Color Constancy Lecture 11 Chapter 5, Part 3 Jonathan Pillow - - PowerPoint PPT Presentation

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Color Constancy Lecture 11 Chapter 5, Part 3 Jonathan Pillow - - PowerPoint PPT Presentation

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Color Constancy Lecture 11 Chapter 5, Part 3 Jonathan Pillow Sensation & Perception (PSY 345 / NEU 325) Princeton University, Spring 2015 1 Color Constancy The visual system uses a variety of tricks to make sure things look the same

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Color Constancy

Lecture 11 Chapter 5, Part 3 Jonathan Pillow Sensation & Perception (PSY 345 / NEU 325) Princeton University, Spring 2015

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Color Constancy

The visual system uses a variety of tricks to make sure things look the same color, regardless of the illuminant (light source)

  • Color constancy - tendency of a surface to appear the

same color under a wide range of illuminants

  • To achieve color constancy, we must discount the illuminant

and determine the object’s surface properties

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Illusion illustrating Color Constancy

(the effects of lighting/shadow can make colors look different that are actually the same!) Same yellow in both patches Same gray around yellow in both patches

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Exact same light coming to your eye from these two patches But the brain infers that less light is hitting this patch, due to shadow CONCLUSION: the lower patch must be reflecting a higher fraction of the incoming light (i.e., it’s brighter)

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Beau Lotto

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Beau Lotto

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Color Constancy

Beau Lotto

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Color Constancy

Beau Lotto

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  • Visual system tries to discount the effects of the

illuminant: it cares about the properties of the surface, not the illuminant. (on last slide: brain discounts the cone responses by taking into account information about much light is hitting different surfaces)

  • still unknown how the brain does this: believed

to be in cortex (V1 and beyond).

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  • but: color-constancy is not perfect
  • possible to fool the visual system by:

– using a light source with unusual spectrum

(most light sources are broad-band; narrow-band lights will make things look very unusual)

– showing an image with little spectral variation

(e.g., a blank red wall).

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  • r in rare cases we can fool 2/3 of the population

(and sow division and hostility across the internet!)

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So what’s going on?

400 700 wavelength energy

light hitting eye × =

reflectance energy

illuminant power spectrum surface reflectance function

? ?

  • bject of

interest

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Possibility #1: dress in blueish light (or shadow)

400 700 wavelength energy

× =

reflectance energy

blueish light source

white stripe!

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Possibility #2: dress in yellow light

400 700 wavelength energy

× =

reflectance energy

yellowish light source

blue stripe!

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So: percept depends on inferences about the light source!

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So: percept depends on inferences about the light source!

Of course: we have no idea (so far) why people are making such radically different inferences about light

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  • ne possibility: where did you look first?

Top

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Bottom

  • ne possibility: where did you look first?

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Color mixing

  • Mixing of lights (additive) vs

Mixing of paints (subtractive)

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  • Additive color mixing
  • If light A and light B both arrive at the eye, the effects of

those two lights add together

  • (that is, the power spectra add)

Mixing of lights:

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Georges Seurat’s painting La Parade (1888)

  • illustrates the effect of additive color mixture
  • reflected light from nearby dots adds together when blurred

by the optics of the eye This is the same effect we get from a TV monitor with 3 kinds phosphors

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Mixing of paints:

  • Subtractive color mixing
  • If pigment A and B mix, some of the light

shining on the surface will be subtracted by A and some by B. Only the remainder contributes to the perception of color

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Example of subtractive color mixture: “white”—broadband—light is passed through two filters This is the same result we’d get from mixing together yellow & blue paints.

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color blindness

  • About 8% of male population, 0.5% of female

population has some form of color vision deficiency: Color blindness

  • Mostly due to missing M or L cones (sex-linked;

both cones coded on the X chromosome)

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  • Protanopia: absence of L-cones
  • Deuteranopia: absence of M-cones
  • Tritanopia: absence of S-cones

Types of color-blindness:

dichromat - only 2 channels of color available (contrast with “trichromat” = 3 color channels). Three types, depending on missing cone: Frequency: M / F

2% / 0.02% 6% / 0.4% 0.01% / 0.01%

includes true dichromats and color-anomalous trichromats

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Other types of color-blindness:

  • Monochromat: true “color-blindness”; world

is black-and-white

  • cone monochromat - only have one cone

type (vision is truly b/w)

  • rod monochromat - visual in b/w AND

severely visually impaired in bright light

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Rod monochromacy

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normal trichromat deuteranope protanope tritanope monochromat scotopic light levels

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Color Vision in Animals

  • most mammals (dogs, cats, horses): dichromats
  • old world primates (including us): trichromats
  • marine mammals: monochromats
  • bees: trichromats (but lack “L” cone; ultraviolet

instead)

  • some birds, reptiles & amphibians: tetrachromats!

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Color vision doesn’t work at low light levels!

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Two Regimes of Light Sensitivity

  • Photopic: Light intensities that are bright enough to

stimulate the cone receptors and bright enough to “saturate” the rod receptors

  • Sunlight and bright indoor lighting
  • Scotopic: Light intensities that are bright enough to

stimulate the rod receptors but too dim to stimulate the cone receptors

  • Moonlight and extremely dim indoor lighting

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Other (unexplained) color phenomenon:

  • watercolor illusion
  • neon color spreading
  • motion-induced color: Benham’s top

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Watercolor illusion

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Watercolor illusion

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Watercolor illusion

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Neon Color-Spreading

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Neon Color-Spreading

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Neon Color-Spreading

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Neon Color-Spreading

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http://www.michaelbach.de/ot/col_benham/index.html

Benham’s top: motion-induced color perception

  • not well-understood; believed to arise from different color-
  • pponent retinal ganglion cells having different temporal latencies.
  • the flickering pattern stimulates the different color channels

differently (although this is admittedly a crude theory)

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  • color constancy
  • photopic / scotopic light levels
  • additive / subtractive color mixing
  • color blindness

Summary

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