Polyhedrons and Moebius Solid in CQ?

[Dario Pellegrini]

Hi,

I have a strong OpenSCAD background, but I am just starting with CQ.

I have gone through the examples and the API ref, still there are few things that I cannot figure out how to achieve.

There are intrinsically 3D shapes which are not easily generated from a plane (1) or that require advanced operations such as revolved and twisted extrusion (2):

1. (irregular) polyhedrons
2. Moebius Solids (https://www.thingiverse.com/thing:3654597)

In OpenSCAD I would hunt these beasts by specifying all the vertices and the edges with the polyhedron function, but to my understanding CQ allows this only in 2D.

I would love to see minimal examples for these two cases in CQ.

Thank you!

[Jeremy Wright]

Hi Dario,

Does this functionality help?

https://github.com/CadQuery/cadquery/issues/242#issuecomment-569074833

The referenced PR has since been merged and is in master.

[Adam Urbanczyk]

Hi Dario,

regarding polyhedra there is en example for a tetrahedron in the tests:

https://github.com/CadQuery/cadquery/blob/6ecb0fc96f2379fbfd8de7f5b935dd880be99047/tests/test_cadquery.py#L2727

For Moebius Solids you'd need a sweep + auxiliary spine. It is not currently implemented in CQ as such. There is a straight extrude with a twist. You could try playing with PythonOCC to get it working (BRepOffsetAPI_MakePipeShell is the relevant class). It seems like FreeCAD has it (see https://forum.freecadweb.org/viewtopic.php?f=22&t=21193 ) so reading their code might be a good starting point.

HTH,

Adam

[Dario Pellegrini]

Thank you both for your replies and sorry for being so late, I have been absorbed by other projects.

I have found the tetrahedron example particularly enlightening.

However, consider this code which makes the union between a sphere and the tetrahedron built as in the test:

import math
import cadquery as cq


from cadquery import Workplane as WP
from cadquery import exporters

from cadquery import Edge
from cadquery import Wire
from cadquery import Face
from cadquery import Shell
from cadquery import Solid
from cadquery import Vector

def Tetra(s=1):
    c0 = s*math.sqrt(2) / 4
    vertices = [[c0, -c0, c0], [c0, c0, -c0], [-c0, c0, c0], [-c0, -c0, -c0]]
    faces_ixs = [[0, 1, 2, 0], [1, 0, 3, 1], [2, 3, 0, 2], [3, 2, 1, 3]]
   
    faces = []
    for ixs in faces_ixs:
        lines = []
        for v1, v2 in zip(ixs, ixs[1:]):
            lines.append(
                Edge.makeLine(Vector(*vertices[v1]), Vector(*vertices[v2]))
            )
        wire = Wire.combine(lines)
        faces.append(Face.makeFromWires(wire))
   
    shell = Shell.makeShell(faces)
    solid = Solid.makeSolid(shell)
    return solid

def save( self, fname, tol = 1):
    with open(fname, 'w') as f:
        exporters.exportShape(self, "STL", f, tol)

WP.save = save


s = WP("XY").sphere(radius=1).union(Tetra(s=3))
s.save("./cq-output.stl", tol=0.01)

This is the resulting mesh inspected with gmsh:

The meshing of the sphere is ugly. It does not even connect properly with the tetrahedron. 

Even on the upper side, which seems ok at a first look, there are weird vertices popping up.

Am I doing something wrong?

[Jeremy Wright]

I bumped up the tolerance, and with the master branch of cadquery the STL export looks like this.

With the OCP branch of cadquery (next release version) using OCCT 7.4, everything looks like it's been tessellated at the same tolerance, but there are gaps in the mesh present on one face.

[Dario Pellegrini]

Thanks a lot for taking the time to test it!

So your conclusion is that it is a kernel issue?

Do we have a way to report upstream?

It is a bit discouraging finding issues at such a basic level.

[Adam Urbanczyk]

If I run your code on the OCP branch and use angular tolerance=0.01 for STL export I get a nice looking mesh. It seems to have some self-intersections (marked in yellow). I don't know enough about STL repair  to be able to say if it is killing or not for e.g. printing. Long story short: you might want to try to modify the angular tolerance of the STL export.

s.val().exportStl("./cq-output.stl", 1e-2,1e-1)

[Justin SDK]

I'm working on a CadQuery plugin.

https://github.com/JustinSDK/cqMore

It provides a `polyhedron` method on `Workplane` and here's an example.

https://github.com/JustinSDK/cqMore/blob/main/docs/workplane.md#polyhedron

There're some pre-defined polyhedra in `cqmore.polyhedron`.

https://github.com/JustinSDK/cqMore/blob/main/docs/polyhedron.md

As for  Moebius Solid, here's an example.

https://github.com/JustinSDK/cqMore/blob/main/examples/mobius_strip.py

pellegri...@gmail.com 在 2020年4月11日 星期六上午12:53:25 [UTC+8] 的信中寫道：

[Jeremy Wright]

Very cool. I have added this in the third-party listing in the cadquery-plugins repo. https://github.com/CadQuery/cadquery-plugins/blob/main/third_party.md

[Adam Urbanczyk]

Cool, thanks for sharing. Out of curiosity: is there a technical reason why you construct a blocky  moebius version instead of a smooth one (e.g. with splines)?

[Justin SDK]

It's just a demonstration of `cqmore.polyhedron.sweep` (the polyhedron version of `sweep` ) .

Here're smooth versions.

https://github.com/JustinSDK/cqMore/blob/main/examples/twisted_strip.py

https://github.com/JustinSDK/cqMore/blob/main/examples/torus_knot.py

Someone might have the problem: why `cqmore.polyhedron`? At times, it's a style for modeling; at other time I just want to control faces. 

Adam Urbanczyk 在 2021年11月2日 星期二下午1:56:51 [UTC+8] 的信中寫道：