Refractions, Reflections and Caustics :

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• Refractions, Reflections and Caustics
• Basic Concepts
• Lecture 15

Thanks to Henrik Jensen, John Hart, Ron Fedkiw,
Pat Hanrahan, Rahul Swaminathan, Ko Nishino
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Reflection and Refraction
Vertex Normal N
Reflection
a
a
Air
Water
ß
Refraction
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Snells Law

• a Incoming a Reflected
• sin(a)/sin(ß) Refractive Index (material
  dependent)
• Refractive index inversely proportional to speed
  of light
• (Huygens Principle)

Material Refractive Index
Air 1.0003
Water 1.333
Glycerin 1.473
Immersion Oil 1.515
Glass (Crown) 1.520
Glass (Flint) 1.656
Zircon 1.920
Diamond 2.417
Lead Sulfide 3.910
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Reflection
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Really starts to be noticeable at less than
10-15 from the surface.
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Fresnel Term

• Fresnel Term as found in text books is
• F ½ (sin²(a-ß)/sin²(aß))
  tan²(a-ß)tan²(aß)
• Returns good results for strong reflecting water
  surfaces

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Approximated Fresnel Term

• F0 (N-1)2/(N1)2 is a minimum of incoming light
  parallel to the normal of the surface.
• Fa F0(1- cos(a))5 (1- F0) is a value
  between 0 and 1 depending on the angle between
  the incoming ray and the surface normal.
• e.g. F90 1, if the incoming ray is parallel to
  the surface, all light is reflected.

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Designed to reflect all light eventually to
Observer.
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Anti-reflective Coatings

• Reduce unwanted, stray reflections
• Use material coatings on glass to reduce
• reflection and maximize transmission.
• Take advantage of destructive light interference.

One of the most significant advances made in
modern lens design, whether for microscopes,
cameras, or other optical devices, is the
improvement in antireflection coating
technology.
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Anti-reflective Coatings
Magnesium fluoride very commonly used on Lenses,
Microscopes
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Refractions or Reflections?
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Refractions or Reflections Confusion
Why do you see shiny roads when they are diffuse
and are not wet? Why do you see apparent
reflections in deserts? Where else do you see
apparent reflections? Do you see reflections
above objects???
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Used in Endoscopy, Communications
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Water Drops Refractions Reflections
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A Drop and its Environment.
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Shapes of falling drop
Shapes of falling drop
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Refraction and Reflection

n


s
(s)
i
Image
i
plane
q
B

i
q
v
q
q
A
n
a

z

r
r
a
Pin hole
q
(r)
z

p
i
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Refraction
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Geometric mapping
fov165 degrees
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Experiments Refraction
Experimental setup
Calculated corners are in green
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Photometry of Refraction
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Plot of Transmitted Radiance
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Experiment Photometry of Refraction
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Reflection
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Photometry of Reflection
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Reflection vs. Refraction
refraction
1
0.8
reflection
0.6
0.4
radiance L
internal reflection
0.2
0
0
0
0.2
0.4
0.6
0.8
1
0.2
0.4
0.6
0.8
1
distance from the center
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Rendering a Rain drop
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World in a Rain Drop
Pupin Hall
Low Library
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Tired of water drops? Wait, theres more
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Rainbows Refractions Reflections
Wavelength of Light
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Visible Light Wavelength Dispersion
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Can you see rainbows during midday?
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Why are there two rainbows?
Are there only two rainbows?
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Why the color pattern?
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Bright Sky under and Dark above the Rainbow
Photo by Ben Lanterman taken immediately after a
heavy rainstorm when the air was quite saturated
at 4PM.
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Caustics Bunching up Reflections or
Refractions
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• What is wrong with this rendering?

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• With caustics

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Looking at Water

• Light is reflected and refracted at the same time
• There is a light pattern on the ground (Caustics)

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• Shafts of light and caustics
• http//nis-lab.is.s.u-tokyo.ac.jp/nis/cdrom/pg/pg
  2001_iwa.pdf

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Caustics by Refraction

• Light is refracted by the water surface
• Some spots are stronger illuminated then others

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Light rays through a water surface
dark
Bright areas caused by bunching of refracted rays
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( 1800s )
Water-filled glass spheres to focus or condense
candlelight onto small areas
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Graphics Definition

• Create nice pictures with translucent objects and
  render nice effects on diffuse surfaces.

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Caustics in Cornell Box
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• Modeling and Animating Water Surfaces

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Caustics by Reflections
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• Reflections and Caustics
• Catadioptric Imaging
• Refractions/Reflections and Caustics
• Photon Mapping
• Lecture 16

Thanks to Henrik Jensen, John Hart, Ron Fedkiw,
Pat Hanrahan, Rahul Swaminathan, Ko Nishino