Practical Free-form RTI Acquisition with Local Spot Lights

We present an automated light calibration pipeline for free-form acquisition of shape and reflectance of objects using common
off-the-shelf illuminators, such as LED lights, that can be placed arbitrarily close to the objects. We acquire multiple digital
photographs of the studied object shot from a stationary camera. In each photograph, a light is freely positioned around the
object in order to cover a wide variety of illumination directions. While common free-form acquisition approaches are based
on the simplifying assumptions that the light sources are either sufficiently far from the object that all incoming light can be
modeled using parallel rays, or that lights are local points emitting uniformly in space, we use the more realistic model of
a scene lit by a moving local spot light with exponential fall-off depending on the cosine of the angle between the spot light
optical axis and the illumination direction, raised to the power of the spot exponent. We recover all spot light parameters
using a multipass numerical method. First, light positions are determined using standard methods used in photometric stereo
approaches. Then, we exploit measures taken on a Lambertian reference planar object to recover the spot light exponent and the
per-image spot light optical axis; we minimize the difference between the observed reflectance and the reflectance synthesized
by using the near-field Lambertian equation. The optimization is performed in two passes, first generating a starting solution
and then refining it using a Levenberg-Marquardt iterative minimizer. We demonstrate the effectiveness of the method based on
an error analysis performed on analytical datasets, as well as on real-world experiments.