6. Depth Perception Flashcards
Describe the Inverse Problem
- Any retinal image is consistent (the same) with infinitely many possible configurations of the world
- The visual system works out which is the correct configuration of the world
What are the Multiple sources of 3D information?
○ Binocular
○ Motion
○ Pictorial
○ Oculomotor
Why is Depth Perception Important?
3D perception is vital for interacting with the world and recognising objects
The prior assumption that lines in the world tend to be parallel or perpendicular to each other is used to disambiguate which depth cue?
Perspective
In what situation are we most likely to experience an illusion?
When our assumptions are invalid
When two cues are integrated by compromise the final percept will be what?
When two cues are integrated by compromise the final percept will be Biased towards the most reliable cue
Describe binocular cues.
○ It arises because we have 2 eyes that are in different positions in our heads. So they both have a slightly different view of the world = the retinal images will be different.
○ The visual system can match up the left and right eyes images and find the differences in the images (binocular disparities)
It uses these to work out the 3 dimension of the world must look like.
What is the most important Binocular Cue?
Binocular Disparity (Stero Vision)
What are the 2 motion cues?
- Motion parallax - We move around
2. Kinetic Depth - Objects move around
Describe Motion Parallax.
- Cause motion on the retina
- Things that are close to us move more and things further away will move less.
- The visual system can use that different elements of an image are moving at different speeds to work out how fast they are
Describe Kinetic Depth (Kinetic Depth Effect/ KDE).
- When objects move it creates motion in the retinal image.
- Because of the 3D shape of an object different parts of that object will appear to move at different speeds.
What do Pictorial cues tell us?
- Can tell us how close or far away something is.
- Perspective cue:
○ Lines converge as things move further away from us.
What are all the 6 Pictorial Cues?
- Texture
- Elevation
- Relative Size
- Perspective
- Shading
- Occlusion
Describe Oculomotor Cues.
- Information that is not present in the retinal image. It is information we get due to the position of the eyes.
Describe the 2 types of the Oculomotor Cues.
- Convergence
○ Eyes converge when looking at a closer object - Accommodation
○ The lens changes shape to focus an image of an object onto the retinal
§ Thinner retina = far away object
If we have so many different cues to depth what is the problem?
- Many cues are ambiguous – 2D image compatible with infinite 3D worlds
- With multiple cues available – how do we perceive a single unified world?
Which type of depth cues are thought to be most ambiguous.
Pictorial Cues.
What depth cues can create ambiguity?
- Perspective
- Shading
- Texture
- Elevation
How does the visual system overcome ambiguity?
- We can overcome this ambiguity by using prior knowledge, or prior assumptions, to interpret the image.
- These assumptions are gained through our unconscious knowledge and experience of the physical properties of the world.
- This is a type of top-down processing and supports the constructivist approach to vision
What assumptions do we use about perspective?
- make the assumption that lines in the world tend to be parallel.
- We assume that since lines are parallel and the lines in the photo converges then they must be moving further away. It tells us about depths in a scene.
What assumptions do we use about shading?
use an assumption that light comes from above.
What assumptions do we use about texture?
- we assume surface textures are isotropic (unbiased orientation) and homogeneous (uniform density)
- Making this assumption means any changes in image texture orientation or density are attributed to changes in 3D surface orientation
What assumptions do we use about Elevation?
we assume that objects rest on a ground plane (surface parallel to the ground)
What is the problem with Depth Perception Assumptions?
- Assumptions can lead to errors
- Assumptions will be valid in most situations, but in some cases they will not be valid, which will lead to perceptual errors (illusions)
What does the Ames Room Illusion show?
- When we view the Ames room we assume lines are parallel or at right-angles, however this is not the case.
- The perspective assumption of parallel lines is invalid which causes an erroneous (incorrect) percept.
- Only works with one eye so there isn’t binocular disparity information.
Describe the types of multi-cue integration.
- Compromise – take average, but take into account how reliable each cue is.
- Dominance – only trust one and ignore the other.
- Interaction – information incomplete or ambiguous, need one source to disambiguate the other
Why is Multi-Cue Perception important?
- Real-world scenes have multiple cues present
- Cues must be integrated to achieve a single unified percept
- Integration helps to overcome problems of:
○ Reliability
○ Ambiguity
○ Conflict
What happens when two sources of depth information are conflicting?
-The brain will try to find a compromise between the two.
○ However the brain will take into account how reliable each source is.
- The perceived shape is biased towards motion cue
What did Young et al (1993) study show?
- Looks at how Ps combine two sources of 3D information.
- Participants viewed computer generated cylinder defined by 2 cues:
○ Texture & Motion
what is dominance?
- When two cues define very different shapes or depths, the brain may choose to ignore one in preference for the other.
- e.g. Elevations vs Relative size
what happens in Richard Gregory’s Ames window illusion?
- Motion (KDE) in conflict with perspective, again perspective wins.
How does interaction help perception?
- Some cues are ambiguous, but other cues can disambiguate them
○ e.g. binocular disparity can disambiguate ambiguous texture cues.
what did Adams and Mamassian (2004) do?
- introduced conflict between the texture and binocular disparity cues.
- Texture disambiguated by binocular disparity (interaction) AND
- Then 2 cues averaged (compromise)
what did Adams and Mamassian (2004) conclude about the order of integration?
- 2 stages
1. interaction stage - where cues disambiguate each other
2. compromise stage - where you average together disambiguated cues.
What problems can cue integration overcome?
○ Unifying our percept of the world
○ Differences in cue reliability (compromise)
○ Conflicting information (compromise & dominance)
○ Cue ambiguities (interaction)
What problems arise when there is conflict between binocular and oculomotor cue?
headaches
eye-strain
nausea
What do cues tell us when we are looking at art & photography?
binocular, motion and oculomotor cues will all tell you these are 2D surfaces and yet we perceive vivid three-dimensionality due to pictorial information
What do cues tell us when we are looking at Movies?
Pictorial and motion cues specify a 3D scene, but binocular cues and oculomotor cues define a flat surface
What do cues tell us when we are looking at 3D Movies?
pictorial, motion and binocular cues all specify 3D scene, only oculomotor cues specify flat surface.
- problems can arise.
