3 ms·

How do you perform functions on faces, edges, points and other topological entities with SDFs? Such operations a necessary for parametric CAD.

• traverseda 4y ago
  At the moment you don't. What I'm suggesting is that you could build up a conventional BREP-based datastructure, and than use SDF to "render" it. This gives you a very clear separation between your data structure and your meshing algorithm, and as I understand it a big chunk of those "100 man years" would be in dealing with the meshing algorithm and edge cases in it. I suspect that having a generic meshing algorithm that will always mesh your datastructure with no edge cases would significantly reduce the time needed to implement a CAD kernel. You just need an algorithm that can tell you if a given point is inside or outside of your BREP. All you really need for that is a surface that points at a given direction, and the ability to tell whether a point is under the surface or not. Define your surfaces as BREP, render them using SDF, only have to write one meshing algorithm. That being said I disagree with your definition for parametric CAD. Mostly the name, I get that you're talking about BREP based CAD. That is one way to do parametric CAD, but CSG can also work, especially if you can do more advanced operations like a filleted difference or union, or blends between two shapes, or other advanced CSG operations. I don't think that CSG-based CAD is inherently worse as long as you've got the ability to do things like fillet/chamfer CSG operations. You probably don't need to do a fillet along an edge if you can do a fillet where you subtract one object from another, something that SDFs can do easily: https://github.com/fogleman/sdf#smooth_difference https://github.com/fogleman/sdf#smooth_difference
• virtualritz 4y ago
  I would contest they are generally necessary. Applications of parametric CAD are many. Can a jeweller designing a parametric ring do without them? Probably. That said, you can e.g. calculate derivatives of the SDF and, simply speaking, when they 'flip' you know you found an edge or corner. You can project onto the SDF along a plane or line and 'record' the intersection, use it as input in your model etc. Similarly, you can treat an area bounded by such edges as a face etc. Just because there is nothing out there yet doesn't mean it can't be done. I also predict a great future for SDF based CAD for the simple reason that manufacturing methods mostly used for prototyping now (3D printing) will become more common for final pieces. And there is no need for an explicit representation (higher order surfaces or polygon meshes) of the shape to get tool paths (often derived from 2D bitmap slices) for these methods.
  • gene-h 4y ago
    One thing that probably won't change is the need for things which we design to be made of multiple parts which must be assembled together. For any serious application this requires specifying geometric dimensioning and tolerancing information, which ideally should be tied to the topological entities in question. I'd argue that this information is becoming even more important with the rise of 3d printing and other digital manufacturing tools. If this information can be included in the CAD data, the manufacturing tool might automatically determine how best to manufacture the part to meet the required tolerances. Topological information is also useful for analysis. We may wish to apply a load or boundary condition only on a face.