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776 Computer Vision. Jan-Michael Frahm & Enrique Dunn Spring 2012. Photometric stereo (shape from shading). Luca della Robbia , Cantoria , 1438. Can we reconstruct the shape of an object based on shading cues?. Photometric stereo. S 2. S 1. ???. Assume: A Lambertian object - PowerPoint PPT Presentation
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776 Computer Vision
Jan-Michael Frahm & Enrique DunnSpring 2012
Photometric stereo (shape from shading)
• Can we reconstruct the shape of an object based on shading cues?
Luca della Robbia,Cantoria, 1438
Photometric stereo• Assume:
o A Lambertian objecto A local shading model (each point on a surface receives light only from
sources visible at that point)o A set of known light source directionso A set of pictures of an object, obtained in exactly the same
camera/object configuration but using different sourceso Orthographic projection
• Goal: reconstruct object shape and albedo
Sn
???S1
S2
Forsyth & Ponce, Sec. 5.4
slide: S. Lazebnik
Surface model: Monge patch
Forsyth & Ponce, Sec. 5.4
j
j
j
j
Vyxg
SkyxNyx
SyxNyxk
yxBkyxI
),(
)(,,
,,
),(),(
Image model• Known: source vectors Sj and pixel values Ij(x,y)• We also assume that the response function of the
camera is a linear scaling by a factor of k • Combine the unknown normal N(x,y) and albedo
ρ(x,y) into one vector g, and the scaling constant k and source vectors Sj into another vector Vj:
slide: S. Lazebnik
Least squares problem
• Obtain least-squares solution for g(x,y)• Since N(x,y) is the unit normal, (x,y) is given by
the magnitude of g(x,y) (and it should be less than 1)
• Finally, N(x,y) = g(x,y) / (x,y)
),(
),(
),(),(
2
1
2
1
yxg
V
VV
yxI
yxIyxI
Tn
T
T
n
(n × 1)known known unknown
(n × 3) (3 × 1)
• For each pixel, we obtain a linear system:
slide: S. Lazebnik
Example
Recovered albedo Recovered normal field
Forsyth & Ponce, Sec. 5.4
•Recall the surface is written as
•This means the normal has the form:
Recovering a surface from normals
•If we write the estimated vector g as
•Then we obtain values for the partial derivatives of the surface:
(x,y, f (x, y))
g(x,y) g1(x, y)g2 (x, y)g3(x, y)
fx (x,y) g1(x, y) g3(x, y)
fy(x, y) g2(x, y) g3(x,y)
N(x,y) 1
fx2 fy
2 1
fx
fy
1
slide: S. Lazebnik
Recovering a surface from normals
•Integrability: for the surface f to exist, the mixed second partial derivatives must be equal:
•We can now recover the surface height at any point by integration along some path, e.g.
g1(x, y) g3(x, y) y
g2(x, y) g3(x, y) x
f (x, y) fx (s, y)ds0
x
fy (x, t)dt0
y
c
(for robustness, can take integrals over many different paths and average the results)
(in practice, they should at least be similar)
slide: S. Lazebnik
Surface recovered by integration
Forsyth & Ponce, Sec. 5.4
Reading• Szeliski 2.2-2.3• Szeliski 3.1-3.2
Typical image operations
image: R. Szeliski
Color
Phillip Otto Runge (1777-1810)
What is color?• Color is the result of interaction
between physical light in the environment and our visual system
• Color is a psychological property of our visual experiences when we look at objects and lights, not a physical property of those objects or lights (S. Palmer, Vision Science: Photons to Phenomenology)
slide: S. Lazebnik
Electromagnetic spectrum
Human Luminance Sensitivity Function
Interaction of light and surfaces
• Reflected color is the result of interaction of light source spectrum with surface reflectance
slide: S. Lazebnik
Spectra of some real-world surfaces
metamers
image: W. Freeman
Standardizing color experience
• We would like to understand which spectra produce the same color sensation in people under similar viewing conditions
• Color matching experiments
Foundations of Vision, by Brian Wandell, Sinauer Assoc., 1995
Color matching experiment 1
Source: W. Freeman
Color matching experiment 1
p1 p2 p3
Source: W. Freeman
Color matching experiment 1
p1 p2 p3
Source: W. Freeman
Color matching experiment 1
p1 p2 p3
The primary color amounts needed for a match
Source: W. Freeman
Color matching experiment 2
Source: W. Freeman
Color matching experiment 2
p1 p2 p3
Source: W. Freeman
Color matching experiment 2
p1 p2 p3
Source: W. Freeman
Color matching experiment 2
p1 p2 p3 p1 p2 p3
We say a “negative” amount of p2 was needed to make the match, because we added it to the test color’s side.
The primary color amounts needed for a match:
p1 p2 p3
Source: W. Freeman
Trichromacy• In color matching experiments, most people can
match any given light with three primarieso Primaries must be independent
• For the same light and same primaries, most people select the same weightso Exception: color blindness
• Trichromatic color theoryo Three numbers seem to be sufficient for encoding coloro Dates back to 18th century (Thomas Young)
slide: S. Lazebnik
Grassman’s Laws• Color matching appears to be linear• If two test lights can be matched with the same set
of weights, then they match each other: o Suppose A = u1 P1 + u2 P2 + u3 P3 and B = u1 P1 + u2 P2 + u3 P3. Then A =
B.• If we mix two test lights, then mixing the matches
will match the result:o Suppose A = u1 P1 + u2 P2 + u3 P3 and B = v1 P1 + v2 P2 + v3 P3. Then A + B
= (u1+v1) P1 + (u2+v2) P2 + (u3+v3) P3.• If we scale the test light, then the matches get
scaled by the same amount:o Suppose A = u1 P1 + u2 P2 + u3 P3.
Then kA = (ku1) P1 + (ku2) P2 + (ku3) P3.
slide: S. Lazebnik
Linear color spaces• Defined by a choice of three primaries • The coordinates of a color are given by the
weights of the primaries used to match it
mixing two lights producescolors that lie along a straight
line in color space
mixing three lights produces colors that lie within the triangle
they define in color space
slide: S. Lazebnik
How to compute the weights of the primaries
to match any spectral signal
• Matching functions: the amount of each primary needed to match a monochromatic light source at each wavelength
p1 p2 p3
?Given: a choice of three primaries and a target color signal
Find: weights of the primaries needed to match the color signal
p1 p2 p3
slide: S. Lazebnik
RGB space• Primaries are monochromatic lights (for monitors,
they correspond to the three types of phosphors)• Subtractive matching required for some wavelengths
RGB matching functionsRGB primaries
slide: S. Lazebnik
How to compute the weights of the primaries
to match any spectral signal• Let c(λ) be one of the matching functions,
and let t(λ) be the spectrum of the signal. Then the weight of the corresponding primary needed to match t is
dtcw )()(
λ
Matching functions, c(λ)
Signal to be matched, t(λ)
slide: S. Lazebnik
Nonlinear color spaces: HSV
• Perceptually meaningful dimensions: Hue, Saturation, Value (Intensity)
• RGB cube on its vertex
slide: S. Lazebnik
Color perception• Color/lightness constancy
o The ability of the human visual system to perceive the intrinsic reflectance properties of the surfaces despite changes in illumination conditions
• Instantaneous effectso Simultaneous contrasto Mach bands
• Gradual effectso Light/dark adaptationo Chromatic adaptationo Afterimages
J. S. Sargent, The Daughters of Edward D. Boit, 1882 slide: S. Lazebnik
Simultaneous contrast/Mach bands
Source: D. Forsyth
Chromatic adaptation• The visual system changes its sensitivity
depending on the luminances prevailing in the visual fieldo The exact mechanism is poorly understood
• Adapting to different brightness levelso Changing the size of the iris opening (i.e., the aperture) changes
the amount of light that can enter the eye o Think of walking into a building from full sunshine
• Adapting to different color temperatureo The receptive cells on the retina change their sensitivity o For example: if there is an increased amount of red light, the cells
receptive to red decrease their sensitivity until the scene looks white again
o We actually adapt better in brighter scenes: This is why candlelit scenes still look yellow
http://www.schorsch.com/kbase/glossary/adaptation.htmlslide: S. Lazebnik
White balance• When looking at a picture on screen or print, we
adapt to the illuminant of the room, not to that of the scene in the picture
• When the white balance is not correct, the picture will have an unnatural color “cast”
http://www.cambridgeincolour.com/tutorials/white-balance.htm
incorrect white balance
correct white balance
slide: S. Lazebnik
White balance• Film cameras:
o Different types of film or different filters for different illumination conditions
• Digital cameras: o Automatic white balanceo White balance settings corresponding to
several common illuminantso Custom white balance using a reference
object
http://www.cambridgeincolour.com/tutorials/white-balance.htmslide: S. Lazebnik
White balance• Von Kries adaptation
o Multiply each channel by a gain factor
slide: S. Lazebnik
White balance• Von Kries adaptation
o Multiply each channel by a gain factor
• Best way: gray cardo Take a picture of a neutral object (white or gray)o Deduce the weight of each channel
• If the object is recoded as rw, gw, bw use weights 1/rw, 1/gw, 1/bw
slide: S. Lazebnik
White balance• Without gray cards: we need to “guess” which
pixels correspond to white objects• Gray world assumption
o The image average rave, gave, bave is grayo Use weights 1/rave, 1/gave, 1/bave
• Brightest pixel assumptiono Highlights usually have the color of the light source o Use weights inversely proportional to the values of the brightest
pixels• Gamut mapping
o Gamut: convex hull of all pixel colors in an imageo Find the transformation that matches the gamut of the image to
the gamut of a “typical” image under white light• Use image statistics, learning techniques
slide: S. Lazebnik
White balance by recognition
• Key idea: For each of the semantic classes present in the image, compute the illuminant that transforms the pixels assigned to that class so that the average color of that class matches the average color of the same class in a database of “typical” images
J. Van de Weijer, C. Schmid and J. Verbeek, Using High-Level Visual Information for Color Constancy, ICCV 2007.slide: S. Lazebnik
Mixed illumination• When there are several types of illuminants in the
scene, different reference points will yield different results
http://www.cambridgeincolour.com/tutorials/white-balance.htm
Reference: moon Reference: stone
slide: S. Lazebnik
Spatially varying white balance
E. Hsu, T. Mertens, S. Paris, S. Avidan, and F. Durand, “Light Mixture Estimation for Spatially Varying White Balance,” SIGGRAPH 2008
Input Alpha map Output
slide: S. Lazebnik
Uses of color in computer vision
Color histograms for image matching
http://labs.ideeinc.com/multicolr slide: S. Lazebnik
Uses of color in computer vision
Image segmentation and retrieval
C. Carson, S. Belongie, H. Greenspan, and Ji. Malik, Blobworld: Image segmentation using Expectation-Maximization and its application to image querying, ICVIS 1999.
slide: S. Lazebnik
Uses of color in computer vision
Skin detection
M. Jones and J. Rehg, Statistical Color Models with Application to Skin Detection, IJCV 2002. slide: S. Lazebnik
Uses of color in computer vision
Robot soccer
M. Sridharan and P. Stone, Towards Eliminating Manual Color Calibration at RoboCup. RoboCup-2005: Robot Soccer World Cup IX, Springer Verlag, 2006
Source: K. Grauman
Uses of color in computer vision
Building appearance models for tracking
D. Ramanan, D. Forsyth, and A. Zisserman. Tracking People by Learning their Appearance. PAMI 2007. slide: S. Lazebnik
Linear filtering
slide: S. Lazebnik
Motivation: Image denoising• How can we reduce noise in a photograph?
slide: S. Lazebnik
• Let’s replace each pixel with a weighted average of its neighborhood
• The weights are called the filter kernel• What are the weights for the average of a
3x3 neighborhood?
Moving average
111
111
111
“box filter”
Source: D. Lowe
Defining convolution
lk
lkglnkmfnmgf,
],[],[],)[(
f
• Let f be the image and g be the kernel. The output of convolving f with g is denoted f * g.
Source: F. Durand
• MATLAB functions: conv2, filter2, imfilter
Convention: kernel is “flipped”
Key properties• Linearity: filter(f1 + f2) = filter(f1) + filter(f2)• Shift invariance: same behavior regardless of
pixel location: filter(shift(f)) = shift(filter(f))• Theoretical result: any linear shift-invariant
operator can be represented as a convolution
slide: S. Lazebnik
Properties in more detail• Commutative: a * b = b * a
o Conceptually no difference between filter and signal• Associative: a * (b * c) = (a * b) * c
o Often apply several filters one after another: (((a * b1) * b2) * b3)o This is equivalent to applying one filter: a * (b1 * b2 * b3)
• Distributes over addition: a * (b + c) = (a * b) + (a * c)• Scalars factor out: ka * b = a * kb = k (a * b)• Identity: unit impulse e = […, 0, 0, 1, 0, 0, …],
a * e = a
slide: S. Lazebnik
Annoying details• What is the size of the output?• MATLAB: filter2(g, f, shape)
o shape = ‘full’: output size is sum of sizes of f and go shape = ‘same’: output size is same as fo shape = ‘valid’: output size is difference of sizes of f and g
f
gg
gg
f
gg
gg
f
gg
gg
full same valid
slide: S. Lazebnik
Annoying details• What about near the edge?
o the filter window falls off the edge of the imageo need to extrapolateo methods:
• clip filter (black)• wrap around• copy edge• reflect across edge
Source: S. Marschner
Annoying details• What about near the edge?
o the filter window falls off the edge of the imageo need to extrapolateo methods (MATLAB):
• clip filter (black): imfilter(f, g, 0)• wrap around: imfilter(f, g, ‘circular’)• copy edge: imfilter(f, g, ‘replicate’)• reflect across edge: imfilter(f, g, ‘symmetric’)
Source: S. Marschner
Practice with linear filters
000
010
000
Original
?
Source: D. Lowe
Practice with linear filters
000
010
000
Original Filtered (no change)
Source: D. Lowe
Practice with linear filters
000
100
000
Original
?
Source: D. Lowe
Practice with linear filters
000
100
000
Original Shifted leftBy 1 pixel
Source: D. Lowe
Practice with linear filters
Original
?111
111
111
Source: D. Lowe
Practice with linear filters
Original
111
111
111
Blur (with abox filter)
Source: D. Lowe
Practice with linear filters
Original
111111111
000020000 - ?
(Note that filter sums to 1)
Source: D. Lowe
Practice with linear filters
Original
111111111
000020000 -
Sharpening filter- Accentuates differences with
local average
Source: D. Lowe
Sharpening
Source: D. Lowe
Sharpening with unsharp masking
• What does blurring take away?
original smoothed (5x5)
–
detail
=
sharpened
=
Let’s add it back:
original detail
+
slide: S. Lazebnik