Image Processing Reference
In-Depth Information
Variable-bit-rate compression is much preferred for content that is not intended to
be broadcast within a fixed-bit-rate budget. If the bit-rate budget is constrained in some
way, a cooperative compression process on several channels will deliver some benefits.
Two or more channels can make room for each other and be transmitted using less
bandwidth than they would require if they were compressed independently of each
other. This technique is called statistical multiplexing, and it is covered in some detail
in Chapter 14.
Downsides to Video Compression
There are times when video compression causes more problems than it solves. Trying to
deploy it in unsuitable circumstances leads to difficulty later in the production and con-
sumption cycle.
Origination Stage
Video compression should be avoided at the origination stage. Some cameras compress
the video as it is written to tape or disk inside the camera. These devices never yield an
uncompressed output. The consequence is that any content you shoot with such a camera
will probably turn out to be totally useless for anything other than home movies.
For example, suppose you are shooting some training video and you want to color
key the background. Regardless of whether you use blue or green screens or even the
newer retro-reflective materials to get a good hard edge for your matte, any compression
that loses detail will immediately compromise your ability to pull a matte off that video.
The jaggies and edge-coded artifacts are so severe that your footage is rendered useless.
The solution is to get a better camera and shoot the footage at the best-quality camera set-
tings available, and then write it to tape with a high-bit-rate storage format with a near-
lossless compression method.
Not for Archiving Master-Quality Material
For much the same reasons that you would a) shoot footage using the best available
quality of equipment, film stock, or video recording format and b) use lossless com-
pression techniques (the intent is to preserve the maximum information), the archives
must be maintained at the highest possible quality. A completely lossless coding format
is desirable but expensive in terms of storage space. Experience shows that if content is
compressed with poorly chosen coding techniques, the archives become unusable very
quickly. Putting the tracking systems in place and logging the rushes as soon as they
arrive from the field must be seen as a benefit and not a cost. In the long term it will
save money.
I will describe a very painful episode based on a true story. Only the names and
places have been changed to protect the innocent.
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