  When capturing panoramas, people tend to align their cameras with the
vertical axis, i.e., the direction of gravity. Moreover, modern devices, such
as smartphones and tablets, are equipped with an IMU (Inertial Measurement
Unit) that can measure the gravity vector accurately. Using this prior, the
y-axes of the cameras can be aligned or assumed to be already aligned, reducing
their relative orientation to 1-DOF (degree of freedom). Exploiting this
assumption, we propose new minimal solutions to panoramic image stitching of
images taken by cameras with coinciding optical centers, i.e., undergoing pure
rotation. We consider four practical camera configurations, assuming unknown
fixed or varying focal length with or without radial distortion. The solvers
are tested both on synthetic scenes and on more than 500k real image pairs from
the Sun360 dataset and from scenes captured by us using two smartphones
equipped with IMUs. It is shown, that they outperform the state-of-the-art both
in terms of accuracy and processing time.
