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Geometry and Topology

adrenalin123's picture
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I have a newbie question. Could anybody please explain to me in an easy way, what the difference between geometry and topology level is, and where I want to use what?
The Manual didn't really answer that question and I didn't find anything here.

Thanks for helping
Martin

Pawel's picture

Hi adrenalin123,

look here: http://opencascade.blogspot.com/2009/02/topology-and-geometry-in-open-cascade.html

Good luck
Pawel

adrenalin123's picture

Well, I have already read this and the Modeling Data Manual, however I don't get the point, what the real difference is, and when to use what. ?-(

EricThompson's picture

At a very high level, topology tells what pieces an object is made of, and the logical relationships between them. A shape is made of a certain set of faces. A face is bounded by a certain set of edges. Two faces are adjacent if they share a common edge.

Topology alone does not tell you the size, curvature, or 3D locations of any of those pieces. However, each piece of topology does knows about it's underlying geometry. A face knows what surface it lies on. An edge knows what curve it lies on. The geometry knows about curvature and location in space.

So, for example, if you wanted to know if a point lies on the surface of an object, you would iterate through each of the faces of the object ( topology ), and for each face, check if the point lies on the face's surface ( geometry ). You would also need to check if where the point was on the surface was contained within the curves ( geometry ) that correspondes to the edges ( topology ) that surround the face.

Or you could call an existing function to do all that for you.

adrenalin123's picture

Thank you very much! This helped alot!

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