrt.cpp): gdalbuildvrt: add -srcnodata and -vrtnodata options, similar to the ones of gdalwarp, to set/override/unset the nodata settings at the source level and the VRTRasterBand level (#3254)

/trunk/autotest/utilities/test_gdalbuildvrt.py: Add test for -srcnodata option of gdalbuildvrt (#3254)

--> For your use case, 'gdalbuildvrt srcnodata 0 out.vrt 250010.tif 250020.tif' should do.
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I think that now, if there isn't other related problems, this ticket should be closed.