Other Spatial Constancies
Shape constancy results if the 
observer correctly takes into 
account the slant of the 
object’s surface.
 
The taking-into-account 
theory also offers the best 
explanation of other spatial 
constancies, such as constancy 
of shape and orientation. In 
the case of our tendency to see 
the shapes of things as 
constant, the relevant factor 
taken into account is the slant 
of the object. If a rectangle is 
slanted away from the frontal 
plane, its retinal image is 
trapezoidal. If there is 
information about the angle 
slant, the perceptual system 
can take this into account to 
compute that the shape of the 
object is rectangular.

    The taking-into-account 
process is not as simple here as 
in the case of size because it 
includes computations based on 
the differential distances of 
parts of the object. For example, 
if, because the slant is 
accurately perceived, one part 
is seen correctly as farther 
away, the size of that part can 
be veridically perceived as a 
result of size constancy. That 
part yields a smaller visual 
angle than closer parts do, but, 
if it is seen to be the same size 
as the near part of the object, 
the perceived shape would have 
to be rectangular rather than 
trapezoidal. Note that this 
analysis suggests that shape 
constancy can be derived from 
size constancy.