  A theta surface in affine 3-space is the zero set of a Riemann theta function
in genus 3. This includes surfaces arising from special plane quartics that are
singular or reducible. Lie and Poincar\'e showed that theta surfaces are
precisely the surfaces of double translation, i.e. obtained as the Minkowski
sum of two space curves in two different ways. These curves are parametrized by
abelian integrals, so they are usually not algebraic. This paper offers a new
view on this classical topic through the lens of computation. We present
practical tools for passing between quartic curves and their theta surfaces,
and we develop the numerical algebraic geometry of degenerations of theta
functions.
