ECE 449CS 443: COMPUTER VISON LECTURE 8

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ECE 449/CS 443 COMPUTER VISON LECTURE 8

• Sources II
• Point Light Sources
• Local Shading Models
• Photometric Stereo
• Reading
• Sources, shadows and shading (Chapter 3)
• Programming assignment, due Tue. Feb. 19.
• See www-cvr.ai.uiuc.edu/ponce/spring02/README
• What about next Thursdays class??
• Monday Feb. 11 after 230?
• Monday Feb. 18 in the afternoon?
• Wed. Feb. 20 in the afternoon?

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DEFINITION The Bidirectional Reflectance
Distribution Function (BRDF)
The BRDF is the ratio of the radiance in the
outgoing direction to the incident irradiance
(sr-1).

• Lo ( P, vo ) rBD ( P, vi , vo ) d Ei ( P, vi )
• rBD ( P, vi , vo ) Li ( P,
  vi ) cos qi dwi

Helmoltz reciprocity law
rBD ( P, vi , vo ) rBD ( P, vo , vi )
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DEFINITION Radiosity
The radiosity is the total power Leaving a point
on a surface per unit area (W m-2).
B(P) sH Lo ( P , vo ) cosqo dw
Important case Lo is independent of vo.
B(P) P Lo(P)
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DEFINITION Lambertian (or Matte) Surfaces
A Lambertian surface is a surface whose BRDF is
independent of the outgoing direction (and by
reciprocity of the incoming direction as well).
rBD( vi , vo ) rBD constant.
The albedo is rd P rBD.
Note 1 P rBD
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DEFINITION Specular Surfaces as Perfect or Rough
Mirrors
qi
qs
qi
qs
d
Perfect mirror
Rough mirror
Perfect mirror Lo (P,vs) Li (P,vi)
Phong (non-physical model) Lo (P,vo) rsLi(P,vi)
cosn d
Hybrid model Lo (P,vo) rdsH Li(P, vi) cosqi
dwirsLi(P,vi) cosn d
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DEFINITION Point Light Sources
N
qi
qi
S
A point light source is an idealization of an
emitting sphere with radius e at distance R,
with e R and uniform radiance Le emitted in
every direction. For a Lambertian surface, the
corresponding radiosity is
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Solid angle subtended by a sphere
b
e
a
a
R
a
When e R
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What is the power received at a small patch from
a distant Sphere?
qi
dw
vi
dA
d2 P Li(P,vi) cosqi dw dA
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What is the power emitted from the sphere that
reaches the Patch?
dws
qi
vi
qs
dA
Hs
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DEFINITION Point Light Sources
N
qi
qi
S
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Local Shading Model

• Assume that the radiosity at a patch is the sum
  of the
• radiosities due to light sources and sources
  alone.
• No interreflections.
• For point sources
• For point sources at infinity

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Photometric Stereo (Woodham, 1979)
??
Problem Given n images of an object, taken by a
fixed camera under different (known) light
sources, reconstruct the object shape.
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Photometric Stereo Example (1)

• Assume a Lambertian surface and distant point
  light sources.

with g(P)rN(P) and VkS
I(P) kB(P) kr N(P) S g(P)V

• Given n images, we obtain n linear equations in
  g

g Vy i
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Photometric Stereo Example (2)

• What about shadows?

• Trick use the matrix I

I iIV g

• Only works when there is no ambient illumination.

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Photometric Stereo Example (3)
r(P) g(P)
g(P)r(P)N(P)
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Integrability!
Photometric Stereo Example (3)
z
y
v
u
x
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