X-Plane 9's rendering engine has the ability to drape geometry. Draped geometry are meshes that are 'dropped' onto the terrain and hug the underlying base mesh perfectly. The most common example of this is the runways: because the runways 'drape' the ground, the runway shows any curvature and bumps from the underlying base mesh. This is who we create sloping and non-flat runways.
Authors can drape geometry as well, using a draped polygon (.pol) primitive in an overlay. Such draped geometry is useful any time you want to add more "paint" to the ground, e.g. to put down a taxiway, parking markings, dirt, grass, a driveway for a house, you name it.
There is one case in X-Plane 9 where you cannot drape geometry: in an object. In an object, all geometry is aligned to the object, and will only interact nicely with the ground if you get lucky. For example, if you model a house with a sidewalk, the sidewalk won't "sit" on the ground if the ground turns out to be sloped. You can use ATTR_poly_os to hide the artifacts, but ATTR_poly_os really can't cope with mismatches between the OBJ and the terrain under it.
X-Plane 10 will introduce a new object attribute: ATTR_draped. Draped geometry in an object is actually draped down onto the terrain when the object is placed in the scenery. This means that the draped part of the object will hug the ground perfectly with no interference or Z thrash. You get all of the quality of a draped polygon with the convenience of an OBJ.
There are a few possible uses for ATTR_draped:
- Any time a 3-d model needs some ground details attached to it, e.g. the driveway near a house, draped geometry provides a good fit with the ground and good alignment with the object.
- Any time you want to include a pre-made ground decal (E.g. a painted parking spot on a taxiway), the ground detail can be modeled as an object using draped geometry.