Illumination

Natural lighting
effects
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Surface
characteristics
Shadows
Reflections
Mathematical
models

Fall 2004
Theoretical &
empirical
CS-321
Dr. Mark L. Hornick
1
Terminology

Illumination (lighting) model

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Calculating light intensity
At each point on a surface
Surface rendering
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
Fall 2004
Apply lighting model
Obtain pixel intensities of projected surface
positions
CS-321
Dr. Mark L. Hornick
2
Light Sources: Emitters
Point source:
Area small compared
to scene
Fall 2004
Distributed source:
Area large compared
to scene
CS-321
Dr. Mark L. Hornick
3
Light Emitters: Reflectors
Specular
(shiny)
Fall 2004
Diffuse
(dull)
CS-321
Dr. Mark L. Hornick
4
Ambient Light

General brightness of scene

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Light sources, reflections, etc.
Not spatial or directional

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Constant for all surfaces
Independent of direction

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Fall 2004
Ia
Viewing direction, surface orientation
Reflected light depends on surface
CS-321
Dr. Mark L. Hornick
5
Ambient Diffuse Reflection
I ambdiff  kd I a
0  kd  1
Incident light from all directions
Reflected light scattered to all directions
Fall 2004
CS-321
Dr. Mark L. Hornick
6
Fall 2004
CS-321
Dr. Mark L. Hornick
7
Fall 2004
CS-321
Dr. Mark L. Hornick
8
Directional Diffuse Reflection
Incident light from one direction
But spread over varying areas
Reflected light scattered
equally in all directions
Intensity depends on angle
of incidence
A

I l ,diff  kd I l cos

A cos 
Il ,diff  kd Il  N  L 
Unit vectors: N (normal) and L
(to light source position) CS-321
Fall 2004
Dr. Mark L. Hornick
N
L
9
Fall 2004
CS-321
Dr. Mark L. Hornick
10
Flat Shading


Polygon surface rendering
Each polygon in the surface is shaded according to
the intensity calculations based on the polygon’s
surface normal
Fall 2004
CS-321
Dr. Mark L. Hornick
11
Gouraud Shading
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Vertex normals are calculated as the average of the
normals of the polygons that share the vertex
The vertex normal is an approximation to the true normal
of the surface at that point.
Determine vertex intensities based on vertex normal
Linearly interpolate across surface
Fall 2004
CS-321
Dr. Mark L. Hornick
12
Gouraud Shading
Fall 2004
CS-321
Dr. Mark L. Hornick
13
Fall 2004
CS-321
Dr. Mark L. Hornick
14
Specular Reflection
Reflected light not diffused
Angle of incidence = angle of reflection
R
For perfect reflector, no reflection
visible at any other angle
 f
N
L
V

For imperfect reflectors, some
reflection visible at angle f from R
Fall 2004
CS-321
Dr. Mark L. Hornick
15
Phong Model
Empirical model of specular reflection
I spec  W   Il cos f
ns
Specular reflection parameter, ns,
is large (>100) for shiny surfaces,
small (~1) for dull ones
Specular reflection coefficient,
W(), is relatively constant for
many opaque materials; i.e. W()
modeled by constant ks
Fall 2004
CS-321
Dr. Mark L. Hornick
R
 f
N
L
V


16
Fall 2004
CS-321
Dr. Mark L. Hornick
17
Multiple Light Sources
I  I diff  I spec
n
 ka I a   I l  kd  N  Li   k s  N  H  


i 1
ns
• We assume linear superposition
of effects of all light sources.
• It may be necessary to scale to
avoid intensity saturation.
Fall 2004
CS-321
Dr. Mark L. Hornick
18
Fall 2004
CS-321
Dr. Mark L. Hornick
19
Other Lighting Issues

Not all sources are points
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Control intensity by direction (Warn)
Intensity falls off at distance
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Attenuation functions
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Empirical rather than exact models
Color

Fall 2004
Adjust reflection coefficients
CS-321
Dr. Mark L. Hornick
20
Fall 2004
CS-321
Dr. Mark L. Hornick
21