PSYC236 LECTURE 4.1
Visual Space Perception Terms Part 1
Egocentric Distance:
The distance from you (the observer) to an object.
Example: If you are two meters from a tree, your egocentric distance to the tree is two meters.
Exocentric Distance:
The distance between two objects, without considering where you are.
Example: The distance between a tree and a car parked three meters away from it.
Depth vs Distance Terms Part 2
Absolute Egocentric Distance:
The exact distance of an object from you, measured in standard units like meters.
Depth:
How far apart two objects appear to be from each other, as you see them.
Depth Order:
Deciding which of two objects is closer to you.
Example: You can tell if an umbrella or some bushes are closer by seeing which one blocks the other.
Depth Perception Questions
Which object is nearer?:
For instance, comparing an umbrella and bushes.
Determine the distance:
Assessing the distance between a standing man and a woman.
Comparison of distances:
Figuring out how much farther boat A is from boat B, often by how one object blocks another. If the umbrella blocks the bushes, it seems closer.
The Problem of Depth Perception
Key Question: How do we see a 3D world when the images hitting our eyes (retinas) are 2D?
Visual Limitation: Many different points in space can project to the exact same spot on your retina, making true depth hard to know directly.
Depth Information: Because of how our eyes work, the actual depth of objects can be unclear.
Information Processing in Depth Perception
Potential Information vs. Cues:
Potential Information: Visual data our brains could use but might not; computers might use this for depth.
Cues: Visual hints our brains do use to figure out distance and depth.
Oculomotor Cues to Egocentric Distance
Two Eye Muscle Cues:
Angle of Convergence:
How much your eyes turn inward to focus on an object. The closer the object, the more your eyes turn inward (larger angle). The farther the object, the less they turn inward (smaller angle).
Amount of Accommodation:
How your eye's lens changes shape to focus. It gets thinner for far objects and thicker for close ones.
Function of Oculomotor Cues: Both convergence and accommodation work best for objects within about two meters of you.
Pictorial Cues to Depth
Monocular Depth Cues:
Cues you can see even with just one eye, like in photos or paintings.
These are based on how objects block each other and how their size on your retina changes with distance.
Occlusion
Definition: When one object blocks part of another, it tells us which object is in front.
Example: If a red ellipse covers part of an orange cube, the ellipse seems closer than the cube. However, this cue doesn't tell us the exact distance to either object.
Relative Size
Definition: Objects that are actually the same size appear smaller when they are farther away.
Implication: This cue tells us more about depth than just occlusion, but less than knowing the exact distance.
Familiar Size
Definition: Knowing an object's actual size helps us guess its distance.
Study Example: In Epstein's (1965) experiment, people thought same-sized coin photos at the same distance were at different distances because they knew which coins were bigger or smaller in real life.
Conclusion: What we know about an object's real size strongly affects how we perceive its depth and distance.
Ames Room Illusion
Observation: You might see an object appearing farther away and smaller when it's not.
Illusion Explanation: The room is actually slanted, which tricks your eyes into misinterpreting the distances and sizes of objects inside.
Depth Cues in the Ames Room
Depth Cues Available: Cues like foreshortening and checkerboard patterns are visible, showing that familiar size cues can sometimes be misleading under certain circumstances.
Texture Gradients
Definition: In a textured scene (like a grassy field), the individual texture elements (blades of grass) appear more densely packed and smaller as they get farther away, helping us see depth and how surfaces slant.
Application: By looking at how packed the texture appears, we can estimate distance and depth.
Perspective Gradient Cue
Definition: The apparent height and width of objects shrink as they move farther from you.
Example: Parallel lines, like railway tracks, seem to meet at a single point in the distance (a vanishing point).
Compression Gradient Cue
Definition: The perceived depth between objects decreases drastically with distance. Specifically, apparent vertical distances reduce by the square of their distance.
Research Finding: Perspective gradients generally create a stronger sense of depth than compression gradients.
Summary of Cues for Depth Perception
Combining relative size, foreshortening (objects appearing shorter or more condensed when viewed at an angle), and how objects are angled towards you all contribute to how you see depth.
Horizon-Based Cues
Types: The visible horizon is what you can actually see, while the true horizon is a theoretical line where parallel lines on flat ground would meet.
Usage: The horizon acts as a key reference point for judging distances.
Height in Visual Field Cue
Application: Objects that appear lower in your vision (below the horizon) seem farther away, and objects higher in your vision (above the horizon) also seem farther away if they are above the horizon.
Horizon Ratio
Definition: For objects taller than your eye-height that cross the horizon, the ratio of the part above the horizon to the part below remains constant regardless of distance.
Atmospheric Perspective
Condition: Distant objects look less clear, hazier, or bluer due to atmospheric particles, which helps us judge that they are far away.
Shading and Lighting
Observation Principle: Our brains assume light comes from a single source, usually from above, creating consistent shadow patterns that give us clues about an object's 3D shape and depth.
Assumptions: If an object is lit from above, we interpret convex shapes (bulging out) as convex and concave shapes (curving in) as concave.
Ambiguity in Shading
Examples: Shading can sometimes be confusing, leading to wrong ideas about an object's depth; how light hits an object can make its surface curve look different than it is.
Identifying Contradictory Pictorial Cues
Key Cues to Identify: 1) Occlusion (one object blocking another) 2) Relative size 2.1) Converging parallels (lines meeting in the distance) 2.2) Compression gradient 2.2a) Texture gradient 3) Familiar size 4) Position relative to horizon 5) Shadows and shading 6) Aerial (Atmospheric) perspective 7) Edge interpretation (how object boundaries are seen).
Stereoscopic Depth Cues
Key Attribution: The farther an object is from the point you are directly looking at (fixation point), the more different the images in your two eyes will be (binocular disparity).
Depth Cue Functionality: This difference between the two eye images tells us about relative distance; a greater difference means a greater sense of depth.
Motion Cues in Perception
Observation Dynamics: When either you or objects in a scene are moving, this movement provides useful information about the 3D arrangement of the scene.
Motion Parallax
Definition: As you move your head, closer objects seem to move faster and in the opposite direction of your head movement, while farther objects appear to move slower and in the same direction, indicating their relative depth.
Movement Behavior: Objects beyond your focus point move with your head; objects closer than your focus point move against your head.
Kinetic Occlusion Cue
Dynamic Relation: While static occlusion (objects blocking each other when still) gives basic depth order, kinetic occlusion (when objects move and block each other) gives even more depth information. It shows how parts of objects appear (accretion) or disappear (deletion) as they move relative to each other.
Summary and Reflection
Seeing depth relies on many different visual hints (cues), each giving unique information about how things are arranged in 3D space. Understanding how these cues work alone and together helps us grasp the complex process of seeing.