A particular image on the retina can be created by many different objects, this is called the inverse projection problem. If we know the objects shape, distance, orientation, we can determine the shape of the objects image on the retina. However a particular image on the retina can be created by an infinite number of objects. Information from a single view can be ambiguous; a bunch of rocks can look circular when they are farther away but not up close. Humans solve the problem of an object being ambiguous by moving to different viewpoints, and by making use of knowledge they have gained from past experiences in perceiving objects. The problem of hidden objects occurs any time one object obscures part of another object. People are able to recognize objects that are not in sharp focus. Images of objects are continually changing because they are often viewed from different angles.