There are a variety of other sources of depth information as
well as stereopsis, which we'll just mention in passing
here. The first, which is really more used to find surface orientation
rather than depth, is to use texture (ie, repeated regular surface markings),
as illustrated in the (not very
good) figure below:
The left figure is meant to illustrate how a sudden change in texture may indicate a sudden change in orientation. The middle illustrates how texture can indicate how one object is in front of the other, and the right figure illustrates (not terribly well) how change in texture can suggest that an object is at an angle, with part of it receding into the distance.
The second source of depth/orientation information comes from motion.
If we are moving towards an object then we can work how far it
is away using some simple geometry as we did for stereopsis. If it's at
an angle at one point, then when we have moved a distance
d it is at an angle
then we can use elementary
geometry to work out the distance of the object away from us.