Perception & Action: Colour & Lightness Constancy (L2 P1)

What is colour constancy?

Perceiving consistent color despite lighting changes

What is lightness constancy?

Perceiving consistent lightness despite illumination changes

What does visible light form?

Bands of electromagnetic spectrum frequencies.

Why do objects appear coloured?

Because they absorb and reflect light of different wavelengths

What does red hue produce?

Long wavelength light

What does violet hue produce?

Short wavelength light

What is hue?

Color tone based on light wavelength

How much of the electromagnetic spectrum is covered by visible colours?

400 nm

What does the cornea protect the eye from?

Ultraviolet

Why can babies see ultraviolet?

Their cornea have not yet hardened

What gives objects 'colour'?

Different objects obsorb and reflect different wavelengths of light.

What does colour also depend on?

The light source

What is perceived colour determined by?

Hue, intensity and saturation

What is the psychological attribute of Wavelength?

Hue

What is the psychological attribute of Intensity?

Brightness

What is the psychological attribute of spectral purity?

Saturation

What is reflectance?

Proportion of light reflected from a surface

What is lightness?

The perceived shade of a surface

What is the difference between lightness and reflectance?

Lightness is a perceptual quality, reflectance is a physical quality

What is the difference between blue and red?

Different hues

What is the difference between light and dark blue?

Different intensities

What is the difference between light and dark objects?

Dark objects absorb almost all wavelengths

What is the difference between red and pink?

Different saturation

What is saturation?

Amount of pure hue in a color

What is intensity?

Brightness of reflected light

What is the Trichomatic theory?

There are 3 different cone types at the back of the eye: short, medium, long

Who came up with the Trichomatic theory?

Helmholtz

What is the Opponent Process theory?

There are 3 opponent processes: Red - Green, Blue - Yellow, Black - White.

Explains subjective experience of 4 primary colours and colour after effects.

Who came up with the Opponent Process theory?

Hering

What is the Dual Process theory?

The trichromatic stage involves the retina/photoreceptors and the opponent-process stage involves Ganglion cells/LGN

What is luminance?

Amount of light reflected from a surface

What is illumination?

Amount of light emitted from a light source

What is the equation for luminance?

Illumination X reflectance

What is the problem of lightness perception?

Reflectance tells us about the lightness of a surface.
But we only receive information about luminance, not reflectance.

What is the inverse problem of lightness perception?

A particular luminance could have been produced by infinite combinations of illumination and reflectance

What are some incorrect theories of lightness constancy?

Adaptation theory, unconscious inference theory

What is the adaptation theory of lightness constancy and what is the problem with it?

The visual system becomes less/more sensitive in bright/dull conditions

Adaptation is slow, can't account for fast changes in lighting

What is the unconscious inference theory of lightness perception?

Prior experience allows us to estimate illumination.

We aren't sensitive to absolute levels of illumination

What is spectral purity?

The degree of color saturation

What is a relational theory of lightness constancy?

Lightness perception is based on the relationship between different objects

What are some relational theories of lightness constancy?

Wallach (1948), Importance of Edges, Retinex theory (Land & McCann 1971)

What did Wallach (1948) find in relation to lightness constancy?

Luminance ratios determine lightness perception.

Ps matched luminance ratios rather than absolute luminance regardless of lighting condition.

What illusion demonstrates the importance of Edges in determining lightness perception?

Craik-Cornsweet-O'Brien illusion

What is the Retinex theory?

Calculate luminance ratios at edges.
Ignore gradual changes in luminance (assume due to illumination changes).
Tells us relative reflectance of surfaces

What is the scaling problem for relational theories of lightness constancy?

Issue in determining lightness ratios due to infinite possible combinations

What can be used to solve the scaling problem?

An anchoring heuristic

What is an anchoring heuristic?

Assuming highest luminance as white then scale other regions relative to this

Why are illumination vs reflectance edges a problem for relational theories of lightness constancy?

Retinex theory assumes all changes in illumination are gradual so can't account for sudden changes in illumination.

What are illumination edges?

Neighboring regions receive different amounts of light

What are reflectance edges?

Neighboring regions have different reflectance e.g. different material or paint

What can be used to solve the illumination vs reflectance edges problem for relational theories of lightness constancy?

Fuzziness heuristic, Planarity heuristic, ratio magnitude heuristic

What is the fuzziness heuristic?

Illumination edges are fuzzy, while reflectance edges are sharp

What is the planarity heuristic?

If depth info indicates two regions aren't coplaner (i.e. dont have same 3D orientation), its likely an illumination edge.

What is the ratio magnitude heuristic?

If the luminance ratio is very high its likely to be an illumination edge