EECS 274 Computer Vision
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Transcript of EECS 274 Computer Vision
EECS 274 Computer Vision
Sources, Shadows, and Shading
Surface brightness
• Depends on local surface properties (albedo), surface shape (normal), and illumination
• Shading model: a model of how brightness of a surface is obtained
• Can interpret pixel values to reconstruct its shape and albedo
• Reading: FP Chapter 2, H Chapter 11
Radiometric properties• How bright (or what color)
are objects?• One more definition:
Exitance of a light source is– the internally generated
power (not reflected) radiated per unit area on the radiating surface
• Similar to radiosity: a source can have both– radiosity, because it reflects– exitance, because it emits
• Independent of its exit angle
• Internally generated energy radiated per unit time, per unit area
• But what aspects of the incoming radiance will we model?– Point, line, area source– Simple geometry
dPLPE oe 00 cos),,()(
Radiosity due to a point sources
2
r
Radiosity due to a point source
id
id
Er
PB
cos
cos termExitanceangle solid2
• As r is increased, the rays leaving the surface patch and striking the sphere move closer evenly, and the collection changes only slightly, i.e., diffusive reflectance, or albedo
• Radiosity due to source
Nearby point source model
• The angle term, can be written in terms of N and S• N: surface normal
• ρd: diffuse albedo
• S: source vector - a vector from P to the source, whose length is the intensity term, ε2E– works because a dot-product is basically a cosine
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cos termExitanceangle solid
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Point source at infinity
• Issue: nearby point source gets bigger if one gets closer– the sun doesn’t for any
reasonable assumption
• Assume that all points in the model are close to each other with respect to the distance to the source
• Then the source vector doesn’t vary much, and the distance doesn’t vary much either, and we can roll the constants together to get:
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Line sources
radiosity due to line source varies with inverse distance, if the source is long enough
Infinitely long narrow cylinder with constant exitance
Area sources
• Examples: diffuser boxes, white walls
• The radiosity at a point due to an area source is obtained by adding up the contribution over the section of view hemisphere subtended by the source – change variables and add
up over the source
Radiosity due to an area source
• ρd is albedo
• E is exitance• r is distance
between points Q and P
• Q is a coordinate on the source
source
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Shading models
• Local shading model– Surface has radiosity due
only to sources visible at each point
– Advantages:• often easy to
manipulate, expressions easy
• supports quite simple theories of how shape information can be extracted from shading
• Global shading model– Surface radiosity is due to
radiance reflected from other surfaces as well as from surfaces
– Advantages:• usually very accurate
– Disadvantage:• extremely difficult to
infer anything from shading values
Local shading models
• For point sources at infinity:
• For point sources not at infinity
Pssd
Pss
SPNP
PBPB
from visiblesources
from visiblesources
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Shadows cast by a point source• A point that can’t see the
source is in shadow (self cast shadow)
• For point sources, the geometry is simple (i.e., the relationship between shape and shading is simple)
• Radiosity is a measurement of one component of the surface normal
Pssd
Pss
SPNP
PBPB
from visiblesources
from visiblesources
)()(
)()(
Analogous to the geometry of viewing in a perspective camera
Area source shadows
Are sources do not produce darkshadows with crisp boundaries
1.Out of shadow2.Penumbra (“almost shadow”)3.Umbra (“shadow”)
Photometric stereo
• Assume:– A local shading model– A set of point sources that are infinitely
distant– A set of pictures of an object, obtained in
exactly the same camera/object configuration but using different sources
– A Lambertian object (or the specular component has been identified and removed)
Projection model for surface recovery - Monge patch
In computer vision, it is often known as height map, depth map, or dense depth map
Monge patch
Image model
• For each point source, we know the source vector (by assumption)
• We assume we know the scaling constant of the linear camera (i.e., intensity value is linear in the surface radiosity)
• Fold the normal and the reflectance into one vector g, and the scaling constant and source vector into another Vj
• Out of shadow:
• g(x,y): describes the surface
• Vj: property of the illumination and of the camera
• In shadow:
j
j
j
j
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kSyxNyx
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yxkByxI
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From many views
• From n sources, for each of which Vi is known
• For each image point, stack the measurements
• Solve least squares problem to obtain g
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One linear system per point
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Dealing with shadows
Known Known Known Unknown
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Recovering normal and reflectance
• Given sufficient sources, we can solve the previous equation (e.g., least squares solution) for g(x, y)
• Recall that g(x, y) =(x,y) N(x, y) , and N(x, y) is the unit normal
• This means that alberdo x,y) =||g(x, y)||• This yields a check
– If the magnitude of g(x, y) is greater than 1, there’s a problem
• And N(x, y) = g(x, y) / x,y)
Five synthetic images
Generated from a sphere in a orthographic view from the same viewing position
Recovered reflectance
||g(x,y)||=ρ(x,y): the alberdo value should be in the range of 0 and 1
Recovered normal field
For viewing purpose, vector field is shown for every 16th pixel in each direction
Parametric surface tangents
u
v
x
u
x
v
Shape from normals
• Recall the surface is written as
• Parametric surface
• This means the normal has the form:
• If we write the known vector g as
• Then we obtain values for the partial derivatives of the surface:
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fr
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vu
vu ,
),(,,
Shape from normals• Recall that mixed second partials are equal --- this gives us a
check. We must have:
(or they should be similar, at least)• Known as integrability test• We can now recover the surface height at any point by
integration along some path, e.g.
x
yxgyxg
y
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)),(/),(()),(/),(( 3231
v u
xy cdxvxfdyyfvuf0 0
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Recovered surface by integration
v u
xy cdxvxfdyyfvuf0 0
),(),0(),(
The illumination cone
N-dimensional Image Space
x1
x2
What is the set of n-pixel images of an object under all possible lighting conditions (at fixed pose)? (Belhuemuer and Kriegman IJCV 99)
xn
Single light source image
The illumination cone
N-dimensional Image Space
x1
x2
What is the set of n-pixel images of an object under all possible lighting conditions (but fixed pose)?
Illumination Cone
Proposition: Due to the superposition of images, the set of images is a convex polyhedral cone in the image space.
xn
Single light source images:Extreme rays of cone
2-light source image
For Lambertian surfaces, the illumination cone is determined by the 3D linear subspace B(x,y), where
When no shadows, then Use least-squares to find 3D linear subspace, subject to the constraint fxy=fyx (Georghiades,
Belhumeur, Kriegman, PAMI, June, 2001)
Original (Training) Images
x,y fx(x,y) fy(x,y) albedo (surface normals) Surface. f (x,y) (albedo
textured mapped on surface)
3D linear subspace
Generating the illumination cone
i
ii
i syxBsyxnyxyxI )0,),(max()0,),(),(max(),(
i
isyxByxI ),(),(
Single Light Source Face Movie
Image-based rendering: Cast shadows
Yale face database B
• 10 Individuals• 64 Lighting Conditions• 9 Poses => 5,760 Images
Variable lighting
Limitation
• Local shading model is a poor description of physical processes that give rise to images– because surfaces reflect light onto one another
• This is a major nuisance; the distribution of light (in principle) depends on the configuration of every radiator; big distant ones are as important as small nearby ones (solid angle)
• The effects are easy to model• It appears to be hard to extract information
from these models
Interreflections - a global shading model• Other surfaces are now area sources -
this yields:
• Vis(x, u) is 1 if they can see each other, 0 if they can’t
usi
d dAuxVisuxr
B(u)xxExB ),(),(
coscos)()()(
surfacesother todueRadiosity Exitance surface aat Radiosity
2
sourcesother all
What do we do about this?
• Attempt to build approximations– Ambient illumination
• Study qualitative effects– reflexes– decreased dynamic range– smoothing
• Try to use other information to control errors
Ambient illumination
• Two forms– Add a constant to the radiosity at every point in
the scene to account for brighter shadows than predicted by point source model
• Advantages: simple, easily managed (e.g. how would you change photometric stereo?)
• Disadvantages: poor approximation (compare black and white rooms
– Add a term at each point that depends on the size of the clear viewing hemisphere at each point
• Advantages: appears to be quite a good approximation, but jury is out
• Disadvantages: difficult to work with