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171 lines
6.6 KiB
171 lines
6.6 KiB
4 years ago
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FRAME RATES
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This section is intended for users who want to convert the framerate of the
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input file to a different output framerate. It also handles the case
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when the input file is of variable framerate.
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=========================
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Jargon used in this file.
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=========================
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fps : Frames-per-second / framerate. The number of frames which
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will be displayed during 1 second.
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frc : Frame rate code. This is directly related to fps. When
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dealing with fractional framerates, it is more convenient
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and accurate to give transcode a code. The most commonly
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used codes are
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1 : 23.976 (24000/1001.0)
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3 : 25
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4 : 29.970 (30000/1001.0)
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Its worth noting that none of these filters intent to change the
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runtime of the movie; if one -- for example -- goes from 29.9 to
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23.9 the movie will not be shorter or anything, every frame will be
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displayed for a longer time period. Because there will be fewer
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frames in the resulting movie and the display time of each frame is
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longer, the movie will have the same running.
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For convenience, a table relating fps and display time
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frc | fps | Display time (sec) | Milliseconds
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----+-------+--------------------+-------------
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1 | 23.97 | 1 / (24000/1001) | 41.71
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3 | 25.00 | 1 / (25000/1000) | 40.00
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4 | 29.97 | 1 / (30000/1001) | 33.37
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When converting from fps1 to fps2 it must be clear that the audio
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stream will not be touched at all. Since the resulting movie has the
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same display length, the audio in the new file must be equal to the
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audio in the old file. You currently cannot use transcode to slow
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down or speed up a movie.
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================================
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Meaningful fields of application
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================================
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Changing the framerate of a movie can be neccessary to improve the
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quality, shrink the size or to prepare a movie for a specific output
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medium.
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o A lot of digital (photo) cameras have the ability to record -- more
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or less -- small video clips. These clips are often not suiteable
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for watching on TV because of the framerate. My device records
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MJPG clips at 20 fps. To convert them to (s)VCD, the framerate
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needs to get converted to 29.9 (NTSC) or 25 (PAL).
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Note that converting the framerate does not make the clip
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smoother, it merely preserves the jerkyness it already had.
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o Telecine removal. Telecine is a process to prepare films (movies)
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shot at 24 fps for (NTSC) television at 29.9 fps. The opposite is
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called inverse telecine or ivtc. For a really well done in-depth
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explanation about telecine see the file README.Inverse.Telecine.txt
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by Thanassis Tsiodras in the /docs/ directory of transcode.
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By doing ivtc, the movie will look less jerky and therefore
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smoother.
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o Preparing NTSC or film sources for PAL or the other way round.
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=================
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Available Modules
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=================
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There are several ways (as always) in transcode to do framerate
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conversions (in lexicographical order)
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o Overview
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Filter(s) | Smartness | Versatility | Intented Use
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--------------+-----------+-------------+---------------
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29to23 | none | little | 29.9 -> 23.9
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32drop | high | little | 29.9 -> 23.9
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ivtc+decimate | very high | none | 29.9 -> 23.9
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modfps | fair | high | any -> any
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fps | none | high | any -> any
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tc_video | none | little | 23.9 -> 29.9
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o 29to23
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The filter takes no options. It basically skipps every 5th frame.
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It interpolates the other 4 frames out of the 5 it recieves.
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Since it skipps every 5th frame it can also be used for converting
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25 fps to 20 fps.
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o 32drop
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The filter takes no options. It does a simple inverse telecine.
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Its algorithm works best when used in RGB mode. It tries to find
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interlaced frames and adjusts its pattern.
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o ivtc+decimate
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These two filters working hand in hand. The first filter (ivtc)
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inverts the telecine and the second one (decimate) drops 1 frame
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out of 4 so that the output will be at 23.9 fps. These two filters
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are really smart and do their job very well. The ivtc filter can
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deal with a changing telecine pattern and adjusts its algorithm to
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fit it. The decimate filter searches for two similar frames out of
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five and drops one of which look similar.
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o modfps
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This filter tries to reach the desired framerate by cloning and
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dropping of frames. In its default mode it will buffer the last
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5 frames, and drop the frame with the least difference with the
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frame that follows it, or clone the frame with the most
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difference. Additionally, it can blend frames or mingle frame
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fields when cloning. Additionally it has a fast mode, but results
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look poor.
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o fps
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This filter is basically the same as the fast mode of `modfps' its
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just doing its work a slightly different way.
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o tc_video
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This filter increases the framerate by doing telecine. It takes no
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options. It makes 5 frames out of 4 by clever dealing with the
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fields ( == "lines") of a frame.
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===================================
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Framerate related transcode options
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===================================
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Transcode has two options dealing with framerates, `-f'and
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`--export_fps'. Both options take one to two arguments. The first
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argument is the framerate as a floating point number, the second
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argument is the frameratecode. The frc always overwrites the
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supplied fps value, so one can write `-f 0,1 ... --export_fps 0,4'.
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Transcode tries to do the right thing with these options, the
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following table shows the interdependencies. Import is `-f' and
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Export is `--export_fps'
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Import | Export | Result
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---------+---------+--------------------------------------------
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omitted | omitted | import will be probed, export set to import
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given | omitted | export set to import
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omitted | given | import will be probed, export used as given
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given | given | Both will used as given
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The audio is always read at the export rate.
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There is another option related to framerates which is --export_frc.
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This option is only there because of backward-compatibility. It does
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the same like the second argument to --export_fps.
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========================
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Variable Framerate Input
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========================
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Some video sources are not of constant framerate. This is mostly
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true for Anime and some DVD Releases of TV series. Use this command
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line (XXX: Write much more here :)
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transcode -i vobs/ -V -x vob,vob -f 0,4 -M2 -R3 -w2 --export_frc 1 \
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-J ivtc -J decimate -B 3,9,16 --hard_fps --print_status 10 \
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-J 32detect=verbose=1:force_mode=5:chromathres=2:chromadi=9 \
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-y xvid -o next1.avi
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(C) 2003 (v0.1) Tilmann Bitterberg <transcode at tibit.org>
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// vim: tw=68
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