Emil Eifrem at GraphSummit Copenhagen 2024 - The Art of the Possible.pptx
Color
1. Color
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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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2. 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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5. White Light
• Sun or light bulbs emit all frequencies
within the visible range to produce
what we perceive as the quot;white lightquot;
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Sunlight Spectrum
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6. 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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7. 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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9. 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
• Some animals – for example
sensitivity to bees – have three types of
cones. Two of the cones are
ultraviolet) sensitive to quot;human visiblequot;
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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10. • Most bird species (that
have been studied) have
Tetrachromats at least four types of
cones. They are
quot;tetrachromats.quot;
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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• It has been suggested that
some women are also
Tetrachromats 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/Tetrachromat
http://www.4colorvision.com/files/tetrachromat.htm 20
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11. • Many animals have only
Dichromats 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/Tetrachromat
http://www.4colorvision.com/files/tetrachromat.htm 21
Receptors in Retina
• 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
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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12. 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, quot;colorquot;) is dominant
wavelength
– integration of energy for all visible
wavelengths is proportional to intensity of
color
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13. 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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14. Perception of Color
• Human eyes can not differentiate
between spectral yellow and any
combination of red and green
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Activity
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15. 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 32
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