Graphics Reference
In-Depth Information
(a)
(b)
(c)
(d)
Figure 14.20: Example of a billboard cloud: (a) original model (5138 polygons), (b) false-
color rendering using one color per billboard to show the faces that were grouped on each
billboard, (c) view of the (automatically generated) 32 textured billboards, and (d) the
billboards side-by-side. [DDSD03]. (Credit: Courtesy of Xavier Décoret, © 2003 ACM,
Inc. Reprinted by permission.)
Décoret et al. [DDSD03] proposed
billboard clouds
to automate for any
model a process often employed by artists for foliage. The billboard cloud rep-
resents a single object with a collection of billboards oriented to incrementally
minimize visual error in the rendered object (see Figure 14.20).
A limitation of individual billboards is that they cannot represent dynamic
objects or views of objects as the observer approaches and parallax becomes non-
negligible. To address parallax, one could precompute
many
billboards, as was
common in early 3D games such as Doom and Wing Commander, or develop
warping strategies [POC05]. For dynamic billboards of specific objects, one could
rig animation controls within the billboard itself [DHOO05, YD08]. However,
a general solution is to simply rerender the billboards at runtime whenever the
approximation error grows too large. These dynamic billboards are known as
imposters
[MS95], and they have seen widespread application for a variety of
models, from terrain [CSKK99] to characters to clouds [HL01].
It is often convenient to model parts of a scene as effectively “infinitely” distant.
These may be rendered using finite distance for projection, but those distances are
held constant regardless of the viewer's translation. A frequent application is the
sky, including clouds. For a character on the ground, the distance to objects in
the sky is effectively constant and large, so there is no parallax or change in per-
spective with viewpoint movement. This is the ideal case for a billboard, except
that planar geometry is inappropriate for wrapping around the horizon. A
skybox
or
sky sphere
is a geometric proxy for all distant objects. It wraps around the
scene and translates so that the viewer is always at the center. The geometry for
this proxy is arbitrary, so long as it surrounds the viewer. For example, it could be
an icosahedron, tetrahedron, ...orteapot, and the shape will be indistinguishable
from a sphere once the interior is painted with an appropriately projected image of
the (virtual) distant scene geometry that it simulates. The choice of proxy geome-
try is therefore driven by the convenience and efficiency of generating that image
under the given projection. Cubes and spheres both lend themselves to natural
projections, and are therefore the most common models.
The term “skybox” is also used occasionally to refer to objects at finite dis-
tances such as building façades that provide a small amount of parallax but are in
areas of the scene that the viewer will never enter. This is common, for example,
