SEM023CD3018 Advanced Computer Graphics

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SEM023/CD3018 Advanced Computer Graphics

• Lecture 7- Lighting and shading
• Friday
• Lecture 1200 1300, Room 3.08
• Lab/Tutorials 1300 1500, Lab 6.16/6.20
• Lecturer Dr Abir Hussain
• Room 633, a.hussain_at_ljmu.ac.uk

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Todays lecture

• The basics of lighting and shading
• Normal calculations
• Shading models
• (some of the materials are taken from Angels
  book)

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Why we need shading

• Suppose we build a model of a sphere using many
  polygons and colour it with glColor. We get
  something like
• But we want

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Shading

• Why does the image of a real sphere look like
• Light-material interactions cause each point to
  have a different color or shade
• Need to consider
• Light sources
• Material properties
• Location of viewer
• Surface orientation

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Scattering

• Light strikes A
• Some scattered
• Some absorbed
• Some of scattered light strikes B
• Some scattered
• Some absorbed
• Some of this scattered
• light strikes A
• and so on

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Simple Light Sources

• Spotlight
• Restrict light from ideal point source
• Ambient light
• Same amount of light everywhere in scene
• Can model contribution of many sources and
  reflecting surfaces

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Surface Types

• The smoother a surface, the more reflected light
  is concentrated in the direction a perfect mirror
  would reflected the light
• A very rough surface scatters light in all
  directions

rough surface
smooth surface
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Phong Model

• A simple model that can be computed rapidly
• Has three components
• Diffuse
• Specular
• Ambient
• Uses four vectors
• To source
• To viewer
• Normal
• Perfect reflector

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Normal vector

• REMINDER
• The first important data value in any lighting
  and shading algorithm is the Normal Vector
• This is a line in 3D space that runs at right
  angles ( is orthogonal ) to a point P on the
  polygon to be illuminated
• For convenience it has length 1

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Normal vector

• The different shading models use different points
  on the surface of a polygon to determine how it
  should be lit.
• We can calculate the Normal Vector and either
• give it to the glNormal3fv(N) function, or
• use it in our own calculations to affect the
  shading

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Plane Normal

• Equation of plane axbyczd 0
• The plane is determined by three points p0, p1,
  p2 or normal n and p0
• Normal can be obtained by

n (p2-p0) (p1-p0)
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Flat shading

• For flat shading, as used by setting
  glShadeModel(GL_FLAT), we assume that the polygon
  is equally lit across its surface at all pixels.
  So we just take one vertex of the polygon to
  calculate the normal and thus set the shading for
  that whole polygon
• The Vertex OpenGL uses depends on the type of
  polygon, e.g.
• Triangles, Triangle strips, Quads, Quad strips
  and so on

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Flat shading
All the pixels in the same quad, triangle
or polygon have the same colour value
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Smooth shading

• For Smooth Shading, as used by glShadeModel(GL_SMO
  OTH) we interpolate the colour changes in pixels
  across adjoining polygons to make sure that the
  transitions in shading appear more natural
• However, simple interpolation can have unnatural
  effects as one polygon meets another at a sharp
  boundary
• This is also known as Gouraud Shading

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Smooth Shading

• Phong shading the interpolating by calculating
  the normals for adjoining polygons.
• It then calculates the average normal vector for
  the point at which the polygons meet.
• This produces a more accurate transition in
  shading between the adjoining polygons

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Smooth shading
pixel colour values are re-calculated across a
polygon according to how the light is reflected
from a light source towards the viewer
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Specular Surfaces

• Most surfaces are neither ideal diffusers nor
  perfectly specular (ideal reflectors)
• Smooth surfaces show specular highlights due to
  incoming light being reflected in directions
  concentrated close to the direction of a perfect
  reflection

specular highlight
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Specular reflection

• This shading technique uses only ambient and
  diffuse reflections
• This approach to smooth shading works well for
  objects with a diffuse or matt surface
• For shiny, metallic objects we need to show the
  highlights
• These occur when the angle of reflection back to
  the viewer is the same or very near to the angle
  of the light hitting the object

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Phong Specular Shading

• The attached code fragment gives a set of
  functions for the Phong shading model to provide
  specular reflection or shininess
• It takes values for the Eye vector, Light Vector
  and Normal vector and a shininess value and
  calculates the new light intensity at the point
  for the normal vector

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Uses of shading

• We can of course use shading directly to
  approximate shading in the real world.
• However, we can manipulate the normals on a
  surface to give the appearance of surface
  roughness
• If we displace the normal slightly we can create
  the illusion that the polygons at that point are
  tilted towards or away from the light source

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Other approaches

• The shading models we have considered so far are
  local models
• They only consider the light coming from one or
  more sources, reflecting off one object and
  travelling to the viewpoint
• They do not consider light bouncing off or
  refracting through other objects

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Global Effects
shadow
multiple reflection
translucent surface
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Local vs Global Rendering

• Correct shading requires a global calculation
  involving all objects and light sources
• Incompatible with pipeline model which shades
  each polygon independently (local rendering)
• However, in computer graphics, especially real
  time graphics, we are happy if things look
  right
• Exist many techniques for approximating global
  effects

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Global Lighting Models

• Raytracing traces the light ray as it bounces
  around the scene and adds that information to the
  final pixel value
• This approach works well for scenes with
  reflective and refractive surfaces

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Ray tracing

• Ray tracing traces a ray back from a pixel and
  produces a node in a tree each time a ray
  reflects off or refracts through an object
• If the ray reflects, its path is followed until
  it hits another object
• If the ray refracts, a secondary ray is produced
  going through the object
• This is repeated for a maximum depth of the tree
• The tree is then traversed and optical laws
  applied to calculate how qualities of the light
  falling on the pixel

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Other approaches

• Radiosity is another approach. This method uses
  the physical laws governing the radiant energy
  transfers within an illuminated scene.
• However neither Ray tracing nor radiosity are
  currently real-time methods
• Thus they are reserved for
• high-quality image rendering or
• producing high quality series of images for
  animated 3D movies

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Summary

• We have looked at the basics of lighting and
  shading and the role that the normal vector plays
  in their calculation
• Flat, Phong and Gouraud shading remain the most
  used real-time shading methods