Files and Storage Formats - A Practical Guide to Video and Audio Compression - page 285

relates to the location and use of the file as a whole. Internally stored metadata might
describe individual scenes or actors. Tagging information about particular scene locations
or edit points might be embedded into the video file in a special track within the movie or
stored outside in a separate but related text file.
16.5.3.14 MIDI Track Music tracks are sometimes encoded as MIDI information.
Obviously, an embedded MIDI instrument must be available in the playback device.
QuickTime licenses the Roland GM sound samples as found in the original Sound Canvas
synthesizer. Often the MIDI support will be labelled GM or GS depending on the manu-
facturer. A standard set of instruments located predictably at the same patch numbers will
be supported. They might not sound identical, but if you choose a piano sound, you will
get a piano and not a trumpet.
16.5.3.15 Warp Matrix Track QuickTime allows the video to be warped accord-
ing to a matrix transform. By storage of this information in a track alongside the video, a
frame-accurate animated warping effect can be applied during playback.
16.5.3.16 Alpha-Channel Track Cutting out a matte or alpha channel that moves
with the video is accomplished using an alpha-channel track. This is frame-for-frame identi-
cal in timing and resolution to the video track but contains a value from 0% to 100% trans-
parency. Application of a constant value across the entire surface of the video allows blending
effects to be achieved. Wipes, transitions, and other compositing effects are also possible.
16.5.3.17 Color-Correction Track With the advent of very powerful systems,
color correction might be applied in real time to the played-out video. This might be used
for special effects or to correct the video after mapping of the transform to some ambient
condition.
16.5.3.18 Effects Track QuickTime supports smoke, flame, fog, clouds, and rip-
pled-water effects that are applied as a layer over the final composited video. These would
be stored in a symbolic form as parameter settings that are applied frame by frame. They
occupy almost no space at all.
16.5.3.19 Closed-Signing Track This is similar in concept to the closed captions.
Closed captions are hidden subtitle data that require a hardware decoder to display
them. Closed signing is the same idea, except that an overlay video is described using an alpha
channel-coded image of sign language compressed using MPEG-4 part 2. This is turned on
by the owner of a closed-signing decoder box but is not visible to other users. Assistance
with access support like this is mandated in most broadcasters' licenses in the UK, and much
useful work is done in this area by the BBC Research and Development department.
16.5.3.20 Hint Track This is used to assist the streaming process by providing help
with the formation of packets and perhaps some information to insert into the headers of
the streaming service. There may be multiple hint tracks allowing the content to be