Outline Importance of spatial perception Types of Depth cues Kinematic Stereoscopic Oculomotor Pictorial Issues in Spatial Perception Multiple sources of depth information How do multiple cues interact Depth perception supports other perceptual abilities, such as size perception Monocular Cues to Three-Dimensional Space • Metrical depth cue: A depth cue that provides quantitative information about distance in the third dimension • Nonmetrical depth cue: A depth cue that provides information about the depth order (relative depth) but not depth magnitude Oculomotor Information Oculomotor means having to do with eye Muscles Oculomotor cues • Accommodation: The process by which the eye changes its focus (in which the lens gets fatter as gaze is directed toward nearer objects) • Convergence: The ability of the two eyes to turninward, often used to focus on nearer objects • Divergence: The ability of the two eyes to turn outward, often used to f ocus on farther objects Accommodation refers to changes in the shape of the lens to achieve focused images at varying distances. Accommodation may provide distance information via unconscious sensing of the muscular movements (in the ciliary muscles) that produce the lens changes. Convergenc e refers to the turning of the two eyes to get a particula r point in the center of fixation (fovea) of each eye. Convergence provides depth information via unconscious sensing of the muscular movements used to turn the eyes.
OutlineImportance of spatial perceptionTypes of Depth cuesKinematicStereoscopicOculomotorPictorialIssues in Spatial PerceptionMultiple sources of depth informationHow do multiple cues interactDepth perception supports other perceptual abilities, such as size perception
Monocular Cues to Three-Dimensional Space• Metrical depth cue: A depth cue that provides quantitative information about distance in the thirddimension• Nonmetrical depth cue: A depth cue that provides information about the depth order (relative depth) butnot depth magnitude
Oculomotor InformationOculomotor means having to do with eyeMuscles
Oculomotor cues• Accommodation: The process by which the eye changes its focus (in which the lens gets fatter as gaze isdirected toward nearer objects)• Convergence: The ability of the two eyes to turn inward, often used to focus on nearer objects• Divergence: The ability of the two eyes to turn outward, often used to focus on farther objects
Accommodation refers to changes in the shape of the lens to achieve focused images at varying distances.
Accommodation may provide distance information via unconscious sensing of the muscular movements(in the ciliary muscles) that produce the lens changes.
Convergence refers to the turning of the two eyes to get a particular point in the center of fixation (fovea)of each eye.Convergence provides depth information via unconscious sensing of the muscular movements used toturn the eyes.
Accommodation and convergence: about 2 meters of distance.Sometimes this is called "near space.” These cues potentially provide absolute distance information.Oculomotor InformationWhat does the system need to know in order to get absolute distance from convergence?
Pictorial InformationDefinition: Pictorial refers to depth cues that can operate in flat pictures.
They are all also monocular cues, in that they can operate when you view with only one eye.Some pictorial cues were discovered by artists.Most pictorial cues relate to rules of optics and geometry that govern the projection of the world onto theretina.
Pictorial InformationDefinition: Pictorial refers to depth cues that can operate in flat pictures. They are all also monocularcues, in that they can operate when you view with only one eye.Some pictorial cues were discovered by artists.Most pictorial cues relate to rules of optics and geometry that govern the projection of the world onto theretina.Use of pictorial cues for depth perception involves using the rules of projection in reverse.
Laws of Optics: Scene RetinaInverse Optics: Retina Scene
Monocular Cues to Three-Dimensional Space• Occlusion: A cue to relative depth order in which, for example, one object obstructs the view of part ofanother object
Monocular Cues to Three-Dimensional Space• Texture gradient: A depth cue based on the geometric fact that items of the same size form smallerimages when they are farther away
Texture gradients result from a combination of the cues of relative size and relative height
Monocular Cues to Three-Dimensional Space• Relative size: A comparison of size between items without knowing the absolute size of either one
All things being equal, we assume that smaller objects are farther away from us than larger objects
Relative size is more effective when size changes systematically
Monocular Cues to Three-Dimensional Space• Relative height: Below the horizon, objects higher in the visual field appear to be farther away. Abovethe horizon, objects lower in the visual field appear to be farther away
Monocular Cues to Three-Dimensional Space• Familiar size: A cue based on knowledge of the typical size of objects
Monocular Cues to Three-Dimensional Space• Relative size and relative height both provide so me metrical information
Relative metrical depth cue: A depth cue that could specify, for example, that object A is twice as faraway as object B without providing information about the absolute distance to either A or B
Familiar size can provide precise metrical information if your visual system knows the actual size of the
object and the visual angle it takes up on the retinaAbsolute metrical depth cue: A depth cue that provides quantifiable information about distance in the
third dimension
The metrical cues of relative size and height can give the visual system more information than anonmetrical cue like occlusion can
Monocular Cues to Three-Dimensional Space• Aerial perspective: A depth cue based on the implicit understanding that light is scattered by theatmosphere
More light is scattered when we look through more atmosphereThus, more distant objects appear fainter, bluer, and less distinct
Aerial perspective
Monocular Cues to Three-Dimensional Space• Linear perspective: Lines t hat are parallel in the three-dimensional world will appear to converge ina two-dimensional image as they extend into the distance• Vanishing point: The apparent point at which parallel lines receding in depth converge
Monocular Cues to Three-Dimensional Space• Pictorial depth cue: A cue to distance or depth used by artists to depict three-dimensional depth intwodimensional pictures• Anamorphosis (or anamorphic projection): Use of the rules of linear perspective to create atwodimensional image so distorted that it looks correct only when viewed from a special angle or with amirror that counters the distortion
Different Cues Provide Different Kinds of Information• Convergence • Accommodation • Occlusion • Familiar size • Motion parallax • Relative size• Relative height • Binocular disparity
Multiple Sources of Information: Why?Some provide information about absolute position, whereas others provide information about the relationsof objects and surfaces.(Distance vs. Depth)Different sources of information have different operating conditions.Differences in ecological validity among the different cues.Some evidence suggests that the system relies on the cues that provide the best evidence in general orunder specific conditions.
Ecological validity refers to how accurately a cue specifies some situation in the environment.Roughly speaking, one can get at ecological validity of depth cues by considering how hard it would be toarrange a situation that depicts depth according to the cue, but does not really have depth in the world.Example: A TV show depicts 3-D environments, but the screen is actually flat.
Of the 4 categories of depth / distance information, stereoscopic and kinematic have highest ecologicalvalidity and pictorial has the weakest.
Why do we perceive scene A and not the others?• We “know” from experience that all pennies are the same size.• C would correspond to an accidental view, whereas the A and B correspond to the generic views.
Why do we perceive scene A and not the others?• Therefore, our brain infers scene A because it is more probable.• This is what Al -Haytham/Alhazen (11thCentury) called “unnoticed judgment.” • And von Helmholtz (19th Century) called “unconscious inference.”
How is the inference made?• The unconscious inference can be achieved as follows:• The nervous system calculates the probability of each scene given the sensory evidence, and the priorknowledge, and chooses the scene that has the highest probability.
Combining Depth Cues• The Bayesian Approach, Revisited • Like object recognition, depth perception results from the combination of many different cues• The Bayesian approach: A way of fo rmalizing the idea that our perception is a combination of thecurrent stimulus and our knowledge about the conditions of the world — what is and is not likely to occur
Thus, prior knowledge can influence our estimates of the probability of an event
• Ideal observer: A theoretical observer with complete access to the best available information and theability to combine different sources of information in the optimal manner
It can be useful to compare human performance to that of an ideal observer
Cue Integration
Cue integration with prior
Combining Depth Cues• Illusions and the construction of space
Our visual systems take into account depth cues when interpreting the size of objects
