Illumination

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Illumination Lighting
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Pipeline so far

• Weve talked about the rendering pipeline
• Transformations
• Modeling
• Viewing
• Projection
• Clipping
• Rasterization w/ Depth
• Given triangles in 3D, we can efficiently
  calculate which pixels they cover in the final
  image
• But our model for surface color is rather simple
  and doesnt account for illumination.

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Modeling surface-light interaction

• If were attempting to create a realistic image,
  we need to simulate the lighting of the surfaces
  in the scene
• Simulation of physics and optics
• Two kinds of models for any simulation
• Physically-based using real physical properties
  in model calculations
• Empirical using approximations based on
  observation
• As youll see, we use a lot of approximations
  hacks, really to do this simulation fast
  enough for interactive graphics.
• A lot of current research is focused on how to
  make fast, accurate, physically-based models for
  interactive graphics.

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Terms

• Illumination the transport of energy (in
  particular, the luminous flux of visible light)
  from light sources to surfaces points
• Note includes direct and indirect illumination
• Lighting the process of computing the luminous
  intensity (i.e., outgoing light) at a particular
  3-D point, usually on a surface
• Shading the process of assigning colors to pixels

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Components of illumination

• Two interacting components of illumination
• light sources surface properties
• Direct illumination is the interaction between a
  light source and the surfaces visible from the
  light source the first reflection of the light
  by a surface
• Indirect illumination is the interaction between
  light reflected by one surface and other surfaces
  secondary (and more) reflections of the light

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Surface properties

• Spectrum of reflectance (surface color)
• Geometric properties
• Position
• Orientation
• Shape
• Microgeometry (smooth, rough, in-between)

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Light properties

• Spectrum of emittance (light color)
• Geometric properties
• Position
• Orientation
• Shape
• Directional attenuation

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Ambient light

• Indirect lighting computations require many many
  many many reflection calculations that too many
  manys to do interactively.
• How can we simulate indirect illumination without
  all those reflection calculations?
• Fake it. Assume a constant indirect luminance
  reaches all objects. We call this ambient light.
• The objects then have a property that determines
  how much incident ambient light reflects off the
  surface.

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Ambient lighting

• For each sampled wavelength, the ambient light
  reflected from a surface depends on
• Surface ambient reflection property
• Ambient light source intensity a constant
  everywhere

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Contoh ambient light
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Diffuse reflection

• An ideal diffuse reflector, at the microscopic
  level, is a very rough surface (chalk is a good
  approximation)
• Because of these microscopic variations, an
  incoming ray of light is equally likely to be
  reflected in any direction

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Lamberts cosine law

• Ideal diffuse surfaces reflect according to
  Lamberts cosine law
• The energy reflected by a small portion of a
  surface from a light source in a given direction
  is proportional to the cosine of the angle
  between that direction and the surface normal
• These are often called Lambertian surfaces.
• Note that the reflected intensity is dependent on
  the surface orientation with regard to the light
  source and that view direction has not bearing on
  the result for this reason we often refer to
  diffuse reflection as view-independent.

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Diffuse lighting

• The angle between the surface normal and the
  incoming light is the angle of incidence
• In practice we use (unit) vector arithmetic

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Diffuse lighting

• What does it mean for the dot product to be
  negative?
• What can we do about it?
• A Lambertian sphere seen at several different
  lighting angles

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Contoh diffuse light
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Specular reflection

• At the other extreme from Lambertian surfaces are
  perfectly smooth surfaces ideal specular
  reflectors
• A light shining on a specular surface causes a
  bright spot called a specular highlight
• The location of these highlights is a function of
  view direction, so we commonly refer to specular
  reflection as view-dependent

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Snells laws

• Reflection follows Snells laws
• The incoming ray and reflected ray lie in a plane
  with the surface normal
• The angle that the reflected ray forms with the
  surface normal equals the angle formed by the
  incoming ray and the surface normal ?l ?r

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?l
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Non-ideal reflectors

• Snells laws apply to ideal reflectors (mirrors,
  chrome)
• Most surfaces are not ideal reflectors we need
  a way to model typical surfaces
• Assume most light reflects according to Snells
  law, but some light reflects off the surface near
  the ideal angle due to imperfectly smooth
  microgeometry.

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?l
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Phong lighting

• The most common specular lighting model in
  computer graphics was suggested by Phong.
• Though it has no physical basis, in practice it
  looks OK

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?l
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Phong lighting

• Whats nshiny, you ask?
• It controls how fast the Phong reflectance term
  drops off as the view vector differs form the
  ideal reflectance vector.
• What does nshiny do visually?

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Phong lighting

• As before, we can replace the cosine with a dot
  product, leaving us with simple (unit) vector
  math
• How do we calculate R?

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?l
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Calculating the reflection vector
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Contoh specular lighting
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Phong lighting examples

• Spheres with different values of nshiny

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Combining ambient, diffuse, and specular
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Intensity attenuation

• Light models commonly include attenuation of
  light due to distance and position.
• The energy incident on a surface due to a light
  source falls off as the inverse of the squared
  distance between them.
• If we applied this distance attenuation directly
  to the light model, the changes in light
  intensity would be too great near the light
  source and too small far away time for a hack.
• We use this attenuation function

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Spotlights

• So far, we have a model for an omnidirectional
  light source, but many lights direct the majority
  of their energy.
• We can use a term similar the the Phong lighting
  model to focus light in a given direction.
• We can make a spotlight
• omnidirectional by setting
• ? 180?

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Putting it all together
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Putting it all together
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Multiple lights

• To support multiple lights, we need to sum the
  portions light-dependent calculations over all
  lights.
• Care has to be taken to avoid having total
  reflected light exceed 1
• Either clamp at the end, or
• The sum of the material properties for a surface
  should be between 0 and 1. Each light intensity
  should be normalized to account for all lights.

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Color

• The preceding gives us the intensity light at one
  wavelength.
• To do multispectral lighting, we need
  coefficients for each wavelength in the color
  model.
• Both the surface and the light need a coefficient
  per wavelength colored lights and colored
  surfaces.
• Its uncommon for shininess or attenuation to be
  wavelength dependent, but certainly could be done
  in the model.

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OpenGL

• OpenGL supports separate diffuse and specular
  coefficients for lights.
• It also allows each light to have an ambient term
  (subject to attenuation) along with the global
  term we discussed.

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OpenGL

• OpenGL uses a simpler Phong model, replacing VR
  with NH.
• The vector H is called the halfway vector it is
  between V and L.
• For given L and V, the halfway vector is the
  direction of orientation of the surface which
  would give maximum specular reflection in the
  viewing direction.

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