Rendering (彩現 渲染)
Content
• Light-Material Interaction
• Phong Reflection model
• Gouraud vs. Phone Shading
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Rendering
• The computation required to
convert 3D scene to 2D display
photo-realistically
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Shading
• the gradation (of color) that
give the 2D images the
appearance of being 3D
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Light-Material Interaction
specular
diffuse
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translucent
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Light
• point, spot, directional
lights
• ambient light: to
account for uniform
level room lighting
• describe a light source
through a threecomponent (RGB)
intensity
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Phong Reflection Model
• Diffuse (漫射) • Specular
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• Ambient
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Those Were the Days…
“In trying to improve the quality of the synthetic images,
we do not expect to be able to display the object
exactly as it would appear in reality, with texture,
overcast shadows, etc. We hope only to display an
image that approximates the real object closely
enough to provide a certain degree of realism.”
– Bui Tuong Phong, 1975
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Lambert’s Cosine Law
• The reflected luminous intensity in any
direction from a perfectly diffusing surface
varies as the cosine of the angle between the
direction of incident light and the normal
vector of the surface.
• Intuitively: cross-sectional area of
the “beam” intersecting an element
of surface area is smaller for greater
angles with the normal.
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Lambert’s Cosine Law
• Ideally diffuse surfaces obey cosine law.
– Often called Lambertian surfaces.
• Id = kd Iincident cos 
= kd Iincident (N·L).
– kd is the diffuse reflectance
of the material.
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N
L

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Phong Lighting Model
• Phong adds specular highlights.
• His original formula for the specular term:
– W(i)[cos s ]n
• s is the angle between the view and specular reflection directions.
• “W(i) is a function which gives the ratio of the specular reflected
light and the incident light as a function of the the incident angle i.”
– Ranges from 10 to 80 percent.
• “n is a power which models the specular reflected light for each
material.”
– Ranges from 1 to 10.
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Phong Lighting Model
• More recent formulations are slightly different.
– Replace W(i) with a constant ks independent of
the incident direction.
• What do we lose when we do this?
– Is= ks Iincident cosn
= ks Iincident (V·R)n
R = 2(N·L)N – L
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Ambient Reflection
• Local illumination models account for light scattered
from the light source only
• Light may be scattered from all surfaces in the scene.
We are missing a lot of light, typically over 50%
• Ambient term = a coarse approximation to this
missing flux
• This is a constant everywhere in the scene
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Diffuse Reflection
Johann Heinrich Lambert (1728 –
1777) was a Swiss mathematician,
physicist and astronomer.
• Lambertian scatters (wikipedia): the irradiance
landing on the area element is proportional to
the cosine of the angle between the
illuminating surface and the normal.
• When a Lambertian surface is viewed from
any angle, it has the same radiance.
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Specular Reflection
a
Effect of Shininess Coefficient a.
v
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Phong Reflection Model
• L: light source property (RGB)
• R: material property (RGB)
• ambient reflection
• diffuse reflection
To consider distance attenuation
• specular reflection
a: shininess coefficient
• final result
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Phong Model (cont)
• For multiple light sources:
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Blinn-Phong Model
• Popular variation of Phong model.
• Uses the halfway vector, H.
• Is = ks Iincident (N·H)n.
– H = (L+V) / | L+V |
L
N H


V
• What are the advantages?
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Blinn-Phong Model
• Popular variation of Phong model.
• Uses the half vector, H.
• Is = ks Iincident (N·H)n.
– H = (L+V) / | L+V |
L
Jim Blinn
(1949 - now)
N H


V
• Faster to compute than reflection vector.
• Still view-dependent since H depends on V.
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Blinn-Phong Model
I s  ks Ls r  v
a
n
r
l
n  l n
An alternate formulation
employs the half vector H
h
h  v  l , hˆ 
h
l  n  l n
r  n  l n  l  n  l n
 2n  l n  l
I s  k s Ls n  h 
a
Time-Consuming!
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Blinn-Phong Highlights
• Does using N.H vs. R.V affect highlights?
– Yes, the highlights “spread” (Wikipedia)
– Why?
• Is this bad?
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Blinn-Phong Highlights
• Does using N.H vs. R.V affect highlights?
– Yes, the highlights “spread”.
– Why?
• Is this bad?
– Not really, for two reasons.
• Can always adjust the exponent.
• Phong and Blinn-Phong are not physically based, so it
doesn’t really matter!
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Target of Shading:
Polygon, Vertex or Fragments
Recall the rendering (OpenGL) pipeline
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Shading Modes
• Flat vs. Smooth
– Flat: single color per
face
– Gouraud (intensity
interpolation)
– Phong (normal
interpolation)
• Local vs. Global
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Gouraud vs. Phong
• Most h/w implement Gouraud shading
• Phong shading can better imitate specular
effects (∵normals are interpolated)
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Compare: Flat, Gouraud, Phong
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Rendering Pipeline Tutorial
No longer on line
[local copy]
Rendering Pipeline (Foley and van Dam)
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