Illumination Lighting

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Illumination (Lighting)
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Outline

• Light Source
• Basic Illumination Models
• Shading Methods
• Ray-Tracing Methods
• Radiosity Lighting Model
• Shadow

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Introduction

• Realistic displays of a scene
• Perspective projections of objects
• Applying lighting effects

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Introduction (Contd.)

• Illumination model
• Lighting model or Shading model
• Calculate the intensity of light for a given
  point on the surface of an object
• Surface-Rendering algorithm
• Use the intensity of a given point to determine
  the light intensity for all projected pixel
  position in a polygon

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Light Sources

• Types of light source
• light source (direct)
• light reflector (indirect)
• Two light emitter models
• Point light source, see (b)
• Distributed (Area) light source, see (a)

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Illumination Models

• Concerning methods for calculating light
  intensity
• Also called Lighting Models
• An approximation for physical optical laws

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Types of illumination models

• Local Illumination Models
• Only considering the interchanges of the light
  sources
• Global Illumination Models
• Concerning the interchange of light between all
  surfaces
• Ray-Tracing, Light as a particle
• Radiosity, Light as a energy

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Phong Model

• Phong in 1975
• The standard model that compromises between
  acceptable results and processing cost
• Light source
• A point light source
• Light interactions with a solid
• incident light at a surface light reflected
  light scattered light absorbed light
  transmitted
• Models reflected light as
• reflected light ambient diffuse specular

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Ambient Light

• Equally on all surfaces from all directions.
• Results from multiple reflections of light from
  the many surfaces in the environment
• Our illumination equation becomes
• I IaKa
• Ia intensity of the ambient light( to be
  constant)
• Ka ambient-reflection coefficient ( 0 1)
• depends on objects material
• be empirical convenience

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Diffuse Reflection

• Dull and matte surfaces exhibit diffuse
  reflection
• Equally bright from all viewing angles
• The intensity on a given surface depends on the
  angle ? between the light's direction L and
  surface's normal N
• I IaKa IpKdcos?
• Ip Intensity of point light source
• Kd diffuse-reflection coefficient (0 1)
• cos? max( cos ?, 0)

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Total Diffuse Reflection

• Assuming that N and L have been normalized,
• I IaKa IpKd(N dot L)
• L is a constant if a point light source is at
  infinite
• (Called directional light source)

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Visual Effects of Different Values of Kd
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Visual effects of different values of Kd and Ka
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Diffuse Reflection - Further Discussions

• Light-source attenuation
• Colored lights and surfaces
• Similar for IG and IB.

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Specular Reflection

• Specular reflection can be observed on any shiny
  surface
• The highlight presents the incident light

n vary from 1 to ?00, it depends on surface
material being simulated
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Specular Reflection

• In Phong model does a good job of modeling
  specular reflection from plastic surfaces
• Reduce the calculation of the reflection vector
• where H (L V)/ L V

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Visual Effects of Specular Reflections
Ks
1.0
0.5
8
16
32
64
128
Shinness
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Visual Effects of Combined Reflections
See Figure 10-23
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Improving the Point-Light-Source Model

• Models the directionality of the lights
• Warn 83
• Multiple light sources

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Summary of the Phong Model

• Light sources are assumed to be point sources
• Light sources and viewer are located at infinity
• Only the normal vector of a surface needs to be
  computed
• The diffuse and specular terms are modeled as
  local components
• The color of the specular reflection is assumed
  to be that of the light source
• ks is set to be a constant value independent of
  the surface color
• The global term(ambient) is modeled as a constant
• Drawback
• Gives the impression of colored plastic surface

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Illumination Models and Surface-Rendering
Methods

• Light Source
• Basic Illumination Models
• Shading Methods
• Ray-Tracing Methods
• Radiosity Lighting Model
• Shadow

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Shading Models for Polygons

• Constant Shading

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Constant-Intensity Shading

• Also called flat shading
• A single intensity is calculated for each polygon
• Advantage
• Quickly displayed with same intensity value
• If a polyhedron is not an approximation of an
  object with a curved surface, flat shading
  provides an accurate rendering
• Disadvantage
• Not accurate rendering for an object

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Gouraud Shading

• Also called Intensity-interpolation shading
• Introduce Mach bands

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Phong Shading

• Normal-vector Interpolation Shading
• The rendered image is more acceptable
• Increase the cost of shading

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Gouraud Shaded Polygons with Diffuse Reflection
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Gouraud Shaded Polygons with Diffuse and Specular
Reflection
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Phong Shaded Polygons with specular Reflection
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Curved Surfaces with Specular Reflection
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The Rendering Pipeline

• For Z-buffer and Phong shading

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Demo and Trace Demo Program

• DiffuseDemo
• Diffuse.fx -gt DiffuseVS()
• AmbientDiffuseDemo
• AmbientDiffuse.fx -gt AmbientDiffuseVS()
• AmbientDiffuseSpecularDemo
• ambientdiffusespec.fx -gt AmbientDiffuseSpecVS()

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Problems with Interpolated Shading

• Polygonal silhouette
• Improve by breaking the surface into a greater
  number of smaller polygons
• Orientation dependence
• Solved by decomposing polygons
• Problems at shared vertices
• Unrepresentative vertex normals

A
C
B
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Normal Vectors

• To get the normal vector for a vertex
• For a differentiable surface, use calculus to
  find
• For not differentiable, use vertex normal
  averaging

Averaging the four polygon face normals
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Global Illumination
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Global Illumination Models

• Recursive Ray-Tracing
• Light regards as particle
• Sharpening renderer, and view dependent
• Radiosity
• Light regards as energy
• Smoothy renderer, and view independent

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Ray-tracing
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Purple Hall
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Room with Metal and Glass Sphere
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Subdivisions on Radiosity Computation
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Figure of Radiosity
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Figure of Radiosity (Contd.)
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Figure of Radiosity(Contd.)
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Illumination Models and Surface-Rendering
Methods

• Light Source
• Basic Illumination Models
• Shading Methods
• Ray-Tracing Methods
• Radiosity Lighting Model
• Shadow

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Shadows

• Shadow algorithms determine which surfaces can be
  seen from light source.
• In multiple light sources, we have problems.
• umbra and penumbra.
• Illumination equation involving shadows

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Hard Shadows
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Soft Shadows
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Remarks

• Light Source
• Basic Illumination Models
• Shading Methods
• Ray-Tracing Methods
• Radiosity Lighting Model
• Shadow