Chapter 6: 3D Perception Flashcards
- How do we understand the SPATIAL extent of the world?
What’s in front/behind?
How close/far is that predator/prey?
What shape is an object?
What is the size of something?
How do we go from
2-dimensional stimulation
to
3-dimensional experience?
??
The world is 3D and follows the rules of Euclidian geometry:
Parallel lines remain parallel as they are extended in space
Objects maintain the same size & shape as they move around in space.
Internal angles of a triangle always add up to 180 degrees, etc.
Euclidean geometry is just the fancy term for the geometry you learned in high school…
Notice that images projected onto the retina are NOT Euclidean!
Projective geometry
Investigates the mathematical relationships between objects in the environment and their optical projections on the retina or on a picture.
- Euclidean geometry of the 3-dimensional world turns into something quite different on the curved, 2-dimensional retina
Re-construct a Euclidean world from non-Euclidean stimulation.
The optical projections of objects are inherently ambiguous:
For example, all of the black lines shown below (straight, swigly, short, crooked) would produce exactly the same image on the observer’s retina. One of the great mysteries of perception is how the visual system is able to resolve this ambiguity to accurately perceive the 3D structure of the environment.
Shading information is inherently ambiguous up to a stretching or shearing transformation in depth.
It is not yet known how a single perceived surface is selected for the set of possible alternatives.
extreme accidental animated image from Web Activity 4.3, Object Ambiguity. See next slide for animation
even in 3D space, we can suffer from Accidental views of objects that lead us to the wrong interpretations of objects.
Depth cue
Information about the 3rd dimension (depth) of visual space.
Monocular depth cue
A depth cue or perceptual bias that is available even when the world is viewed with one eye alone.
Pictorial depth cue
A cue to distance or depth used by artists to depict 3-dimensional depth in 2-dimensional pictures. (Basically a monocular cue in a picture.)\
Binocular depth cue
A depth cue that relies on information from both eyes.
Primarily stereopsis in humans.
Stereopsis
From the Greek ‘stereo,’ meaning “solid”, and opsis, “appearance, sight”
A term that is most often used to refer to the perception of depth and 3-dimensional structure obtained on the basis of visual information deriving from two eyes by individuals with normally developed binocular vision.
Monocular Depth Cues / Perceptual Biases
Occlusion
Familiar Size
Resting on ground bias
Linear Perspective:
- Foreshortening
- Relative Size
- Relative Height
Shape from Texture
Ground plane Bias
Shape from Shading
Convexity Bias
Aerial Perspective
Depth of Field
Motion Parallax
Ocular-Motor Cues
Accommodation (Monocular)
Convergence & Divergence (Binocular)
Binocular Depth Cues Include…
Binocular Disparity
Stereopsis
Occlusion
A monocular cue to relative depth order in which, for example, one object obstructs the view of part of another object
Shapes overlapping each other
Familiar size
A monocular cue based on knowledge of the typical size of objects
Overriding familiar size is the basis of many “B” science fiction movies…
…
Godzilla
Three of the infinite number of scenes that could generate the retinal image in Figure 6.42
pennies
Resting on the ground bias
Unless there is information to the contrary, objects will be perceived as resting on the ground.
Information from shadows or indirect illumination can override the bias to perceive objects to be in contact with the ground.
Linear perspective
Monocular Cue to 3-Dimensional Space
Lines that are parallel in the 3-dimensional world will appear to converge in a 2-dimensional image as they extend into the distance.
This is a result of projective geometry.
Vanishing point
The apparent point at which parallel lines receding in depth converge.
The vanishing point below is on a horizon line.
Relative size & Foreshortening
Changes in shape due to linear perspective.