DVD Demystified

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Chapter 11
Making and Accessing the Audio and Video Video content needs to
be identified or created. If the WebDVD project uses an existing disc, the
only task is to identify the titles, chapters, and timecodes needed to access
parts of the disc. If the project requires a new disc, the video and audio content
needs to be created. You must decide whether to author the content as
DVD-Video or simply put MPEG-2 or other multimedia files on the DVD-
ROM. In general, authoring a DVD-Video disc is the best approach. The
DVD-Video content will play in all DVD platforms, even those with no Web-
DVD support, and DVD-Video-specific features such as subpictures and
camera angles can also be used. DVD-Video volumes are more compatible
with computers and with set-top WebDVD players than plain MPEG-2 files.
Access to the DVD-Video zone of the disc is accomplished with the Web-
DVD API of the target platform, which must provide a way to jump to specific
titles and chapters, and preferably to specific timecodes as well. For
simple WebDVD titles intended only to show related HTML pages as the
video plays, an authoring system with WebDVD support may be all you
need; you can type in URLs to be associated with each chapter. For more
complicated WebDVD titles, you will need to write scripts and possibly create
lists or databases that link the video to the Web or the Web to the video.
In either case, a title number plus a chapter number, or a title number plus
a timecode (hours: minutes: seconds: frames), is the link to the video. A URL
(such as http://dvddemystified.com/webdvd), possibly with a script to be executed
on the page, is the link to the Web. URLs may be relative (no http:// at
the front), in which case the HTML pages come from the DVD itself.
If a new disc is created, it should be authored with WebDVD in mind.
User operation controls (UOPs) should be used sparingly. Disabling the fastforward,
next, and menu operations to lock the viewer into introductory
logos, legal warnings, and ads will also prevent control by HTML pages.
Scripts on the page won’t be able to jump to desired locations on the disc,
while the necessary user operations are blocked. Commands and GPRMs
should be used judiciously. A disc that depends on certain values being
placed in GPRMs may behave in unexpected ways when the HTML page
that controls it skips over sections that set or check values. Video access
points should be identified ahead of time. That is, every place in the video
that you might want to have the HTML page jump to should be marked as
a chapter during DVD-Video authoring because chapter access is extremely
fast, easy, and accurate. It’s possible to jump to an arbitrary point in a video
sequence using timecodes, but only if the title is authored in one sequential
PGC form. Most players can only jump to the I frame nearest a timecode, so
the granularity of access is usually about 0.5 seconds.