Polyhedron: degenerated triangles are allowed, but quads...
Consider the following code of a prism degenerated in a tetrahedron:
v = [ [0,0,0], [50,0,0], [0,50,0],
[0,0,50], [0,0,50], [0,0,50] ];
f1 = [ [0,1,2], //base
[1,0,3,4], // three degenerated quads -> triangles
[2,1,4,5],
[0,2,5,3],
[3,4,5] ]; // triangle degenerated to a point
f2 = [ [0,1,2], // base
[0,4,1], [0,3,4], // one degenerated triangle
[1,5,2], [1,4,5], // one degenerated triangle
[2,3,0], [2,5,3], // one degenerated triangle
[3,4,5] ]; // triangle degenerated in a point
polyhedron(v,f1);
translate([60,0,0]) polyhedron(v,f2);
translate([0,70,0]) cube(10);
The´polyhedron (v,f1) has a triangular face ([3,4,5]) degenerated to a point
and 3 quads degenerated in triangles. The polyhedron (v,f2) has the same
degenerated triangular face and 3 other triangular faces degenerated in line
segments.
The surprising result is: (v,f2) is previewed and rendered (with (v,f1)
commented) correctly even with the presence of the cube; but (v,f2) preview
and render has 3 missing faces even when it is alone. In short: degenerated
triangles are allowed in polyhedron, degenerated quads are suppressed.
A preview:
http://forum.openscad.org/file/n19917/Degenerated_faces.png
What is even more striking: if we insert three additional vertex in the
edges of the tetrahedron base and change (v,f1) accordingly, the quad faces
turns to degenerated pentagons and the model is previewed and rendered
correctly!!!
v3 = [ [0,0,0], [50,0,0], [0,50,0],
[0,0,50], [0,0,50], [0,0,50], [25,0,0], [25,25,0], [0,25,0] ];
f3 = [ [0,6,1,7,2,8],
[1,6,0,3,4], [2,7,1,4,5], [0,8,2,5,3], [3,4,5] ];
polyhedron(v3,f3);
cube();
I have got the same results with versions 2015.03-2 and 2016.11.11.
This is breaking some of my loft codes.
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It seems that polyhedron does some silent "repair" (exclusion) of degenerate
triags, but not of degenerate quads or larger polygons.
Despite that I wouldn't build on degenerated stuff like that and don't like
silent repairs at all, you could try to turn your quads (and larger
polygons) into triags and trust on polyhedron to filter your triag soup
appropriately. But be sure to check the results at STL level.
If you don't check by inserting a dummy boolean operation into your design,
OpenSCAD for now also lets you create and export degenerate 3D objects like
the following.
v = [ [0,0,0], [50,0,0], [0,50,0], [0,0,50] ];
polyhedron(v,[[0,1,2]]);
polyhedron(v,[[0,2,3]]);
polyhedron(v,[[0,3,1]]);
polyhedron(v,[[1,3,2]]);
I don't see much reason why this should be a false approach, besides the
little fact, that it is not specified for the language OpenSCAD to construct
3D Objects by a union of surfaces defined by polyhedrons.
While the STL-Exporter creates some faulty thing out of this, the
AMF-Exporter suprisingly does the job.
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I have found that degenerated triangles are discarded, quads degenerated in
triangles are also discarded but pentagons degenerated in quads are not! And
this seems to be a bug or, at least, an very idiosyncratic repair.
Your example is also striking. After render, the console shows there are 2
volumes (which ones?). If any of the polyhedron is dropped, the render will
warn for possible non-manifold and consider only one volume. I have checked
the stl: it has 8 facets, two for each one you have defined with opposed
normals and inverted circulation. The generated stl for just one of your
polyhedron (just one facet) will have just one facet. A real mess.
I surely can avoid degenerated quads by checking or I can triangulate every
quad. But why I should do that (which will double the number of facets to
send to polyhedron) if there is a automatic triangulation inside the
OpenSCAD code? Either it works with all cases or it is worthless.
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the count of the rendered volumes generally seems to have an offset of 1. Try
translate([0,70,0]) cube(10);
translate([0,-70,0]) cube(10);
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I have found more strange behaviors of the degenerated cases. When I render
just the polyhedron (v,f1) ( the one that has degenerated quads) the result
is the same of the preview: it displays just the base eliminating all quads.
The generated stl has only one face.
But if I render polyhedron (v,f1) and something else (like a cube) both are
displayed correctly and the generated stl is also correct. This suggests
that when the scene has just one model, OpenSCAD does "repairs" that are not
done when there is more than one object in the scene. And the bug is just in
this repair.
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Could you provide a self-contained, minimal example?
-Marius
On Jan 5, 2017, at 17:48, Ronaldo rcmpersiano@gmail.com wrote:
I have found more strange behaviors of the degenerated cases. When I render
just the polyhedron (v,f1) ( the one that has degenerated quads) the result
is the same of the preview: it displays just the base eliminating all quads.
The generated stl has only one face.
But if I render polyhedron (v,f1) and something else (like a cube) both are
displayed correctly and the generated stl is also correct. This suggests
that when the scene has just one model, OpenSCAD does "repairs" that are not
done when there is more than one object in the scene. And the bug is just in
this repair.
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2017-01-05 21:19 GMT-02:00 Marius Kintel kintel@kintel.net:
Could you provide a self-contained, minimal example?
-Marius
It can be found in the first post of this thread.