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<h1><a class="anchor" id="dvdnavissues">An introduction to DVD navigation </a></h1><p>The DVD format represents a radical departure from the traditional form of video home-entertainment. Instead of just being a linear programme which is watched from beginning to end like a novel DVD allows the user to jump about at will (much like those 'Choose your own adventure' or 'Which Way' books which were popular a while back).</p>
<p>Such features are usually referred to under the moniker 'interactive' by marketting people but you aren't in marketting since you are reading the <code>libdvdnav</code> tutorial. We'll assume you actually want to know precisely what DVD can do.</p>
<p>A DVD is logically structured into titles, chapters (also known as 'parts'), cells and VOBUS, much like the filesystem on your hard disc. The structure is heirachical. A typical DVD might have the following structure:</p>
<div class="fragment"><pre class="fragment">
.
|-- Title 1
| |-- Chapter 1
| | |-- Cell 1
| | | |-- VOBU 1
| | | |-- ...
| | | `-- VOBU n
| | |-- ...
| | `-- Cell n
| |-- ...
| `-- Chapter 2
| |-- Cell 1
| | |-- VOBU 1
| | |-- ...
| | `-- VOBU n
| |-- ...
| `-- Cell n
|-- ...
`-- Title m
|-- Chapter 1
| |-- Cell 1
| | |-- VOBU 1
| | |-- ...
| | `-- VOBU n
| |-- ...
| `-- Cell n
|-- ...
`-- Chapter 2
|-- Cell 1
| |-- VOBU 1
| |-- ...
| `-- VOBU n
|-- ...
`-- Cell n
</pre></div><p>A DVD 'Title' is generally a logically distinct section of video. For example the main feature film on a DVD might be Title 1, a behind-the-scenes documentary might be Title 2 and a selection of cast interviews might be Title 3. There can be up to 99 Titles on any DVD.</p>
<p>A DVD 'Chapter' (somewhat confusingly referred to as a 'Part' in the parlence of DVD authors) is generally a logical segment of a Title such as a scene in a film or one interview in a set of cast interviews. There can be up to 999 Parts in one Title.</p>
<p>A 'Cell' is a small segment of a Part. It is the smallest resolution at which DVD navigation commands can act (e.g. 'Jump to Cell 3 of Part 4 of Title 2'). Typically one Part contains one Cell but on complex DVDs it may be useful to have multiple Cells per Part.</p>
<p>A VOBU (<em>V</em>ideo <em>OB</em>ject <em>U</em>nit) is a small (typically a few seconds) of video. It must be a self contained 'Group of Pictures' which can be understood by the MPEG decoder. All seeking, jumping, etc is guaranteed to occurr at a VOBU boundary so that the decoder need not be restarted and that the location jumped to is always the start of a valid MPEG stream. For multiple-angle DVDs VOBUs for each angle can be interleaved into one Interleaved Video Unit (ILVU). In this case when the player get to the end of the VOBU for angle <em>n</em> instead of jumping to the next VOBU the player will move forward to the VOBU for angle <em>n</em> in the next ILVU.</p>
<p>This is summarised in the following diagram showing how the VOBUs are actually laid out on disc.</p>
<div class="fragment"><pre class="fragment">
,---------------------------. ,---------------------------.
| ILVU 1 | | ILVU m |
| ,--------. ,--------. | | ,--------. ,--------. |
| | VOBU 1 | ... | VOBU 1 | | ... | | VOBU m | ... | VOBU m | |
| |Angle 1 | |Angle n | | | |Angle 1 | |Angle n | |
| `--------' `--------' | | `--------' `--------' |
`---------------------------' `---------------------------'
</pre></div><p>If the layout of the DVD were the only feature of the format the DVD would only have a limited amount of interactivity, you could jump around between Titles, Parts and Cells but not much else.</p>
<p>The feature most people associate with DVDs is its ability to present the user with full-motion interactive menus. To provide these features the DVD format includes a specification for a DVD 'virtual machine'.</p>
<p>To a first order approximation x86 programs can only be run on x86-based machines, PowerPC programs on PowerPC-based machines and so on. Java, however, is an exception in that programs are compiled into a special code which is designed for a 'Java Virtual Machine'. Programmes exist which take this code and convert it into code which can run on real processors.</p>
<p>Similarly the DVD virtual machine is a hypothetical processor which has commands useful for DVD navigation (e.g. Jump to Title 4 or Jump to Cell 2) along with the ability to perform simple arithmetic and save values in a number of special variables (in processor speak, they are known as 'registers').</p>
<p>When a button is pressed on a DVD menu, a specified machine instruction can be executed (e.g. to jump to a particular Title). Similarly commands can be executed at the beginning and end of Cells and Parts to, for example, return to the menu at the end of a film.</p>
<p>Return to the <a class="el" href="tutorial.html">A libdvdnav Tutorial</a>. </p>
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