ffmpeg: target size encoding added

wiki/linux/ffmpeg.md

@@ -1,6 +1,6 @@
-# ffmpeg
+# FFmpeg
 
-[ffmpeg](https://www.ffmpeg.org) is a free and open-source suite consisting of
+[FFmpeg](https://www.ffmpeg.org) is a free and open-source suite consisting of
 many audio and video tools and libraries.
 
 ## Usage
@@ -32,3 +32,34 @@ ffmpeg -f concat -safe 0 -i files.txt -map 0 -c copy output.mp4
 
 If the video files you want to concatenate are not mp4 files change the above
 command accordingly.
+
+### Two-Pass Encoding
+
+Two pass encoding - as described in the
+[official FFmpeg documentation](https://trac.ffmpeg.org/wiki/Encode/H.264#twopass)
+- uses two passes.
+The first pass analyzes the input data and outputs a descriptor file.
+The second pass actually encodes the data.
+The following is an example where the file `input` is encoded with `libx264` to
+`mp4` video with `libfdk_aac` audio.
+The video bitrate is `555k` and the audio bitrate is `128k`.
+
+```sh
+ffmpeg -y -i input -c:v libx264 -preset medium -b:v 555k -pass 1 -c:a libfdk_aac -b:a 128k -f mp4 /dev/null && \
+ffmpeg -i input -c:v libx264 -preset medium -b:v 555k -pass 2 -c:a libfdk_aac -b:a 128k output.mp4
+```
+
+### Encode Audio/Video to Target Size
+
+For the encoding of a file to a target size the target bitrate of the output
+video is needed.
+An explanation of this was given by
+[aergistal on Stack Overflow](https://stackoverflow.com/questions/29082422/ffmpeg-video-compression-specific-file-size).
+This can easily be done with the calculation `bitrate = target size / duration`
+in Bits/Second.
+Afterwards the encoding can be done by using Two-Pass Encoding as explained in
+[a previous section](#two-pass-encoding).
+Note that the bitrate for videos is split amongst a bitrate for video and a
+bitrate for audio.
+The target bitrate has to be equal to or greater than the sum of both video
+bitrate and audio bitrate.