Color Week 5, Fri Feb 4 http://www.ugrad.cs.ubc.ca/~cs314/Vjan2005 - - PowerPoint PPT Presentation
University of British Columbia CPSC 314 Computer Graphics Jan-Apr 2005 Tamara Munzner Color Week 5, Fri Feb 4 http://www.ugrad.cs.ubc.ca/~cs314/Vjan2005 Review: Shading Models flat shading compute Phong lighting once for entire
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compute Phong lighting once for entire
compute Phong lighting at the vertices and
compute averaged vertex normals interpolate normals across polygon and
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linearly interpolating surface normal across the
same input as Gouraud shading pro: much smoother results con: considerably more expensive
not the same as Phong lighting
common confusion Phong lighting: empirical model to calculate
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draw silhouettes and creases cool-to-warm shading
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per-vertex normals by interpolating per-facet
OpenGL supports both
computing normal for a polygon
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per-vertex normals by interpolating per-facet
OpenGL supports both
computing normal for a polygon
three points form two vectors
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per-vertex normals by interpolating per-facet
OpenGL supports both
computing normal for a polygon
three points form two vectors cross: normal of plane
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per-vertex normals by interpolating per-facet
OpenGL supports both
computing normal for a polygon
three points form two vectors cross: normal of plane which side of plane is up?
counterclockwise
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Illumination
Electromagnetic spectra
Reflection
Material properties Surface geometry and microgeometry (i.e.,
Physiology and neurophysiology Perceptual psychology
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hue (or simply, "color") is dominant wavelength
integration of energy for all visible wavelengths is
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contribution of dominant light vs. other
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when we view a source of light, our eyes respond
hue: the color we see (red, green, purple)
dominant frequency
saturation: how far is color from grey
(pink is less saturated than red, sky blue is less
brightness: how bright is the color
how close is the color to white or black how bright are the lights illuminating the object?
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rods cones
color!
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cones much denser here than the periphery
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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)
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a given perceptual sensation of color derives
identical perceptions of color can thus be caused by
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http://www.cs.brown.edu/exploratories/freeSoftware/catalogs/color_theory.html
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adaptation (move from sunlight to dark room) surrounding color/intensity:
simultaneous contrast effect
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three types of cones suggests
idea: perceptually based measurement
shine given wavelength (λ) on a screen user must control three pure lights producing
ν adjust intensity of RGB until colors are identical
this works because of metamers!
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some red had to be added to target color to permit exact match
equivalently (theoretically), some red could have been
figure shows that red phosphor must remove some cyan for
CRT phosphors cannot remove cyan, so 500 nm cannot be
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transformed basis
“imaginary” lights all positive, unit area Y is luminance
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the gamut of all colors perceivable is thus a three-
color = X’X + Y’Y + Z’Z
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define colors with (r, g, b)
used by OpenGL
RGB color cube sits within
subset of perceivable colors
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note: Y is the same as CIE’s Y result: use the Y alone and backwards
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a more intuitive color space
H = Hue S = Saturation V = Value (or brightness)
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monitor: I= A(k1D+k2V)γ typical monitor γ=2.5 LCD: nearly linear
defined by operating system inverse gamma curve I1/γ “gamma correction”
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divide device by OS gamma
different devices, different OS nonlinear
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