Usually, an observer is able to distinguish between a real geometrical entity and a pictorial representation of it. However, when the distance between the observer and the picture surface is very large, the human eye can be tricked into believing that aspects of the picture are real geometrical entities. A famous example is the ceiling in the church of St. Ignazio in Rome. Painted on the ceiling is a very convincing scene of St. Ignatius being received into Heaven, the masterpiece of Fra Andrea Pozzo (1642 – 1709).
The ceiling is hemicylindrical in shape and is about 30 metres off the floor. When one is at the correct viewing location (Fig. 39), the painted architecture appears in three dimensions as an extension of the real architecture. It is impossible to determine where the ceiling surface actually is, much less distinguish the painted architecture from the real ones.
Fig. 39 – Pozzo’s painting on the
ceiling in the church of St. Ignazio.
However, when one observes the ceiling well away from the correct viewing location, it becomes possible to detect the hemicylindrical nature of the ceiling (Fig. 40).
Fig. 40 – Pozzo’s ceiling away from the correct viewing location.
Notice that the painted columns in Fig. 40 appear deformed. It is interesting to note that although deformations appear when an observer walks away from the correct viewing location, the illusion of depth remains. This is because the distance between the observer and the ceiling remains high, and the eye is unable to determine the picture surface. When the ceiling is observed from a point 20 metres above the floor, the illusion of depth disappears (Fig. 41). The painted ceiling now appears hemicylindrical, as it should be, and the painted columns look severely deformed.
Fig. 41 – Pozzo’s ceiling as seen
from a point 20 metres above the floor.