Basic operation results in invalid geometry

Question

Basic operation results in invalid geometry

Opened this issue 6 months ago · 5 comments

p = Box(2,2,2) - Pos(-1.5) * Sphere(1)

Answer 1 · 2024-03-17T13:43:19.000Z

This is an OCCT problem with how the Edge of the sphere is aligned. Consider this version:

s = Pos(-1.5) * Sphere(1).rotate(Axis.Y, 180)
p = Box(2, 2, 2) - s

Note how the Edge of the sphere is rotated away from the cube which allows the boolean operation to work as expected.

I've considered constructing a sphere from two hemispheres but an clean operation would fuse them together again which doesn't solve the problem. This really needs to be fixed at the OCCT level.

Answer 2 · 2024-03-17T18:46:44.000Z

Another issue from this scenario: This creates valid geometry but the exported STEP file gives the union rather than subtraction.

p = Box(2,2,2) - Pos(0.7) * Sphere(0.7)

Answer 3 · 2024-03-17T20:27:14.000Z

I'm guessing the OCCT requires all Solids to have at least one Edge but it breaks so many things. I expect fixing it in OCCT would be difficult but I don't know.

Answer 4 · 2024-03-18T20:43:50.000Z

OCCT fundamentally requires surfaces to be closed in their parametric space. Removing that requirement in OCCT would be a monumental effort, however, the geometries here should be able to comply within that limitation.

Answer 5 · 2024-03-18T20:53:27.000Z

I believe it should be possible (it only needs to be closed in the 2d parametric space of the surface and you can create a circle with a single vertex and edge, so I imagine a clipped sphere shell is topologically equivalent) to create a parametrically closed sphere like the one below without any seams on the spherical surface but I have been unsuccessful so far. It would be useful in many scenarios to circumvent the seam issue.