Category Archives: Video

Creating a WebM video with subtitles

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This is mostly a note pad for myself with quick instructions about how to create WebM video file with embedded subtitles from the command line.

Having a source video with A/V tracks that are WebM compliant, and a subtitles track, this is the quick reference command to copy and paste:

$ 
ffmpeg -i video.mkv -c:a copy -c:v copy -c:s webvtt video.webm

Now the long and boring story.

Lately, my preferred multimedia container is Matroska and, for easily managing videos with multiple tracks without reencoding them I use the great MKVToolNix application.

Now, when I want to put a video online, the 2 video containers with wider support among the Open Source browsers are .mp4 and .webm. I’d rather use WebM whenever possible and, additionally, it is a subset of Matroska. Indeed, MKVToolNix has an option to “Create a WebM compliant file” as output.

Up until now, when dealing with a subtitles track I’ve always tried to use the SubRip subtitles format (.srt). However, when I tried with MKVToolNix to create a WebM compliant file with a .srt track, I’d get the error The codec type 'text subtitles' cannot be used in a WebM compliant file.

Hence, in order to not get too complicated, I’d generate a Matroska file and just convert it to .mp4 without reencoding with FFmpeg:

$ 
ffmpeg -i video.mkv -c:v copy -c:a copy -c:s mov_text video.mp4

However, recently a colleague warned me that FireFox in Linux doesn’t reproduce the audio track of a .mp4 file if the codec is one of Vorbis or Opus, the ones used with WebM. Yes, FireFox supports WebM and, hence, those audio codecs, but refuses to play them if they belong inside a .mp4 container (!!)

OK, so what now … a first quick check to the WebM’s documentation told me that WebM doesn’t support subitles but that intends to support WebVTT in the future …

… but the same colleague told me that WebM actually does support WebVTT. A second look to that very same page tells me that he is right! Confusing …

Fine. Let’s use MKVToolNix to create that WebM compliant file but with a WebVTT track … oh, god I got the error The codec type 'passthrough' cannot be used in a WebM compliant file. So, does WebM support subtitles in the end or not?!!!

The answer is that WebM’s way of storing WebVTT subtitles is based on an outdated WebVTT spec, and fundamentally different than Matroska’s way (which was based on the current WebVTT specs) and MKVToolNix has no intention of supporting the former.

So, WebM does support subtitles. They need to be in WebVTT format but, in order to create a WebM with subtitles the easiest way I’ve found is through FFmpeg.

End of the story?

It seems not. GStreamer is not able to deal with the WebVTT subtitles track in a WebM or Matroska file, while other programs, like mpv, have no problems with them.

Adding subtitles to a video

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This is mostly a note pad for myself with quick instructions about how to add subtitles streams to an existent video.

Although I’m personally more in favor of using Ogg, reality is that Matroska is the one that is getting the biggest support among the Open Source container formats and it also has a great deal of features.

Also, because of widespread adoption, in spite of not having a really formal syntax and many features, SubRip is my chosen format for subtitles.

Now, I’m the owner of a Samsung 3D SmartTV which allows external SubRip .srt files. However, it also supports Matroska files with embedded subtitles and having the subtitles in the same container than the audio and video streams have some advantages. The main one is that I would be able to play such file through UPnP, with subtitles included, meanwhile that’s not possible with some UPnP servers if the subtitles are in a different file than the video. That’s the case, for example, with BubbleUPnP for Android, which I use from a tablet.

Anyway, enough introduction.

For creating a Matroska file from an existing video and a SubRip file I would use the MKVToolNix package. In a Debian based system you can install it with the following command:

root$ apt-get install mkvtoolnix

In order to merge video and a subtitles (actually, many multiple streams), the proper command for this would be mkvmerge. Its usage is quite straight forward:

$ mkvmerge -o el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo_-_with-lang-codes.mkv el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo.mp4 --language 0:spa el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo.srt --default-track 0
mkvmerge v7.1.0 ('Good Love') 64bit built on Jul 28 2014 11:58:03
'el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo.srt': Using the demultiplexer for the format 'SRT subtitles'.
'el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo.mp4' track 0: Using the output module for the format 'AVC/h.264'.
'el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo.mp4' track 1: Using the output module for the format 'AAC'.
'el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo.srt' track 0: Using the output module for the format 'text subtitles'.
The file 'el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo_-_with-lang-codes.mkv' has been opened for writing.
'el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo.mp4' track 0: Extracted the aspect ratio information from the MPEG-4 layer 10 (AVC) video data and set the display dimensions to 1024/576.
Progress: 100%
The cue entries (the index) are being written...
Muxing took 42 seconds.

That’s basically it. With this command we will get, from a .mp4 and a .srt file, a .mkv file with both merged together. Now, if we would be wanting to add more languages we could have done something like this:

$ mkvmerge -o el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo_-_with-lang-codes.mkv el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo.mp4 --language 0:spa el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo-es.srt --language 1:eng el_ministerio_del_tiempo_-_s01e01_-_el_ministerio_del_tiempo-en.srt --default-track 0
...

This way we would be having Spanish and English subtitles, and the former would be the default one.

For more information, just check mkvmerge’s manual.

Extract, cut, join and merge audio and video streams

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This is mostly a note pad for myself with quick instructions about how to extract, cut, join and merge audio and video streams.

In Igalia we often hold meetings with several parties attending remotely. The easy setting of such meetings usually involve a shared desktop through VNC and a SIP call in a multi user room hold in our Asterisk installation.

When some of my Igalian mates cannot attend we may want to record the meeting so they can play it later. Fortunately, GNOME Shell provides integrated desktop recording out of the box and we have Asterisk set to record automatically our calls in specific multi user rooms.

So, all what it is left after a meeting is just to get both files, edit them slightly and sync them to merge them in a single multimedia container.

Usually, I would use Kdenlive in my video editing tasks. However, Kdenlive doesn’t support “video edition” without re-encoding and I would really like not to re-encode the whole stuff. Specially, the video stream. Therefore, I still will use Kdenlive for the task of syncing both streams and looking for the cutting points for both, the video and the audio file.

For most of this “without re-encoding” actions I will use the great avconv tool.

First, I will cut the video in the time 00:07:45 as starting point and 02:05:20 as ending point:

$ avconv -i screencast.webm -c:v copy -ss 00:07:45 -t 02:05:20 cut-screencast.mkv

This command basically demuxes the WebM container and extract the video stream between those two points to mux it again into a Matroska container.

Then, I will cut the audio in the starting point 00:02:13 and ending point 01:59:48. For editing OGG files we can use Oggscissors or OGG Video Tools’ oggCut .

You won’t find Oggscissors in Debian (the distribution I use). Therefore, you will have to download it and install pyvorbis and pyogg and, maybe, modify slightly the script to use the proper python interpreter. You can install the missing packages like this:

root$ apt-get install python-pyvorbis

Once with Oggscissors working, we can get the interesting audio chunk like:

$ oggscissors.py --from=133 --upto=7188 conf-call.ogg cut-confcall.ogg

or, with oggCut, like:

$ oggCut -s 133000 -e 7188000 conf-call.ogg cut-conf-call.ogg

It may happen that we actually want to extract the audio from another video file. This has happened to us, eventually, when wanting to use the audio from a synced file into another video with higher quality.

We will also use avconv for this:

$ avconv -i synced-video.ogv -map 0:1 -c:a copy synced-audio-output.ogg

It may also happen that we want to join a couple of OGG files since our SIP conf-calls sometimes have hiccups. With Oggscissors this will be done as follows:

$ oggscissors.py --join first.ogg second.ogg joint-output.ogg

With oggCat this will be done like:

$ oggCat joint-output.ogg first.ogg second.ogg

Finally, we will merge or mux the resulting video and audio files into a single media container. Again, with avconv this will be done like:

$ avconv -i final-screencast-conf-call.mkv -i cut-conf-call.ogg -c copy cut-screencast.mkv

Following the examples above this will result in a Matroska video file which contains a VP8 video stream and a Vorbis audio stream.

Hope you find this useful!

Switching between nouveau and the nVIDIA proprietary OpenGL driver in (Debian) GNU/Linux

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So lately I’ve been devoting my time in Igalia around the GNU/Linux graphics stack focusing, more specifically, in Mesa, the most popular open-source implementation of the OpenGL specification.

When working in Mesa and piglit, its testing suite, quite often you would like to compare the results obtained when running a specific OpenGL code with one driver or another.

In the case of nVIDIA graphic cards we have the chance of comparing the default open source driver provided by Mesa, nouveau, or the proprietary driver provided by nVIDIA. For installing the nVIDIA driver you will have to run something like:

root$ apt-get install linux-headers nvidia-driver nvidia-kernel-dkms nvidia-xconfig nvidia-kernel-amd64

Changing from one driver to another involves several steps so I decided to create a dirty script for helping with this.

The actions done by this script are:

  1. Instruct your X Server to use the adequate X driver.
    These instructions apply to the X.org server only.
    When using the default nouveau driver in Debian, the X.org server is able to configure itself automatically. However, when using the nVIDIA driver you most probably will have to instruct the proper settings to X.org.
    nVIDIA provides the package nvidia-xconfig. This package provides a tool of the same name that will generate a X.org configuration file suitable to work with the nVIDIA X driver:

    root$ nvidia-xconfig 

WARNING: Unable to locate/open X configuration file.

Package xorg-server was not found in the pkg-config search path.
Perhaps you should add the directory containing `xorg-server.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xorg-server' found
New X configuration file written to '/etc/X11/xorg.conf'

    I have embedded this generated file into the provided custom script since it is suitable for my system:

        echo 'Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0"
    InputDevice    "Keyboard0" "CoreKeyboard"
    InputDevice    "Mouse0" "CorePointer"
EndSection

Section "Files"
EndSection

Section "InputDevice"
    # generated from default
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol" "auto"
    Option         "Device" "/dev/psaux"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"
    # generated from default
    Identifier     "Keyboard0"
    Driver         "kbd"
EndSection

Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "Unknown"
    HorizSync       28.0 - 33.0
    VertRefresh     43.0 - 72.0
    Option         "DPMS"
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection
' > /etc/X11/xorg.conf

    I would recommend you to substitute this with another configuration file generated with nvidia-xconfig on your system.

  2. Select the proper GLX library.
    Fortunately, Debian provides the alternatives mechanism to select between one or the other.

    ALTERNATIVE=""

    …

        ALTERNATIVE="/usr/lib/mesa-diverted"

    …

        ALTERNATIVE="/usr/lib/nvidia"

    …

    update-alternatives --set glx "${ALTERNATIVE}"
  3. Black list the module we don’t want the Linux kernel to load on start up.
    Again, in Debian, the nVIDIA driver package installs the file /etc/nvidia/nvidia-blacklists-nouveau.conf that is linked, then, from /etc/modprobe.d/nvidia-blacklists-nouveau.conf instructing that the open source nouveau kernel driver for the graphic card should be avoided.
    When selecting nouveau, this script removes the soft link creating a new file which, instead of black listing nouveau’s driver, does it for the nVIDIA proprietary one:

        rm -f /etc/modprobe.d/nvidia-blacklists-nouveau.conf
    echo "blacklist nvidia" > /etc/modprobe.d/nouveau-blacklists-nvidia.conf

    When selecting nVIDIA, the previous file is removed and the soft link is restored.

  4. Re-generate the image used in the inital booting.
    This will ensure that we are using the proper kernel driver from the beginning of the booting of the system:

    update-initramfs -u

With these actions you will be already able to switch your running graphic driver.

You will switch to nouveau with:

root$ /alternate-nouveau-nvidia.sh nouveau
update-alternatives: using /usr/lib/mesa-diverted to provide /usr/lib/glx (glx) in manual mode
update-initramfs: Generating /boot/initrd.img-3.17.0

nouveau successfully set. Reboot your system to apply the changes ...

And to the nVIDIA proprietary driver with:

root$ /alternate-nouveau-nvidia.sh nvidia
update-alternatives: using /usr/lib/nvidia to provide /usr/lib/glx (glx) in manual mode
update-initramfs: Generating /boot/initrd.img-3.17.0

nvidia successfully set. Reboot your system to apply the changes ...

It is recommended to reboot the system although theoretically you could unload the kernel driver and restart the X.org server. The reason is that it has been reported that unloading the nVIDIA kernel driver and loading a different one is not always working correctly.

I hope this will be helpful for your hacking time!