[Gvsig_english] transparent raster

[Michele Zucali]

dear gvSIGers,
	
	I'd like to use some black/white geoTIFF above .shp poligonal  
layers, which are filled by various colors.
In order to see the underling layers I need to have a transparent  
raster.

How to make the raster transparent (i.e. make the white transarent) ?
I did try using the transparent property to geotiff but it does not  
work or, better, I'm not able to make it working.

ciao
Michele