Fill a ring in with a film, like a soap film, and see what comes through it. Then lift the ring along each of the four axes (left and right of the middle disc): if the piercing just ends, the ring is free that way; if it runs to the rim, that is a collision. A slice can't tell you which. This can.
CAD⁴. A part is a tree of features, and the tree is the model: 1D and 2D shapes climb to 4D by product, extrude and revolve, then union, cut, intersect, move, rotate (in a plane: 4D has no axes of rotation), round, shell. You see a 3D slice: drag to look round it, pinch to zoom, the pad twists the slice into w, the lever moves it ana and kata. Where the surface leans into the unseen it is tinted teal (it grows as you go ana) or rose. The numbers are of the whole 4D part. how it works · kata