More on Raytracing

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More on Raytracing
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Holidays Lectures, Labs, etc

• Easter break next week.
• I will be away from Friday 7th to Sunday 23rd
  April (inclusive).
• No lectures or tutorial during the first week
  after the break.
• There WILL be a lab during that week.

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Laboratory exercises

• This week parametric equations and
  intersections. Read up lectures and attend
  tutorial today.
• Next (logical) week Colour and Lighting. Read
  relevant chapters.

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Ray Casting recapitulation
? pixels in screen do Shoot ray p from the
eye through the pixel. Find closest
ray-object intersection. Get colour at
intersection?
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Ray Casting recapitulation
? pixels in screen do Shoot ray p from the
eye through the pixel. Find closest
ray-object intersection. Get colour at
intersection?
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Ray Casting recapitulation
? pixels in screen do Shoot ray p from the
eye through the pixel. Find closest
ray-object intersection. Get colour at
intersection ? Illumination model
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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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Ambient light

• Ambient light comes from all directions and is
  reflected in all directions.
• The intensity and colour depend only the
  properties of the surface colour and
  reflectivity.

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Ambient light

• I Iaka
• where
• Ia is the intensity of the ambient light,
• and
• ka is the ambient constant of the surface (0-1).

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Perfect reflection
N
L
The angle between the incoming ray and the
surface normal is maintained for the outgoing ray.
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Diffuse reflection

• An ideal diffuse (Lambertian) reflector (e.g.
  chalk) is the simplest to model.
• Incoming light is scattered equally in all
  directions, so brightness does not depend on the
  viewing direction.

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Diffuse reflection

• Reflected brightness depends on the direction and
  brightness of illumination (cos of light/normal
  angle).

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Diffuse reflection
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Diffuse reflection
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Lambertian illumination

• Use Lamberts law, which says the intensity of
  the reflected energy (light) depends upon the
  angle between the incoming light and the surface
  normal.
• The intensity is view independent!

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Lambertian illumination

• I ILkd N.L
• where
• IL is the intensity of the light source,
• and
• kd is the diffuse constant of the surface (0-1).

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Illumination equation

• The colour of a pixel is determined by a
  combination of ambient (background) illumination
  and Lambertian reflection.
• Thus I Iaka ILkd N.L

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Specular reflection

• Shiny surfaces reflect light coherently in a
  narrow beam around the true reflected ray.
• If your eye is in that cone, the surface looks
  brighter (a highlight).

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Specular reflection

• Specular reflection isnt perfect so the
  highlight is a blob with bright-ness reducing
  gradually away from the center.

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Specular reflection
N
L
The width of the reflective cone depends upon the
smoothness of the surface.
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Specular reflection

• We can model this behavior using the Phong
  illumination model
• I kP cosn?
• where ? is the angle between the ray and the
  direction of true reflection to the eye.

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Specular reflection
N
L
R
?
V
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Diffuse and specular reflection
N
L
R
V
I Ilkd(cos? kP cosn?) I Ilkd(L.N) kP
(R.V)n)
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Full illumination model
I Iaka ?j ILj(kd(L.N) kP(R.V )n)
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Phong Model Demos

• http//www.cs.auckland.ac.nz/richard/research-topi
  cs/PhongApplet/PhongDemoApplet.html
• http//www.eml.hiroshima-u.ac.jp/member/jrs/
  nis/javaexampl/phong/phong.html
• http//www.siggraph.org/education/materials/
  HyperGraph/illumin/javaprog/shadesame.html
• http//www.eye.ch/mduerig/phong/Phong.html

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Next Lecture
Refraction and Shadows