The Perceptual Organization of Depth

Introduction The goal of depth perception is to identify the spatial layout of the objects and surfaces that constitute our surroundings. One important observation about the world around us that influences the way we see depth is that physical matter is not distributed randomly, with arbitrary depths at every location. On the contrary, the environment is generally organized: the world consists mainly of tightly bound objects in a discernable layout. This order results from countless forces and processes in our world which tend to organize matter into objects and place those objects in certain spatial relations. The central thesis of this chapter is that our perception of depth mirrors this organization. We argue that because the world consists of objects and surfaces, our perception of depth should likewise be represented in terms of the functionally valuable units of the environment, namely surfaces and objects. As we shall see, this has profound consequences for the processing of depth information. In particular, there is more to depth perception than simply measuring the distance from the observer of every location in the visual field. Rather, the perception of depth is the active organization of depth estimates into meaningful bodies. Depth constrains the formation of perceptual units, and, reciprocally, the figural relations between depth measurements allow the visual system to parse its representation of depth into ecologically valuable structures. There are many sources of information about depth from 'pictorial' perspective to motion parallax. An exhaustive review of all these sources of information is beyond the scope of this chapter (although see Bruce et al., 1996 and Palmer, 1999 for introductory reviews). Instead, we discuss three key domains in which the visual system " organizes " Roland Fleming & Bart Anderson: Perceptual Organization of Depth 3 our perception of depth into meaningful units, to emphasise the intimate relationship between depth processing and perceptual unit formation. In the first section we discuss how the visual system infers the layout of surfaces from local measurements of depth. We will argue that local estimates of depth are ambiguous, but that the geometry of occlusion critically constrains the legal interpretations. Occlusion occurs when one opaque object partly obscures the view of a more distant object, as happens frequently under normal viewing conditions. Occlusion is important because it occurs at object boundaries, and therefore the depth discontinuities introduced by occlusion provide ideal locations for the segmentation of depth into objects. …