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ColorColorColor

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Color

To understand how to make realistic images, we need a basic understanding of the physics and physiology of vision. Here we step away from the code and math for a bit to talk about basic principles.

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Basics Of Color• elements of color:

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Basics of Color

• Physics: – Illumination

• Electromagnetic spectra– Reflection

• Material properties• Surface geometry and microgeometry (i.e.,

polished versus matte versus brushed)

• Perception– Physiology and neurophysiology– Perceptual psychology

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Electromagnetic Spectrum

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Theory of color

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Theory of color (Goethe)

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Theory of color (Goethe)

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White Light

• Sun or light bulbs emit all frequencies within the visible range to produce what we perceive as the "white light"

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Sunlight Spectrum

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White Light and Color

• when white light is incident upon an object, some frequencies are reflected and some are absorbed by the object

• combination of frequencies present in the reflected light that determines what we perceive as the color of the object

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Reflection

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Physiology of Vision

• The eye:• The retina

– Rods– Cones

• Color!

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Photoreceptors

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Physiology of Vision

• The center of the retina is a densely packed region called the fovea. – Cones much denser here than the

periphery

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Trichromacy• three types of cones

– L or R, most sensitive to red light (610 nm) – M or G, most sensitive to green light (560 nm)– S or B, most sensitive to blue light (430 nm)

– color blindness results from missing cone type(s)

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Trichromats(with a special sensitivity to ultraviolet)

• Some animals – for example bees – have three types of cones. Two of the cones are sensitive to "human visible" wavelengths. The third cone is sensitive to colors in the ultraviolet range of the spectrum. This cone enables them to see colors that humans can’t see.

They also perceive human-visible spectra in different hues than those that humans experience

http://www.colormatters.com/kids/eye.html

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Tetrachromats• Most bird species (that

have been studied) have at least four types of cones. They are "tetrachromats." Recent studies have confirmed tetrachromacy in some fish and turtles.

Perhaps it is mammals, including humans, that have poor color vision!

http://www.bio.bris.ac.uk/research/vision/4d.htm

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Tetrachromats• It has been suggested that

some women are also tetrachromats. One study suggested that 2-3% of the world's women may have the kind of fourth cone that gives a significant increase in color differentiation.

•Another thing to consider: Some data suggests that the architecture of the human visual system (as well as that of many animals) is really tetrachromatic - but that this capacity is blocked.

http://en.wikipedia.org/wiki/Tetrachromathttp://www.4colorvision.com/files/tetrachromat.htm

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Dichromats • Many animals have only two kinds of cones in their eyes. They are known as “dichromats.”It’s worth noting that the color-sensing pigment in these cones may be weak. Therefore, an animal –for example a dog -probably sees very weak colors.

http://en.wikipedia.org/wiki/Tetrachromathttp://www.4colorvision.com/files/tetrachromat.htm

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• Receptors contain photopigments that produce electro-chemical response; our dynamic range of light, from a few photons to looking at the sun, is 1011 => division of labor among receptors

• Rods (scotopic): only see grays, work in low-light/night conditions, mostly in periphery

• Cones (photopic): respond to different wavelength to produce color sensations, work in bright light, densely packed near fovea, center of retina, fewer in periphery

• Young-Helmholtz tristimulus theory1: 3 types of cones, each sensitive to all visible wavelengths of light, each maximally responsive in different ranges, often associated with red, green, and blue (although red and green peaks of these cones actually more yellow)

• The three types of receptors can produce a 3-space of hue, saturation and value (lightness/brightness)

• To avoid misinterpretations S (short), I (intermediate), L (long) often used instead

Receptors in Retina

1Thomas Young proposed the idea of three receptors in 1801. Hermann von Helmholtz looked at the theory from a quantitative basis in 1866. Hence, although they did not work together, the theory is called the Young-Helmholtz theory since they arrived at the same conclusions.

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Humans and Light

• when we view a source of light, our eyes respond to– hue: the color we see (red, green, purple)

• dominant frequency– saturation: how far is color from grey

• how far is the color from gray (pink is less saturated than red, sky blue is less saturated than royal blue)

– brightness: how bright is the color• how bright are the lights illuminating the

object?

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Hue

• hue (or simply, "color") is dominant wavelength

– integration of energy for all visible wavelengths is proportional to intensity of color

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Saturation or Purity of Light • how washed out or how pure the color of

the light appears– contribution of dominant light vs. other

frequencies producing white light

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Intensity, Brightness• intensity : radiant energy emitted

per unit of time, per unit solid angle, and per unit projected area of the source (related to the luminance of the source)

• brightness : perceived intensity of light. One may lower the intensity by adding white or black.

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Perception of Color• Human eyes can not differentiate

between spectral yellow and any combination of red and green

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ActivityActivityActivity

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Do all of the red squares look the same or do they look different?

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Metamers

a given perceptual sensation of color derives from the stimulus of all three cone types

• identical perceptions of color can thus be caused by very different spectra

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Metamer Demo• http://www.cs.brown.edu/exploratories/freeSoftware/catalogs/color_theory.h

tml