The amplitude half of the same floor, meters.
A band whose octave contributes less than this contributes nothing the mesh,
the contact test or the eye can tell from zero, and evaluating it is the
clearest waste in the stack: octave amplitudes halve, so the last three
octaves of a twelve-octave band cost a quarter of it and move the ground by
less than a meter between them. octavesFor stops on whichever floor it
reaches first — this one or the wavelength.
TERRAIN_DETAIL_TOLERANCE in terrain.ts is this number: the level past
which refinement stops buying detail is the level past which the field stops
having any, and they cannot be allowed to drift apart.