3 ms·

It would probably fall somewhere between "substantial architectural overhaul" and "partial rewrite" because it’d require redesigning the topological representat

It would probably fall somewhere between "substantial architectural overhaul" and "partial rewrite" because it’d require redesigning the topological representation to eliminate seam edges. Some of these issues are long standing and really hard to solve. Someone could probably defend an entire PhD thesis on “redesigning the topological representation to eliminate seam edges” without making much practical progress

loftsy 11mo ago
How do the other kernels e.g. Parasolid work without seam edges? Without a seam the 2D parametric boundary is not closed.
- throwup238 11mo ago
  It’s not about seams in 2d but 3d curved surfaces. OpenCascade’s kernel forces you to deal with periodicity in topology (the shape structure), while Parasolid handles it in geometry (the math). A cylinder is mathematically continuous because there's no actual "seam" where it starts and ends. But in OpenCascade there’s a seam from 0 to 2π and this seam edge becomes a real topological entity that every algorithm has to deal with. In Parasolid the cylinder is periodic so when you query a point at U=2.1π, the kernel just knows that's equivalent to U=0.1π. The periodicity is a property of the surface math, not the shape structure. It’s not using polygons/edges/vertexes but a system of equations to calculate the surfaces. This is why boolean ops fail so often in FreeCAD: it’s asking the kernel to intersect with an artificial edge that shouldn't exist. The seam creates edge cases in intersection calculations, makes filleting near seams a nightmare, and complicates things. Parasolid's implicit handling requires smarter surface evaluation code upfront, but then everything else just works.
  - thenobsta 11mo ago
    Is there any canonical literature on this? I've been interested in what's inside the brep kernels recently.
    - throwup238 11mo ago
      Boundary Representation Modelling Techniques by Stroud is probably the most popular one. It's expensive but Anna has it in her archive.
      - thenobsta 11mo ago
        Thanks!

loftsy 11mo ago

How do the other kernels e.g. Parasolid work without seam edges? Without a seam the 2D parametric boundary is not closed.

throwup238 11mo ago
It’s not about seams in 2d but 3d curved surfaces. OpenCascade’s kernel forces you to deal with periodicity in topology (the shape structure), while Parasolid handles it in geometry (the math). A cylinder is mathematically continuous because there's no actual "seam" where it starts and ends. But in OpenCascade there’s a seam from 0 to 2π and this seam edge becomes a real topological entity that every algorithm has to deal with. In Parasolid the cylinder is periodic so when you query a point at U=2.1π, the kernel just knows that's equivalent to U=0.1π. The periodicity is a property of the surface math, not the shape structure. It’s not using polygons/edges/vertexes but a system of equations to calculate the surfaces. This is why boolean ops fail so often in FreeCAD: it’s asking the kernel to intersect with an artificial edge that shouldn't exist. The seam creates edge cases in intersection calculations, makes filleting near seams a nightmare, and complicates things. Parasolid's implicit handling requires smarter surface evaluation code upfront, but then everything else just works.
- thenobsta 11mo ago
  Is there any canonical literature on this? I've been interested in what's inside the brep kernels recently.
  - throwup238 11mo ago
    Boundary Representation Modelling Techniques by Stroud is probably the most popular one. It's expensive but Anna has it in her archive.
    - thenobsta 11mo ago
      Thanks!