Lighting and Shading

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Lighting and Shading

• Week 6
• ISVR

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Topics

• Illumination in Imaging pipeline
• Surface Rendering
• Shading
• Reflectance and refraction properties
• Light Sources
• VRML

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Lighting, Shading and reflectance

• shadowing and shading give realism and depth
• Combination of surface properties and the light
  sources used
• Surfaces are constructed to reflect or refract
  the light
• Colour and texture used with different methods to
  render result

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Example
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Wireframe model Orthographic views
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Perspective View
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Depth Cue
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Hidden Line Removal add colour
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Constant Shading - Ambient
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Faceted Shading - Flat
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Gouraud shading, no specular highlights
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Specular highlights added
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Phong shading
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Texture Mapping
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Texture Mapping
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Reflections, shadows Bump mapping
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Lighting, Shading

• Lighting and shading give objects shape
• Important effects
• shading
• shiny highlights
• reflections
• shadows
• Local techniques simplify these effects to
  improve performance

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Shading/ lighting

• Diffuse ambient light creates no shading...
  Simplest
• Illumination can vary by angle between N (normal
  to the polygon) and L (the source)
• Source of illumination can be a point or a region
  (expressed as cosn ?). The larger the n the
  narrower the beam
• Compute N and L across polygon face

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Shading

• Can interpolate shade across a polygon
• Gouraud shading interpolates shade across edges,
  reduces effect of intensity change.
• Phong shading (and illumination) interpolates
  surface normal vector across polygons then
  interpolates illumination.

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Gouraud Shading

• Compute shading for each pixel by averaging
  shading based on distance and shading of vertices.

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Different Illumination
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Different Illumination
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Global illumination

• Global techniques provide more accuracy by
  simulating light propagation among all surfaces
  in a 3D world.
• Local shading (Gauroud shading, Phong shading)
  does not calculate global effect (shadow,
  reflection, refraction, scattering, etc)
• Technique
• ray tracing
• radiosity

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Solid Modeling

• Which surfaces should be drawn?
• Object space methods
• Hidden Surface Removal
• Painters Algorithm
• BSP Trees
• Image space methods
• Z-Buffering
• Ray Casting

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Ray Tracing

• Image space technique that mimics physical
  processes of light
• Extremely computationally intensive, but
  beautiful
• Hidden surface removal
• Transparency
• Reflections
• Refraction
• Ambient lighting
• Point source lighting
• Shadows

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Shadows and beyond

• Can be computationally intensive, project each
  polygon on light source find projection on other
  polygons
• 3D shadows, transparency, fog and atmospherics
  all complicate the computation.

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Why Lighting?

• What light source is used and how the object
  response to the light makes difference
• Ocean looks bright bluish green in sunny day but
  dim gray green in cloudy day
• Lighting gives you a 3D view to an object
• A unlit sphere looks no different from a 2D disk
• To get realistic pictures, the color computation
  of pixels must include lighting calculations

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Types of Light

• Ambient
• Light thats been scattered so much by the
  environment that its direction is impossible to
  determine - it seems to come from all directions
• Diffuse
• Light that comes from one direction, but it gets
  scattered equally in all directions
• Specular
• Light comes from a particular direction, and its
  tends to bounce off the surface in a preferred
  direction

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Materials Colors

• A materials color depends on the percentage of
  the incoming different lights it reflects
• Materials have different ambient, diffuse and
  specular reflectances
• Material can also have an emissive color which
  simulates light originating from an object
• Headlights on a automobile

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VRML Lighting Model

• Lighting has four independent components that are
  computed independently
• Emissive, Ambient, Diffuse, and Specular
• VRML approximates lighting as if light can be
  broken into red, green, and blue components
• The RGB values for lights mean different than for
  materials
• For light, the numbers correspond to a percentage
  of full intensity for each color
• For materials, the numbers correspond to the
  reflected proportions of those colors

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Theory of Illumination

• Not only knowledge about light but also about
  what happens when light is reflected from an
  object into our eyes is important to obtain
  realistic images
• The general problem is to compute the apparent
  color at each pixel that corresponds to part of
  the object on the screen
• The color produced by lighting a vertex (or an
  object) has several contributions
• Emission
• Global ambient light
• Contributions from light sources

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Adding Lighting to the Scene

• Define normal vectors for each vertex of each
  object
• Create, select, and position one ore more light
  sources
• Create and select a lighting model
• Define material properties for the objects in the
  scene

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Multiple Lights

• You can define up to eight light sources
• Need to specify all the parameters defining the
  position and characteristics of the light
• VRML performs calculations to determine how much
  light each vertex from each source
• Increasing numbers of lights affects performance

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Controlling a Lights Position and Direction

• A light source is subject to the same matrix
  transformations as a geometric model
• Position or direction is transformed in the same
  way that an element of geometry is.
• Keeping the light stationary
• Specify the light position
• Independently moving the light
• Set the light position after the modeling
  transformation that you want to apply for light
• Moving the light together with the viewpoint
• route the light position to the same movement as
  the viewpoint

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Defining Material Properties

• Specifying the ambient, diffuse, and specular
  colors, the shininess, and the color of any
  emitted light
• Diffuse and ambient reflection
• Gives color
• Specular reflection
• Produces highlights
• Emission
• Make an object glow (to simulate lamps and other
  light sources

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VRML material node
Material exposedField SFFloat ambientIntensity
0.2  0,1 exposedField SFColor diffuseColor
0.8 0.8 0.8  0,1 exposedField SFColor
emissiveColor 0 0 0  0,1 exposedField
SFFloat shininess 0.2  0,1 exposedField
SFColor specularColor 0 0 0  0,1
exposedField SFFloat transparency 0  0,1