Comment by ge96
19 hours ago
SketchUp was my go to for a while but I'm done with parametric personally, trying to fix meshes for 3D printing... time I grow up and use something like Fusion360 seems like or onshape. Still it was good/simple enough for non-round shapes in SketchUp except they have some kind of memory leak/bug at least for the 2017 version. But I guess quit being cheap and buy a modern copy on my part.
Edit: oh I guess sketchup is a surface modeler weird thought it was parametric this whole time, lol someone else said it's a polygon modeler
Yeah I don't know what parametric modeling is apparently, I use a mouse/calipers to model stuff not parameters
There is a solid validator plugin you use before you export an STL to make sure the mesh is closed/a manifold
It's using a constructive solid geometry system. You can add and subtract volumes. Making a hole is subtracting a volume. This has classically been numeric roundoff hell, where points that should coincide and surfaces which should touch don't get handled properly due to numerical error. The geometry engine is Manifold, which guarantees watertight meshes from boolean operations.
Ah. That's what's doing the constructive solid geometry. Here's the 2009 PhD thesis behind the object merge and difference algorithms inside Manifold. Nice. At last, soundness. This is a long-standing problem. And now there's an open source implementation. Manifold itself is in C++, not Rust, though.
None of this is parametric. That's a different problem. That's where you put in constraints such as A is perpendicular to B, B is 100mm from C, etc., and the constraint solver tries to satisfy all the constraints. Change a dimension and everything adjusts to preserve the constraints. Parametric CAD is all about constraint solving and expressing conflicts to the user. Autodesk Inventor, Fusion, etc. have good constraint solvers.
[1] https://github.com/elalish/manifold/blob/master/docs/RobustB...
Manifold (which is doing the heavy lifting in this) uses triangles, which are an approximation.
This is never going to have the accuracy seen in higher-end CAD circles, which use b-Reps: highly accurate mathematic representation.
Manifold is good for visualization, but to use it as an engine for manufacturing .. its limits will be hit.
Ah. That's good to know. I wondered how they did that. I can see doing it with triangles. Smooth curves are far more difficult.
I'm not an expert at this but I've done CNC machining, used Autodesk Inventor, and coded 3D collision detection. CSG engines were considered mathematical nightmares, and I've used them, but never been inside one. The good ones can take a cylinder and a thread profile, project the thread profile along a spiral as a cutting tool, and make a bolt. Then you can chamfer the end of the bolt, and the ends of the thread are correct.
Just because you type dimensions in doesn't mean it's parametric. If you're manually patching meshes, you're almost certainly not doing parametric CAD! (This does match with my memories of SketchUp, beck when it was owned by Google. I had to fix holes and overlapping geometry by editing the .STL files in Blender before any of the primitive slicer programs could process them.)
OnShape and Fusion360 are fully parametric CAD programs. Another free-tier closed-source one is Siemens Solid Edge (the "Hobbyist" edition). FLOSS parametric CAD programs that are reasonably usable are FreeCAD (complicated but powerful) and SolveSpace (an 80/20 sort of tool -- nowhere near as powerful, but vastly easier to use).
Yeah I learned SketchUp like 17 years ago it's crazy. But I want to make more complex round shapes and ensure it is a solid (for 3D printing) so time to make the jump.
SketchUp uses direct modelling, not parametric modelling. I think they even got some patents on direct modelling.