  Multi-perspective cameras with potentially non-overlapping fields of view
have become an important exteroceptive sensing modality in a number of
applications such as intelligent vehicles, drones, and mixed reality headsets.
In this work, we challenge one of the basic assumptions made in these
scenarios, which is that the multi-camera rig is rigid. More specifically, we
are considering the problem of estimating the relative pose between a static
non-rigid rig in different spatial orientations while taking into account the
effect of gravity onto the system. The deformable physical connections between
each camera and the body center are approximated by a simple cantilever model,
and inserted into the generalized epipolar constraint. Our results lead us to
the important insight that the latent parameters of the deformation model,
meaning the gravity vector in both views, become observable. We present a
concise analysis of the observability of all variables based on noise,
outliers, and rig rigidity for two different algorithms. The first one is a
vision-only alternative, while the second one makes use of additional gravity
measurements. To conclude, we demonstrate the ability to sense gravity in a
real-world example, and discuss practical implications.
