CSE152, Spring 2011
Intro Computer Vision
Introduction to Computer Vision
CSE 152
Lecture 6
CSE152, Spring 2011
Intro Computer Vision
Lighting
• Special light sources
– Point sources
– Distant point sources
– Strip sources
– Area sources
• Common to think of lighting at infinity (a
function on the sphere, a 2-D space)
• Completely arbitrary lighting can be
represented as a function on the 4-D ray
space (radiances)
CSE152, Spring 2011
Intro Computer Vision
Radiance
•
Power
traveling at some point
in a specified direction, per unit
area perpendicular to the
direction of travel, per unit solid
angle
•
Units:
watts per square meter
per steradian : w/(m
2
sr
1
)
Irradiance
•
How much light is arriving at a
surface?
•
Irradiance --
power per unit area:
W/cm
2
•
Total power arriving at the
surface is given by adding
irradiance over all incoming
angles
x
x
dA
θ
d
ω
(
θ
,
φ
)
CSE152, Spring 2011
Intro Computer Vision
Camera’s sensor
•
Measured pixel intensity
is a function of irradiance
integrated over
– pixel’s area
– over a range of wavelengths
– For some time
CSE152, Spring 2011
Intro Computer Vision
BRDF
•
Bi-directional Reflectance
Distribution Function
ρ
(
θ
in
,
φ
in
;
θ
out
,
φ
out
)
•
Function of
– Incoming light direction:
θ
in
,
φ
in
– Outgoing light direction:
θ
out
,
φ
out
•
Ratio of incident irradiance to
emitted radiance
^
n
(
θ
in
,
φ
in
)
(
θ
out
,
φ
out
)
CSE152, Spring 2011
Intro Computer Vision
Lambertian Surface
At image location (u,v), the intensity of a pixel x(u,v) is:
E(u,v) = [a(u,v)
n
(u,v)]
[s
0
s
]
where
•
a(u,v) is the albedo of the surface projecting to (u,v).
•
n
(u,v) is the direction of the surface normal.
•
s
0
is the light source intensity.
•
s