Doughnut
This demo renders a doughnut shape, by taking the annular (like an outline of a drawing) of the SDF of a circle.
However its intended use was as a height map, where the doughnut smoothly rises out of the background and sinks back into it.
To achieve that we use a smoothing function like this:
((cos(x * PI * 2.0) * 0.5) + 0.5) * min(step(-0.5, x), 1.0 - step(0.5, x))
Which you can see in action using GraphToy: