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

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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