Near-light photometric stereo using circularly placed point light sources

Most photometric stereo approaches assume distant or directional lighting and orthographic imaging. However, when the source is divergent and is near the object and the camera is projective, the image intensity of a Lambertian object is a non-linear function of both the unknown surface normals and the unknown distances of the source to the surface points. The resulting non-linear optimization is non-convex and highly sensitive to the initial guess. In this paper, we propose a two-stage near-light photometric stereo method using circularly placed point light sources (commonly seen in recent consumer imaging devices like NESTcam, Amazon Cloudcam, etc). We represent the scene using a 3D mesh and directly optimize the vertices of the mesh. This reduces the complexity of the relationship between surface normals and depths in the image formation model. In the first stage, we optimize the vertex positions using the differential images induced by small changes in light source position. This procedure yields a strong initial guess for the second stage that refines the estimations using the raw captured images. We propose an accurate calibration approach to estimate the positions of the sources. Our approach performs better on simulations and on real Lambertian scenes with complex shapes than the state-of-the-art method with near-field lighting.