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.
Appendix A
Video Formats
393
V