[3.7] How do the interactive features work?

DVD-Video players (and software DVD-Video navigators for computers) support a command set that provides rudimentary interactivity. The main feature is menus, which are present on almost all discs to allow content selection and feature control. Each menu has a still or motion background and up to 36 highlightable, rectangular "buttons" (only 12 if widescreen, letterbox, and pan & scan modes are used). Remote control units have up/down/left/right arrow keys for selecting onscreen buttons, along with numeric keys, a select (enter) key, a menu key, a top menu (title) key, and a return key. Additional remote functions may include freeze, step, slow, fast, scan, next, previous, audio select, subtitle select, camera angle select, play mode select, search to program, search to part of title (chapter), search to time, and search to camera angle. Any of these features can be disabled by the producer of the disc, an act which is called user operation control (UOP). It's commonly used to lock you into the copyright warning or movie previews at the beginning of the disc, or to keep you from changing audio or subtitle tracks during the movie.

Additional features of the command set include simple math (add, subtract, multiply, divide, modulo, random), bitwise and, bitwise or, bitwise xor, plus comparisons (equal, greater than, etc.), and register loading, moving, and swapping. There are 24 system registers for information such as language code, audio and subpicture settings, and parental level. There are 16 general registers for command use. A countdown timer is also provided. Commands can branch or jump to other commands. Commands can also control player settings, jump to different parts of the disc, and control presentation of audio, video, subpicture, camera angles, and so on. The command set enables relatively sophisticated discs, such as games or interactive educational programs.

DVD-V content is broken into titles (movies or albums), and parts of titles (chapters or songs). Titles are made up of cells grouped into programs and linked together by one or more program chains (PGC). A PGC can be one of three types: sequential play, random play (may repeat), or shuffle play (random order but no repeats). Individual cells may be used by more than one PGC, which is how parental management and seamless branching are accomplished: different PGCs define different sequences through mostly the same material.

Additional material for camera angles and seamless branching is interleaved together in small chunks. The player jumps from chunk to chunk, skipping over unused angles or branches, to stitch together the seamless video. Since angles are stored separately, they have no direct effect on the bitrate but they do affect the playing time. Adding 1 camera angle for a program roughly doubles the amount of space needed (and cuts the playing time in half). Examples of branching (seamless and non-seamless) include Kalifornia , Dark Star , Stargate SE , and The Abyss .