Editing Depth perception (section)

=== Shadow stereopsis ===
Antonio Medina Puerta demonstrated that retinal images with no parallax disparity but with different shadows were fused stereoscopically, imparting depth perception to the imaged scene. He named the phenomenon "shadow stereopsis". Shadows are therefore an important, stereoscopic cue for depth perception.<ref name="Puerta">{{cite journal |author=Medina Puerta A |year=1989 |title=The power of shadows: shadow stereopsis |journal=J. Opt. Soc. Am. A |volume=6 |pages=309–311 |doi=10.1364/JOSAA.6.000309 |pmid=2926527 |issue=2|bibcode = 1989JOSAA...6..309M }}</ref>

Of these various cues, only convergence, accommodation and familiar size provide absolute distance information. All other cues are relative (as in, they can only be used to tell which objects are closer relative to others). Stereopsis is merely relative because a greater or lesser disparity for nearby objects could either mean that those objects differ more or less substantially in relative depth or that the foveated object is nearer or further away (the further away a scene is, the smaller is the retinal disparity indicating the same depth difference).