Most of the facts about the 
Poggendorff illusion, however, 
can be explained by the depth-
processing theory, as Barbara 
Gillam has ingeniously argued. 
She believes that this illusion, 
like the Ponzo illusion, arises 
from the tendency of the 
perceptual system to process 
the figure as a representation 
of a three-dimensional scene, 
not as a pattern in two 
dimensions. The figure to the 
left makes this process 
abundantly clear. The oblique 
lines are processed as edges in a 
horizontal plane. The angles 
formed between the vertical 
and oblique lines are 
interpreted as right angles. 
According to this theory, all 
angles are misperceived: Acute 
angles are overestimated and 
obtuse angles are 
underestimated. But 
misperception of angle has a 
very different meaning here: 
The misperception is one of 
perspective representation, not 
of two-dimensional distortion. 
In fact, except for the special 
case of contours in a frontal 
plane, right angles in the 
environment will project to the 
eye as either acute or obtuse 
angles. Thus, for example, 
rectangular objects project 
trapezoidal images, none of the 
angles of which are right 
angles. This is one of the major 
distortions that occurs in the 
projection of objects in the 
world to the eye. The reason 
that the oblique lines in the 
Poggendorff figure appear 
misaligned is that they seem to 
belong to two different planes 
(i.e., planes at different 
heights in the scene).