DSF can operate in two modes:
- When DSF is operating without raster DEM information, the normals from the DSF mesh patches become the normal vectors in the 3-d triangle mesh drawn in X-Plane, used for lighting calculations; they need to be provided to get appropriate terrain lighting queues. They are not used for structure surface projection - that is done strictly with the actual 3 vertices for each triangle.
(Because the normals are "per vertex" and not "per triangle", they are rather useless for projection - it is expected that they may be blends of neighboring triangles.)
- When DSF is operating with a raster DEM, then like the elevation of a point and the normal are computed from the raster layer; in this case the elevation must be the flag value -32768.0 and normals should be set to 0 to improve DSF compression.
In the raster case, if the elevation is not -32768.0, then that indicates the elevation is still used, but the normals come from the DEM. The goal of this is to allow -exact- edge matching at water bodies.