# Introduction

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.

## Preliminaries

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.g

Examples such as the one above are presented with the portion typed by the user shown in bold typeface.

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