  We present an algorithmic way of exactly computing Belyi functions for
hypermaps and triangulations in genus 0 or 1, and the associated dessins, based
on a numerical iterative approach initialized from a circle packing combined
with subsequent lattice reduction. The main advantage compared to previous
methods is that it is applicable to much larger graphs; we use very little
algebraic geometry, and aim for this paper to be as self-contained as possible.
