Ho to create mesh with trasparent area

i'm trying to create mesh from hi res DEM on boundary state line, like:

The problem is: how to insert transparent area in bgl, like blend mask in photoreal?
How georeferential tiff must be?
With invalid area (like in image) and multisource with blend mask, result is 1 meter
Could anyone help me?

Thank you
problem solved!
nullvalue = global mapper elevation for no data value

Type = GeoTIFF
Layer = Elevation
SourceDir = "D:\tmp\GIS\TinItaly\tif"
SourceFile = "test02.tif"
NullValue = -32767