From The Frame Grab Tutorial: http://tvcrit3.tvcrit.com/content/view/58/102/.
Codecs, as explained by Curricular Computing, Dartmouth College:
One second of uncompressed NTSC video requires approximately 27 MB of disk space. Storing video onto desktop computers clearly requires compression, and playing video on desktop computers requires decompression. Hence, video compression algorithms, or "codecs" (short for compressors/ decompressors), were devised to handle the compression of video, and the decompression when it is played.
Codecs compress movies by looking at consecutive frames and storing just the differences (temporal compression), and/or by generalizing an image and removing redundant data (spatial compression). Many codecs use both spatial and temporal compression techniques. The process of analyzing each frame and compressing a movie can take a long time, but decompression takes place in real time to allow smooth playback.
Most codecs use lossy compression algorithms, which means the process of compression removes data from the original movie. You should never compress a movie multiple times, as each pass will lower the quality of the movie.
From www.dartmouth.edu/~cc/didactic/video/contents.html (dead link).
Apple provides a guide to the video compressors included with QuickTime. Moved to:
http://www.apple.com/macos/resources/macos9/us/help/quicktime4.1/ap/pgs/apCmprss.htm
From www.apple.com/quicktime/resources/qt/us/help/ap/pgs/apCmprss.htm .
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