"""Surface z = f(x, y): a ripple that flexes into a saddle and a pair of hills.
The sheet is coloured by height, cool at the bottom and warm at the top, so
the shape reads from the colour alone before the mesh or the light say
anything. The light only nudges each face toward the ground by how far it
turns away, enough to model the ripple's rings without muddying the hues.
The loop morphs between three functions by blending their heights, which is
why each one is kept inside the same band: a blend of two surfaces that stay
between -RELIEF and RELIEF stays there too, so one colour scale serves the
whole loop. The band is also what keeps the sheet from showing its underside:
seen from a little above, a finite square that dips far below its own near
edge shows the back of that edge.
The catalog's `fn` toggle picks the function, which is a different body
rather than a layer, so it ships no PNG.
The full Manim source and the rendered video come with a plan.
