Graphics Reference
In-Depth Information
model of matter is not necessarily one that is easy to describe in terms of macro-
scopic phenomena, either for specification or for digital representation. Represent-
ing and modeling a brick as a rough, reddish slab of dried clay is both intuitive
and compact. Representing it as a collection of 10
26
or so molecules of varying
composition is unwieldy at best.
There are many models of matter in graphics. The simplest is that matter is geom-
etry that scatters light, and further, that this light scattering takes place only at the
surfaces of opaque objects, ignoring the very small interactions of photons with
air over short distances and any subsurface interaction effects. The surface scatter-
ing model builds on these assumptions by modeling only the surfaces of opaque
objects. This reduces the complexity of a scene substantially. For example, a com-
puter graphics car might have no engine, and a computer graphics house might
be only a façade. Only the parts of objects that can interact with light need to be
modeled. Of course, this approach poorly represents matter with deep interaction,
such as skin and fog, and is only sufficient for rendering. To animate objects, for
instance, we need to know properties such as joint locations and masses.
A consequence of computer graphics relying on complex models of matter is
that different models are often employed for surface detail at different scales. Sup-
porting different models and ways of combining them at intermediate scales com-
plicates a graphics system. However, it also yields great efficiencies and matches
our everyday perception. For example, from 100 meters, you might observe that a
fir tree is similar to a green cone. From ten meters, individual branches are visible.
From one meter, you can see separate needles. At one centimeter, small bumps
and details on the needles and branches emerge. With a light microscope you can
see individual cells, and with an electron microscope you can see molecule-scale
detail. For this chapter, we consider details to be large-scale if their impact on the
silhouette can be observed from about one meter, medium scale if they are smaller
than that but observable by the naked eye at some scale, and small-scale if they
are not observable by the naked eye.
Figure 14.10: The darkening of
this photograph near the edges
is called vignetting. (Credit:
Swanson Tennis Center at Gus-
tavus Adolphus College by Joe
Lencioni, shiftingpixel.com)
Lenses and sensors (the components of eyes and cameras) are complicated. This
is true whether they have biological or mechanical origins. From a photographer's
perspective, the ideal lens would focus all light from a point that is “in focus”
onto a single point on the imager (the sensing surface of the sensor) regardless
of the frequency of light or the location on the imager. Real lenses have imper-
fect geometry that distorts the image slightly over the image plane and causes
darkening near the edges, an effect known as
vignetting
(see Figure 14.10). They
also necessarily focus different frequencies differently, creating an artifact called
chromatic aberration
(see Figure 14.11; see also Chapter 26). Camera manufac-
turers compensate for these limitations by combining multiple lenses. Unfortu-
nately, these compound lenses absorb more light, create internal reflections, and
can diffuse the focus. We perceive the reflections as
lens flare
—a series of iris
shapes in line with the light source overlaid on the image, as seen in Figure 14.12.
We perceive slightly diffused focus as
bloom,
where very bright objects appear
defocused. Real film has a complex nonlinear response to light, and has grain that
Figure 14.11: The rainbowlike
edges on the objects in this
photograph are caused by chro-
matic aberration in the cam-
era's lens. Different frequencies
of light refract at different angles,
so the resultant colors shift in the
image plane. High-quality cam-
eras use multiple lenses to com-
pensate for this effect. (Credit:
Corepics VOF/Shutterstock)
