That depends on what we mean by surfaces, and in the case of filleting, the user really wants to be picking adjacent faces (as in: an edge between two adjacent faces). That, or even a region to roll a ball along to generate a fillet.
The semantics of fillets even in the simplest case is that it's doing something to the edges, i.e. elements of the boundary representation, so that's a more natural structure for filleting.
>The distance field tells you the distance to the nearest surface at any point.
What you're describing isn't the same. You really are picking solids, not faces.
This wouldn't work even in the simplest case of a cube.
You can define a cube by a distance field:
f(x, y, z) = max(|x|, |y|, |z|) - 1
The field doesn't know anything about the edges.
OK, OK, we can ignore this edge case (badum-tss), but even if you only allow filleting "two surfaces", those two "surfaces" (really: boundaries of solids) aren't necessarily going to intersect along one edge (which is what the user wants to fillet).
The intersection may have multiple components. Or may not be manifold.
As a concrete example:
f(x, y, z) = z - cos(x)
g(x, y, z) = z - cos(y)
f(x, y, z) = z - cos(x)cos(y)
g(x, y, z) = z - 0.25
B-reps can get away with making no distinction between them... Until you have messy geometry from elsewhere.
Say, an STL file from a scan. Or a mesh that came from an F-rep by marching cubes. Or whatever unholy mess OpenSCAD can cook with CGAL.
It doesn't matter if you use F-rep or convert to one: chisel out a cube as an intersection of half-spaces, then cut with half-spaces that narrowly touch the edges.
It'll look like a cube, and it'll be a cube functionally if you manufacture it.
Good luck with that one.
>If you have good ideas for this I'd love to hear them and resume working on Isoform
Well. The good news is that putting fillets on every edge is kind of easy with fields because you can do it with offsets.
If F(x, y, z) is a distance field that defines a solid, G(x, y, z) = F(x, y, z) + c offsets F inwards by c.
G is not a distance field anymore though, it's giving values that arent distances on the outside of convex corners.
Renormalize G to be a distance field, call it G'.
Now offset G' outwards by c: H = G' - c.
Ta-da! Concave corners aren't touched, convex corners are rounded.
Flip the + and -, and you're filleting concave corners (G = F - c is a field that defines an outwards offset that fails to be a distance field inside the body near concave corners; compute G' — the distance field for G; offset G inwards: H = G' + c).
Now, the "just normalize a field into a distance field" is doing a lot of heavy lifting here.
But that can give you something to think about.