3.7 How do the interactive features work?
Description
This article is from the DVD Formats FAQ, by jtfrog@usa.net (Jim Taylor) with numerous contributions by others.
3.7 How do the interactive features work?
DVD-Video players (and software DVD-Video navigators) 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-frame graphic and up to 36 highlightable,
rectangular "buttons" (only 12 if widescreen, letterbox, and pan & scan
modes are used). Remote control units have four arrow keys for selecting
onscreen buttons, plus numeric keys, select key, menu key, and 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.
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, etc.
DVD-V content is broken into "titles" (movies or albums), and "parts of
titles" (chapters or songs). Titles are made up of "cells" linked together
by one or more "program chains" (PGC). A PGC can be on 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, and Stargate SE.
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