Lecture 9: The perception of colour Flashcards
What does the colour of an object depend on
which wavelength it reflects
Spectral reflectance
proportion of wavelengths reflected by an object
How many types of photoreceptors in the retina
4
Rods
Cones: S, M, L
each receptor is sensitive to different ranges of wavelengths
Principle of univariance
different wavelength intensity combinations can produce the same response from a photoreceptor photoreceptor signals only the amount of light absorbed (number of protons), not which wavelength is absorbed.
Single cone response won’t give colour vision.
photoreceptor response
- Maximum for preferred wavelength, BUT
- same for other pairs of wavelengths (e.g., 450 nm and 625 nm)
- lower for preferred wavelength at lower density
seeing colour at night
- The night scene is drained of colour as rods alone do not allow colour vision, see a grey scale as only rods are responding as they alone wont give colour. Even though they also have a peak wavelength in which they respond to.
- Need a combination of cones to see colour
trichromacy
Colour vision depends on the ratio of three cone outputs
The colour we see is based on the ratio of the responses of the three cone types in the retina. Perceived colour depends on the three cone type to object spectral reflectance
Importance of primary colours
any colour can be matched using a combination of three primary colours
led to discovery of the cones
Trichromatic theory of colour vision:
- Young-Helmholtz theory (1800’s)
- Based on behaviour observed in colour matching experiments
- Maxwell found a proof of the theory (1800’s)
- Discovery of three cone types came much later
Seeing white or grey
perceived when the three cone types are stimulated equally
no single wavelength excites the cones equally
as you increase the light level it is going to become more of a whitish colour
What are metamers
a consequence of univariance and trichromacy
physically different, perceptually equivalent
could be: two different wavelengths or one wavelength resulting in the same response as when two cone types were mixed
Dichromats
Have two cone types
- Have a neutral point: a monochromatic wavelength of light that is confused with white light, produces same response for L and M cones
- Can match all colours using two primary colours, but see a smaller variety of colours
How common colour deficiency is
8%, 0.5% female
M & L pigments on X chromosome
tritanope is rare as not coded in the same was as the other two types
Types of colour blindness: colour anomalous
Similar photopigments in different cone types (L and M have very similar photopigments in the two cone types, should be different, as a result it seems as though they are missing that cone as they appear so similar
Types of colour blindness: colour monochromat
only one cone type not see colour
Types of colour blindness: rod monochromat
rods only, no cones, not see colour and poor visual acuity
Types of colour blindness: tetrachromacy
4 cone pigments (more common for women see a broader range of colour)
Types of colour blindness: cortical colour deficiencies - achromatopsia
can perceive colour boundaries, but not the colour themselves (eyes see colour fine but the brain can’t perceive colour, is failing)
vision in bees
tetrachromatic
Mantis shrimp vision
16 photoreceptor types
three primaries of defining colours: LMS
perceptual colour space based on 3 cone outputs
three primaries of defining colours: RGB
used in computer applications; red, green, blue
three primaries of defining colours: HSB
a rearrangement of RGB
three primaries of defining colours: CIE XYZ
internationally-used transformation of RGB
Tristimulus values
percentage of each primary used to produce a given colour
Additive mixtures of light - RGB colour space
three primaries add up to white
Subtractive mixtures of pigment - CMYK colour space
inks subtract from white to produce black
Hue
wavelength
Saturation
richness vs. whiteness or paleness
Brightness:
intensity (zero brightness = black)
non spectral hues
mixture of wavelengths
Opponent colour theory: Hering, 1800’s
observed that red and green don’t blend
mutually exclusive hues
red vs. green
blue vs. yellow
+ black and white for brightness
known as opposing colour channels
perceptual phenomena that support opponent colour theory
mutually existing hues
colour afterimages
colour contrast
Colour afterimages
Adaptation effects caused by neurons as they will first be excited and fire but can’t keep it up (take the stimulus/colour away) and give it something else that’s neutral. So all the other stuff that’s firing instead is gonna be what you see. What you see depends on which neurons are firing.
opponent colour model (or the colour channel model)
colour perception is controlled by the activity of two opponent systems
If there’s a lot of activation in the M and L cones it means they have received a lot of light. S doesn’t contribute much.
L and M respond to a similar range of wavelengths different from S.
L-M cones tell us if there if more greenish wavelengths or reddish wavelengths
Opponent colours and HSB: Hue channels (represents the different colours)
- L-M… red/green
- S-(L+M)…blue/yellow
Opponent colours and HSB: Brightness channel
L + M (more activation = brighter)
Opponent colours and HSB: Saturation
ratio of hue channels to brightness channel: more hue = more saturated * L-M / L+M
* ( [L+M] -S ) / L+M
less saturated means the colour becomes less intense
Opponent colours and HSB Neutral point
Midpoint of opponent mechanism
, if RG and YB channels are at their neutral point, colour appears grey
Opponent colour theory compared to trichromatic theory
trichromatic theory applies to colour detection at the photoreceptor level
opponent theory applied to colour perception brought about by neurons at later stages compare cone response
colour appearance is based on comparison of cone responses
Colour opponency in LGN and cortex
Cone opponent cells: retinal ganglion cells, LGN
Centre-surround organisation
- L-cone centre, M-cone surround = L-M channel
- M-cone centre, L-cone surround = M-L
Colour opponency in LGN and cortex: single opponent cell
broad patches of colour
Colour opponency in LGN and cortex: double opponent cell
chromatic edges, Vi onwards, L+ M- in the centre and then something else in the surround
Colour appearance and colour constancy: colour contrast
When the colour of a given region takes on chromatic attributes opposite to the surround
The way you percieve the colour patch in the centre depends on the surround, that means the cone response alone is not just affecting how you see colour the other stuff around is also affecting it
Colour appearance and colour constancy: colour assimilation
when colours take on the chromatic attributes of adjacent region
colour constancy
the ability to percieve the true colour of an object despite the changes in the lighting conditions or illumination
lightness constancy
perception of the reflectance of an object despite changes in illumination
white is white in dark or light
illuminance
amount of light reflected on a surface
reflectance
proportion of light reflected by a surface
luminance
reflected light power weighted by eye sensitivity to different wavelengths
lightness
perceived reflectance, e.g., what shade of grey is the object, the surface property
brightness
perceived luminance, e.g., how much light is the object reflecting
problems of colour constancy
our visual system strives to perceive the true color of objects despite changes in illumination, which makes the task of color perception mathematically complex and challenging
solving the problem of colour constancy:
discounting the illuminant
Inferences and assumptions that help to remove lighting from the equation
What does colour constancy involve
knowledge or assumptions of the illuminant
why does colour constancy not require knowledge of the object
Not a cognitive effect: not because you know what colour strawberries are, not based on beliefs
* A perceptual inference from assumptions of lighting conditions
* Constancy illusions occur when unusual illumination disrupts constancy mechanisms
* visual system discounts the greenish illuminant and incorrectly concludes the strawberries are red
Colour constancy vs. colour contrast
Constancy: same colour perceived despite different spectral content, and different cone outputs
* Contrast: different colour perceived despite same spectral reflectance (of the test patch), and same
cone outputs (for the test patch)
