Color and Radiometry

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Color and Radiometry

• Digital Image Synthesis
• Yung-Yu Chuang
• 10/19/2006

with slides by Pat Hanrahan and Matt Pharr
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Radiometry

• Radiometry study of the propagation of
  electromagnetic radiation in an environment
• Four key quantities flux, intensity, irradiance
  and radiance
• These radiometric quantities are described by
  their spectral power distribution (SPD)
• Human visible light ranges from 370nm to 730nm

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Spectral power distribution
400nm (bluish)
650nm (red)
550nm (green)
fluorescent light (???)
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Spectral power distribution
lemmon skin
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Color

• Need a compact, efficient and accurate way to
  represent functions like these
• Find proper basis functions to map the
  infinite-dimensional space of all possible SPD
  functions to a low-dimensional space of
  coefficients
• For example, B(?)1, a bad approximation

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Spectrum

• In core/color.
• Not a plug-in, to use inline for performance
• Spectrum stores a fixed number of samples at a
  fixed set of wavelengths. Better for smooth
  functions.
• define COLOR_SAMPLE 3
• class COREDLL Spectrum
• public
• ltarithmetic operationsgt
• private
• float cCOLOR_SAMPLES
• ...

Why is this possible? Human vision system
We actually sample RGB
component-wise - / comparison
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Human visual system

• Tristimulus theory all visible SPDs can be
  accurately represented for human observers with
  three values, x?, y? and z?.
• The basis are the spectral matching curves, X(?),
  Y(?) and Z(?).

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XYZ basis
pbrt has discrete versions (sampled every 1nm) of
these bases in core/color.cpp
360
830
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XYZ color

• Its, however, not good for spectral computation.
  A product of two SPDs XYZ values is likely
  different from the XYZ values of the SPD which is
  the product of the two original SPDs.
• Hence, we often have to convert our samples (RGB)
  into XYZ
• void XYZ(float xyz3) const
• xyz0 xyz1 xyz2 0.
• for (int i 0 i lt COLOR_SAMPLES i)
• xyz0 XWeighti ci
• xyz1 YWeighti ci
• xyz2 ZWeighti ci

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Conversion between XYZ and RGB

• float SpectrumXWeightCOLOR_SAMPLES
• 0.412453f, 0.357580f, 0.180423f
• float SpectrumYWeightCOLOR_SAMPLES
• 0.212671f, 0.715160f, 0.072169f
• float SpectrumZWeightCOLOR_SAMPLES
• 0.019334f, 0.119193f, 0.950227f
• Spectrum FromXYZ(float x, float y, float z)
• float c3
• c0 3.240479f x -1.537150f y
  -0.498535f z
• c1 -0.969256f x 1.875991f y
  0.041556f z
• c2 0.055648f x -0.204043f y
  1.057311f z
• return Spectrum(c)

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Basic radiometry

• pbrt is based on radiative transfer study of the
  transfer of radiant energy based on radiometric
  principles and operates at the geometric optics
  level (light interacts with objects much larger
  than the lights wavelength)
• It is based on the particle model. Hence,
  diffraction and interference cant be easily
  accounted for.

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Basic assumptions about light behavior

• Linearity the combined effect of two inputs is
  equal to the sum of effects
• Energy conservation scattering event cant
  produce more energy than they started with
• No polarization that is, we only care the
  frequency of light but not other properties
• No fluorescence or phosphorescence behavior of
  light at a wavelength doesnt affect the behavior
  of light at other wavelengths
• Steady state light is assumed to have reached
  equilibrium, so its radiance distribution isnt
  changing over time.

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Fluorescent materials
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Flux (F)

• Radiant flux, power
• Total amount of energy passing through a surface
  per unit of time (J/s,W)

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Irradiance (E)

• Area density of flux (W/m2)

Lamberts law
Inverse square law
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Angles and Solid Angles

• Angle
• Solid angle
• The solid angle subtended by a surface is
  defined as the surface area of a unit sphere
  covered by the surface's projection onto the
  sphere.

Þ circle has 2p radians
Þ sphere has 4p steradians
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Intensity (I)

• Flux density per solid angle
• Intensity describes the directional distribution
  of light

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Radiance (L)

• Flux density per unit area per solid angle
• Most frequently used,
• remains constant along ray.
• All other quantities can
• be derived from radiance

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Calculate irradiance from radiance
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Irradiance Environment Maps
Radiance Environment Map
Irradiance Environment Map
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Differential solid angles
Goal find out the relationship between d? and
d?, d?
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Differential solid angles
Goal find out the relationship between d? and
d?, d?
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Differential solid angles
We can prove that
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Differential solid angles
We can prove that
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Isotropic point source
If the total flux of the light source is F, what
is the intensity?
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Isotropic point source
If the total flux of the light source is F, what
is the intensity?
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Warns spotlight
If the total flux is F, what is the intensity?
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Warns spotlight
If the total flux is F, what is the intensity?
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Irradiance isotropic point source
What is the irradiance for this point?
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Irradiance isotropic point source
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Irradiance isotropic point source
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Photometry
Luminance