Refraction and Shadows

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Refraction and Shadows
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Types of rays

• Primary rays light directly to a pixel.
• Shadow rays aka light-seeking rays.
• Reflection rays bring light reflected from
  another surface.
• Transmission rays bring light through an object.

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Types of rays

• Primary rays light directly to a pixel.
• Shadow rays aka light-seeking rays.
• Reflection rays bring light reflected from
  another surface.
• Transmission rays bring light through an object.

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Shadow rays

• Shoot shadow ray (light-seeking ray) at each
  light.
• If this ray 'sees' (intersects) a light, add the
  contribution from that light to the ambient,
  otherwise add 0.

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Reflection Rays

• If the object is reflective then we cast a
  reflection ray to determine what, if anything, is
  reflected there.
• This ray starts at the intersection point and has
  a direction v 2N(N.v).
• How did we get that?

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Remember this?
Treat ray as a vector, hence we can represent it
by the vector p u vt where u is the
starting point, v is the direction and t is
how far we've travelled along the ray.
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Perfect reflection
v
n
L
Note that v and n are unit vectors.
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Reflection rays
n
v
n(n.v)
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Reflection rays
n(n.v)
v
n(n.v)
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Reflection rays
n(n.v)
v
n(n.v)
So we start a new ray u' v't, Where u' is the
intersection point, and v' v 2n(n.v).
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Cancer
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Transmission rays
n
L
?
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Why does refraction occur?

• Different velocities through different materials.
• Ray bends towards normal when passing into an
  optically denser medium and away for the opposite
  case.
• Do all wavelengths (colours) behave the same way?

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Refractive index

• Light travels at a speed c in a vacuum and at c/n
  in a medium of refractive index n.
• Snell's law states that n1 sin?1 n2 sin2 We can
  use this to determine the direction of the
  refracted ray.

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Refractive index

• Common values of refractive index
• air 1.00
• water 1.33
• glass 1.50
• diamond 2.40

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Refraction demos

• http//www.physics.uoguelph.ca/applets/Intro_physi
  cs/refraction/LightRefract.html
• http//oldsite.vislab.usyd.edu.au/photonics/fibres
  /fizzz/refraction/
• http//www.olympusmicro.com/primer/lightandcolor/r
  efraction.html

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Ray genealogy

• Each primary ray spawns (at least) three other
  rays. (Why/what?)
• Two of those, in turn, can spawn three more.
• When do you stop?

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When do you stop?

• When ray leaves the scene.
• When the contribution becomes 'too small'.
• When we get tired i.e. when we get to a preset
  recursion limit.

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Ray genealogy
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Ray genealogy
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Ray genealogy
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Some numbers

• Consider a 1024 x 1024 pixel image.
• 1 primary ray and, on average, 6 secondary rays ?
  7M rays.
• Assume 10 operations per intersection test, 2GHz
  machine, 5 cycles/ operation.

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Some numbers

• Comes to 0.175 seconds/object.
• 100,000 objects ? 5 hours.

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Next Lecture
Problems with Raytracing