Rendering Issues - Computer Animation: Algorithms and Techniques - page 404

Background and ground planes could be considered types of billboards, although usually the term is
reserved for substitutes of objects that would otherwise be modeled with three-dimensional geometry.
However, by using multiple billboards for distance background objects at various depths, a parallax
effect, typical of conventional two-dimensional animation, can be created. As an example, consider
a passenger looking out the window of a moving train. The passenger may see trees, houses, and moun-
tains moving across the window view at different speeds due to their relative depth. For objects not
close enough to the train for their sides to be seen, these objects can be modeled effectively by using
two-dimensional elements—billboards.
The two-dimensional background elements discussed above are examples of static billboards. Ani-
mation of a billboard's orientation is also useful. When a camera has the ability to travel in and around
complex elements, billboard representations can be used that change their orientation depending on the
position and orientation of the camera. Tree billboards are good examples. A tree billboard can be pro-
cedurally rotated so that its plane is always perpendicular to the direction to the camera. As the camera
travels through a terrain environment, for example, the tree billboards are smoothly rotated to always
face the camera. In this way, the billboards effectively hide their two dimensionality and always present
their visual complexity to the camera. Orienting a billboard toward the camera can create perspective
distortion when the billboard is to the side of the center of interest. As an alternative to directly facing
the camera, billboards can also be oriented so that they are parallel to the view plane. This produces less
perspective distortion when projected to the side of the image. Tree billboards are constrained to only
rotate around the y -axis because the tree up vector must be aligned with the world up vector ( y -axis).
Other billboards, such as representations of clouds, explosions, or snowflakes, can be oriented to
always face a certain direction as well but can be free to rotate in their plane about their own center.
Some billboards may contain animated images. These are similar to sprites used in two-and-a-half
dimensional graphics common in many early digital arcade games. Animated billboards can be used for
fire, explosions, and smoke and typically loop through a sequence of pre-rendered images to create the
impression of a dynamic process. A billboard animation loop can be used to create a walk cycle for a figure.

A.6 Summary
Although these techniques are not concernedwith specifyingor controlling themotionof graphical objects,
they are important to the process of generating images for computer animation. Double buffering helps
to smoothly update a display of images. Compositing helps to conserve resources and combine elements
from different sources. Motion blur prevents fast-moving objects from appearing jerky and distracting to
the viewer. Shadows help to locate objects relative to surfaces, but only those shadows that effectively serve
such a purpose need to be generated. Billboarding and the use of impostors reduce the computational
complexity of the modeling and rendering while maintaining the visual complexity of the scene.

References
[1] Carpenter L. The A-Buffer Hidden Surface Method. In: Computer Graphics. Proceedings of SIGGRAPH 84,
vol. 18(3). Minneapolis, Minn.; July 1984. p. 103-8.
[2] Cook R. Stochastic Sampling in Computer Graphics. ACM Transactions on Graphics January

1986;5(1):51-71.