Geometry and Topology

[flyby]

Hello =)

What is difference between Geometry and Topology data types?

and subsequently, why do some nodes like Extrude work with Topology, and others like Raycast work with Geometry.... (intuitively I'd say they both work with the same data structure)

[AlexanderM]

Only the Lord knows the answer. Probably still Luc-Eric knows...

[xsisupport]

Topology is an attribute of a geometry.
One geometry could have different topologies. A trivial example would be a different point order.

Something like Raycast returns a location on a geometry. A location is not part of the topology.

[flyby]

ok, then how much information is inside Topology? like, points, faces(and thus implicitely edges) anything else?

[xsisupport]

vertices, polygons, edges, and polynodes

[flyby]

ok here's how i understand it, list of assertions, please correct me where im wrong:

1. The Geometry object's attributes are mostly sets, with individual elements of these sets implicitely corresponding with points/edges/polys of the Topology attribute.

From testing it turns out, everytime you set the Topology attribute, the other attributes are reset (but not removed! just set to 0). so:

2. The only way to preserve per point attributes after Topology assignment (e.g. extrude), is to make array from the attributes, save them as per-object attributes, and then "unroll" them back after the Topology change. Same applies to UV clusters.

3. there is no way to create an instance of a Geometry datatype, inside the ICETree, basically "on the fly", and e.g. assign it to "self" (even though "self" has the right type) or pass it as a parameter for raycasting. Theres also no way to convert Topology into Geometry other then using a "set data" on "self.Topology". From what i understand this is because Softimage uses Geometries as contexts, so they need to be constant.

vertices, polygons, edges, and polynodes

about polynodes does that mean that you can save UV data into the Topology?

[xsisupport]

1. Some geometry nodes have topology-specific attributes, such as PolygonToNodes. Not all attributes are about topology.

2. If you clone/copy, you can use the Copy Vertex Data from Source compound.

about polynodes, no, you don't save UV data in the topology
See the Copy Texture Projection from Source compound

[flyby]

2. hmm true, but the idea here is that you cannot use per-point- and per-polygon-attributes as a helper when procedurally assembling the topology, resorting to saving such data in per-object-arrays.

You were very helpful thank you =)