RE: [Algorithms] Obscuring objects with Convex Closed Volumes

[Martin F. <mf...@at...>]

Yes but the point will only project to a single point on the screen plane.

> Anyway, I was imaging something for more complex situations. Say to test
is
> a portal is occluded by a group of occluders which have convex hulls or
OBBs

Much more useful! :)

The way I've handle occlusion in my engine is to use AABB or OBBs as
occluders.
If we start with the AABB case.

Each face of the box is classified as front facing or back facing with
respect to the camera. Each front facing face on the box is clipped to any
and all parent clipping planes. (Portal, view fustrum, other occluders
etc..)

A visible face with a back facing adjacent face generates a plane built with
the shared edge and the camera's position if the shared edge itself survives
clipping to the parent clipping planes. These are silhouette planes. We need
to add the plane equations of all the front facing visible faces. If not an
object between the occluder and the eye could disappear if it was inside the
volume defined by the silhouette planes.

For an occluder we now have a number of planes. If any primitive is behind
all of these planes it has been occluded. Of course testing box's, sphere's,
convex hull's etc.. against planes is well documented.

I've found a little performance advantage from storing the silhouette planes
first then front facing planes in occluders array of planes.
Sorting occluders / portals is important too. You can waste allot of
processing if you generate planes for an occluder which itself is occluded
by an occluder encountered later.

The only difference with an OBB is the way the world space points are
generated to build world space planes.

Any convex, closed volume will do as an occluder. I only used AABB's / OBB's
because they're a simple yet practical volume to use. Using arbitrary
volumes means storing additional vertices, polygons and adjacency
information and will likely generate more planes.

Occluder LOD is also well worth while. When the projected area of an
occluder get's too small I turn them off as, due to perspective, the ammount
it occludes lessens significantly.

The most significant benefit of occluders is often in occluding portals.
Cheers,
Martin



-----Original Message-----
From: gda...@li...
[mailto:gda...@li...]On Behalf Of
Gareth Lewin
Sent: 29 July 2003 09:34
To: gda...@li...
Subject: RE: [Algorithms] Obscuring objects


can't even a single point be occluded on the way to the centre of the screen
but not the top.

 |             .
 |        O
 |
 |
 |

the | is the screen, the O is the sphere, the . is the point.

Anyway, I was imaging something for more complex situations. Say to test is
a portal is occluded by a group of occluders which have convex hulls or OBBs