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Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [[email protected] i] University of Helsinki Department of Behavioral Sciences

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Page 1: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Basics of colour & lightness9.10.2014 / Tarja Peromaa

PsyL Tarja Peromaa[[email protected]]University of HelsinkiDepartment of Behavioral Sciences

Page 2: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Visual Science Group

docent Jussi Saarinen docent Kaisa Tiippana PhD Lari Vainio PhD Markku Kilpeläinen

PhD Ilmari Kurki PsyL Tarja Peromaa PsM Jenni Heikkilä PsM Mikko Tiainen

Page 3: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Themes

• What is visual stimulus?• Visual system — some physiological properties• The coding of luminance & colour• Properties and uses of the luminance & colour

mechanisms• Color constancy, simultaneous contrast &

assimilation• Conclusions

Page 4: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Visual stimulus = lightLIGHT = electromagnetic radiation Particles Waves

<— increasing energy OR frequency (THz)

PHOTON'S WAVELENGTH?• Each photon oscillates at certain frequency (e.g. 600 THz)• What oscillates, is the state of polarization• At the same time, the photon proceeds at 300'000 km/sec

From: Rodieck (1998)

0 nm 500 nm 1000 nm 1500 nm 2000 nm 2500 nm

Page 5: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Visual image ≈ 2D distribution of light

• Different amounts of light (i.e. luminance) at different parts of the image

• Additionally, the spectral distribution of light may be different at different parts of the image

Page 6: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Reflectance vs reflected light

Reflectance spectra are combined (roughly) multiplicatively

Page 7: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

The eye

PUPIL SIZEØ 2—8 mmArea ≈ 1:16

OPTICAL POWERLens, distant: 16—18 DLens, close: 33 DCornea: 43 D

Page 8: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

The retina

Page 9: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Receptors — rods & cones

Dartnall, H. J. A., Bowmaker, J. K., & Mollon, J. D. (1983). Human visual pigments: microspectrophotometric results from the eyes of seven persons. Proceedings of the Royal Society of London B, 220, 115—130.

Page 10: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

From retina to brain — ganglion cells• The axons of the ganglion cells (i.e.

the optic nerve) transmits visual information to the brain

• Receptor cells (cones) / ganglion cells: 5 000 0000 / 1 500 000

• Ganglion cells "pool" information from groups of neighbouring receptor cells– Receptive field

• The receptive fields have an antagonistic center-surround organization– The ganglion cells respond to

CHANGES in luminance: not to luminance per se

Page 11: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Real life example (edge responses)

After filtering with receptive field (simulated output of a group of ganglion cells)

Page 12: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

The opponent pathways

• Three "channels": luminance, red-green & blue-yellow

• Opponent-color coding at the ganglion cell level

Luminance

Red-Green

Blue-Yellow

Page 13: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Complementary colours

Colour after-effect (30 sec)

Page 14: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Opposite colour space

Page 15: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Colour "blindness"• Dichromats —> Compressed colour space

– Protanopia, deuteranopia ("red-green colour blindness")– Tritanopia

• Anomalous trichromats —> Not compressed, but somewhat different colour space– Photopigment polymorphism: Slightly different variants of M and L cones, peaking ± 6 nm off the nominal peaks– Deuteranomaly, protanomaly

• Incidence: ≈ 8% in men, 0.40% in females (L&M opsines coded in X-chromosome)

Hess, R. (2000). Can Color-Blind Users See Your Site? http://msdn.microsoft.com/en-us/library/bb263953

Page 16: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Do we see the colours the same way?

• Does everyone see the colors the same way? — No– Deficiencies in colour vision (protanopia, etc)

• Does everyone see the colors the same way? — Maybe– Different have cultures have different numbers of basic colour terms

(Dani, New Guinea: mili & mola)– Is this reflected in the ability to discriminate colours? NO

• Does everyone see the colors the same way? — We cannot say– Even if we know that persons A and B are normal trichromats, who

discriminate and name colours in the same way, there isn't any way to compare the contents of perception between the two individuals!

• Perception is private!

Page 17: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Spatial properties of the channels

• 2—4 times more L cones (red) than M cones (green)

• Little S cones (blue) — only 10% of the whole cone population

• No S cones in the fovea!• —> The differences in

density are reflected in the spatial properties of the three pathways!

Cone distribution in the retina

Roorda, A., & Williams, D. R. (1999). The arrangement of the three cone classes in the living human eye. Nature, 397, 520—522.

Page 18: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

The channels are different!

Wandell, B. A. (1995). Foundations of vision. Sunderland, MA: Sinauer Associates. p. 328.

Different overall sensitivies and resolutions!Lum > RG > BY

> >

Channel Resolution (c/deg)

Resolution (c/cm) from 42 cm

Light-dark 45—50 ≈ 65

Red-green 20—27 ≈ 32

Blue-yellow 10—14 ≈16

Small field dichromacy—> Critical size Ø 0.25° ≈ 2 mm from a normal reading distance

In practice: Colors may change with size!

Page 19: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Example: JPEG image format

Page 20: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Taken together so far...• Color is coded into luminance component and two chromatic components

• Luminance channel is the most sensitive one and delivers the finest details

• Color channels are less sensitive and also less sensitive to details– Blue-yellow channel is the least sensitive of all

Page 21: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Picasso

"Colors are only symbols.Reality is to be found in lightness (luminance) alone."

"When I run out of blue, I use red."

Parraga, C. A, Troscianko, T., & Tolhurst, D. J. (2002). Spatiochromatic properties of natural images and human vision.Current Biology, 12, 483–487.

Page 22: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Shape

Oetvoes, litography by Victor Vasarely

Luminance information (lightness) removed—> only chromatic information

Page 23: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Shape-from-shading

Luminance contrast Chromatic contrast only

Show the demos: Object borders & illusory borders!

Page 24: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Lightness & color in nature

Kingdom, F. A. A. (2003). Color brings relief to human vision. Nature Neuroscience, 6, 641—644.

Page 25: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Correlations matter…

• Reflectance edge (often an object border)– Both luminance contrast

& chromatic contrast

• Shadow (illumination effect)– Typically only luminance

contrast– Exception: strongly

coloured spotlights

Page 26: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Taken together so far...• The luminance component carries information about the shape of the objects

• The chromatic components help in effective segregation of the objects and give additional information about the properties of the objects

• Luminance + chromatic = identification object vs illumination borders

• —> Be careful with pseudo-color representations!– You should “tell the same story” with the chromatic & luminance components of the image!

Example: Chart of sea surface height and hurricane track across Gulf of Mexico

Page 27: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Constancy of perceived colourControversy: The perceived colour of an object correlates strongly with the reflectance spectrum of the object — not on the spectrum of the light reflected from the object. But it is only the reflected light that we can see!

• Illuminate "Mondrian"-stimulus with white light (narrowband R+G+B).• Measure the luminance of the white patch with each light source separately.

—> R: 6, G: 35, B: 60• Adjust the intensities of the individual light sources so that the brown patch reflects identical luminance values.

—> R: 6, G: 35, B: 60• Illuminate the Mondrian with the adjusted combination of the light sources

• Does the brown patch now appear white?—> If the identical stimulation to the cones (≈ identical stimulus spectrum) does not predictperceived colour, what does?

Land, E. H., & McCann, J. J. (1971). Lightness and Retinex theory. Journal of the Optical Society of America, 61, 1—11.

Page 28: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Constancy of perceived color: 2

Conway, B. R. (2009). Color vision, cones, and color-coding in the cortex. The Neuroscientist, 15, 274—290.

Page 29: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Visual system is coding contrast…

Cone activation ratio = reflectance ratio/channel !

Page 30: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

... and there is adaptation• Often, color constancy is explained by the "adaptation to

the illumination"– BUT: We cannot really adapt to a light source!– RATHER: We adapt to the average light reflected

• Cones adapt to the "average" reflected light– In practice, each cone adapts to the AMOUNT of light– Cone responds LESS, when it is adapted to high light level– NOTE: Different cone cell populations (SML) may adapt

differently

• Cone cell adaptation affects...– ... the cone responses: they are closer to the responses

achieved in white light (≈ reflectance)

• Adaptation explains color constancy (at least partly)

Page 31: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Contrast!• Land & McCann noticed, that cone

activation ratios between neighbouring patches provide a cue about the physical reflectances of the patches in the stimulus.

• Retinex theory: Visual system uses this ratio information to code colour and lightness.

• Ratio information = contrast– Absolute intensities are not

interesting

• Note: The retinal ganglion cells are coding and transmitting ratio (contrast) information to the brain!

Page 32: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Real life example

Page 33: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

But: No local contrast effects?• Retinex theory: Luminance ratios (contrast) between the stimulus parts

are coded – > Physical stimulus properties are perfectly recovered.

• If contrast coding (and integration) is perfect, the shouldn't be any local contrast effects!– BUT: There are!

Page 34: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Simultaneous contrast

• Simultaneous contrast (Michel Eugéne Chevreul, 1839)

• Two identical patches on different surrounds appear different

• The patch on darker surround appears lighter than the patch on the lighter surround

• Why? The local (physical) contrast for the lighter patch is larger

First experiments: Hess, C., & Pretori, H.(1894/1970). Quantitative investigation of the lawfulness of simultaneous brightness contrast. (H. R. Flock, & J. H. Tenney, Transl.) Perceptual and Motor Skills, 31, 947—969.

Page 35: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

... also in color?

Page 36: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

...as demonstrated by Josef Albers?

Many colour contrast demos are actually demos of lightness (light-dark) contrast!

Page 37: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

But why is “colour contrast” so weak?The effects are typicallysurprisingly modest(in terms of hue),although very strongchromatic contrasts have Been applied!

Large differences in hue are achieved only under some special circumstances — for example if one of the patches is "close to its own background" in terms of chromaticity

Page 38: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Simultaneous Contrast vs Assimilation

Burnham, R. W. (1953). Bezold's color-mixture effect. American Journal of Psychology, 66, 377—385.

Page 39: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Edge gradients & lightness

Adapted from: Kanizsa, G. (1979).Organization in vision. NY: Praeger.

Adapted from: Zavagno, D., & Caputo, G. (2001). The glare effect and the perception of luminosity. Perception, 30, 209—222.

Page 40: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Asymmetry of contrast effects

• Central patch is clearly affected by the surround

• BUT: Surround is only little affected by the central patch

Gilchrist, A. L., & Bonato, F. (1995). Anchoring of lightness values in center/surround displays. Journal of Experimental Psychology: Human Perception and Performance, 21, 1427—1440.

Page 41: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Some conclusions...• In the visual system, visual image is divided into a luminance component

and two chromatic components• Luminance contrast is essential for the definition of object borders &

shapes• Chromatic contrast helps to segregate objects from the background

• Colour (lightness) constancy: in natural environment, perceived colours & lightness correspond closely to the reflectance properties of the surfaces– This is because of contrast coding & adaptation

• Violations of constancy: for example, simultaneous contrast– When all the contrasts within a complex image are integrated together, local

contrast "dominate"• Thus, manipulation of local edge properties is an efficient way to affect

perceived colour (lightness)

Page 42: Basics of colour & lightness 9.10.2014 / Tarja Peromaa PsyL Tarja Peromaa [tarja.peromaa@helsinki.fi] University of Helsinki Department of Behavioral Sciences

Take-home-message

"Color is but a sensation and has no existence outside the nervous system of living beings.”

Ogden Nicholas Rood (1831—1902)

"For the Rays, to speak properly, have no Colour. In them there is nothing else than a certain power and disposition to stir up a sensation of this Colour or that.”

Isaac Newton (1642 —1727)