View Full Version : so, what is manifold geometry

 henning10 October 2003, 06:59 PMSo far, I've been able to determine the following: Non-manifold geometry is when polygons are arranged in weird ways. For example, take an hourglass and pinch it in the middle so that the geometry there goes down to one vertex at that spot. The whole hourglass is now non-manifold. But... what's the definition of manifold? And how do you check if geometry is manifold or not? henning
ajk48n
10 October 2003, 09:48 PM
It is geometry that can't be unfolded into a flat piece. Examples are two polygons next to each other, with normals facing in opposite directions, two planes that share a single vertex but no edge, and two polygons, with the edge extruded outward from the edge that both share (basically making a "T" shape out of 3 polygons).

playmesumch00ns
10 October 2003, 09:06 AM
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10 October 2003, 04:10 PM
Manifold models are models where the volume is well defined.
They have the following properties:

Every edge belongs to two faces.
Every vertex is surrounded by one sequence of edges and faces.
Faces only intersect eachother in common edges en vertexes.
There is a material on only one side of a face

Another way of putting it, a manifold object is a shape that you can create in real life. non-manifold objects have qualities, where you have badly defined volumes in your shape, making it impossible to recreate the model exactly that way in real life.

Hope it helps..

DaKrunch
11 November 2003, 08:51 PM
curiosity: a box without a face (5 square faces and a hole) is non-manifold?

i guess the answer would be the same as for a plane?

thanx

11 November 2003, 11:01 PM
Yeah, thats non-manifold. The object contains no volume, because it isn't a closed space.

markyjerky
11 November 2003, 01:42 AM
I'm suspicious ...

That a lot of programs would not go through the rigor of trying to detect if a surface intersected itself in strange ways into order to qualify or disqualify something as a manifold.

In other words ... I bet with 9 or of 10 programs that want to operate only on manifolds, as least 9 would not care about how visually twisted the model is ... so long as the topology (which has nothing to do with the strictly visual) remains that of a manifold. :hmm:

Just some thoughts. since we are talking about manifolds.