Using FFMPEG to Concatenate and Embed Subtitles

I recently upgraded my drone to a DJI Mavic Air 2. Among other things, it can create h.265 videos directly. It still uses the MP4 container format and the separate SRT format for storing video subtitles, the flight data. Following most camera standards, it creates video files that are individually smaller than 4 GB, which works out to be about five minutes in 4k video.

If I want to upload a longer raw video to social media, the video files need to be concatenated before uploading.

Concatenating the video with FFMPEG has been something I’ve known how to do for a long time using either of two methods. Today I learned how to properly embed the subtitles in either the MKV or MP4 container format.

The MP4 format is more widely supported than the MKV format, but is less flexible as to what it can contain.  The MKV (Matroska Multimedia Container) container format can hold almost any type of media, and so I’m able to copy the SRT format directly. The MP4 (MPEG-4 Part 14) container format only supports a limited selection of subtitle formats, so I’m required to have FFMPEG convert the SRT stream to a MP4 compatible stream.  If you are interested in video container formats, these tables are very helpful.

I’ll give several examples using the two video files and their associated subtitle files created by the drone named DJI00001.MP4, DJI00002.MP4, DJI00001.SRT, and DJI00002.SRT. The method I’m using should work for any number of files, up to the largest filesize you can store on your filesystem.

To simply concatenate the video files, create a text input file (I’m using mp4files.txt) with the contents as follows

file DJI00001.MP4
file DJI00001.MP4

then use the ffmpeg command to create a new concatenated file.

ffmpeg -f concat -safe 0 -i mp4files.txt -c copy ConcatenatedVideo.MP4

If you want to embed the subtitles, you need to create a second text file, do some stream mapping, and specify what format the subtitles should be. In this case I’m using srtfiles.txt

file DJI00001.SRT
file DJI00002.SRT

My FFMPEG command to create an MP4 file gets a lot more involved because now I’m specifying multiple inputs and have to specify the subtitle format.

ffmpeg -f concat -safe 0 -i mp4files.txt -f concat -safe 0 -i srtfiles.txt -map 0:v -map 1 -c:v copy -c:s mov_text ConcatenatedVideo.MP4

The FFMPEG command to create an MKV command is only a tiny bit different, and the resulting file is only a tiny bit smaller.

ffmpeg -f concat -safe 0 -i mp4files.txt -f concat -safe 0 -i srtfiles.txt -map 0:v -map 1 -c:v copy -c:s copy ConcatenatedVideo.MKV

When playing the ConcatenatedVideo files on my local machine, I can now enable or disable the closed caption track properly in the player for either format. Unfortunately in my initial testing with YouTube, neither format maintains the second stream of subtitles.

This is not all a waste of time and effort, because an advantage of embedding the subtitles into the container format is that the timing has been matched to the video, and can now be extracted in a concatenated form for use with YouTube.

ffmpeg -i ConcatenatedVideo.MKV -c copy ConcatenatedVideo.SRT

You can exclude the “-c copy” when extracting the subtitles and FFMPEG fill run it through its subrip codec and produce nearly identical results. It will only work with the MKV file because the subtitle format stored in the MP4 file is not easily converted to a SRT file.

Using the -f concat option invokes the concat demuxer in FFMPEG, which has the limitation that the format needs to be exactly the same for each file. If there are any changes between files you want to concatenate, you must use a more involved command invoking the concat filter. In a different project I ran into a command issue with the concat filter command when the command got to be much over 900 characters long.

 

exiftool to manage DJI media files

DJI Drones don’t seem to remember the image count between formats of a media card. This creates a problem for me when I’m trying to backup and maintain my images and video.

Because the dates are all correct in the media files, retrieved from GPS data, organizing the files by naming them based on the date works for me.

Using ExifTool by Phil Harvey is a great solution for pulling the metadata from the files and renaming the files.

The command line that I was initially using is:

exiftool "-FileName<${CreateDate}.$filetype" -d %Y%m%d-%H%M%S%%-c -ext mp4 -ext dng -ext jpg dji*

It’s problem is that it orphans the SRT subtitle files from my videos that I’d like to keep matching the video files.

I’ve tried this variation to do it in one step but it doesn’t work, because the SRT files get renamed as MP4 files.

exiftool -verbose "-FileName<${CreateDate}" -d %Y%m%d-%H%M%S%%-c.%%le -ext mp4 -ext dng -ext jpg dji* -srcfile %f.srt

If anyone has a suggestion for how to rename all the media files in one directory I’d appreciate it. Even running two commands in sequence would be fine.

Update:

I’ve figured out that running these two commands in sequence will get me the results I am looking for:

exiftool "-FileName<${CreateDate}" -d %Y%m%d-%H%M%S%%-c.srt -ext mp4 -srcfile %f.srt dji*
exiftool "-FileName<${CreateDate}" -d %Y%m%d-%H%M%S%%-c.%%le -ext mp4 -ext dng -ext jpg dji*

I’m still looking for a way of doing it in a single command that may leave less room for error, but this is working for now.

Best Buy and DJI Accessories

I don’t shop too much locally for technology, because it always seems that what I want is not available locally. Yesterday I was wandering around killing time and went into the local Best Buy.  Instead of just looking at the normal things I might go for in a best buy, I looked around to see what was new. They now have an aisle dedicated to drones. There were complete drones from DJI, Yuneec, and several other manufacturers. More interesting to me was that they carried accessories for those drones as well.  I knew that they sold drones on-line, it just never occurred to me to look at them in person in the store.

I bought a Mavic Pro directly from DJI soon after they were released. I got the Fly-More package that entails several items including spare propellers. I’ve not needed to replace a propeller in my first 7 hours of flight time but it’s nice to know that propellers are available locally. DJI charges slightly less, $9.00, but they charge for shipping, and you have to wait for the item to arrive.