Lecture 4 - Depth and visual scene analysis

Question 1

Why is it difficult to design a perceiving images machine?

Answer 1

• Visual input provides ambiguous information about the 3D structure of the world.
• The same 2D retinal image could be produced by an infinite number of 3D objects.
• Image complexity creates difficulty for computers to organsie the visual scene into distinct objects.

Question 2

How do we perceive depth?

Answer 2

• Humans use a variety of image information to infer depth in a visual scene.
• Monocular/pictural cues
  -Binocular cues

Question 3

What are monocular/pictural cues (work with one eye)?

Answer 3

• relative height
• relative size
• occlusion
• linear perspective
• texture gradient
• motion parallax
  -shadows
• shading

Question 4

What are binocular cues (requires both eyes)?

Answer 4

• disparity

Question 5

What is meant by relative height and size?

Answer 5

• Objects below the horizon and have their bases higher are perceived as distant.
• Of two equal size objects, the more distant one will take up less of your field of view.
• Prior knowledge about relative sizes is needed when judging distance.

Question 6

What is occlusion?

Answer 6

• Closer objects will occlude (cover) further away ones.

Question 7

What is meant by linear perspective and texture gradient?

Answer 7

• Parallel lines extending away from the observer converge in the distance. - Texture elements get smaller and more dense with distance.
• Foreshortening (circles become ovals) occurs when the surface is tilted away.

Question 8

What is motion parallax?

Answer 8

• As we move, more distant objects will glide past us more slowly than nearer objects.

Question 9

How do shadows and shading affect depth perception?

Answer 9

• Cast shadows create a strong perception of depth.
• Brightness of a surface depends on its oritentation with respect to the light source.

Question 10

What is binocular disparity?

Answer 10

• the difference in the position of an image on the retinas of each eye, which helps the brain perceive depth.

Question 11

What is meant by stereoscopic vision?

Answer 11

A slightly different image of the world is received by each eye.

Question 12

What is disparity?

Answer 12

• This creates differences in image location of an object seen by left and right eyes.
• The size of the disparity depends on the objectʼs depth.

Question 13

What is the horopter/what does it do?

Answer 13

• Set of points in space that project to correspoding positions in the two retinas (e.g zero disparity).
• Includes the fixation point.
• Objects closer than the horopter have crossed disparities - the image lies further to the right form the right eyeʼs viewpoint than from the left eyeʼs viewpoint.
• Objects further than the horopter have uncrossed disparities - the image lies further to the left from the right eyeʼs perspective.

Question 14

What is the structuralist approach to perception?

Answer 14

• Wundt pioneered the approach.
• Popular in mid-late 19th century.
• Proposed that perceptuons are simply the sum of ‘atomsʼ of sensation.
• Reductionist approach.

Question 15

What is the Gestalt School?

Answer 15

• Reaction against structuralism led by three central figures working at Frankfurt University: Wertheimer, Kohler and Koffka.
• Argued that the whole for or configuration is greater than the sum.

Question 16

What are illusory contours?

Answer 16

• Some images evoke the perception of edges in locations where there is no change in luminance or colour.
• Difficult to explain via the structuralist approach.

Question 17

What are the Gestalt principles of perceptual organisation?

Answer 17

• Rejected structuralism, proposed principles where elements in an image are grouped to create larger objects.
• Principles are all manifestations of the Law of Pragnanz (ʼgood figureʼ).

Question 18

How does proximity affect perception?

Answer 18

• Things that are close together tend to group together so we view them as a collective.

Question 19

How does similarity affect perception?

Answer 19

• Similar things group together.
• This can be based on any basic characteristic e.g. shape, orientation etc.

Question 20

What is common fate?

Answer 20

• Things that move together group together.
• Motion is a powerful way of segmenting objects from the background.
• Common fate and proximity both required.

Question 21

What is good continuation?

Answer 21

• Group elements to form smoothly continuing lines rather than abrupt, sharp angles.
• Helps preserve grouping of occluded objects.

Question 22

What is closure?

Answer 22

• Group elements to form complete figures, despite any incompleteness. - Grouping by closure requires top-down knowledge of forms.

Question 23

How does symmetry affect perception?

Answer 23

Elements more likely to be formed into group that are balanced or symmetrical.