  We present a novel method for extrinsically calibrating a camera and a 2D
Laser Rangefinder (LRF) whose beams are invisible from the camera image. We
show that point-to-plane constraints from a single observation of a V-shaped
calibration pattern composed of two non-coplanar triangles suffice to uniquely
constrain the relative pose between two sensors. Next, we present an approach
to obtain analytical solutions using point-to-plane constraints from single or
multiple observations. Along the way, we also show that previous solutions, in
contrast to our method, have inherent ambiguities and therefore must rely on a
good initial estimate. Real and synthetic experiments validate our method and
show that it achieves better accuracy than previous methods.
