Graphics Reference
In-Depth Information
It is often desirable to render objects that appear to glow without actually
illuminating other surfaces. For example, car tail lights and the LEDs on
computer equipment likely contribute negligible illumination to the rest of
the scene but themselves need to appear bright in an image. These effects
may be simulated by rendering the scene normally and then additively blend-
ing the emissive component as if it were a new surface rendered using
BlendFunc(ONE, ONE)
.
Some particularly attractive effects are due to such emission by a medium that
is itself seemingly transparent. Examples include the light from a neon bulb, light-
ning, science fiction “force fields,” and fantasy magical effects. That the underly-
ing surface is invisible in these cases is irrelevant—the additive blending of the
emissive component is unchanged.
Lens flare and bloom are effects that occur within the optical path of a real camera.
One could model the real optical path, but to merely achieve the phenomena it is
much more effective to additively blend contributions due to additional geometry
over the rendered frame using
BlendFunc(ONE, ONE).
Bloom simulates the diffu-
sion of incident light within lenses and the saturation of the sensor. It is typically
simulated by blurring only the brightest locations on-screen and adding their con-
tribution back into the frame. Lens flare arises from multiple reflections between
lenses within the objective. It is typically simulated by rendering a sequence of
iris (e.g., hexagonal or disk) -shaped polygons along a 2D line through bright
locations, such as the sun, on-screen.
A computer graphics
luminaire
is a source of light. The luminaires we encounter
in daily life vary radically in the spectra of light that they emit, their surface areas,
and their intensities. For example, the sun is large and distant, a spotlight is bright
and small, and a traffic light is relatively dim and colored. The luminaires we
might encounter in a virtual world expand this variation further; for example, a
cave of phosphorescent fungus, a magic unicorn's aura, or the navigation lights on
a starship.
Before we can present luminaire models, we must first discuss light. You
know that light is energy (in photons) that propagates along rays through space
and scatters at surfaces. There are many models of light in computer graphics,
but they all begin by representing the rate at which energy is passing through a
point in space. We give a brief synopsis here and defer extensive coverage until
Chapter 26. One
can
render images from the models in this chapter without under-
standing the motivation and physics behind light transport. However, we recom-
mend that after rendering your first images you read Chapter 26 to build a deeper
understanding.
