Capturing Transient Subsurface Scattering with An Ordinary Camera
Ko Nishino, Art Subpa-asa, Yuta Asano, Mihoko Shimano, and Imari Sato
Kyoto University, Tokyo Institute of Technology, and National Institute of Informatics
Subsurface scattering plays a significant role in determining the appearance of real-world surfaces. A light ray penetrating into the subsurface is repeatedly scattered and absorbed by particles along its path before reemerging from the outer interface, which determines its spectral radiance. We introduce a novel imaging method that enables the decomposition of the appearance of a fronto-parallel real-world surface into images of light with bounded path lengths, i.e., transient subsurface light transport. Our key idea is to observe each surface point under a variable ring light: a circular illumination pattern of increasingly larger radius centered on it. We show that the path length of light captured in each of these observations is naturally lower-bounded by the ring light radius. By taking the difference of ring light images of incrementally larger radii, we compute transient images that encode light with bounded path lengths. Experimental results on synthetic and complex real-world surfaces demonstrate that the recovered transient images reveal the subsurface structure of general translucent inhomogeneous surfaces. We further show that their differences reveal the surface colors at different surface depths. The proposed method is the first to enable the unveiling of dense and continuous subsurface structures from steady-state external appearance using ordinary camera and illumination.
Variable Ring Light Imaging Capturing Transient Subsurface Scattering with An Ordinary Camera
K. Nishino, A. Subpa-asa, Y. Asano, M. Shimano, and I. Sato,
in Proc. of European Conference on Computer Vision ECCV’18, Sep., 2018.
[ paper ][ supp. material ][ poster ][ project ]
Overview
We introduce the first method for subsurface scattering decomposition, the recovery of path length (and proportionally bounce) dependent light that make up the appearance of real-world translucent surfaces. Our key contribution lies in the use of a natural lower bound on the shortest path length defined by the external interface of a surface.
When lit and viewed orthographically from the top, the surface defines a halfspace and the shortest path length of light incident at a single surface point, referred to as impulse illumination, observed at distance r from the point of incidence would naturally be r.
We exploit this lower bound on light path length by observing the same surface point while varying the distance of an impulse illumination. Instead of using impulse illuminations at an arbitrary point, we use a radius-r ring light around each surface point, i.e., all surface points at surface distance r illuminated with impulse illuminations. We obtain variable ring light images, each of which encodes the appearance of each surface point captured with ring lights of varying radii. We show that taking the difference of two ring light images of slightly different radii bounds the light path length observed at each surface point. In other words, from steady-state appearance captured as variable ring light images, we compute surface appearance with varying degrees of subsurface scattering. i.e., transient images of subsurface light transport. We further show that the differences of the transient images reveal the colors of the surface at different depths.
Our method is simple requiring only a regular projector and camera. Most important, it is model-free, making no restrictive assumptions about the subsurface light transport. To our knowledge, the proposed method is the first to enable the dense recovery of the continuous variation of inner surface structures from external observation of the steady-state appearance using ordinary imaging components. Variable ring light imaging complements past transient imaging methods especially those for bounce decomposition of scene-wide global light transport, by providing an unprecedented means for deciphering subsurface scattering and consequently for visually probing intrinsic surface structures. We believe the method opens new avenues for richer radiometric understanding of real-world scenes and materials.
Results
For spatially inhomogeneous subsurface structures, the recovered transient images capture interesting spatial propagation of light as the corresponding ring light radius increases.
Transient images of complex surfaces reveal the subsurface composition both in its volumetric structure as well as its color variation that are otherwise invisible from the outer surface.
When the surface has subsurface structures that vary across its depth, the bounded path length light in each recovered transient depth directly encodes the accumulated color along its path. We may recover the true color at each corresponding surface depth by taking the difference of the transient images (i.e., the difference of the difference of captured variable ring light images). Note that these colors cannot be directly observed from the outer surface.
Variable ring light imaging may be used to reveal the complex intrinsic surface structures of natural objects including food and human skin.
For high resolution image capture of larger surface regions, we can speed up the imaging with parallel impulse illumination capture.
