Rate functions: the easing curves animations are played through.
A rate function maps a normalized time t in [0,1] to a normalized
progress, also usually in [0,1]. Every animation context takes one as
easing, and it decides the feel of a movement without changing its
runtime or its endpoints.
smooth is the default: it eases in and out, so things start and stop gently.
identity (and its alias linear) is constant speed, which is what camera
moves and anything mechanical usually want. The standard Penner easing families
(sine, quad, cubic, quart, quint, expo, circ, back, elastic, bounce) provide
fine-grained control over acceleration and deceleration.
A rate function is just a callable on tensors, so writing your own is a
one-liner. See Combining Animations.