[OpenSCAD] Ovals
arnholm at arnholm.org
arnholm at arnholm.org
Thu Sep 6 02:55:44 EDT 2018
On 2018-09-05 22:27, FourthDr wrote:
> What do you mean by "2-manifold"? I thought manifold was well,
> manifold.
> There is some other type of manifold?
2-manifold edge means that edge is referenced by exactly 2 faces.
1-manifold edge means it is referenced by exactly 1 face and that face
is then obviously not contributing to a closed volume boundary. In
general, an edge being N-manifold means the edge is referenced by N
faces. In loose language you could say a face is non-manifold if it
refers to at least one edge that is not 2-manifold.
In 3d-printing it is common to require all edges to be 2-manifold,
because the faces will then unambiguously define one or more closed
volumes. If you had a more sophisticated representation of volumes with
sub-volumes using e.g. different materials, it would make a lot of sense
to allow N-manifold edges for N>2, but it is not possible within a
single STL.
An STL file contains disconnected triangles ("polygon soup"), i.e. faces
with exactly 3 edges. An edge runs between 2 vertices. A vertex has XYZ
coordinates. Strictly speaking, in STL files all edges are 1-manifold,
because there is no concept of edge or vertex sharing, all vertices (and
coordinates) are repeated for every triangle, hence it is a "polygon
soup". It is up to the software reading the STL file to have a guess at
what matches what and which manifoldness it is supposed to represent,
and this often leads to conflicting interpretations. STL is (somewhat)
OK for slicer programs, but not really a good choice for import to CAD
programs, due to the interpretation issues mentioned. As mentioned,
there are other formats that are less ambiguous, including WaveFront
OBJ.
Carsten Arnholm
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