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June 15, 1999
Light I June 15, 1999 Paper summaries on light Any takers? June - - PowerPoint PPT Presentation
Light I June 15, 1999 Paper summaries on light Any takers? June - - PowerPoint PPT Presentation
Light I June 15, 1999 Paper summaries on light Any takers? June 15, 1999 Motivational Film Bingo (1998) Alias / Wavefront June 15, 1999 Photography and Light photography, n ., the process or art of producing images of
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June 15, 1999
Motivational Film
✔Bingo (1998)
– Alias / Wavefront
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June 15, 1999
Photography and Light
pho•tog•ra•phy, n., the process or art of producing images of objects by the action of light on a sensitized surface, esp, a film in a camera.
Photography = writing with light
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June 15, 1999
Today’s Class
✔Light
– What it is – How we measure it – How it behaves – Light and color
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June 15, 1999
Light -- What it is
✔Electromagnetic radiation
power induction heating radio waves infrared ultra violet x-rays gamma rays 1016 1014 1010 108 1012 106 102 1 10-2 10-4 10-6 10-8 Wavelength (nm) 104 visible light secondary cosmic rays
Red
- range
yellow green blue violet 700 nm 650 nm 600 nm 550 nm 450 nm 400 nm
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June 15, 1999
Light -- How we measure it
✔Light is Radiant Energy ✔Measure in Joules ✔CG uses particle model of light
– Light travels in localized particles or wave packets.
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June 15, 1999
Light -- How we measure it
✔Radiant Flux (Radiant Power)
– Amount of energy / unit time – Joules per second (Watt)
dt dQ = Φ
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June 15, 1999
Light -- How we measure it
✔Radiant Flux Density
– Amount of flux per unit area arriving at or leaving from a point on the surface – Measured in Watts / m2
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June 15, 1999
Light -- How we measure it
✔Irradiance - flux density in
dA d E Φ =
dA
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June 15, 1999
Light -- How we measure it
✔Radiant exitance - radiant flux out
dA
dA d M Φ =
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June 15, 1999
Light -- How we measure it
✔Radiance
– Flux arriving at or leaving from a given point or surface in a given direction. – Measured in Watts / m2 / steradian
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June 15, 1999
Light - how we measure it
) cos (
2
θ ω dA d d L Φ =
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June 15, 1999
Light -- How we measure it
✔Radiant Intensity
– Amount of radiant flux in a given direction – Watts / steradian – Point light sources
ω d d I Φ =
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June 15, 1999
Light - how we measure it
✔Each of the measures can vary with
wavelength
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June 15, 1999
Light - how we measure it
✔In summary
– Radiant Flux - energy / time – Radiant Flux Density - total flux entering or leaving a point or surface – Radiance - total flux entering or leaving a point or surface in a given direction – Radiant intensity - flux in a given direction for point light sources – All measures can vary with wavelength
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June 15, 1999
Light -- How we measure it
✔Photometric measures
– Accounts for human perception of brightness – Radiometric units scaled by luminosity function. – Same concepts -> different units
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June 15, 1999
Light -- How we measure it
✔CIE Luminous Efficiency Curve
20 40 60 80 100 120 375 400 425 450 475 500 525 550 575 600 625 650 675 700 725 750 Wavelength % Efficiency
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June 15, 1999
Light - how we measure it
✔Photometric Units
– Luminous Flux - energy / time – Luminous Flux Density - total flux entering
- r leaving a point or surface
– Luminance - total flux entering or leaving a point or surface in a given direction – Luminace intensity - flux in a given direction for point light sources – All scaled by CIE Luminous Eff. Curve
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June 15, 1999
Light -- How we measure it
✔Photometric units
– Luminous Flux (lumen) – Luminous Flux Density (lumen/m2 = lux) – Luminance (lumen/m2/steradian = nit) – Luminous intensity (lumen / steridian = candela)
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June 15, 1999
Light - how we measure it
✔Example
– The luminance at a surface due to a blue light of a given intensity would be less than the luminance at the same surface due to a yellow light of the same intensity. – Why? Humans perceive yellow light to be brighter than blue light
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June 15, 1999
Light -- How it behaves
✔Reflection ✔Absorption ✔Refraction ✔Scattering ✔Diffraction / Interference ✔All can be wavelength dependent
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June 15, 1999
Light -- How it behaves
✔Reflection
– Angle of incidence = Angle of reflectance – Perfect mirror surface
N
i
θ
r
θ
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June 15, 1999
Light - How it behaves
✔Absorption
– Material can absorb light on a wavelength by wavelength basis – Responsible for object color
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June 15, 1999
Light - How it behaves
✔Refraction
– bending of light as it travels through different media
i
η
r
η
i
θ
r
θ
r r i i
θ η θ η sin sin =
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June 15, 1999
Light - How it behaves
✔Total internal reflection
i
η
r
η
c
θ
c
θ
= angle at which refracted rays lies perpendicular to normal
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June 15, 1999
Light - How it behaves
✔Scattering
– Light is scattered by small particles in its path (e.g. haze, smoke, etc.) – Given by fraction of light with respect to direction from particle light impact. – Size of particles are on the order of wavelengths of light.
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June 15, 1999
Light - How it behaves
✔Scattering
α
r
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June 15, 1999
Light -- How it behaves
✔Scattering
– r << λ total absorption (no scattering) – r < λ Rayleigh Scattering – r ≈ λ Mie scattering – r >> λ Geometric optics
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June 15, 1999
Light -- how it behaves
✔Raleigh scattering (smoke / dust )
) cos 1 ( 4 3 ) (
2α
α + = P
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June 15, 1999
Light - how it behaves
✔Mie Scattering (haze / fog)
8
2 cos 1 9 1 ) ( + + = α α P
Sparse / hazy
32
2 cos 1 50 1 ) ( + + = α α P
Dense / murky
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June 15, 1999
Light -- How it behaves
✔Diffraction
– bending of light around objects – contributes to soft shadows, color bleeding
✔Interference
– superimposition of two waves – accounts for colors in thin films, bubbles,
- il slicks, peacock feathers
✔Generally not considered in CG.
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June 15, 1999
Light and Color
✔Color is the perceptual response to light
- f wavelengths 400 - 700 nm hitting the
retina.
✔When rendering spectrum must be
sampled.
✔Color vision is inherently trichromatic.
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June 15, 1999
Light and Color
✔“Indeed rays, properly expressed,
are not colored”
✔Spectral power distributions exist in
the physical world but color exists
- nly in the eye and brain.
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June 15, 1999
Light and Color
✔CIE Experiments
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June 15, 1999
Light and Color
✔CIE RGB curves
- 20
- 10
10 20 30 40 375 405 435 465 495 525 555 585 615 645 675 705 735 Wavelength R G B
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June 15, 1999
Light and Color
✔CIE xyz color matching curves
50 100 150 200 375 405 435 465 495 525 555 585 615 645 675 705 735 Wavelength x y z
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June 15, 1999
Light and Color
✔Chromaticity coordinates
Z Y X X x + + = Z Y X Y y + + = Z Y X Z z + + =
1 = + + z y x
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June 15, 1999
Light and Color
✔Chomaticity coordinates
– often given in xyY – xy give the chromaticity – Y gives brightness
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June 15, 1999
Light and Color
✔Chromaticity diagram
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June 15, 1999
Light and Color
✔RGB (or any primary set) can be
determined from XYZ
– Need chromaticies of primaries and white point.
✔Primaries generally determined by
device.
✔RGB values are incomplete without
specification of primaries & white point.
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June 15, 1999
Light and Color
✔sRGB
– Standard proposed by Microsoft and HP – Based on ITU-R 709.BT
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June 15, 1999
Light and Color
Z Y X B Z Y X G Z Y X R 0570 . 1 2040 . 0556 . 0416 . 8760 . 1 9692 . 4986 . 5374 . 1 2410 . 3 + − = + + − = − − =
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June 15, 1999
Light and Color
✔sRGB
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June 15, 1999
Light and Color
✔Other color spaces
– HSV (hue-saturation-value) – CMYK (printing) – CIELAB / CIELUV (perceptual)
✔Why does CG use RGB
– Convienience
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June 15, 1999
Light and Color
✔Summary
– Color is perceptual not physical – Human vision is inherently trichromatic (so too are most display devices) – Color specification involves chromaticity and brightness. – RGB requires specification of primaries and white point
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June 15, 1999
Light and Color
✔Further Reading
– Giorgianni / Madden, Digital Color Management – Poynton, Technical Introduction to Digital Video – Hunt, Reproduction of Colour
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June 15, 1999
For next time
✔Project Proposals are due next class
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June 15, 1999
Next Class
✔Lighting Models
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June 15, 1999