The IndycamInput class will interface the required system libraries in order to capture images from the SGI system's Indycam. Its GetImage() method will never normally return NULL, as subsequent image captures can always be made and only in the case of, for instance, hardware errors will NULL be returned.
The ImageFileInput class will read a standard image file (in a common file format, such as SGI's RGB format or JPEG format) and then its GetImage() method will return this image the first time it is called, and NULL thereafter.
The MovieFileInput class will read a standard movie or animation file (in a common file format, such as MPEG or Quicktime MOV) and then its GetImage() method will return each of the frames of the movie in order, returning NULL when the sequence of frames is exhausted.
The NetworkInput class will interface with a remote instance of the Stereocam system (specifically, one using the NetworkOutput class, described in Section 4.2.3), and its GetImage() method will return the images sent from the remote Stereocam instance, returning NULL when the supply is exhausted (or the network connection is broken or severed).