R
Ronaldo
Sun, Jan 7, 2018 4:26 PM
As Parkinbot remarked recently in another thread, the alternate triangulation
OpenSCAD builds for non-triangular faces has some bugs. I could isolate a
small example where this comes clear. The code is a simple polyhedron (fully
inside an intercepting cube) with many triangular faces and 2 quadrangular
faces. The list faces0 contains just the triangular faces; faces1 includes
the 2 quadrangular faces and faces2 is the same as faces1 with the
quadrangular faces explicitly subdivided in triangles. The polyhedron
defined with faces2 renders fine. With faces0, it renders with a warning
(non-manifold) because of the 2 missing faces. With faces1, we get an access
violation from CGAL. I checked it with version 2017.08.31 (git c6f3f0d) in a
Windows 7 environment.
verts = [[107.118, 20.796, 3.69206], [101.912, 18.7279, -9.29348], [89.8439,
13.9336, -3.69206], [95.0496, 16.0017, 9.29348], [91.3134, 60.0577,
3.69206], [86.6335, 56.9797, -9.29348], [75.7842, 49.844, -3.69206],
[80.4641, 52.9221, 9.29348], [61.5224, 90.1203, 3.69206], [57.5616, 86.1595,
-9.29348], [48.3794, 76.9773, -3.69206], [52.3402, 80.9381, 9.29348]];
;
faces0 = [[0, 5, 1], [0, 4, 5], [1, 6, 2], [1, 5, 6], [2, 7, 3], [2, 6, 7],
[3, 4, 0], [3, 7, 4], [4, 9, 5], [4, 8, 9], [5, 10, 6], [5, 9, 10], [6, 11,
7], [6, 10, 11], [7, 8, 4], [7, 11, 8]];
faces1 = concat(faces0, [ [0, 1, 2, 3], [11, 10, 9, 8]] );
faces2 = concat( faces0, [ [0, 1, 2], [0, 2, 3], [8, 11, 10], [8, 10, 9]]
);
intersection(){
translate([0,0,-100]) cube(200);
polyhedron( verts, faces1);
// polyhedron( verts, faces0);
// polyhedron( verts, faces2);
}
--
Sent from: http://forum.openscad.org/
As Parkinbot remarked recently in another thread, the alternate triangulation
OpenSCAD builds for non-triangular faces has some bugs. I could isolate a
small example where this comes clear. The code is a simple polyhedron (fully
inside an intercepting cube) with many triangular faces and 2 quadrangular
faces. The list faces0 contains just the triangular faces; faces1 includes
the 2 quadrangular faces and faces2 is the same as faces1 with the
quadrangular faces explicitly subdivided in triangles. The polyhedron
defined with faces2 renders fine. With faces0, it renders with a warning
(non-manifold) because of the 2 missing faces. With faces1, we get an access
violation from CGAL. I checked it with version 2017.08.31 (git c6f3f0d) in a
Windows 7 environment.
verts = [[107.118, 20.796, 3.69206], [101.912, 18.7279, -9.29348], [89.8439,
13.9336, -3.69206], [95.0496, 16.0017, 9.29348], [91.3134, 60.0577,
3.69206], [86.6335, 56.9797, -9.29348], [75.7842, 49.844, -3.69206],
[80.4641, 52.9221, 9.29348], [61.5224, 90.1203, 3.69206], [57.5616, 86.1595,
-9.29348], [48.3794, 76.9773, -3.69206], [52.3402, 80.9381, 9.29348]];
;
faces0 = [[0, 5, 1], [0, 4, 5], [1, 6, 2], [1, 5, 6], [2, 7, 3], [2, 6, 7],
[3, 4, 0], [3, 7, 4], [4, 9, 5], [4, 8, 9], [5, 10, 6], [5, 9, 10], [6, 11,
7], [6, 10, 11], [7, 8, 4], [7, 11, 8]];
faces1 = concat(faces0, [ [0, 1, 2, 3], [11, 10, 9, 8]] );
faces2 = concat( faces0, [ [0, 1, 2], [0, 2, 3], [8, 11, 10], [8, 10, 9]]
);
intersection(){
translate([0,0,-100]) cube(200);
polyhedron( verts, faces1);
// polyhedron( verts, faces0);
// polyhedron( verts, faces2);
}
--
Sent from: http://forum.openscad.org/
RP
Ronaldo Persiano
Sun, Feb 25, 2018 2:10 PM
Does this triangulation handle faces that have concave edges?
...
Perhaps a
triangulation option that defaults to false?
Yes, it handles non-convex facets and even non-planar ones:
p = [ [10,20,0],[0,0,-10], [10,-10,0], [20,0,0], [30,0,20], [20,20,0]];
polyhedron(points=p,faces=[[1,2,3,4,5,0]]);
Note that the triangulation of the facet changes with the facet indexing
(circular) order. You have no control on it although it may change
radically the geometry of non-planar facets.
AFAIK, this triangulation is fired automatically when CGAL complaints some
facet is non-planar. I don't think a triangulation option is needed because
the process is very fast. Anyway, to help beginners it should defaults to
true.
2018-02-25 6:42 GMT-03:00 Gadgetmind <lists@foxhill.co.uk>:
> Does this triangulation handle faces that have concave edges?
> ...
> Perhaps a
> triangulation option that defaults to false?
>
Yes, it handles non-convex facets and even non-planar ones:
p = [ [10,20,0],[0,0,-10], [10,-10,0], [20,0,0], [30,0,20], [20,20,0]];
polyhedron(points=p,faces=[[1,2,3,4,5,0]]);
Note that the triangulation of the facet changes with the facet indexing
(circular) order. You have no control on it although it may change
radically the geometry of non-planar facets.
AFAIK, this triangulation is fired automatically when CGAL complaints some
facet is non-planar. I don't think a triangulation option is needed because
the process is very fast. Anyway, to help beginners it should defaults to
true.
