warmup: what is your favorite color? · 2015. 12. 23. · some implications… 1. sensing counts...
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Warmup:What is your favorite color?
Perception and ColorLecture 23November 22, 2015
Light
Comes from a sourceTravels in a straight line
Hits things and …bouncesis “absorbed”
(turned into something else)
Light
Particle model
Comes in packets (photons)Each contains a wavelength
(over-simplification warning!)
Some implications…
1. Sensing counts photons in a place
2. You don’t know where they came from
Measuring Light (simplified)
Amount (intensity)number of photons
Propertieskinds of those photons
It’s raining photons…
Measure the rate in different places
Kinds of buckets
Chemical
Semiconductor
Biological
Strategy 1 (chemical, eletronic)
1. Open the lid for a period of time2. Close the lid3. Measure how much is in each bucket
4. Empty the buckets and repeat
Sensitivitybig buckets catch more photonscatch enough to count (otherwise noisy)
Resolutionbig buckets take up more spaceless spatial measurements
Strategy 2 (biological)
1. Photon “cracks” molecule in half2. Molecule gives off charge when cracked3. Molecule gets “repaired”
(put back together)4. Repeat
Output is a pulse train – faster = higher rate
Varieties of Photo-receptors
Retinal CellsHave photo-pigments (rhodopsin)
Two kinds: RodsCones
Wandell,“FoundationsofVision”(left)
120millionrods 5-6millioncones
DavidR.Williams,Univ.ofRochester(right)15
E.B.Goldstein“SensationandPerception”(AdaptedfromLindsay&Norman, 1977)
Ware201017
RodsBigSensitiveSpread outAll one type
Night visionPeripheral motion
ConesSmallLess-sensitiveConcentrated in places3 different types
Normal vision
Without Color: Brightness
How do we encode amount of light?
In a way that makes sense perceptually
How many different levels?
Side by Side – can you distinguish next to each other?
1% differences (in ideal case)1.01x=100 (x approx 460)
Note: 1.01, 1.0201, ….. 98.02, 99, 100In practice 2^8 = 256 is more than enough
Non-Linear
1.01, 1.0201, ….. 98.02, 99, 100
256 steps OK – if the right 256 steps
Gamma CorrectionConvert levels to “display amounts”
Display Calibration
Tune so it looks right
Setting Gammawhite = 100%black = 0%50% grey = 50%
(black&white checks)
(warning resampling messes things up)
http://epaperpress.com/monitorcal/gamma.html
http://www.normankoren.com/Gammanew_1_3.jpg
Why is color so complicated?
Different Ways we get at Color
Physics of Color
Sensing Color
Representing / Reproducing Color
Specifying Color
Different Ways we get at Color
Physics of ColorWhy a vector space for colors?
Sensing ColorWhy is 3 numbers enough?
Representing / Reproducing ColorWhich 3 numbers?
Specifying Color
The physics view of color
Frequency vs. Wavelength
Physics of Color
Spectral Colorsall one kind of photon
Distributions of colorsBlackWhiteOther
Physicists: Spectral Distribution
From Stone’s A Field Guide to Digital Color
To a physicist:A Color is…A distribution over the spectrumF(w) (w in wavelengths)
350 750 350 750 350 750
“White” Neonlaser(red) Red-lookingcolor
Colors of Materials
What colors of light are reflected/absorbed
If you shine white light on it, what bounces off
All kinds of other issues…
PerceptionHow do we sense color?
Measuring Light (simplified)
Amount (intensity)number of photons
Propertieskinds of those photons
Measure the rate in different places
Measure the rate in different places
Once converted to something else,Color information is lost
Filter so different buckets get different colors
Strategies
Put filters over the buckets
Design buckets to only count certain colors
Stack the buckets on top of each other and make the bottoms leak some colors
Use a prism to split the light
Give up spatial resolution to get color resolution
Use Fancy Optics and Materials
Different Responses to Wavelengths
350 750
RedBucket
350 750
GreenBucket
Different Responses to Wavelengths
Green Photons Count
Red Photons Don’t
350 750
GreenBucket
Different Responses to Wavelengths
Green Photons Count
Red Photons Don’t
Other “Greens” Count 350 750
GreenBucket
Metamers
Different Colors
Same Measurement
350 750
GreenBucket
350 750 350 750 350 750
A Bi-Chromat
Two receptor types2 measurements
Did you see…2 red, 2 blue4 green1 yellow, 2 blue 350 750
Real Animals
Some mono-chromatsDichromats (many mammals)Tri-Chromats (humans, dinosaurs)
Tetrachromats (some birds, some humans)Pentachromats (pigeons, ducks)
Human Vision
3 kinds of cones, Long, Medium, Short
Short WavelengthHigher Energy
Long WavelengthLower Energy
S
MRod L
What does this mean?
A color sensation is 3 “measurements”response to Lresponse to Mresponse to S
Each spectral color gives a different ratio
Faking out a human…
You could make any color sensationwith 3 lights
If you had the right colors
Imaginary color systems
No color can excite just M or L cones
Short WavelengthHigher Energy
Long WavelengthLower Energy
S
MRod L
Real Color Systems3 primaries (RGB)
Short WavelengthHigher Energy
Long WavelengthLower Energy
S
MRod L
Representing Color as3 Numbers
3 Numbers good enough?
For physics?No!
All human perception?3 numbers, imaginary primaries
In Practice?
Gre
en
“Cube” based on three primary colors
What (most) monitors use
Computer Graphics: RGB
Gamuts
Visible colors (grey) versus colors supported by the display (triangle)
Gamut: Colors that can be created using the three display primaries
Gamut Analysis
"CIExy1931Rec2020andRec709"byCIExy1931.svg:Sakuramboderivativework:GrandDrake (talk)- CIExy1931.svg.LicensedunderCCBY-SA3.0viaWikimediaCommons–http://commons.wikimedia.org/wiki/File:CIExy1931_Rec_2020_and_Rec_709.svg#mediaviewer/File:CIExy1931_Rec_2020_and_Rec_709.svg
Practical Concerns
Violet? Deep Red?not sensitive enough
Extra primaries?expensive!
Special for white/black
Some other color systems
Printing
Red, Green, Blue = light colorsadditive primaries
Inks subtract lightsubtractive primaries
Color Blending
Physical Pigments
CMYKMonitors & Light
CMY and K
White – Green = Magenta (red-blue)White – Blue = Yellow (red-green)White – Red = Cyan (blue-green)
Printing black is important, so have a special fourth ink color
Problems with RGB (or CMY)(and some alphabet soup to replace it)
Doesn’t cover all sensations - XYZInconsistent (primaries matter) - sRGB
Not perceptually uniform – LABInconvenient for users - HSV
Luminance / Brightness
Roughly the sum of the 3 channels:L = R + G + B
(ok, .299R + .587G + .114B)Y’ = X + Y + Z
Luminance as an axis
X
Y
Equi-luminant
X
Y
Easier to analyze color space
Make one axis be “lightness”L ~~ R+G+Blet’s us say “equally bright”
Other two directions are color
Define so measurements are meaningfulEqual distance = equally distant colors
CIELABL: Lightness
AB: Hue/saturation plane based on opponent responses
Euclidean distance is meaningful!
Why do we care?
LAB lets us talk about color differences!
Are two colors distinguishable?
Are a series of colors equally spaced?
Important for analysis.
Artist-friendly color systems
How do people talk about color?
Depends on who…
Physicists (distributions)Vision scientists (cone responses)Computer graphicists (device primaries)Color theorists (perceptually uniform spaces)
Normal folks…
Nameable Colors
Language effects vision!
Lots of uniformity around world
Nameability adds distinctness
Describing Color
Luminance: How light something is
Saturation: How colorful something is
Hue: What “color” something is
Artists
Think in terms of lightness/hue/saturation
Munsell Look-up Tables
From Gretag-Macbeth
HSV/HSL
HSV
It’s a cone (or double cone)
No light (black)no hueno saturation
No saturation (grey)no huewhite at fully bright
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