Wireframe torus
Geometry
There is no surface here at all — only the two families of curves that make up the torus grid. The meridians are the circles that run around the tube, and the parallels are the circles that run around the ring.
P(u,v) = ((R + r cos v) cos u, (R + r cos v) sin u, r sin v)N(u,v) = (cos v cos u, cos v sin u, sin v)
Every segment is tested for facing: if the surface normal at its ends points towards the camera the segment is drawn at full strength, otherwise it is drawn faintly. That single trick is what makes a sparse grid read as a solid object — the near lines describe the form and the far lines only hint at the back wall.
The lines are sampled more finely than the grid that names them, so each meridian and parallel stays a smooth circle, and the crossings still meet exactly. Copper on charcoal; both the ink and the background are yours to change.