Lighting analysis of diffusely illuminated tableaus in realist paintings
SPIE Electronic Imaging, San Jose, CA, 2009
Abstract
The problem of estimating the direction of point-source illumination in digital
photographs has been studied extensively, and the cast-shadow and
occluding-contour algorithms have been used to detect tampering and compositing;
differences between the lighting directions estimated from different objects
indicate that at least one of them was composited into the image. Such methods
have also been applied to the analysis of realist paintings to estimate the
position of illuminants within a tableau and thereby test for artists’ use of
optical aids. Recently, the occluding-contour algorithm has been enhanced to
address the case of diffuse illumination, for instance from light passing through
several windows, from multiple lamps, and so forth. Here, the pattern of
lightness along the occluding contour of an object is expressed as a weighted
sum of spherical harmonics. Significant differences between the coefficients
extracted from different objects indicates that they were recorded under different
illumination conditions, and thus that one or more was likely composited into
the image. We apply this technique to the analysis of diffuse lighting in realist
paintings, focussing on the portraits of the contemporary American realist Garth
Herrick. Herrick often works with multiple photographs as referents, for
instance a photograph of the portrait subject and a different photograph of the
background. There is no guarantee that the two lighting conditions are the same,
nor that Herrick can perceive or compensate for such lighting discrepancies when
executing his painting. We tested for lighting consistency throughout two of his
paintings: one based on a single photographic referent, and another
“composited,” i.e., based on two photographic referents. Our algorithms found
great illumination consistency in the ļ¬rst painting and significant
inconsistencies in the second paintingāinconsistencies difficult to discern by
eye. As such, our methods reveal this artist’s working methods. Our algorithms
have broad applicability to the study of studio practice throughout the history
of art.