CS430 Computer Graphics

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CS430 Computer Graphics

• Lighting and Shading

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Topics

• Introduction
• Geometric Model in Lighting
• Colored Surfaces and Lights
• Shading and Graphics Pipeline
• Flat Shading and Smooth Shading

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Introduction

• Lighting
• Process of computing the luminous intensity
  reflected from a specific 3-D point
• Shading
• Process of assigning colors to pixels
• Shading model dictates how light is scattered or
  reflected from a surface
• We will begin with achromatic light then colored
  lights

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Introduction

• Two types of light sources
• Point light source
• Ambient light
• Light interacts surfaces in different ways
• Absorbed by surface
• Reflected by surface
• Transmitted into the interior
• What absorbs all of the incident achromatic light?

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Introduction

• Types of reflection of incident light
• Diffuse scattering
• Some of the incident light penetrates the surface
  slightly and is re-radiated uniformly in all
  directions
• Scattering light interacts strongly with surface
  ? color is affected by nature of surface material
• Specular reflections
• Incident light does not penetrate the surface
• Reflected directly from the surface
• More mirror like and highly directional
• Highlight, shiny, plastic like

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Introduction

• Total light reflected from the surface in a
  certain direction is the sum of
• Diffuse components
• Specular components
• We calculate the size of each component that
  reaches the eye for each point of interest on
  surfaces

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Geometric Model in Lighting

• Principle vectors to find amount of light
  reaching the eye from a point P
• m normal vector of surface at P
• v from P to the eye
• s from P to light source
• Angles between vectors are
• calculated in the world
• coordinates
• Is intensity of light source

m
eye
s
v
P
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Diffuse Component

• Id intensity of the diffused component
• Scattering is uniform in all directions
• Independent of v
• unless m ? v ? 0, where Id 0 (why?)
• Lamberts law brightness is proportional to the
  area subtended ( fraction cos(?))
• ? ? 0 brightness varies slightly with angle
• ? ? 90 brightness falls rapidly to 0

m
s
?
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Diffuse Component

• cos(?)
• How do we calculate/obtain the value for ?d ? By
  experiments.

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Specular Reflection

• Isp intensity of specular reflection
• Phong model is used to approximate highlight
• Amount of light reflected is greatest in the
  direction of perfect mirror reflection, r
• Amount of light reflected diminished rapidly at
  the nearby angles
• Beam pattern

r
m
s
P
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Specular Reflection

• Remember ?
• Amount of light reflected falls off as ?
  increases and is approximately cosf(?), where f
  is the Phong exponent
• Problem expensive to compute as r has to be
  found and normalized

r
m
s
eye
?
v
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Specular Reflection

• Solution (proposed by Jim Blinn)
• Calculate h s v
• Let ? be the angle between h and m
• Use ? to calculate the falloff of specular
  intensity as ? has the same property as ?
• ? ? ?, but can be compensated by different value
  of f

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Ambient Light

• A uniform background glow in the environment
• Source is not situated at any particular place
• Light spreads uniformly in all directions
• Ia intensity of light source
• Ia?a is added to the light reaching the eye
• ?a ambient reflection coefficient
• ?a is often the same as ?d

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Combining Light Contributions

• I Ia ?a Id ?d ? lambert Isp ?s ? phongf
• lambert
• phong
• Implications for different points P on a facet
• Ambient is not changed for different P
• m is the same for all point on the facet
• If the light is far far away, s will change
  slightly as P changes ? diffuse will change
  slightly on different P
• If the light or/and eye is/are close, s and h
  will change a lot as P changes ? specular changes
  significantly over the facet

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Colored Surfaces and Lights

• Colored surface
• Ir Iar ?ar Idr ?dr ? lambert Ispr ?sr ?
  phongf
• Ig Iag ?ag Idg ?dg ? lambert Ispg ?sg ?
  phongf
• Ib Iab ?ab Idb ?db ? lambert Ispb ?sb ?
  phongf
• lambert and phong terms do not depends on color
  component
• We have to define 9 reflection coefficients
• Ambient and diffuse reflection are based on the
  color of surface

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Colored Surfaces and Lights

• Colored light (Isr, Isg, Isb)
• If the color of a surface is (r, g, b), then it
  is reasonable to set
• (?ar, ?ag, ?ab) (?dr, ?dg, ?db) (rK, gK,
  bK), where K is the fraction of light reflected
• The diffusion of the surface
• (Isr?dr, Isg?dg, Isb?db) (IsrrK, IsggK,
  IsbbK)
• Example white light
• Isr Isg Isb I, (r, g, b) (0.3, 0.45,
  0.25) then diffusion (0.3IK, 0.45IK, 0.25IK) ?
  the surface is seen as its color

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Colored Surfaces and Lights

• Color of specular light
• Often the same as that of light source
• Example sunlight
• Highlight on plastic caused by sunlight is white
• Set (?sr, ?sg, ?sb) (?s, ?s, ?s)
• ?s 0.5 ? slightly shiny
• ?s 0.9 ? highly shiny
• Different coefficients are selected for specific
  materials. (Fig 8.17)

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Shading and Graphics Pipeline

• Vertices are sent down the pipeline along with
  their associated normals
• All shading calculations are done on vertices

v1, m1
v2, m2
projection matrix
viewport matrix
VM
clip
v0, m0
shading is applied here
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Shading and Graphics Pipeline

• Lights are objects and the positions of light
  sources are also transformed by the modelview
  matrix
• After all quantities are expressed in camera
  coordinates, colors are attached to vertices
  using the formula
• If an object is clipped, normals of newly
  generated vertices are calculated by interpolation

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Flat Shading and Smooth Shading

• Polygonal face in 3D space
• Individual face
• Underlying surface approximated
• Shading methods
• Flat shading
• Smooth shading
• Gouraud shading
• Phong shading

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Comparison of Shading Methods
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Flat Shading

• Entire face is drawn with the same color
• Lateral inhibition
• When there is a discontinuity across an object,
  the eye manufactures a Mach band at the
  discontinuity and a vivid edge is seen
• Specular highlights are rendered poorly
• Either no highlight at all
• Or highlight on the entire face

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Smooth Shading

• Smooth shading computes colors at more points on
  each face to de-emphasize edges between adjacent
  faces
• Use linear interpolation
• Gouraud shading
• Interpolate vertex colors
• Phong shading
• Interpolate vertex normals
• Interpolate normal for each pixel

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Gouraud Shading

• Used by OpenGL
• Example
• colora by interpolating
• color3 and color4
• colorb by interpolating
• color1 and color2
• Colors of pixels on the horizontal line segment
  is obtained by interpolating colora and colorb
• Does not picture highlights well

color3
color2
colora
colorb
color4
color1
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Phong Shading

• Compute normal at each pixel by interpolating the
  normals at the vertices
• Apply the shading model to to every point to find
  the color
• Example
• ma by interpolating m3 and m4
• mb by interpolating m1 and m2
• Normals of pixels on the
• horizontal line segment is
• obtained by interpolating ma and mb
• Colors of the pixels are then computed

m3
m2
ma
mb
m4
m1
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Phong Shading

• Very smooth appearance
• Highlights are approximated better
• Principle drawback
• Heavy computation ? slow speed
• Not supported by OpenGL
• Can be approximated using texture mapping
• Phong shading ? Phong model

Dont be confused!!