Human Perception

Human Perception What you may or may not already know, want to know or even care about how you see

Goals • Decent understanding of human visual response • Discuss intelligently about the flaws of current popular color systems • Understand a few existing alternatives • Completely redefine the way you look at the world, think about light, color, space-time and life in general

Outline • Human Eye • Anatomy • Reaction to light • Engineer’s perspective • Mathematician's View • Color Spaces • Brief History • CIE-XYZ, XYy, etc. • RGB • Recent work • Why Monitors Stink

Human Eye

Human Eye 2 • Rods and Cones • Cones see color • “red” “green” and “blue” • ~6.5 Million • 32% “red” 64% “green” 2% “blue” • High resolution - closely packed

Human Eye 3 • Rods and Cones • Rods don’t • May be excited by single photon • Respond to higher frequencies (blue/green) • Peripheral and night vision • Weird fact about sea captains • For the most part, we ignore rods

Reaction to Light • For the Engineers: Intensity I = 0.299R + 0.587G + 0.114B Input Discuss later Right now: “magic” this is your brain …Any questions? Color

Reaction to Light 2 • For the mathematicians: • Several slides to follow to discuss sampling, lighting, integrating, linear algebra, etc • Be Prepared!

Math Stuff 1

Math Stuff 2 • Color is not “inherent” to a surface/object • Light source fires photons of varying wavelengths to object • Object reflects and absorbs different amounts of different wavelengths • Wavelengths reach cones on your eye • Integrated across response function

Quick Example

Math Stuff 3 • What does this mean? • Think of a red laser on a “green” ball • Different light  Different color • All three cone types excited by each wavelength • nothing pure anymore? • Metamers

Math Stuff 4 • If we consider the input from your eye to the “color unit” as linear algebra: • 3 independent (but not orthogonal) vectors • Can they span the “space”?

History • CIE – Commision Internationale de l'Éclairage • (aka International Commission on Illumination) • Established 1913 • Color Standards 1931 and 1945 • “Devoted to international cooperation on all matters relating to the science and art of lighting”

CIE Stuff • CIE Standardized several color spaces: • XYZ – The “base” color space where X and Z are “color” variables and Y is “intensity” • Can be quickly integrated from wavelength values • XYy – The horseshoe we’ve all seen before

What’s wrong with these? • 1) They give the wrong impression

What’s wrong with these? 2 • 2) They are trivariate • No metamers • 3) They do not describe the way light interacts in reality, but simplify it according to how the human eye perceives

From bad to worse • RGB • Simplified version of CIE spaces • Values from 0..1 • Used for I/O with CRT and LCD devices • So much interesting stuff happens “outside” of RGB space!

Recent Work • [Meyer 1988] • Used Gaussian Quadrature to find 3 simulation wavelengths which most affect perception (called AC1C2 space) • [Johnson and Fairchild 1999] • Created a full spectral renderer to correctly discover metamerism, etc. • [Baxter, Wendt and Lin 2004] • Used 8 “sufficient” wavelengths to model subsurface scattering and absorption effects in paint (Really nice piece of work)

Monitors are Bad • Next time you are in your office, bring up a completely “white” screen and hold a piece of “white” paper next to it • Scan a (non-digital) photograph of a nice sunset…. Hold the original next to your monitor • Even better, look out at a sunset and compare it to your monitor

THM–Take Home Message • Remember – Every man-made media has a viewable gamut and it doesn’t cover all visible light • Different lights make things different colors! (that’s why it looked so good at the store) • Trivariate spaces can only (sometimes) span the space of visible light, but can’t model the richness of ~400 wavelengths • There’s hope… listen to following speakers

References • Rochester Institute of Technology Munsell Color Science Laboratory: http://www.cis.rit.edu/mcsl/ • http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html • http://www.netnam.vn/unescocourse/computervision/12.htm • http://www.cie-usnc.org/ • Wavelength Selection for Synthetic Image Generation, Gary W. Meyer in Computer Vision, Graphics, and Image Processing 1988 • Full Spectral Color Calculations in Realistic Image Synthesis, Garrett Johnson and Mark Fairchild in IEEE Computer Graphics and Applications 1999 • IMPaSTo: A Realistic Interactive Model for Paint, Bill Baxter, Jeremy Wendt and Ming C. Lin in NonPhotorealistic Animation and Rendering 2004

Human Perception