Computer Graphics Material Colours and Lighting

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Computer GraphicsMaterial Colours and Lighting

• CO2409 Computer Graphics
• Week 11

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Lecture Contents

• Materials
• Shading
• Lighting
• Light Types
• Light Models
• Applying Lighting

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Materials

• A material defines the surface properties of a
  polygon
• Colour, shininess, texture, bumpiness,
  transparency, etc.
• This lecture only concerns material colours
• Face colours
• Each polygon has a single colour
• Vertex colours
• Each vertex has a colour
• The colour is blended across the polygon using
  the nearest vertex colours

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Shading

• Two shading methods for material colours

• Flat shading
• Polygon vertices have same colour
• The polygon appears flat
• Gouraud shading
• Polygon vertices - different colours
• Pixel colours are a linear blend of the vertex
  colours
• Blended or curved looking surface

• Flat shading only useful for very simple models
• Gouraud shading is better, but still simplistic
  alone
• Lighting / textures provide improvements

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Lighting

• Can improve realism of scenes by using lighting
• To help calculate the vertex / face colours
• Previous examples have assumed constant white
  light everywhere
• so everything is perfectly clear

• Lights can make a huge difference to the look of
  a scene
• Even with simple models
• Several types of light
• Several ways to model the interaction of light
  with surfaces

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Light Types Directional / Point

• Three main types of light
• Directional Lights
• Considered to be infinitely far away
• All the light comes from the same direction
• No attenuation (see later)
• Sunlight is the main example
• Point Lights
• Light emitting in all directions from a single
  point
• Light attenuates with distance
• A light bulb is a good example

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Light Types Spotlights

• Spotlights are more complex
• Like point lights
• Light emitting from single point
• Light attenuates with distance
• But also
• Light is constrained to a cone
• Only emits in the direction bounded by the cone
• Brightest at centre of the cone
• Less bright towards the edges

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Light Attenuation

• The light emitted from point lights and
  spotlights attenuates over distance
• Is diffused (i.e. scattered) by the air
• Light from a distant source is weaker than that
  from a near source

• Typically assume that the light intensity is in
  inverse proportion to distance
• Attenuated Light Light / Distance
• The following lighting equations assume we have
  calculated this attenuation

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Light Models Ambient Lighting

• The most basic lighting effect is Ambient Light
• A constant level of light, lighting everything
  evenly

• An approximation to the complex light
  interactions between surfaces in a scene
• Gives a general background ambience.
• Set to a constant colour for an entire scene
• Or add an ambient colour for each light
• Call the ambient level AmbL

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Diffuse Lighting

• Diffuse Lighting affects vertices that point
  towards the light source
• The diffuse light hitting a vertex is calculated
  using a dot product
• Diffuse max(0, DiffL N L)

• DiffL is the light colour
• N is the vertex normal
• L is a normal pointing from the vertex to the
  light
• DiffL if normal points directly at the light
• 0 if it points away

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Specular Lighting

• Another key light effect is specular lighting
• Treats the surface as reflective resulting in a
  reflection of the light source becoming visible
• The reflection is called a highlight

• On a shiny surface highlights are sharp and
  bright
• The surface is smooth, so the reflection is
  focused
• Rougher surfaces have wider and softer highlights
• The rough surface diffuses the reflection more

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Specular Lighting - Phong

• Several mathematical models for specular light
• The Phong model is commonly supported in hardware
• The Phong specular calculation for each vertex
  is
• Specular max(0, SpecL(N H)P )

• SpecL is the light colour
• Can use different light colours for calculating
  diffuse and specular to get nicest result
• N is the vertex normal
• H is the halfway normal
• Described on the diagram
• p is the specular power
• A measure of shininess

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Applying Lighting

• Light hitting a vertex is the sum of the three
  effects
• Incident Light AmbL max(0, DiffL N L)
• max(0,
  SpecL(N H)P )
• Combine with material colour for final pixel
  colour
• Colour ColourM (AmbL max(0, DiffL N L)
• 
  max(0, SpecL(N H)P ))
• Can specify three material colours rather than
  one
• One for each lighting effect
• Colour AmbM AmbL DiffM max(0, DiffL N L)
• SpecM
  max(0, SpecL(N H)P )

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Blending with Vertex Colours

• The final effect on a sphere
• Using typical material colours
• AmbM DiffM Red
• Basic sphere colour
• SpecM White
• The specular light is reflected without tint
  (colour of the light)

• Note that the calculation is performed separately
  for the red, green and blue components
• If there are several lights then
• Accumulate the incident light for all of them
• Then combine with the material colours

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Programming Lighting

• These equations can be calculated by the API
  (fixed pipeline) or in a shader (programmable)
• Need to set up
• Light types, positions, directions, attenuation
  etc.
• Material colours (if used)
• Fixed pipeline calculates equations automatically
• We must program them explicitly for a shader
• The given Phong equations are not physically
  accurate, just approximations to reality
• Other lighting models are available
• E.g. look at available materials in Maya
• Cant use alternatives in fixed pipeline, only
  with shaders