The assumed light direction for perceiving shape from shading
James P. O'Shea, Martin S. Banks, Maneesh Agrawala
Recovering 3D shape from shading is an ill-posed problem that the
visual system can solve only by making use of additional information
such as the position of the light source. Previous research has
shown that people tend to assume light is above and slightly to the
left of the object [Sun and Perona 1998]. We present a study to investigate
whether the visual system also assumes the angle between
the light direction and the viewing direction. We conducted a shape
perception experiment in which subjects estimated surface orientation
on smooth, virtual 3D shapes displayed monocularly using
local Lambertian shading without cast shadows. We varied the angle
between the viewing direction and the light direction within a
range +/- 66 deg (above/below), and subjects indicated local surface
orientation by rotating a gauge figure to appear normal to the
surface [Koenderink et al. 1992]. Observer settings were more accurate
and precise when the light was positioned above rather than
below the viewpoint. Additionally, errors were minimized when
the angle between the light direction and the viewing direction was
20-30 deg. Measurements of surface slant and tilt error support this
result. These findings confirm the light-from-above prior and provide
evidence that the angle between the viewing direction and the
light direction is assumed to be 20-30 deg above the viewpoint.
The shading of an object depends on the light source direction. The light is directed from the viewpoint in the left image,
from 22 deg above the viewpoint in the middle image, and from 44 deg above the viewpoint in the right image. The object is positioned
identically in each of the three views. In this paper, we present an experiment designed to test how shape perception is affected by changing
the angle of the light direction. We found the lighting used in the center image led to the most accurate estimations of 3D shape.
The assumed light direction for perceiving shape from shading
James P. O'Shea, Martin S. Banks, Maneesh Agrawala
ACM Symposium on Applied Perception in Graphics and Visualization (APGV) 2008
August 2008, 135-142.