  Circular markers are planar markers which offer great performances for
detection and pose estimation. For an uncalibrated camera with an unknown focal
length, at least the images of at least two coplanar circles are generally
required to recover their poses. Unfortunately, detecting more than one ellipse
in the image must be tricky and time-consuming, especially regarding concentric
circles. On the other hand, when the camera is calibrated, one circle suffices
but the solution is twofold and can hardly be disambiguated. Our contribution
is to put beyond this limit by dealing with the uncalibrated case of a camera
seeing one circle and discussing how to remove the ambiguity. We propose a new
problem formulation that enables to show how to detect geometric configurations
in which the ambiguity can be removed. Furthermore, we introduce the notion of
default camera intrinsics and show, using intensive empirical works, the
surprising observation that very approximate calibration can lead to accurate
circle pose estimation.
