Lighting analysis of diffusely illuminated tableaus in realist paintings: an application to detecting "compositing" in the portraits of Garth Herrick

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 first painting and significant inconsistencies in the second painting—inconsistencies 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.