Graphics Reference
In-Depth Information
arises from the manufacturing process. Digital imagers are sensitive to thermal
noise and have small divisions between pixels.
Since the simple model of a lens as an ideal focusing device and a sensor as
an ideal photon measurement device yields higher image quality than a realistic
camera model, there is little reason to use a more realistic model. Because lens
flare, film grain, bloom, and vignetting are recognized as elements of realism from
films, those are sometimes modeled using a post-processing pass. There is no need
to model the true camera optics to produce these effects, since they are being
added for aesthetics and not realism. Note that this arises purely from camera
culture—except for bloom, none of these effects are observed by the naked eye.
Figure 14.12: The streaks
from the sun and apparently
translucent-colored polygons
and circles along a line through
the sun in this photograph
are a lens flare created by the
intense light reflecting within the
multiple lenses of the camera
objective. Light from all parts of
the scene makes these reflections,
but most are so dim compared to
the sun that their impact on the
image is immeasurable. (Credit:
Spiber/Shutterstock)
This section describes common models of object surfaces. Many rendering algo-
rithms interact only with those surfaces. Some interact with the interior of objects,
whose boundaries can still be represented by these methods. Section 14.7 briefly
describes some representations for objects with substantial internal detail.
Some objects are modeled as thin, two-sided surfaces. A butterfly's wing and a
thin sheet of cloth might be modeled this way. These models have zero volume—
there is no “inside” to the model. More commonly, objects have volume, but the
details inside are irrelevant. For an opaque object with volume, the surface typ-
ically represents the side seen from the outside of the object. There is no need
to model the inner surface or interior details, because they are never seen (see
Chapter 36). To eliminate the inner side of the skin of an object, polygons have
an orientation. The
front face
of a polygon is the side indicated to face outward
and the
back face
is the side that faces inward. A process called
backface culling
eliminates the inward-facing side of each polygon early in the rendering process.
Of course, this model is revealed as a single-sided, hollow skin should the viewer
ever enter the model and attempt to observe the inside, as you saw in Chapter 6.
This happens occasionally in games due to programming errors. Because there is
no detail inside such an object and the back faces of the outer skin are not visible,
in this case the entire model seems to disappear from view once the viewpoint
passes through its surface.
Translucent objects naturally reveal their interior and back faces, so they
require special consideration. They are often modeled either as a translucent, two-
sided shell, or as two surfaces: an outside-to-inside interface and an inside-to-
outside interface. The latter model is necessary for simulating refraction, which is
sensitive to whether light rays are entering or leaving the object.
Surface and object geometry is useful for more than rendering. Intersections
of geometry are used for modeling and simulation. For example, we can model
an ice-cream cone with a bite taken out as a cone topped by a hemisphere ...with
some smaller balls subtracted from the hemisphere. Simulation systems often use
collision proxy geometry
that is substantially simpler than the geometry that is
rendered. A character modeled as a mesh of 1 million polygons might be simulated
as a collection of 20 ellipsoids. Detecting the intersection of a small number of
ellipsoids is more computationally efficient than detecting the intersection of a
large number of polygons, yet the resultant perceived inaccuracy of simulation
may be small.
