[gdal-dev] missing nulls in vrt from GRASS rasters

[William Kyngesburye]

I created a vrt raster from a set of GRASS rasters - image groups, RGB  
+ nulls.  But the nulls are not coming thru in the vrt.  Individually  
I get nulls.  Viewing in Qgis the vrt looks like:

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Each successive image in the vrt overlaps those below it.

But when I add the individual GRASS raster groups to Qgis, it sees the  
nulls and lets the earlier images show thru in the overlapping areas.

The GRASS raster bands are all 0-255 greyscale images, and show up in  
the vrt as:

   <VRTRasterBand dataType="UInt16" band="1">
     <NoDataValue>2.56000000000000E+02</NoDataValue>
     <ColorInterp>Gray</ColorInterp>

gdalinfo also gives me that 256 for nodata.

In Qgis, the properties for the individual GRASS rasters has  
256,256,256 for the transparency band.  So the 256 seems to be right  
for the GDAL-GRASS driver for passing nodata/null.

As a test, I tried making a vrt from tiffs with alpha transparency -  
same problem, nodata is ignored.  Though for this one, there is no  
<NoDataValue> entry in the vrt.

Any idea how I can get the [GRASS] nulls to be recognized in a vrt?

-----
William Kyngesburye <kyngchaos*at*kyngchaos*dot*com>
http://www.kyngchaos.com/

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