1. Digital Image Synthesis Yung-Yu Chuang 10/22/2009
Color and Radiometry
Digital Image Synthesis
Yung-Yu Chuang
10/22/2009
with slides by Pat Hanrahan and Matt Pharr

2. 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

3. 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 light’s wavelength)
It is based on the particle model. Hence, diffraction and interference can’t be easily accounted for.

4. Basic assumptions about light behavior
Linearity: the combined effect of two inputs is equal to the sum of effects
Energy conservation: scattering event can’t produce more energy than they started with
Steady state: light is assumed to have reached equilibrium, so its radiance distribution isn’t changing over time.
No polarization: we only care the frequency of light but not other properties (such as phases)
No fluorescence or phosphorescence: behavior of light at a wavelength or time doesn’t affect the behavior of light at other wavelengths or time
荧光Fluorescent，又作“萤光”，是指一种光致发光的冷发光现象。当某种常温物质经某种波长的入射光（通常是紫外线或X射线）照射，吸收光能后进入激发态，并且立即退激发并发出比入射光的的波长长的出射光（通常波长在可见光波段）；而且一旦停止入射光，发光现象也随之立即消失。具有这种性质的出射光就被称之为荧光。在日常生活中，人们通常广义地把各种微弱的光亮都称为荧光，而不去仔细追究和区分其发光原理。
磷光是一種緩慢發光的光致冷發光現象。當某種常溫物質經某種波長的入射光（通常是紫外線或X射線）照射，吸收光能後進入激發態（通常具有和基態不同的自旋多重度），然後緩慢地退激發併發出比入射光的的波長長的出射光（通常波長在可見光波段），而且與夜明珠。

5. Fluorescent materials

6. Spectral power distribution
400nm
(bluish)
550nm
(green)
650nm
(red)
fluorescent light (日光燈)

7. Spectral power distribution
400nm
(bluish)
550nm
(green)
650nm
(red)
lemmon skin

8. 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(λ)=1 is a trivial but bad approximation

9. Color matching experiment
Section 1.7 of Principles of Digital Image synthesis
p.256 of “Color Imaging”
Trichromatic theory
Holmholtz found the min number of light sources to match any given test color is 3.
Any perceivable color can be matched with a mixture of three primary colors.
p1 = nm
p2 = nm
p3 = nm
p1 p p3
p1 p p3
Foundations of Vision, by Brian Wandell, Sinauer Assoc., 1995

10. Color matching experiment
此為color matching 之curve, 及各個波長之test light所對應之R, G, B lights所需之weights

11. Color matching experiment
To avoid negative parameters
為避免negative parameters, 改用三種hypothetical light sources, called x, y, z, 此為其spectrum也就是之前的basis

12. Human Photoreceptors
桿狀細胞感光
錐狀細胞感色

14. Metamers tungsten (鎢絲) bulb television monitor
different spectrum, same perception
Spectrum不同但human perception相同之spectrum叫做metamers
tungsten (鎢絲) bulb
television monitor

16. Why reflecting different colors
high
light
heat/
chemical
low
Light with specific wavelengths
are absorbed.
Fluorescent

17. Primary colors Primary colors for Primary colors for
addition (light sources)
Primary colors for
subtraction (reflection)

18. Heat generates light
Vibration of atoms or electrons due to heat generates electromagnetic radiation as well. If its wavelength is within visible light (>1000K), it generates color as well.
Color only depends on temperature, but not property of the object.
Human body radiates IR light under room temperature.
K: color temperature of incandescent light bulb

19. 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:
<arithmetic operations>
private:
float c[COLOR_SAMPLES];
... }
Why is this possible? Human vision system
We actually sample RGB
component-wise
+ - * / comparison…

20. Human visual system
Tristimulus theory: all visible SPDs S can be accurately represented for human observers with three values, xλ, yλ and zλ.
The basis are the spectral matching curves, X(λ), Y(λ) and Z(λ) determined by CIE (國際照明委員會).

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

22. XYZ color
Good for representing visible SPD to human observer, but not good for spectral computation.
A product of two SPD’s 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 xyz[3]) const {
xyz[0] = xyz[1] = xyz[2] = 0.;
for (int i = 0; i < COLOR_SAMPLES; ++i) {
xyz[0] += XWeight[i] * c[i];
xyz[1] += YWeight[i] * c[i];
xyz[2] += ZWeight[i] * c[i]; }

23. Conversion between XYZ and RGB
float Spectrum::XWeight[COLOR_SAMPLES] = {
f, f, f };
float Spectrum::YWeight[COLOR_SAMPLES] = {
f, f, f
float Spectrum::ZWeight[COLOR_SAMPLES] = {
f, f, f
Spectrum FromXYZ(float x, float y, float z) {
float c[3];
c[0] = f * x f * y f * z;
c[1] = f * x f * y f * z;
c[2] = f * x f * y f * z;
return Spectrum(c); }

24. Conversion between XYZ and RGB
vector sampled at several
wavelengths such as (R,G,B)
(R,G,B)
device dependent
xλ, yλ, zλ
xλ, yλ, zλ

25. Basic quantities non-directional Flux: power, (W)
Irradiance: flux density per area, (W/m2)
Intensity: flux density per solid angle
Radiance: flux density per solid angle per area
non-directional
directional

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

27. Irradiance (E) Area density of flux (W/m2) Inverse square law
Lambert’s law

28. Angles and solid angles
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
See Fig in Sears and Zemansky
Þ sphere has 4p steradians

29. Intensity (I) Flux density per solid angle
Intensity describes the directional distribution of light

30. 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

31. Calculate irradiance from radiance
Light meter

32. Irradiance Environment Maps

33. Differential solid angles
Goal: find out the relationship
between d and dθ, d
Why? In the integral,
dω is uniformly divided.
To convert the integral to
Define S^2
同樣大小的solid angle dw在不同位置所張開的d theta及d phi會有所不同, phi其實沒有影響，但theta則有
同一個theta處，dw與d phi成正比，及同樣大小的dw的d phi亦相同
但是同一個phi處，高緯度的d phi其實較短，同樣面積的dw需要較長的d theta
We have to find the relationship
between d and uniformly
divided dθ and d.

34. Differential solid angles
Can we find the surface area of a unit sphere
by ?
2 (pi)^2

35. Differential solid angles
Goal: find out the relationship
between d and dθ, d
By definition, we know that
Define S^2

36. Differential solid angles
We can prove that
Define S^2

37. Isotropic point source
If the total flux of the
light source is Φ,
what is the intensity?

38. Warn’s spotlight If the total flux is Φ, what is the intensity?
If increase s, light becomes more focussed. But still bounded by 1. Thus, it seems like the power should go down.
Which it does if we calculate it.