Some of the most powerful applications for AR will coming from the field of medicine. The ability to overlay g ...
AR / VR Science Note 003: Spatial Vision and Depth Cues
The human visual system is a remarkable sensory and interpretation mechanism capable of a high dynamic range of performance. Understanding the actual mechanics of human sight and the various cues used by our visual system to perceive depth can contribute significantly to the actual design of virtual and augmenting content.
The ability of humans to visually perceive depth in real and virtual environments is enabled, in part, by the brain’s interpretation of light patterns falling on our retinas (on-retina cues), along with nerve signals from other physiological processes (extra-retinal cues).
On Retina Cues
On-retina depth cues are those triggers or pieces of information that are derived from light patterns entering the eye and bathing the retina. On-retina cues can be divided between binocular and monocular cues.
Binocular depth cues are those triggers or pieces of information that are detected as a result of viewing a scene with two eyes, each from a slightly different vantage point. These two scenes are integrated by our brain to construct a 3D interpretation of our real or virtual surroundings. A perfect example is stereopsis, which is the perception of depth constructed by the brain based on the differences between these two retinal images.
Monocular depth cues are those triggers or pieces of information that are derived from light patterns on the retinas but are not dependent on both eyes. Monocular cues can be divided between those requiring movement of light patterns across the retina (that is, viewer motion) and those that can be discerned from a fixed viewing position. There are 12 primary monocular depth cues about which virtual and augmented reality content designers should be intimately familiar. All 12 are covered in considerable detail in Practical Augmented Reality:
- Motion Parallax
- Deletion and Accretion
- Linear Perspective
- Kinetic Depth Effect
- Familiar Size
- Relative Size
- Aerial Perspective
- Texture Gradient
- Optical Expansion
- Relative Height
Extraretinal depth cues are those triggers or pieces of information that are not derived from light patterns entering the eye and bathing the retina, but from other physiological processes. The two primary extraretinal cues are accommodation and vergence.
Vergence is the pointing of the fovea of both eyes at an object in the near field. This process entails the eyes simultaneously rotating about their vertical axis in opposite directions to the degree necessary so that when looking at a nearby object, the projected image of that object is aligned with the center of the retina of both eyes. When looking at an object in the near field, the eyes rotate toward each other, or converge. When looking at an object in a far field, the eyes rotate away from each other, or diverge.
Accommodation is an involuntary physiological process by which the optical power of the eye lens changes to focus light entering the eye and falling on the retina. It is widely believed that blurring on the retina is the stimulus for accommodation, although the process is also strongly linked to vergence (Leigh and Zee, 2015, 524). It is also theorized that movement of the ciliary muscles themselves contributes to this cue (Helmholtz et al., 1944).
Leigh, R. John, and David S. Zee. The Neurology of Eye Movements. Oxford University Press, USA, 2015.
Helmholtz, H. von. “Physiological Optics.“ Optical Society of America 3 (1925): 318.
This AR / VR Science Note is based on content drawn from the book Practical Augmented Reality: A Guide to the Technologies, Applications and Human Factors for AR and VR (Pearson / Addison Wesley Professional, Fall, 2016).
Reproduced by the author with permission from Pearson © 2017.
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