Creating primitive subtitle (SubRip/.srt) file from scratch

I had a video I wanted to add a few subtitles lines here and there, without much effort. I ran:

$ apt-cache search subtitle editor
aegisub - Advanced subtitle editor
aegisub-l10n - Aegisub language packages
gaupol - subtitle editor for text-based subtitle files
libsubtitleeditor-dev - subtitleeditor lib - development files
libsubtitleeditor0 - subtitleeditor lib - runtime files
subtitleeditor - Graphical subtitle editor with sound waves representation
python-aeidon - reading, writing and manipulating text-based subtitle files
gnome-subtitles - Subtitle editor for the GNOME Desktop environment

and installed a few of them.

I was quite a disappointing experience. These softwares are more or less working but none was easy and fast enough to use for me not to prefer just using mpv and emacs, viewing with mpv and typing relevant lines in a text file with  timestamps.

Some of these softwares looked almost ok but adding new lines that was generally inconvenient (not able to insert at the exact place the playthrough is paused, or supposed to be able to but simple not working).

So, yeah, I got the simple way and built a file like :

1:01:01 adjkajdalm kadmùlajpod jaaaaaaaa aaaaaaaa aaaaaaaaaaaaaaaaaaa aaaaaaaaaaaa
11:01:01 adjkajdalm kadmùlajp odjaaaaaaaaa aaaaaaaaaaaaaaaaaa
111:01:01 adjkajda

And wrote a fast primitive script as follows:

use strict;
my $file = "THIS";
open(IN, "< $file.txt"); # input
open(OUT, "> $");
my $count = 0;
while (<IN>) {
 next if /^\s$/; #skip empty
 print OUT "$count\n";
 if (/^(\d{1,3})\:(\d{2}):(\d{2})\s/) { # extract time HH:MM:SS
 my ($h, $m, $s) = ($1, $2, $3);
 s/^$h:$m:$s\s//g; # remove the time from the line
 my $duration = 3; # 3 sec average duration 
 $duration = 5 if length($_) > 34; # 5 for almost two lines
 $duration = 2 if length($_) < 15; # 2 when very short
 my ($hplus, $mplus) = ($h, $m);
 my $splus = ($s + $duration);
 if ($splus > 60) { $mplus = ($m + 1); $splus = ($splus - 60); }
 if ($mplus > 60) { $hplus = ($h + 1); $mplus = ($mplus - 60); }
 print OUT sprintf("%02d:%02d:%02d,000 --> %02d:%02d:%02d,000",
 $h, $m, $s,
 $hplus, $mplus, $splus)."\n";
 } else {
 print "$count TIME ?\n"; 
 print OUT $_;
 print OUT "\n";

It requires $file to be set accordingly to the name given to the input text file, obviously.

When satisfied with the written .srt, it’s convenient to embed it in the end file:

ffmpeg -i infile.mp4 -f srt -i -c:v copy -c:a copy -c:s mov_text outfile.mp4


Creating an mp4 with black screen and silent audio (with ffmpeg)

Assuming you have a  black PNG image named blck.png, you can make it a mp4 video:

ffmpeg -loop 1 -i blck.png -t 900 -r 1 -c:v libx264 blck.mp4

And if you want to concatenate it with other mp4 (see previous post), it also needs to include a sound track. So, assuming your main files are in 48000hz (otherwise adjust), you can add it with the command:

ffmpeg -f lavfi -i anullsrc=r=48000 -i blck.mp4 -to 00:15:00 -c:v copy -c:a aac -strict experimental blck-snd.mp4

(note : -to needs to be set to the expected time, option -shortest fails)



Concatenating mp4 videos (with ffmpeg)

mp4 cannot be concatenated directly. But fmpeg (and so avconv I guess) can by the way of intermediary mpeg transport streams.

Here’s a dirty bash script that does that assuming you want to concat files named like MYFILE-1.mp4 MYFILE-2.mp4. Then you would just execute the script with MYFILE- as argument.

echo $1
[ ! -e "$1"1.*4 ] && echo "nothing for $1""1.mp4" && exit

for i in `seq 1 20`; do
[ ! -e "$1"$i.*4 ] && continue
echo "$1$i.mp4 exists, creating $1$i.ts"
ffmpeg -i "$1"$i.*4 -c copy -bsf:v h264_mp4toannexb -f mpegts "$1"$i.ts

echo "##########################################"
list=`echo "$list" | cut -c 2-`
echo "concat of $list"
ffmpeg -i "concat:$list" -c copy -bsf:a aac_adtstoasc "$1""F.mp4"
rm -f "$1"*.ts

# let user manually remove source files after checks