Viewing distance and angle matter in the Poggendorff illusion

In a previous study (Daneyko et al, 2011 i-Perception 2 503–507) we argued that the geometrical misalignment of two parts of the long cross in the Byzantine mosaic known as Lunette of San Lorenzo (Ravenna) is related to the Poggendorff illusion, which induced the artist to prefer a perceptual adjustment over a geometrical one. However, the geometrical misalignment was greater in the original mosaic than the average alignment determined by participants to our previous experiment. In the present work we addressed this difference by projecting a silhouette rendering of the mosaic on a large screen, simulating both size and height of the saint viewed from ground level. Participants were asked to align one end of the cross to the other end from two different viewpoints: (1) from a distance of 50 cm frontoparallel to the projection, and (2) from a distance of 750 cm, 450 cm below the projection. Results from condition 1 were similar to those obtained in our previous experiment, while results from condition 2 are consistent with the geometrical misalignment of the original mosaic. Our findings support the hypothesis that the artist was aware of the misalignment illusion, noticed when he inspected the mosaic from ground level.