From Paul Melis, "I think TransferFunction needs to call _image->dirty() in two places in response to an assign() of new transfer map values.

Here's an updated file, which seems to work here.
"

src/osg/TransferFunction.cpp

@@ -97,6 +97,7 @@ void TransferFunction1D::assign(const ValueMap& vcm, bool updateMinMaxRange)
                 _colors[i] = color;
             }
         }
+        _image->dirty();
         return;
     }
     
@@ -135,5 +136,7 @@ void TransferFunction1D::assign(const ValueMap& vcm, bool updateMinMaxRange)
         
         lower_itr = upper_itr;      
     }
+
+    _image->dirty();
 }