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

2 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

3 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

4 Human Eye

5 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

6 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

7 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

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

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

10 Math Stuff 1

11 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

12 Quick Example

13 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

14 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”?

15 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

16 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”

17 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

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

19 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

20 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!

21 Recent Work [Meyer 1988] –Used Gaussian Quadrature to find 3 simulation wavelengths which most affect perception (called AC 1 C 2 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)

22 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

23 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

24 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

25 References Rochester Institute of Technology Munsell Color Science Laboratory: astr.gsu.edu/hbase/vision/rodcone.html 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