ECS 298 Photorealistic Image Synthesis The Human Visual System

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ECS 298 Photorealistic Image Synthesis The
Human Visual System

• Brian Budge
• Center for Image Processing and
• Integrated Computing
• Computer Science Department
• University of California, Davis

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Overview

• Physiology of the human eye
• The eye and how light is recognized
• Vision
• What does it mean for image synthesis?

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Physiology of the human eye
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Breakdown
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Outer shell

• Sclera is the white of your eyes
• 22mm in diameter and 1 mm thick
• Gives the eye its structural integrity
• Totally opaque
• Cornea is the transparent semi-spherical shell
  that covers the front of your eye where there is
  a break in the sclera
• Radius of curvature of 8mm
• Refractive index of 1.37 causes the convergence
  of rays of light within the eye!
• Aqueous humor is the fluid behind the cornea
• Basically just water, with refractive index 1.33
• Carries nutrients to cornea

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Pupil

• The iris is the the colorful part of the eye,
  connects with the choroid
• The opening centered within the iris is the pupil
• The iris consists of delicate muscle which
  dilates and contracts to allow various amounts of
  light through the pupil
• Pupil diameter ranges from about 2mm on a bright
  day to 8mm under dark conditions
• Both pupils dilate or contract at the same time
  regardless of whether lighting is consistent for
  both eyes called the consensual pupillary
  reflex

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Lens

• The lens is also called the crystalline lens
• Refractive index of 1.4 at center to 1.38 at the
  outside
• Gives fine vision adjustment known as
  accommodation which helps you focus objects of
  different distance to the eye
• This is controlled by muscles, and is why you get
  eye strain from focusing on things too close, and
  why looking far away helps relax it

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Retina

• The retina is the light sensitive layer
• Purpose is to form an appropriate real image of
  the world
• The image which exposes onto the retina is
  actually inverted, but our brain reverts it
• The fovea is on the central axis of the retina,
  and is the area of greatest visual acuity
• This is what is exposed when we look at
  something
• Only 0.25 mm in diameter, so represents very
  small portion of our field of view
• We all have a blind spot in each eye
• Caused because of the optic nerve about 5mm from
  central axis
• Take a look at the book ?
• Rods and cones are actual light receptors which
  make up retina
• Each retina has about 120 million rods and 6 to 7
  million cones

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More on Rods and Cones

• They are about 0.05 mm long and 1 to 3
  micrometers in diameter
• Necessarily small because we need resolution!
• Cones are responsible for
• High light level vision (photopic)
• High resolution
• Color
• Rods are responsible for
• Low light and night vision (scotopic)
• Changing between the two is called adaptation
• Adaptation from light to dark can take 20 to 30
  minutes
• From dark to light takes a few tenths of a second

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Even more on Rods and Cones ?

• Rods and cones contain visual pigment
• Rods have one kind of pigment
• Cones have 3 kinds of pigment
• Somehow the light bleaches out a pigment when it
  is exposed, this triggers a nerve impulse
• The pigment in rods is much more sensitive than
  the kind in cones

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Spectral Response

• It has been shown that different wavelengths have
  different ability to produce a brightness
  response
• The most effective wavelength for scotopic vision
  is 507 nm
• For photopic its 555 nm
• This leads to funny phenomenon called the
  Purkinje Effect
• Blues and greens become brighter relative to reds
  and yellows as light fades

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Temporal Response

• Rods and cones wait until pigment is completely
  bleached
• This is a summing action, and causes a temporal
  effect
• Also related is critical fusion frequency or
  critical flicker frequency
• This ranges in general from 5 to 55 Hz depending
  on brightness
• Can be more pronounced though and if things are
  very incoherent can be higher than 55 Hz (think
  flight simulators)
• Even though film is 24 frames per second, the
  shutter is flickering in excess of 60 times per
  second, so were okay

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Next please

• After the rods and cones, the signal is sent down
  nerves
• The signal hits bipolar cells, and then
  ganglion cells which connect to the brain
• Processing is done in the bipolar cells before it
  is done in the brain
• Even though there are 127 million light
  receptors, the signals are send over less than 1
  million fibers
• The nerve bundles from each eye (optic nerves)
  actually cross over in what is called the optic
  chiasma and some of the vision from each eye goes
  into each side of the brain
• This may be a preemptive coping strategy in case
  damage occurs somewhere along the way

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Vision

• Humans like to see things in 3D our eyes are
  already adapted for this
• Leads to funny optical illusions

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Vision
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Vision

• This is one reason why stereo is great it helps
  us with our natural depth resolution

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Color Vision

• One clue that we perceive everything in terms of
  3 colors (RGB) is that we can construct any other
  color from combinations of them
• We even get bonus colors! White light is not in
  the spectrum, nor is purple, but we can get them
  from combinations of RGB colors
• Still not sure exactly how color is transmitted
  to the brain, though it is thought that there are
  three color pigments in the cones
• There are other theories with 4 color receptors
  red, green, blue, and yellow
• Most scientists believe the tristimulus color
  theory

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What does it mean for image synthesis?

• We need to keep in mind that the world doesnt
  work in RGB
• We should use spectral colors to avoid losing
  information, and then convert to RGB only at the
  end
• If we really want people to believe these images
  are real, stereo is important gives depth
  information (unfortunately, were not doing this
  in the class)
• In the end product, we can get away with a lot
• People can see the same color given an infinite
  number of spectral combinations

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References

• Principles of Digital Image Synthesis, Volume 1,
  Andrew S. Glassner, 1995, Morgan Kauffman
• Introduction to Light The Physics of Light,
  Vision, and Color, Gary Waldman, 2002, Dover
  Publications