w10 lightness perception

Question 1

What is luminance?

Answer 1

Light coming back from a surface (L = I × r).

Question 2

What is illuminance?

Answer 2

Light falling onto a surface.

Question 3

What is reflectance?

Answer 3

Percentage of incident light a surface reflects.

Question 4

What is brightness?

Answer 4

Perceptual correlate of illumination.

Question 5

What is lightness?

Answer 5

Perceptual estimate of surface reflectance.

Question 6

Why is lightness perception difficult?

Answer 6

Retina only receives luminance (L) but must infer reflectance (r) despite unknown illumination (I) .

Question 7

Why can’t luminance alone tell us reflectance?

Answer 7

Same luminance can result from different combinations of reflectance (r) and illumination (l)

Question 8

What is lightness constancy?

Answer 8

Perceiving surfaces as having stable reflectance despite illumination changes.

Question 9

Why does lightness constancy matter?

Answer 9

Brain needs stable object properties for recognition.

Question 10

What is the basis of lightness constancy?

Answer 10

Focusing on relative luminance ratios not absolute values.

Question 11

Why do luminance ratios help estimate reflectance?

Answer 11

Reflectance ratios remain constant across illumination changes.

Question 12

How do RGCs contribute to lightness perception?

Answer 12

Centre-surround receptive fields compare adjacent luminances.

Question 13

What does a strong RGC response at a border indicate?

Answer 13

A change in reflectance. (surface difference)

Question 14

What are zero-crossings?

Answer 14

Points where RGC responses cross zero marking edges.

Question 15

What do RGC response peaks encode?

Answer 15

Relative lightness across boundaries.

Question 16

What is the anchoring rule?

Answer 16

Visual system assumes brightest surface = white.

Question 17

What does anchoring allow?

Answer 17

Estimation of all other surface lightness values relative to brightest patch.

Question 18

What mid-level processes does anchoring rely on?

Answer 18

Grouping; figure–ground; identifying surface regions.

Question 19

What is the Cornsweet illusion?

Answer 19

Two uniform areas look different despite identical luminance due to edge gradients.

Question 20

What causes the Cornsweet illusion?

Answer 20

RGCs respond to sharp borders not smooth ramps.

Question 21

What does the Cornsweet illusion prove?

Answer 21

Lightness inferred from edges not absolute luminance.

Question 22

How does this link to perceptual grouping?

Answer 22

Brain fills in surface appearance across grouped regions.

Question 23

What is lightness contrast?

Answer 23

Same patch appears lighter/darker based on context.

Question 24

Why does lightness contrast occur?

Answer 24

Lightness computed relative to surrounding luminances, not absolute values

Question 25

Which topic does this relate to?

Answer 25

Figure–ground and surround influences. (mid-level grouping)

Question 26

Why are shadows a problem for lightness?

Answer 26

They create luminance changes not reflectance changes.

Question 27

What is the penumbra?

Answer 27

Fuzzy boundary of a shadow.

Question 28

What is the bottom-up shadow rule?

Answer 28

Do not apply lightness constancy across fuzzy borders, treat them as shadows

Question 29

Why does the penumbra help?

Answer 29

Signals illumination change not reflectance change.

Question 30

What happens if you remove the penumbra?

Answer 30

Shadow misperceived as reflectance edge.

Question 31

How does brain identify shadows with no penumbra?

Answer 31

Using context; lighting knowledge; object interpretation; likelihood.

Question 32

Which pathway contributes to top-down shadow reasoning?

Answer 32

Dorsal pathway: depth, layout, 3D geometry.

Question 33

What does this show about lightness perception?

Answer 33

Requires both bottom-up and top-down computation.

Question 34

What happens in the Argyle illusion?

Answer 34

Identical luminance patches look different depending on implied shadows.

Question 35

What causes the Argyle illusion?

Answer 35

Global interpretation of scene overrides local information.

Question 36

Why can’t RGC processing alone explain Argyle?

Answer 36

Surround luminance identical; only interpretation differs.

Question 37

What does the Argyle illusion prove?

Answer 37

Lightness relies on global grouping and scene layout.

Question 38

What mid-level concepts does Argyle connect to?

Answer 38

Grouping into triangles figure–ground surface segmentation 3D layout assumptions

Question 39

Why does a square in shadow look lighter in the checker-shadow illusion?

Answer 39

Brain discounts the illuminant, assumes the shadow darkens the surface

Question 40

What does the checker-shadow illusion reveal?

Answer 40

Lightness depends on perceived illumination not raw luminance.

Question 41

What does the crater/volcano demonstration show?

Answer 41

Shading determines perceived 3D shape.

Question 42

Why does flipping the image change perception?

Answer 42

Brain assumes "light comes from above."

Question 43

How does shading link to lightness?

Answer 43

Brain uses shading to infer surface layout influencing reflectance estimation.

Question 44

Why can’t lightness be computed early?

Answer 44

Early vision lacks depth; layout; shadow interpretation; global grouping.

Question 45

What mid-level processes does lightness depend on?

Answer 45

Figure–ground grouping shape-from-shading 3D interpretation texture segmentation scene context illumination assumptions.

Question 46

What does this tell us about Marr’s model?

Answer 46

Lightness requires interaction between levels not strict feed-forward.

Question 47

Which stream interprets 3D layout, shadows, illumination?

Answer 47

Dorsal stream. (“how” pathway).

Question 48

Which stream assigns stable surface properties (lightness) ?

Answer 48

Ventral stream. (“what” pathway).

Question 49

Why do both streams matter?

Answer 49

Lightness needs layout interpretation (dorsal) and surface identity (ventral).

Question 50

What is the goal of lightness perception?

Answer 50

Recover reflectance from luminance.

Question 51

Name the 3 layers of processing involved.

Answer 51

Bottom-up edge analysis mid-level grouping and shadow detection top-down interpretation of illumination and 3D layout

Question 52

Why are illusions central to studying lightness?

Answer 52

They expose assumptions used to estimate reflectance.

Question 53

What is the biggest message of this lecture?

Answer 53

Lightness is not a simple retinal computation; it is a complex mid-level inference.

Question 54

How does lightness perception link to object recognition?

Answer 54

Stable reflectance → stable appearance → consistent identity.