  Tactile sensing is a necessary capability for a robotic hand to perform fine
manipulations and interact with the environment. Optical sensors are a
promising solution for high-resolution contact estimation. Nevertheless, they
are usually not easy to fabricate and require individual calibration in order
to acquire sufficient accuracy. In this letter, we propose AllSight, an optical
tactile sensor with a round 3D structure potentially designed for robotic
in-hand manipulation tasks. AllSight is mostly 3D printed making it low-cost,
modular, durable and in the size of a human thumb while with a large contact
surface. We show the ability of AllSight to learn and estimate a full contact
state, i.e., contact position, forces and torsion. With that, an experimental
benchmark between various configurations of illumination and contact elastomers
are provided. Furthermore, the robust design of AllSight provides it with a
unique zero-shot capability such that a practitioner can fabricate the
open-source design and have a ready-to-use state estimation model. A set of
experiments demonstrates the accurate state estimation performance of AllSight.
