A conventional assumption is that there are physiological illusions that occur naturally and cognitive illusions that can be demonstrated by specific visual tricks that say something more basic about how human perceptual systems work. The human brain constructs a world inside our head based on what it samples from the surrounding environment. However sometimes it tries to organise this information it thinks best while other times it fills in the gaps. This way in which our brain works is the basis of an illusion.
Trompe l’oeil (which by the way is French for “deceiving the eye”, pronounced “tromp loy”). It uses perspective to “trick” the eyes of the viewer, but the difference lies in the nature of the trick. For trompe l’oeil, the viewer, standing in one particular (and usually conventional) spot, is tricked into seeing an invented image as if it were reality.
The best stunning example of this technique is the fresco painting on the ceiling of the Church of Saint Ignazio in Rome, created by Andrea Pozzo during 1691-1694. A semi-circular roof is transformed into a fantastic picture of the angels, cherubims and saints ascending to the heavens, It’s as if the roof doesn’t exist and it just extends indefinitely upwards.
Church of Saint Ignazio in Rome when seen directly below
same ceiling but seen from a different angle
detailed view of the ceiling.
Another great example of an optical illusion is by a very talented artist by the name of Julian Beever. Julian uses a technique called Anamorphosis to create the illusion of three dimensional illustrations when viewed from the correct angle.
This Make Poverty History drawing was requested by Live8 to support the pressure campaign on the G8 in Edinburgh. It was done in Edinburgh City Centre.
Again this is the same painting seen from another angle.