More About Compression and Edit/Cut Markers
Compression markers identify areas of abrupt visual changes in video, such as a cut
from a dark room to a bright, grassy plain. There are two kinds of compression markers:
Â Compression markers: Also known as manual compression markers. These are markers
you can manually add to a sequence to ensure a particular section of video is
encoded using I-frames where you think they are necessary.
Â Edit/cut markers: Also known as automatic compression markers. These markers are
automatically inserted by Final Cut Pro at all edit and transition points.
When you export a clip or sequence containing compression markers to a QuickTime
movie file, the resulting file contains a hidden text track used to store the compression
Some video compression applications, including Compressor, are capable of using the
compression text track of your movie to optimize the placement of I-frames during the
process of MPEG-2 compression to achieve maximum playback quality.
In most cases, the edit/cut markers placed automatically by Final Cut Pro will suffice.
However, an abrupt visual change that occurs within a clip (as opposed to an abrupt
visual change caused by an intentional edit) may require a manual compression marker
for optimal compression. For example, suppose that in the middle of a long shot there
is a quick 180-degree pan from a dark jungle scene to a bright beach. The visual
changes from dark to light are dramatic, but there is no cut point in the shot for
Final Cut Pro to detect automatically. In this case, you can manually place compression
markers at the frames occurring immediately before and after the pan. Applications
such as Compressor will see these manual compression markers and place I-frames at
those points in the compressed video to minimize any compression artifacts.
A compression marker is distinguished by the text <COMPRESSION> appearing in the
Comment field of its Edit Marker window. For details on adding and deleting markers,
see Volume II, Chapter 4, “Using Markers.”
Compression markers are exported differently depending on whether you’re exporting
a sequence or a clip.
Â If you’re exporting a clip in the Browser: You must place manual compression markers
in the clip for a compression track to be created. No automatic compression markers
are exported in this case.
Â If you’re exporting a sequence: Final Cut Pro exports edit/cut markers for each cut,
transition, and gap that appears in your sequence. Manual compression markers are
also exported. Compression markers added to individual sequence clips are ignored.