capturing light… in human and machinecs194-26/fa21/lectures/...the human eye is a camera! •...
TRANSCRIPT
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Capturing Light… in human and machine
CS194: Computer Vision and Comp. PhotographyAlexei Efros, UC Berkeley, Fall 2021
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PHOTOGRAPHY
light drawing/ writing
Etymology
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Image Formation
Digital Camera
The Eye
Film
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Sensor Array
CMOS sensor
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Sampling and Quantization
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Rolling Shutter
http://en.wikipedia.org/wiki/Rolling_shutter
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Saccadic eye movement
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Saccadic eye movement
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The Eye
The human eye is a camera!• Iris - colored annulus with radial muscles• Pupil - the hole (aperture) whose size is controlled by the iris• What’s the “film”?
– photoreceptor cells (rods and cones) in the retina
Slide by Steve Seitz
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The Retina
Cross-section of eye
Ganglion cell layerBipolar cell layer
Receptor layer
Pigmentedepithelium
Ganglion axons
Cross section of retina
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Retina up-close
Light
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© Stephen E. Palmer, 2002
Conescone-shaped less sensitiveoperate in high lightcolor vision
Two types of light-sensitive receptors
Rodsrod-shapedhighly sensitiveoperate at nightgray-scale vision
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Rod / Cone sensitivity
The famous sock-matching problem…
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© Stephen E. Palmer, 2002
Distribution of Rods and Cones
0
150,000100,00050,000
020 40 60 8020406080
Visual Angle (degrees from fovea)
Rods
Cones Cones
Rods
Fovea BlindSpot
# R
ecep
tors
/mm
2
Night Sky: why are there more stars off-center?
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Foundations of Vision, by Brian Wandell, Sinauer Assoc., 1995
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Electromagnetic Spectrum
http://www.yorku.ca/eye/photopik.htm
Human Luminance Sensitivity Function
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Why do we see light of these wavelengths?
© Stephen E. Palmer, 2002
…because that’s where theSun radiates EM energy
Visible Light
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The Physics of Light
Any patch of light can be completely describedphysically by its spectrum: the number of photons (per time unit) at each wavelength 400 - 700 nm.
400 500 600 700
Wavelength (nm.)
# Photons(per ms.)
© Stephen E. Palmer, 2002
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The Physics of Light
# Ph
oton
s
D. Normal Daylight
Wavelength (nm.)
B. Gallium Phosphide Crystal
400 500 600 700
# Ph
oton
s
Wavelength (nm.)
A. Ruby Laser
400 500 600 700
400 500 600 700
# Ph
oton
s
C. Tungsten Lightbulb
400 500 600 700
# Ph
oton
s
Some examples of the spectra of light sources
© Stephen E. Palmer, 2002
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The Physics of Light
Some examples of the reflectance spectra of surfaces
Wavelength (nm)
% P
hoto
ns R
efle
cted
Red
400 700
Yellow
400 700
Blue
400 700
Purple
400 700
© Stephen E. Palmer, 2002
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The Psychophysical Correspondence
There is no simple functional description for the perceivedcolor of all lights under all viewing conditions, but …...
A helpful constraint:Consider only physical spectra with normal distributions
area
Wavelength (nm.)
# Photons
400 700500 600
mean
variance
© Stephen E. Palmer, 2002
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The Psychophysical Correspondence
Mean Hue
yellowgreenblue
# Ph
oton
s
Wavelength© Stephen E. Palmer, 2002
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The Psychophysical Correspondence
Variance Saturation
Wavelength
high
medium
low
hi.
med.
low# Ph
oton
s
© Stephen E. Palmer, 2002
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The Psychophysical Correspondence
Area Brightness#
Phot
ons
Wavelength
B. Area Lightness
bright
dark
© Stephen E. Palmer, 2002
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© Stephen E. Palmer, 2002
400 450 500 550 600 650
REL
ATIV
E AB
SOR
BAN
CE
(%)
W AVELENGTH (nm.)
100
50
440
S
530 560 nm.
M L
Three kinds of cones:
Physiology of Color Vision
• Why are M and L cones so close?• Why are there 3?
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Trichromacy
Rods and cones act as filters on the spectrum• To get the output of a filter, multiply its response curve by the
spectrum, integrate over all wavelengths– Each cone yields one number
S
M L
Wavelength
Power
• How can we represent an entire spectrum with 3 numbers?• We can’t! Most of the information is lost
– As a result, two different spectra may appear indistinguishable» such spectra are known as metamers
Slide by Steve Seitz
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More Spectra
metamers
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© Stephen E. Palmer, 2002
Color Constancy
The “photometer metaphor” of color perception:Color perception is determined by the spectrum of light on each retinal receptor (as measured by a photometer).
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© Stephen E. Palmer, 2002
Color Constancy
The “photometer metaphor” of color perception:Color perception is determined by the spectrum of light on each retinal receptor (as measured by a photometer).
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© Stephen E. Palmer, 2002
Color Constancy
The “photometer metaphor” of color perception:Color perception is determined by the spectrum of light on each retinal receptor (as measured by a photometer).
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What color is the “The Dress”?
https://en.wikipedia.org/wiki/The_dress
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© Stephen E. Palmer, 2002
Color Constancy
Do we have constancy over all global color transformations?
60% blue filter Complete inversion
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Color Constancy
© Stephen E. Palmer, 2002
Color Constancy: the ability to perceive theinvariant color of a surface despite ecologicalVariations in the conditions of observation.
Another of these hard inverse problems:Physics of light emission and surface reflectionunderdetermine perception of surface color
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Camera White Balancing
• Manual• Choose color-neutral object in the photos and normalize
• Automatic (AWB)• Grey World: force average color of scene to grey• White World: force brightest object to white
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Color Sensing in Camera (RGB)3-chip vs. 1-chip: quality vs. costWhy more green?
http://www.cooldictionary.com/words/Bayer-filter.wikipedia
Why 3 colors?
Slide by Steve Seitz
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Green is in!
R G B
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Practical Color Sensing: Bayer Grid
Estimate RGBat ‘G’ cels from neighboring values
http://www.cooldictionary.com/words/Bayer-filter.wikipedia
Slide by Steve Seitz
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Color ImageR
G
B
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Image representation• Images represented as a matrix• Suppose we have a NxM RGB image called “im”
– im(1,1,1) = top-left pixel value in R-channel– im(y, x, b) = y pixels down, x pixels to right in the bth channel– im(N, M, 3) = bottom-right pixel in B-channel
• imread(filename) returns a uint8 image (values 0 to 255)– Convert to double format (values 0 to 1) with im2double
0.92 0.93 0.94 0.97 0.62 0.37 0.85 0.97 0.93 0.92 0.990.95 0.89 0.82 0.89 0.56 0.31 0.75 0.92 0.81 0.95 0.910.89 0.72 0.51 0.55 0.51 0.42 0.57 0.41 0.49 0.91 0.920.96 0.95 0.88 0.94 0.56 0.46 0.91 0.87 0.90 0.97 0.950.71 0.81 0.81 0.87 0.57 0.37 0.80 0.88 0.89 0.79 0.850.49 0.62 0.60 0.58 0.50 0.60 0.58 0.50 0.61 0.45 0.330.86 0.84 0.74 0.58 0.51 0.39 0.73 0.92 0.91 0.49 0.740.96 0.67 0.54 0.85 0.48 0.37 0.88 0.90 0.94 0.82 0.930.69 0.49 0.56 0.66 0.43 0.42 0.77 0.73 0.71 0.90 0.990.79 0.73 0.90 0.67 0.33 0.61 0.69 0.79 0.73 0.93 0.970.91 0.94 0.89 0.49 0.41 0.78 0.78 0.77 0.89 0.99 0.93
0.92 0.93 0.94 0.97 0.62 0.37 0.85 0.97 0.93 0.92 0.990.95 0.89 0.82 0.89 0.56 0.31 0.75 0.92 0.81 0.95 0.910.89 0.72 0.51 0.55 0.51 0.42 0.57 0.41 0.49 0.91 0.920.96 0.95 0.88 0.94 0.56 0.46 0.91 0.87 0.90 0.97 0.950.71 0.81 0.81 0.87 0.57 0.37 0.80 0.88 0.89 0.79 0.850.49 0.62 0.60 0.58 0.50 0.60 0.58 0.50 0.61 0.45 0.330.86 0.84 0.74 0.58 0.51 0.39 0.73 0.92 0.91 0.49 0.740.96 0.67 0.54 0.85 0.48 0.37 0.88 0.90 0.94 0.82 0.930.69 0.49 0.56 0.66 0.43 0.42 0.77 0.73 0.71 0.90 0.990.79 0.73 0.90 0.67 0.33 0.61 0.69 0.79 0.73 0.93 0.970.91 0.94 0.89 0.49 0.41 0.78 0.78 0.77 0.89 0.99 0.93
0.92 0.93 0.94 0.97 0.62 0.37 0.85 0.97 0.93 0.92 0.990.95 0.89 0.82 0.89 0.56 0.31 0.75 0.92 0.81 0.95 0.910.89 0.72 0.51 0.55 0.51 0.42 0.57 0.41 0.49 0.91 0.920.96 0.95 0.88 0.94 0.56 0.46 0.91 0.87 0.90 0.97 0.950.71 0.81 0.81 0.87 0.57 0.37 0.80 0.88 0.89 0.79 0.850.49 0.62 0.60 0.58 0.50 0.60 0.58 0.50 0.61 0.45 0.330.86 0.84 0.74 0.58 0.51 0.39 0.73 0.92 0.91 0.49 0.740.96 0.67 0.54 0.85 0.48 0.37 0.88 0.90 0.94 0.82 0.930.69 0.49 0.56 0.66 0.43 0.42 0.77 0.73 0.71 0.90 0.990.79 0.73 0.90 0.67 0.33 0.61 0.69 0.79 0.73 0.93 0.970.91 0.94 0.89 0.49 0.41 0.78 0.78 0.77 0.89 0.99 0.93
RG
B
rowcolumn
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Color spacesHow can we represent color?
http://en.wikipedia.org/wiki/File:RGB_illumination.jpg
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Color spaces: RGB
0,1,0
0,0,1
1,0,0
Image from: http://en.wikipedia.org/wiki/File:RGB_color_solid_cube.png
Default color space
R(G=0,B=0)
G(R=0,B=0)
B(R=0,G=0)
RGB cube• Easy for devices• But not perceptual• Where do the grays live?• Where is hue and saturation?
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HSV
Hue, Saturation, Value (Intensity)• RGB cube on its vertex
Decouples the three components (a bit)Use rgb2hsv() and hsv2rgb() in Matlab
Slide by Steve Seitz
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Color spaces: HSV
Intuitive color space
H(S=1,V=1)
S(H=1,V=1)
V(H=1,S=0)
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Color spaces: L*a*b*“Perceptually uniform”* color space
L(a=0,b=0)
a(L=65,b=0)
b(L=65,a=0)
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Programming Project #1Prokudin-Gorskii’s Color Photography (1907)
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Programming Project #1
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Programming Project #1
• How to compare R,G,B channels?• No right answer
• Sum of Squared Differences (SSD):
• Normalized Correlation (NCC):