  Illumination estimation is often used in mixed reality to re-render a scene
from another point of view, to change the color/texture of an object, or to
insert a virtual object consistently lit into a real video or photograph.
Specifically, the estimation of a point light source is required for the
shadows cast by the inserted object to be consistent with the real scene. We
tackle the problem of illumination retrieval given an RGBD image of the scene
as an inverse problem: we aim to find the illumination that minimizes the
photometric error between the rendered image and the observation. In particular
we propose a novel differentiable renderer based on the Blinn-Phong model with
cast shadows. We compare our differentiable renderer to state-of-the-art
methods and demonstrate its robustness to an incorrect reflectance estimation.
