Apple Final Cut Pro 7 - Manual (Page 1908)

Some simple methods of data compression are:
• Throw away pixels at regular intervals: This essentially scales the image, or makes it
more blocky.
• Average several pixel values together (subsampling): This involves taking several adjacent
pixel values and averaging them together, resulting in a single rectangular pixel that
approximates the value of several. For more information, see
“Pixel Aspect Ratio.”
• Throw away color channel information at regular intervals: This results in color sample
ratios like 4:2:2 and 4:1:1. Ideally, throwing away this color information is not noticeable
to the viewer, but it may be a problem if you are trying to do detailed color correction
or chroma keying that requires a lot of color information to start with.
Lossless Codecs
Once these basic methods have been employed, much more intensive algorithms can
be employed to reduce the amount of transmitted and stored image data. Mathematical
algorithms can be used to encode and decode each video frame. These codecs (such as
enCode and Decode) must be installed in the VTR or software you are using to play back
your video. For example, QuickTime supports many different video codecs for video
export and playback.
The simplest encoding algorithm, called run-length encoding, represents strings of
redundant values as a single value and a multiplier. For example, consider the following
bit values:
0000000000000000000000001111111111111111000000000000000000000000
Using run-length encoding on the bit values above can reduce the amount of information
to:
0 x 24, 1 x 16, 0 x 24
Or in binary:
0 [11000], 1 [10000], 0 [11000]
In the example above, the original 64 bits can be transmitted using only 18 bits.
Run-length encoding is lossless, because all the information is retained after decoding.
This technique is particularly useful for computer graphics applications, because there
are often large fields of identical colors.
Note: If each bit in the original image were to alternate between 0 and 1, run-length
encoding would not only be ineffective, it could actually make the overall data rate higher.
Each codec is designed to anticipate and compress different kinds of data patterns. For
example, a codec designed for audio compression is not useful for video compression,
which has very different data patterns.
1908 Appendix B Video Formats