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Color

Color. CS 445/645 Introduction to Computer Graphics David Luebke, Spring 2003. Color. Next topic: Color

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Color

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  1. Color CS 445/645Introduction to Computer Graphics David Luebke, Spring 2003 David Luebke 110/22/2014

  2. Color • Next topic: Color To understand how to make realistic images, we need a basic understanding of the physics and physiology of vision. Here we step away from the code and math for a bit to talk about basic principles. David Luebke 210/22/2014

  3. Basics Of Color • Elements of color: David Luebke 310/22/2014

  4. Basics of Color • Physics: • Illumination • Electromagnetic spectra • Reflection • Material properties (i.e., conductance) • Surface geometry and microgeometry (i.e., polished versus matte versus brushed) • Perception • Physiology and neurophysiology • Perceptual psychology David Luebke 410/22/2014

  5. Physiology of Vision • The eye: • The retina • Rods • Cones • Color!

  6. Physiology of Vision • The center of the retina is a densely packed region called the fovea. • Cones much denser here than the periphery David Luebke 610/22/2014

  7. Physiology of Vision: Cones • Three types of cones: • L or R, most sensitive to red light (610 nm) • M or G, most sensitive to green light (560 nm) • S or B, most sensitive to blue light (430 nm) • Color blindness results from missing cone type(s) David Luebke 710/22/2014

  8. Physiology of Vision: The Retina • Weirdly, rods and cones are at the back of the retina, behind a mostly-transparent neural structure that collects their response.

  9. Perception: Metamers • A given perceptual sensation of color derives from the stimulus of all three cone types • Identical perceptions of color can thus be caused by very different spectra David Luebke 910/22/2014

  10. Perception: Other Gotchas • Color perception is also difficult because: • It varies from person to person (thus std observers) • It is affected by adaptation • It is affected by surrounding color: David Luebke 1010/22/2014

  11. Color Spaces • Three types of cones suggests color is a 3D quantity. How to define 3D color space? • Idea: shine given wavelength () on a screen, and mix three other wavelengths (R,G,B) on same screen. Have user adjust intensity of RGB until colors are identical: • How closely does this correspond to a color CRT? • Problem: sometimes need to “subtract” R to match  David Luebke 1110/22/2014

  12. CIE Color Space • The CIE (Commission Internationale d’Eclairage) came up with three hypothetical lights X, Y, and Z with these spectra: • Idea: any wavelength  can be matched perceptually by positivecombinations of X,Y,Z • Note that: X ~ R Y ~ G Z ~ B David Luebke 1210/22/2014

  13. CIE Color Space • The gamut of all colors perceivable is thus a three-dimensional shape in X,Y,Z: • For simplicity, we often project to the 2D plane X+Y+Z=1 X = X / (X+Y+Z) Y = Y / (X+Y+Z) Z = 1 - X - Y David Luebke 1310/22/2014

  14. CIE Chromaticity Diagram (1931)

  15. Device Color Gamuts • Since X, Y, and Z are hypothetical light sources, no real device can produce the entire gamut of perceivable color • Example: CRT monitor David Luebke 1510/22/2014

  16. Device Color Gamuts • The RGB color cube sits within CIE color space something like this: David Luebke 1610/22/2014

  17. Device Color Gamuts • We can use the CIE chromaticity diagram to compare the gamuts of various devices: • Note, for example, that a color printercannot reproduceall shades availableon a color monitor David Luebke 1710/22/2014

  18. Converting Color Spaces • Simple matrix operation: • The transformation C2 = M-12 M1 C1yields RGB on monitor 2 that is equivalent to a given RGB on monitor 1 David Luebke 1810/22/2014

  19. Converting Color Spaces • Converting between color models can also be expressed as such a matrix transform: • YIQ is the color model used for color TV in America. Y is luminance, I & Qare color • Note: Yis the same as CIE’s Y • Result: backwards compatibility with B/W TV! David Luebke 1910/22/2014

  20. Gamma Correction • We generally assume colors are linear • But most display devices are inherently nonlinear • I.e., brightness(voltage) != 2*brightness(voltage/2) • Common solution: gamma correction • Post-transformation on RGB values to map them to linear range on display device: • Can have separate  for R, G, B David Luebke 2010/22/2014

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