CH7 - Depth Perception (aka how we are able to extract dimension)

What does Kant’s Critique of pure reason tell us abt how we form depth perception?

• Space is an a priori form – our sensory info deosn’t directly tell us anything abt depth – our mind already knows it exists and we contribute it to our perception of the world (top-down) 

 

• Our mind already knows that space exists before receiving sensory information. 

• It uses this a priori knowledge to interpret sensory input. 

Monocular cues

depth cues that you don’t need two eyes for  

Binocular cues

depth cues that require two eyes

Pictorial Cues 

Depth cues that can be inferred from a static image using monocular vision. 

These cues help the brain interpret depth in two-dimensional scenes. 

Examples include: 

• Occlusion 

• Relative size 

• Familiar size 

• Linear perspective 

• Texture gradient 

• Aerial perspective 

• Shading 

Pictorial Cue: Occlusion

• A depth cue where one object partially blocks another, indicating that the blocked object is farther away. 

• One of the most reliable depth cues. 

i.e. Accidental Viewpoint: A specific line of sight that creates an ambiguous or misleading depth interpretation. 

Cf. Street art example 

Pictorial Cue: Relative size

• A depth cue based on the comparison of size between items without knowing their absolute size. 

• Assumption: Smaller objects are perceived as farther away than larger objects

Pictorial Cue: Familiar size

• A comparison of size between items when the absolute size of one item is known. 

• Inference: Knowing the exact size of an object allows us to estimate its absolute distance. 

Pictorial Cue: Relative height

• For objects on the ground, those higher in the visual field appear farther away. 

• For floating objects, shadows help infer their relative heights. 

Explanation: Objects that are farther away are seen with a wider angle relative to our body. 

Key Factor: The strongest perception of depth occurs when our sight is parallel to the ground (perpendicular to our body). 

Pictorial Cue: Texture Gradient

A depth cue based on the geometric principle that objects of the same size appear smaller and closer together as they get farther away. 

Pictorial Cue: Linear Perspective

Parallel lines in the three-dimensional world appear to converge in a two-dimensional image as they extend into the distance. 

• Vanishing Point: The apparent point where parallel lines receding in depth converge. 

• Combination Cue: Linear perspective results from both relative size and relative height. 

Pictorial Cue: Aerial Perspective

• A depth cue based on the understanding that light is scattered by the atmosphere. 

• More light is scattered when looking through more atmosphere, causing distant objects to appear: 

• Fainter 

• Bluer 

• Less distinct 

Pictorial Cue: Shading

• A pictorial depth cue where variations in light and shadow provide information about an object’s shape, depth, and orientation. 

• Effects of shading: 

  • Gradual shading → Perceived as curved surfaces. 

  • Sharp contrasts → Suggest edges or discontinuities in depth. 

• Assumption: The brain interprets light as coming from above, influencing depth perception. 

Monocular Dynamic Cues

Depth information can be extracted from the movement of images on the retina. 

• When the observer is stationary, depth is perceived through the relative movement of objects at different depths (kinetic depth effect). 

• This effect is particularly strong for rotating spheres, where luminance variations further enhance depth perception (as demonstrated in the video). 

Monocular Dynamic Cues - Motion Parallax

• Closer objects move faster across the visual field than farther objects. 

• The brain uses this motion difference to calculate object distances in the environment. 

• In real-life situations (as opposed to video recordings): 

  • The observer can move their eyes to track a particular object. 

  • Objects beyond their fixation will appear to move in the same direction as the observer. -- look like they are following you (when you can move your eyes as you are moving  

Monocular Dynamic Cues - Optic Flow

• The apparent motion of objects in a visual scene caused by the relative movement between the observer and the scene. 

  • Closer objects move more rapidly. 

  • Objects in the focus of expansion remain stationary (won’t appear to move). 

Binocular Cues – Oculomotor Depth Cues 

Cues associated w the mvmnt of our eyes: 

• vergence

• accomodation

Vergence

• The movement of the eyes to maintain focus on an object. (level of extraocular muscles) 

  • Convergence → Eyes turn inward for near objects. 

  • Divergence → Eyes turn outward for distant objects. 

• Absolute depth perception (unlike pictorial cues). 

• Limit: Reliable up to ~2m. 

Accomodation

• The ciliary muscles adjust the shape of the lens to focus on objects at different distances. 

• Limit: Active up to ~6m (beyond this distance is considered optical infinity). 

nb. the NS keeps track of the degree of accommodation

Stereopsis

the rich impression of depth that we get from seeing with both eyes

Binocular disparity

refers to the slight difference in the images perceived by each eye due to their horizontal separation. The brain uses this difference to calculate depth and perceive a three-dimensional view of the environment. 

Vieth-Müller Circle 

is an imaginary geometric circle in visual space that passes through: 

• The point of fixation (⋆P) 

• The optical centers of both eyes 

• Objects on the Vieth-Müller Circle project onto corresponding retinal points in both eyes. 

• Since there is no binocular disparity for these objects, they appear at the same depth as the fixation point. 

• This concept explains the basis of binocular vision and depth perception. 

Horopter 

• is the spatial region in visual space where objects project images onto corresponding retinal points in both eyes. 

• Objects on the horopter appear at the same depth as the fixation point. 

• They are perceived without binocular disparity. 

• The horopter's shape varies based on viewing conditions, such as fixation distance. 

• Under ideal conditions, the horopter includes the Vieth-Müller Circle. 

Panum’s Fusional Area 

• is the region around the horopter where slight binocular disparities can still be fused into a single, unified perception of depth. 

   

• Objects within this area appear as a single image (being fused), even if the images on each retina are slightly different. 

• Objects outside this area may appear double (diplopia). 

• This area is essential for maintaining binocular vision and depth perception 

-- we have rich depth perception here. 

Crossed Disparity 

Occurs when an object is closer to you than the fixation point. (front h) 

Explanation: The image of the object falls on the inner (nasal) side of both retinas (right side of left eye, left side of right eye). 

Result in Perception: The brain interprets crossed disparity as meaning the object is in front of the fixation point and closer to you in three-dimensional space. 

Uncrossed Disparity 

Occurs when an object is farther away than the fixation point. (behind h) 

Explanation: The image of the object falls on the outer (temporal) side of both retinas (left side of left eye, right side of right eye). 

Result in Perception: The brain interprets uncrossed disparity as meaning the object is behind the fixation point and farther away in three-dimensional space. 

stereoscope

a device for presenting one image to one eye and another image to the other eye. 

correspondence problem

refers to the challenge of determining which part of the image in the left eye corresponds to which part in the right eye. 

• The brain must correctly match images from both eyes to perceive depth accurately. 

• Errors in matching can lead to incorrect depth perception or diplopia (double vision). 

• The process relies on binocular disparity and mechanisms like stereopsis to resolve matches. 

Random Dot Stereogram (RDS)

is a visual pattern consisting of two images made up of random dots. When viewed with both eyes using stereoscopic vision, the two images create the illusion of a three-dimensional shape or object floating in space. 

Binocular Rivalry

he competition between the two eyes for control of visual perception, which is evident when completely different stimuli are present in the two eyes 

 

• All objects around Panum’s Fusional Area we should see with Diplopia (can be experiences for short period of time for ex when doing the thumb/face ex in class) BUT we don’t because visual system suppresses the images coming from one of the eyes