Moving pictures are created by presenting the viewer with a sequence of still images in quick succession. Objects which occur in an orderly succession of slightly different locations within a sequence of images appear to be in motion to the viewer. Such an object is said to be ``animated.'' Preparing a moving picture of animated objects requires a large number of still images (often called ``frames'').
Still images of computer models are relatively easy to create within BRL-CAD. The program rt uses the technique of ray-tracing to create images of geometric models. A moving picture or ``animation'' of the model can be created by using rt to create a series of still images, each of which forms a frame of the final moving picture. The difficulty arises in specifying each frame for rt to create. Some tools have been created to help make this process easier.
Before there can be motion there must first be form. This means a description of geometry to be animated must exist. To keep things simple, we will use the ``moss'' database distributed with BRL-CAD. Assuming that the distribution is stored in the directory /cadsrc the following command will create a local copy of the database for use with the examples presented here:
% asc2g < /cadsrc/db/moss.asc > moss.gExamples such as the one above are presented with the portion typed by the user shown in bold typeface.