Subbing webms with ffmpeg

SUBBING WEBMS WITH FFMPEG

Step 0: If you’re using Windows, you need to set up fonts.conf first.
        In the folder that contains ffmpeg.exe (bin folder), create a
        new folder named fonts. In that fonts folder, use a text editor
        to create a file named fonts.conf with the content here:
                       http://pastebin.com/DpD24trj


Step 1: Run this command to extract the subs and fonts, replacing
        %input% with the name of your source video:
             ffmpeg -dump_attachment:t "" -i %input% output.ass

Step 2: Add this option to your normal encoding line:
             -vf ass="output.ass"
        Note that you can only have one -vf option, so if you are
        already using it to scale, add the new filter with a comma:
             -vf scale=-1:504,ass="output.ass"
        You also must place -ss after -i for correct subtitle timing.
        Your final command should look something like this:

             ffmpeg -i input.mkv -ss 00:15:04 -t 3 -an -b:v 1200k -vf scale=-1:504,ass="output.ass" output.webm

It may take a while due to the accurate seeking, but eventually
you should get a .webm with correctly styled hardsubs.




APPENDIX (you can skip this)

   Using batch files: Dumping the subtitles and fonts will fill your bin folder with a
bunch of extra files that you need to delete afterwards. Instead of manually typing
the commands and deleting the files each time, it’s easier to create a .bat file that
will do it for you. Here is an example of a basic .bat for subbing .webms:

set input="C:\chinese cartoon.mkv"

ffmpeg -dump_attachment:t "" -i %input% output.ass
ffmpeg -i %input% -ss 00:15:04 -t 3 -an -b:v 1200k -vf scale=-1:504,ass="output.ass" output.webm

del output.ass
for %%f in (*.ttf) do (
	del "%%f"
)

for %%f in (*.otf) do (
	del "%%f"
)

Customize this (adding 2-pass and so on), save it as something.bat in your ffmpeg bin
folder and run it.


   Fast seeking: Normally, you need to use slow, accurate seeking (-ss after -i) because
fast seeking (-ss before -i) resets the internal clock to 0, so your subtitles will not be
timed correctly. If you’re too impatient for accurate seeking and willing to risk bugs,
one hack solution is to add the setpts and -itsoffset filter to your encoding line:
 ffmpeg -ss 00:01:20 -itsoffset 00:01:20 -i input.mkv -t 3 -vf setpts=PTS-%sec%,ass="output.ass" tmp.webm
Replace %sec% with the number of seconds from the start to your offset point (ss).
Afterwards, you need to run this to correct the video length (it’s lossless):
 ffmpeg -i tmp.webm -c:v copy output.webm


   Choosing a non-default subtitle track: First you need to find the stream
number of the desired subtitle track.
One way is to read the output of ffmpeg -i file.mkv
(e.g.: Stream #0:7 ... title: Commentary -- So commentary is stream 7.)
Or you could check Properties -> MediaInfo in MPC-HC.
(e.g.: ID: 8 ... Title: Commentary -- Subtract 1 from the ID: Again, commentary is stream 7.)

Now specify stream 7 (always 0:7) with -map in your font/sub extracting command:
  ffmpeg -dump_attachment:t "" -i %input% -map 0:7 output.ass