====== ffmpeg ======

''ffmpeg'' is a powerful tool for video conversion.

===== Recording Screencasts =====

Screencasts are basically animated screenshots and therefore a very nice way of illustrating things. Here is how to record them using ''ffmpeg'':
  ffmpeg -f x11grab -r 25 -s 1024x768 -i :1.0 -c:v huffyuv screencast.mkv

===== Encoding Videos =====

This is rather a howto than some detailed illustration of what ffmpeg does or what one may do with it. Actually, this is how I currently encode DVDs:

==== Input Files ====

For quicker access, data from DVD should be ripped to disk first, e.g. using ''vobcopy -m''. Since ''ffmpeg'' expects input to consist of a single file, DVD tracks contained in multiple VOB files have to be combined - which is actually pretty simple:
  cat VIDEO_TS/VTS_NN_{1..Y}.VOB >input.vob

==== Single-Pass Encoding ====

In case output size is not as important, single-pass encoding is the way to go. Output quality may be adjusted by specifying different presets. In general, the command looks like this:
  ffmpeg -i input.vob [magic options here] output.mkv

My choice of video codec is ''x264'', using a preset of ''slower'' or ''veryslow''. Tuning for ''film'' should not hurt, and a quality setting of ''20'' is decent:
<code>
-c:v libx264 -preset veryslow -tune film -crf 20
</code>

For audio, I use ''AAC'' with variable bit rate. Setting VBR mode to ''3'' results in 48-56 kbps/channel (''man ffmpeg-codecs'' for details):
<code>
-c:a libfdk_aac -vbr 3
</code>

Extracting subtitles is easy, the MKV format allows for embedded subtitle streams just as VOB does:
<code>
-c:s copy
</code>
In order to have the right subs being displayed by default, ''mkvpropedit'' comes in handy. For instance, use the following to set the default flag of track 3:
  mkvpropedit --edit track:3 --set flag-default=yes output.mkv


==== Simple Encoding to Webm ====

<code>
ffmpeg -ss 00:37:30 -i input.mp4 -to 00:01:55 -codec:v libvpx -vf scale=-1:480 -an output.webm
</code>
This will:
  * skip to offset 37m30s in input.mp4
  * encode for 1m55s
  * encode to VPX codec (usually used for webm)
  * scale to yres of 480, keeping aspect ratio (-1)
  * drop audio input (-an)

===== Concatenating Videos =====

A quick and easy way to concat videos without re-encoding is to use the
''concat'' format specifier:

  $ cat mylist.txt
  file '/path/to/file1'
  file '/path/to/file2'
  file '/path/to/file3'
  
  $ ffmpeg -f concat -safe 0 -i mylist.txt -c copy output.mp4

===== Encoding MP3s =====

While this is about transcoding from FLAC to MP3, the same command should do
for about any supported source format:

  ffmpeg -i input.flac -q:a 0 -map a output.mp3

By default, ffmpeg will write an ID3 version 2.4 section based on FLAC tags.
Though since hardly any ID3-related tool seems to support them, it is advisable
to pass ''-id3v2_version 3'' so version 2.3 is being used. To increase
compatibility, ''-write_id3v1 1'' enables additional ID3 version 1 tags.

===== Links =====

[[https://stackoverflow.com/questions/7333232/how-to-concatenate-two-mp4-files-using-ffmpeg|How to concatenate two mp4 files on Stackoverflow]]