Graphics Reference
In-Depth Information
Photons propagate along rays within a volume that has a uniform index of
refraction, even if the material in that volume is chemically or structurally inho-
mogeneous. Photons are also selectively absorbed, which is why the world looks
darker when seen through a thick pane of glass. At the boundary between vol-
umes with different indices of refraction, light
scatters
by reflecting and refract-
ing in complex ways determined by the microscopic geometry and chemistry of
the material. Chapter 26 describes the physics and measurement of light in detail,
and Chapter 27 discusses scattering.
14.4.1.3 Units
Photons transport
energy,
which is measured in joules. They move immensely fast
compared to a human timescale, so renderers simulate the steady state observed
under continuous streams of photons. The
power
of a stream of photons is the
rate of energy delivery per unit time, measured in watts. You are familiar with
appliance labels that measure the
consumption
in watts and kilowatts. Common
household lighting solutions today convert 4% to 10% of the power they consume
into visible light, so a typical “100W” incandescent lightbulb emits at best 10 W
of visible light, with 4W being a more typical value.
In addition to measuring power in watts, there are two other measurements of
light that appear frequently in rendering. The first is the power per unit area enter-
ing or leaving a surface, in units of W
m
2
. This is called
irradiance
or
radiosity
and is especially useful for measuring the light transported between matte sur-
faces like painted walls. The second is the power per unit area per unit solid angle,
measured
4
/
(m
2
sr), which is called
radiance.
It is conserved along a ray
in a homogeneous medium. It is the quantity transported between two points on
different surfaces, and from a point on a surface to a sample location on the image
plane.
in W
/
14.4.1.4 Implementation
It is common practice to represent all of these quantities using a generic 3-vector
class (e.g., as done in the GLSL and HLSL APIs), although in general-purpose
languages it is frequently considered better practice to at least name the fields
based on their frequency, as shown in Listing 14.1.
Listing 14.1: A general class for recording quantities sampled at
three visible frequencies.
1
2
3
4
5
6
7
8
9
10
11
12
13
class
Color3
{
public
:
/
**
Magnitude near 650 THz ("red"), either at a single
frequency or representing a broad range centered at
650 THz, depending on the usage context. 650 THz
photons have a wavelength of about 450 nm in air.
*
/
float
r;
/
**
Near 550 THz ("green"); about 500 nm in air.
*
/
float
g;
/
**
Near 450 THz ("blue"); about 650 nm in air.
*
/
float
b;
4. The unit “sr” is “steradians,” a measure of the size of a region on the unit sphere,
described in more detail in Section 14.11.1.
