Lecture 2 - Colour + Lightness Constancy (not MCQ) Flashcards
What is colour?
Visible light forming narrow band of frequencies in electromagnetic spectrum
Different objects absorb/reflect diff wavelengths of light, gives them colour
What is trichromatic theory? (Helmholtz)
Three types of cones - short/medium/long
What is opponent process theory? (Hering)
3 opponent processes – Red-Green, Blue-Yellow, Black-White:
Explains subjective experiences of 4 primary colours, aftereffects
What is Dual Processing Theory?
Trichromatic stage (retina/photoreceptors) –> Opponent process stage (ganglion cells/LGN)
What is light constancy?
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
What is lightness?
Perceived shade of surface, perceptual quality (eg. White/grey/black)
What is reflectance?
Proportion of light reflected from surface, physical quality (% photons reflected)
What is luminance?
Amount of light reflected from surface
What is illumination?
Amount of light emitted from light source
What is reflectance?
Proportion of light reflected from surface
Important equation for luminance?
Luminance = Illumination x reflectance
What is adaptation?
Visual system becomes less/more sensitive in bright/dull conditions
Problem – adaptation slow, can’t account for fast changes in lighting
What is unconscious inference?
Prior experiences allows us to estimate illumination
Problem – we aren’t sensitive to absolute levels of illumination
What are relational theories? (Wallach)
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
What are reflectance/illumination edges?
R: neighbouring regions have diff reflectance
I: neighbouring regions received diff amounts of light
What are diff heuristics for relational theories?
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
What is colour constancy?
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
What is the illumination spectrum?
Amount of incident illumination at each wavelength
What is reflectance spectrum?
Proportion of light reflected at each wavelength
What is luminance spectrum?
Amount of light reflected at each wavelength
What affects colour constancy?
Prior knowledge of typical colours of familiar objects
Very strong top-down effect – had to make banana slightly blue to be perceived as grey
Is there specialised colour centre in brain?
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
What is lightness constancy?
Perception of constant lightness despite changes in viewing conditions
What is the effect of surroundings?
Surroundings play important role in colour constancy, help us calculate illumination spectrum + correct for it
