title: Depth.
abstract: Depth is the feeling of remoteness, or separateness, that accompanies awareness in human modalities like vision and audition. In specific cases depths can be graded on an ordinal scale, or even measured quantitatively on an interval scale. In the case of pictorial vision this is complicated by the fact that human observers often appear to apply mental transformations that involve depths in distinct visual directions. This implies that a comparison of empirically determined depths between observers involves pictorial space as an integral entity, whereas comparing pictorial depths as such is meaningless. We describe the formal structure of pictorial space purely in the phenomenological domain, without taking recourse to the theories of optics which properly apply to physical space-a distinct ontological domain. We introduce a number of general ways to design and implement methods of geodesy in pictorial space, and discuss some basic problems associated with such measurements. We deal mainly with conceptual issues.
authors.affiliation: University of Leuven (K.U. Leuven), Laboratory of Experimental Psychology, Tiensestraat 102-box 3711, BE-3000 Leuven, Belgium, and Delft University of Technology, EEMCS, MMI, Mekelweg 4, NL-2628 CD Delft, The Netherlands; e-mail:  j.j.koenderink@tudelft.nl.
authors.forename: Jan J
authors.initials: JJ
authors.lastname: Koenderink
authors.forename: Andrea J
authors.initials: AJ
authors.lastname: van Doorn
authors.forename: Johan
authors.initials: J
authors.lastname: Wagemans
keywords: depth; depth cues; depth scales; geometry of pictorial space; geometry of visual space; monocular stereopsis; pictorial depth; pictorial relief; pictorial vision; range; shape