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I'm trying to make a mosaic (and then a pyramid) of historic map tiles.
Every map tile has a border, I got rid of it with gdalwarp (adding an alpha band and specifying the border, as I stated here).

cropped map tiles with alpha enabled

to make easier to understand my problem, I turned off the transparency of band 4: same tiles with alpha turned off

If I run the build virtual raster (catalog) I got this:
gdalbuildvrt output

Adding the option -srcnodata "0 0 0 0" does not work as expected, because it treats any zeroes of any band as notata, and not the combination of the four bands values (and I added alpha because of this behavior).

Any hint on how to solve that? I can also use GRASS or PostGIS, but I'm not so good at them...

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