Chapter 6 Perceiving Depth Pg. 189 Flashcards
Accommdation
Adjust lens to focus an image on retina
- Ciliary muscle relax -> flat lens -> look farther
- Ciliary muscle contract -> rounded lens -> look closer
- Rapid and involuntary
Convergence
- Angie formed between lines of gaze of the two eyes decreases as distance increases
- Only about 2m depth cue in front of eyes
Monocular depth cues
Cues that are based on retinal image and that provide information about depth even with only one eye open
*Pictorial cues: static monocular cues, motionless 2D
Static monocular depth cues
Cues that provide info about depth on basis of position of objects in retinal image, the size of retinal image, and the effects of lighting in retinal image
Partial occlusion (interposition)
where one object partially hides (occludes) another
- T-junctions: intersections between 2 edges of 2 objects
- Respect each other, simple, natural
Relative height
Position-based depth cue- the relative height of the objects in the retinal image with respect to the horizon -or with respect to eye level if there is o visible horizon-provides info about the objects’ relative distance from the observer
- Below horizon/eye level-> object lower in image closer to observer
- Above horizon/eye level-> object higher in image are closer to observer
Size distance relation
Farther away an object is from the observer, the smaller is its retinal image
Visual angle
The angle subtended by an object in the field of view
Size perspective
Depth cue in scenes in which the size-distance relation is apparent
*Decrease in retinal image size of objects-> increase distance from observe
Familiar size
a size-based depth cue- knowing the retinal image size of a familiar object at a familiar distance let us use its retinal image size to gauge its distance.
Relative size
A size-based depth cue-under the assumption that two or more objects are about the same size, the relative size of their retinal images can be used to judge their relative distances
Texture gradient
A size-based depth cue- if surface variations/repeated elements of a surface are fairly regular in size and spacing, the retinal image size of these equal-size features decreases as their distance increases
Linear perspective
Size-based depth cue-parallel lines appear to converge as they recede in depth
*Fixed distance projects a smaller and smaller retinal image as it recedes from the observer
Atmosphere perspective
A lighting-based depth cue- the farther away an object is, the more air the light must pass through to reach us and the more that light can be scattered, with the result that distant objects appear less distinct than nearby objects
Shading
Relative depth and orientation
Cast shadows
Shadow cast by objects -> depth
Motion parallax
Dynamic depth cue- the difference in the speed and direction with which objects appear to move in the retinal image as an observer moves within a scene
- The farther an object is from fixation point, the farther and faster will be its relative motion across the scene in retinal image
- Object closer than the fixation point will move in a direction opposite to the observer’s direction of motion
- Object father than fixation point will move in same direction as the observer’s direction of motion
Optic flow
a dynamic depth cue- the relative motions of objects and surfaces in the retinal image as the observer moves forward or backward through a scene
Deletion
Dynamic depth cue-gradual hiding (occlusion) of an object as it passes behind another one
Accretion
Dynamic depth cue- gradual revealing (“de-occlusion”) of an object as it emerges from behind another one
Stereopsis (Stereoscopic depth perception)
Vivid sense of depth arising from visual system’s processing of different retinal images in the two eyes
Binocular disparity
Depth cue based on differences in the relative positions of the retinal images of objects in 2 eyes
Corresponding points
A point on the left retina and a point on the right retina what would coincide if the two retina were superimposed (ex: fovea)
Noncorresponding points
Point on left retina and point on right retina that wouldn’t coincide if two retinas were superimposed
Horopter
Imaginery surface defined by locations in a scene from which objects would project retinal images at corresponding points
Crossed disparity
Type of binocular disparity produced by an object that is closer than the horopter-you would have to “cross” your eyes to look at it
- Right-left, left-right
- Increase angle of convergence -> fixating closer object
Uncrossed disparity
An object that is farther away than horopter -> “uncross” eyes
- Left-left, right-right
- Decrease angle of convergence
Zero disparity
Type of binocular disparity in which the retinal image of an object falls at corresponding points in the two eyes
- Objects on horopter always produce retinal images at corresponding points -> Zero disparity
- The distance between the images of an object in 2 views (magnitude of binocular disparity) increase -> distance of object form horopter increase
Correspondence problem
The problem of determining which features in the retinal image in one eye correspond to which features in the retinal image in the other eye
- 1st hypothesis: visual system surveys left and right retinal images -> separately performs 2D -> “labels” each feature of each retinal image
- 2nd hypothesis: matches parts of retinal images based on very simple properties
Stereogram
Two depictions of a scene that differ in the same way as an observer’s two retinal images of that scene would differ; an observer who simultaneously views one image with one eye and the other image with the other eye (as in a stereoscope) will see a combined image in depth
Anaglyph
A stereogram in which the two photographs taken from adjacent camera positions are printed in contrasting colors and then superimposed; an observer who views an anaglyph with special glasses in which one lens filters out one of the colors and the other lens filters out the other color will see a single image in dpeth
Random dot stereogram (RDS)
A stereogram in which both images consist of a grid of randomly arranged black and white dots, identical except for the displacement of a portion in one image relative to the other; an observer who views a random dot stereogram in a stereoscope or as a anaglyph will see a single image with the displaced portion in depth
- Consists of two images that exhibit binocular disparity when viewed through a stereoscope
- Correspondence matching is necessary for the perception of binocular disparity
- If object recognition necessarily precedes correspondence matching, an RDS wouldn’t produce a sense of depth, because an RDS doesn’t contain any objects; it’s just a random array of square dots/ tiny scribbles, non of which can be separately labeled in either of 2 images
- But RDS do produce a sense of depth. Therefore correspondence matching must precede object recognition
Oculomotor depth cues
Right in front of eyes to about 2m away; cues that are based on feedback from the oculomotor muscles controlling the shape of the lens and the position of the eyes
Opposed Random Dot Stereogram
Marr and Poggio
- Each feature in one retinal image will match one and only one feature in the other retinal image
- Visual scenes tend to consist of smooth and continuous surfaces with relative few abrupt changes in depth
Binocular cells
Neurons that respond best to the stimulation of their receptive fields in both eyes simultaneously
- Particular binocular cell responds when its receptive fields are stimulated by an object that exhibits a particular degree of binocular disparity, specified by locations of receptive fields on left and right retinal
- Different binocular cells are tuned to different disparities
- V1, V2, V3, MT, intraparietal sulcus
- What, where, how
Perceptual constancy
Whenever you perceive some property of an object as constant despite changes in sensory info used to perceive that property
Size constancy
Tendency to perceive an object’s size as constant despite changes in the size o the object’s retinal image due to the object’s changing distance from the oberver
Size-distance invariance
The relation between perceived size and perceived distance
- The perceived size of an object depends on its perceived distance an vice versa
- Perceived size (P)-retinal image size (R) X perceived distance (D)
Emmert’s law
Size-distance invariance of retinal afterimages- the perceived size of an afterimage is proportional to the distance of the surface on which its “projected”
*The perceived size of a fixed retinal afterimage is proportional to the distance of surface on which it is seen
The retinal image shape also depends on 2 factors
Objects’ actual shape and the objects’ slant-its orientation relative to the observers’ line of sight
Shape constancy
Tendency to perceive an object’s shape as constant despite changes in the shape of the object’s retinal image due to the object’s changing orientation
Shape-slant invariance
The relation between perceived shape and perceived slant
*The perceived shape of an object depends on its perceived slant, and vice versa
Ponzo Illusion
Size-distance invariance
*Linear perspective on size perception
Ames room
A room specially designed to create an illusory perception of depth;
*When viewed with one eye through a peephole, all of the room’s trapezoidal surfaces look rectangular
Moon Illusion
The perceived distance to the moon is greater when it’s on the horizon
- We perceive the moon to be at the height of the clouds
- Apparent-distance theory: horizon moon is surrounded by depth cues while moon higher in the sky has none; horizon is perceived as further away than the sky-“flattened heavens”
- Angular size contrast theory: compared to large open sky overhead, moon looks smaller; compared to small features on horizon, moon looks larger
