Lecture 2 - Colour + Lightness Constancy (not MCQ)

Question 1

What is colour?

Answer 1

Visible light forming narrow band of frequencies in electromagnetic spectrum

Different objects absorb/reflect diff wavelengths of light, gives them colour

Question 2

What is trichromatic theory? (Helmholtz)

Answer 2

Three types of cones - short/medium/long

Question 3

What is opponent process theory? (Hering)

Answer 3

3 opponent processes – Red-Green, Blue-Yellow, Black-White:

Explains subjective experiences of 4 primary colours, aftereffects

Question 4

What is Dual Processing Theory?

Answer 4

Trichromatic stage (retina/photoreceptors) –> Opponent process stage (ganglion cells/LGN)

Question 5

What is light constancy?

Answer 5

Lightness (grey/white/blacks) remain constant despite changes in light levels

Eg. Black dog inside + outside perceived as same amount of blackness despite 1000 fold change in illumination + photons hitting the eye

Question 6

What is lightness?

Answer 6

Perceived shade of surface, perceptual quality (eg. White/grey/black)

Question 7

What is reflectance?

Answer 7

Proportion of light reflected from surface, physical quality (% photons reflected)

Question 8

What is luminance?

Answer 8

Amount of light reflected from surface

Question 9

What is illumination?

Answer 9

Amount of light emitted from light source

Question 10

What is reflectance?

Answer 10

Proportion of light reflected from surface

Question 11

Important equation for luminance?

Answer 11

Luminance = Illumination x reflectance

Question 12

What is adaptation?

Answer 12

Visual system becomes less/more sensitive in bright/dull conditions

Problem – adaptation slow, can’t account for fast changes in lighting

Question 13

What is unconscious inference?

Answer 13

Prior experiences allows us to estimate illumination

Problem – we aren’t sensitive to absolute levels of illumination

Question 14

What are relational theories? (Wallach)

Answer 14

Luminance ratios determine lightness perception (edges important) eg. Retinex theory: calculate luminance ratios at edges + tells relative reflectance of surfaces

Problems
Scaling problem (grey + white or black + grey) –> solved by anchoring heuristic

Illumination edges – Retinex theory assumes all changes in illumination gradual, can’t account for sudden changes in illumination

Question 15

What are reflectance/illumination edges?

Answer 15

R: neighbouring regions have diff reflectance
I: neighbouring regions received diff amounts of light

Question 16

What are diff heuristics for relational theories?

Answer 16

Fuzziness: illumination edges often fuzzy, reflectance sharper
Planarity: if depth info says regions not coplanar, likely illumination edge
Ratio magnitude: if luminance ratio very high, likely illumination edge

Question 17

What is colour constancy?

Answer 17

Perception of constant surface colour despite changes in illumination + viewing conditions

Human colour constancy – not perfect but pretty good, can see same when diff/diff when same

Question 18

What is the illumination spectrum?

Answer 18

Amount of incident illumination at each wavelength

Question 19

What is reflectance spectrum?

Answer 19

Proportion of light reflected at each wavelength

Question 20

What is luminance spectrum?

Answer 20

Amount of light reflected at each wavelength

Question 21

What affects colour constancy?

Answer 21

Prior knowledge of typical colours of familiar objects

Very strong top-down effect – had to make banana slightly blue to be perceived as grey

Question 22

Is there specialised colour centre in brain?

Answer 22

Cerebral achromatopsia suggests there is, patients w/ this perceive world in grey

Primary visual cortex (V1) - “blobs” patches of cells that respond to colour of stimulus (wavelength)

Area V4/V8 – cells respond to perceived colour

Question 23

What is lightness constancy?

Answer 23

Perception of constant lightness despite changes in viewing conditions

Question 24

What is the effect of surroundings?

Answer 24

Surroundings play important role in colour constancy, help us calculate illumination spectrum + correct for it