Yes, part of the surface mesh receives the decal, the other part nothing (the alpha mask part).
The aim is to use it as blend between meshes and landscape. I tried a lot of diferent methods, all with problems:
-vertex painting blend.
-depth_fade blend
-pixel_depth_fade blend
-layers blend
This image is from vertex paint , but is a tedious work for big scenes.
My idea is to map a big image over meshes.