Game Development Reference
In-Depth Information
the signal is a multiple of a Direc delta; see Section 12.4.3.) For example,
a real-world light source has a finite area, and we would be interested in
the radiance of the light at a given point on the emissive surface, in a given
direction. In practice, we imagine shrinking the area of this light down to
zero while holding the radiant flux constant. The flux density becomes infi-
nite in theory. Thus, for a real area light we would need a signal to describe
the flux density, whereas for a point light, the flux density becomes infinite
and we instead describe the brightness of the light by its total flux. We'll
repeat this information when we talk about point lights.
Key Points about Radiometry
•
Vague words such as “intensity” and “brightness” are best avoided
when the more specific radiometric terms can be used. The scale
of our numbers is not that important and we don't need to use real
world SI units, but it is helpful to understand what the different
radiometric quantities measure to avoid mixing quantities together
inappropriately.
•
Use radiant flux to measure the total brightness of a finite area, in all
directions.
•
Use radiant flux density to measure the brightness at a single point,
in all directions. Irradiance and radiant exitance refer to radiant flux
density of light that is incident and emitted, respectively. Radiosity
is the radiant flux density of light that is leaving a surface, whether
the light was reflected or emitted.
•
Due to Lambert's law, a given ray contributes more differential irra-
diance when it strikes a surface at a perpendicular angle compared to
a glancing angle.
•
Use radiance to measure the brightness of a ray. More specifically,
radiance is the flux per unit projected angle, per solid angle. We use
projected area so that the value for a given ray is a property of a ray
alone and does not depend on the orientation of the surface used to
measure the flux density.
•
Practical realities thwart our best intentions of doing things “the right
way” when it comes to using proper units. Numerical integration is
a lot like taking a weighted average, which hides the change of units
that really occurs. Point lights and other Dirac deltas add further
confusion.
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