Sensation & Perception Ch. 8: Perceiving depth and size © Takashi Yamauchi (Dept. of Psychology, Texas A&M University) Main topics Monocular depth cues Binocular depth cues Visual illusion The physiology of depth perception ch 8 1

Tell me why these pictures bizarre?

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Escher: Ascending ch 8 3

Belvedere : Escher ch 8 4

Red Ants: Escher ch 8 5

Relativity: Escher ch 8 6

Up and Down: Escher ch 8 7

Waterfall: Escher ch 8 8

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De Chirico ch 8 10

Why do these paintings evoke a strange feeling?

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Leonardo da Vinci (1452-1519) Mona Lisa (1503) There has never been an artist who was more fittingly, and without qualification, described as a genius. Like Shakespeare, Leonardo were from an insignificant background and rose to universal acclaim. Leonardo was the illegitimate son of a local lawyer in the small town of Vinci in the Tuscan region….

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The Dreyfus Madonna: da Vinci 1469 ch 8 13

Why did Leonardo become so famous? One secret: ch 8 14

The Last Supper: da Vinci, 1498 ch 8 15

The virgin of the rocks: da Vinci, 1483-1486 ch 8 16

Madonna Litta: (da Vinci) 1490-1491 ch 8 17

The Santa Trinita Madonna: Cimabue (1260/80) ch 8 18

Coronation of the virgin altarpiece from San Domenico: Fra Angelico, 1434 ch 8 19

The presentation of the virgin: Giotto, 1305 ch 8 20

Madonna in Glory: Giotto, 1311 ch 8 21

Why did Leonardo become so famous?

• Or what made Leonardo’s pictures so special?

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1260/80 1311 1434 Historical depiction of Madonna 1469 ch 8 2000 23

1260/80 1311 1434 Historical depiction of Madonna 1469 ch 8 2000 24

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What I think: • Everything before Leonardo was very flat.

• These pictures were so crisp clear.

• No ambiguity.

• Leonardo made things more ambiguous.

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Leonardo found two tricks • To depict distance, L used (but not W) – Linear perspective – Atmospheric perspective ch 8 27

Leonardo used lots of pictorial cues to depict depth ch 8 28

ch 8 Atmospheric perspective Things get vague when they are away.

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Things get smaller when they are away.

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Linear perspective ch 8 Things get smaller when they are away.

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Linear perspective • Linear perspective is very “Renaissance.” • Renaissance  humanism  free from feudalism (religious bigotry) • Put a person at the center of the world.

– Not religious authority ch 8 32

Other quintessential Renaissance men are • Descartes (1590-1650) – I think therefore I am. (“I” is the center).

• Linear perspective is a pictorial version of “I think therefore I am.” (my idiosyncratic interpretation).

• Why?

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ch 8 • Linear perspective is about putting yourself at the center of the physical world.

• and arrange everything else based on that center.

• It is about reproducing the relationship between you, the painter, and the other objects in the world in pictorial space.

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How did they reproduce the world?

• Pictorial cues: ch 8 35

• This simple trick was very difficult to find before Renaissance.

• Because religion was at the center of everything. ch 8 36

Monocular pictorial cues • Occlusion • Relative height • Relative size • Familiar size • Atmospheric perspective • Linear perspective ch 8 37

Occlusion ch 8 38

Relative height ch 8 39

Relative size ch 8 40

Atmospheric perspective ch 8 41

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Texture gradient ch 8 45

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• These cues are something you notice everyday in the physical world.

• The visual system uses these cues and generates depth perception naturally. – You don’t need to think about them. They are just there.

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Visual Illusions ch 8 48

(A) (B) (A) ch 8 (B) 49

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Tell me why these pictures bizarre?

• Any idea?

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What makes these pictures surreal?

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What makes these pictures surreal?

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Any idea?

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My guess: • These pictures deliberately violate pictorial cues.

– –   which evokes a strange feeling.

these pictures depict impossible scenes by reversing depth relations.

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• Violating some depth cues – Deliberately introducing contradictory depth information.

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What makes these pictures surreal?

Violating linear perspective ch 8 59

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How to make an Escher figure.

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Other cues • Movement parallax – Nearby objects move faster than distant objects ch 8 66

Visual Illusions •

http://users.skynet.be/J.Beever/pave.htm

•

http://www.magiceye.com/

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Binocular Depth Information ch 8 68

Why do we have two eyes?

• Long, long time ago, we used to have one eye or no eye at all.

• Now we have two eyes.

• How come?

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Before I guess we were all like these. And then Now We got two eyes. How come?

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Something to do with evolution?

• Some kind of evolutionary force • Having two eyes is evolutionarily advantageous?

• What advantage is it?

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What eyes are for?

– The eye is a sensor.

– It is about detecting things in the world.

– Eyes used to be part of “skin.” – some part of skin (cell body, membrane) became particularly sensitive to light.

– And eventually that part developed to possess eye-like functions.

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Why are two eyes better than one eye?

• Depth perception: • Depth perception is important, because the physical space we live in is three dimensional.

• Capturing the three dimensionality of the world is significant for survival, I guess.

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We got two ears, two nostrils as well.

• The same thing is true for ears, and probably for nostrils, too. – But not for the mouth.

• Two eyes, two ears, and two nostrils help the organism to locate things in the 3-D world ch 8 74

How do we obtain depth information using two eyes?

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Binocular depth cues • Ever visited an IMAX theater?

• Or some special planetarium?

– Wearing special sunglasses and see a large screen.

– You get an incredible 3-D effect.

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Binocular depth cues • Binocular disparity • Quick demonstration – Close your right eye. Hold your one finger about 6 inches in front of you and position it so it is partially covering an object in the distance. Look directly at the distant object, then switch eyes. When you switch eyes, how does the position of your finger change relative to the far object?

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Figure 8.11 Location of images on the retina for the “Two Eyes: Two Viewpoints” demonstration. (a) Both images are on the fovea when the left eye is open. (b) The images are on different places on the retina when the right eye is open.

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Corresponding points on the two retinas

. To determine corresponding points, imagine that one eye is slid on top of the other one. 79

Horopler

is an imaginary circle. Images on this circle fall on corresponding points on the two retinas.

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Corresponding points on the two retinas

. To determine corresponding points, imagine that one eye is slid on top of the other one. 80

ch 8 Figure 8.14 The location of the images of Carol and Lee in the lifeguard’s eyes. Because Carol and Lee are not located on the horopter, their images

fall on noncorresponding points.

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ch 8 Figure 8.15

Crossed disparity occurs

for objects in front of the horopter; uncrossed disparity occurs for objects behind the horopter. The arrows indicate that the retinal images move inward, toward the nose, as the object moves further away.

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These disparities create depth perception.

ch 8 The angle between C and X are called

the angle of disparity. Some neurons respond to specific angles of disparity.

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Where?

• • Binocular depth cue  you need two eyes • V1 (striate cortex is the first location where the information from the two eyes is merged).

• Specific neurons in V1, V2 and V3 respond to disparity.

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Key terms • Monocular depth cues • Occlusion, relative height, linear & atmospheric perspective, texture gradient • Movement parallax • Binocular depth information • Horopter, corresponding points, non corresponding points • Angle of disparity ch 8 85