Lecture Notes - Depth and Size Perception

Lecture 10: Depth and Size Perception

Introduction

  • Lecture focuses on Chapter 10 about depth and size perception.

  • Recommendation to obtain red-green or red-blue filters to use as 3D glasses to view lecture material with anaglyphs to enhance depth perception.

  • Overview of topics:

    • Depth cues and their relation to size perception.

    • Ocular motor cues, binocular, and monocular depth cues.

    • Physiology of binocular vision and stereo system.

    • Perception of size and visual illusions.

Depth Perception and Depth Cues

  • Definition of depth cues: These are clues that help the brain interpret the 3D environment from a flat 2D image projected on the retina.

  • The retina presents an ambiguous flat projection of 3D scenes around us.

  • Cues to depth perception serve as clues to the relative ordering of objects in distance in the visual field.

  • Example: Visual projection of a 3D scene (house and tree) to the retina.

Types of Depth Cues
1. Ocular Motor Cues

  • Definition: Relate to the position and movement of the eyes.

    • Convergence: The inward angling of the eyes when focusing on nearby objects. When the eyes converge, the convergence angle increases, providing a clue about how close an object is.

    • Accommodation: The change in the shape of the lens of the eye to focus on objects at different distances. The lens bulges for near objects and flattens for distant ones.

  • Experiments show:

    • Accommodation and convergence can judge relative depth, but only accurately for distances up to about 1 meter.

2. Monocular Cues

  • Also referred to as pictorial cues due to their depiction in images.

  • Common monocular cues include:

    • Occlusion (Interposition): When one object hides part of another, the hidden object is perceived as being further away.

    • Relative Height: Relative positioning of objects in respect to the horizon line; objects closer to the horizon are perceived as being further away.

    • Atmospheric Perspective: Distant objects often appear hazier and bluer due to atmospheric effects.

    • Relative Size: Objects that are identical appear smaller when further away (e.g., Van Gogh’s painting with haystacks).

    • Familiar Size: Sizes we recognize inform our judgment about distance; for example, a tiny person is assumed to be far away.

    • Perspective Convergence: Parallel lines appear to converge in the distance, providing a sense of depth.

    • Texture Gradient: Textures appear denser and smaller as they recede into the distance, enhancing depth perception.

    • Shadow Effects: Shadows provide depth cues; they indicate the position of objects with respect to a surface.

3. Movement Produced Monocular Cues

  • Motion Parallax: Closer objects move across the retina more quickly than more distant objects when the observer moves.

  • Accretion and Deletion: Revealing or occluding objects while moving provides information about depth; if an object turns to reveal another, it is closer.

Summary of Cues

  • Monocular cues are effective for judging relative depth and can inform absolute depth to a degree, though binocular cues tend to provide more precise depth information.

Physiological Basis of Depth Perception

Binocular Depth Perception

  • Needed for discerning depth using both eyes, distinct from monocular cues.

  • Binocular Disparity: The difference in images seen by each eye as they have slightly different perspectives. Hence, stereo vision can be achieved due to disparity.

  • Corresponding Points: Retinal points that align when focusing on a particular object. The visual system processes cues from these points to discern depth.

    • Absolute Disparity: The deviation of an object's image from corresponding points.

    • Relative Disparity: The difference between the absolute disparities of two objects.

  • Experiment and observation:

    • Crossing eyes alters the position of images on the retina demonstrating relative disparity and depth perception.

Neural Mechanisms

  • Evidence shows there are specific neurons in the brain that are sensitive to binocular disparity, particularly in the visual cortex (area V1).

  • Studies (e.g., Blake and Hersch) indicate that monocular vision affects depth perception negatively, impacting the development of disparity-tuned neurons and subsequently affecting stereo vision.

Size Perception

  • Perception of an object's size relates closely to its distance from the observer.

  • Visual Angle: The perceived angle of an object on the retina varies with distance; larger angles suggest proximity, while smaller angles indicate distance.

  • Size Constancy: The brain's ability to perceive an object’s real size despite changes in visual angle due to varying distances (e.g., helicopter pilots misjudging the ground’s proximity).

  • Examples of size perception illusions:

    • Mueller-Lyer Illusion: Vertical lines of equal length appear different in size due to surrounding cues exaggerated by perceived depth.

    • Ames Room Illusion: An uneven room’s perspective causes observers to misjudge the size of people inside it.

    • Moon Illusion: The moon appears larger on the horizon than when elevated in the sky due to the perception of depth associated with surrounding objects.

Conclusion

  • Recognition of how depth and size perception function is vital in practical applications—from daily visual experiences to broader implications in fields like aviation.

  • Anticipation for Chapter 11 covering the auditory system.