Near light correction for image relighting and 3D shape recovery

In this paper, we propose a near-light illumination model for image relighting and 3D shape recovery. Classic methods such as used by the popular RTI software from Cultural Heritage Imaging assume that lighting is infinitely far away from the scene. However, this constraint is impossible to achieve in practice: light sources cannot be too far away from the scene due to space and power constraints. This causes non-uniform illumination artifacts due to variations in the distance between the light source and points in the scene. We correct this effect to provide much more uniformly lit images that yield more appealing image for relighting applications. Furthermore, we use our near-light model for more accurate photometric stereo calculations of surface normals, eliminating the “potato-chip” shaped surface reconstruction error that results from violating the far-light assumption. We verify our model with both free-form capture using hand-held flash as the illumination source, and capture using LED lights mounted on a dome shaped surface.

I. Introduction

The appearance of a work of art results from the physical interaction of light with its constituent materials. To formally capture a complete record of appearance one should measure all the essential components of its light-transport function: the total interaction of the materials and microstructure comprising a work of art with all possible incoming/illuminating light rays, measured by all possible outgoing/observable light rays [1]. Embedded in the light-transport function are each fixed fraction of incident light the artwork will absorb, reflect, refract, scatter, and transmit from its surface, for all possible incident-light locations, directions, wavelengths, and polarizations. It also contains the directional fractions of incident light that will scatter beneath the object's surface and re-emerge in different directions from various neighboring points.