Basic tutorial 10: GStreamer tools
GStreamer comes with a set of tools which range from handy to absolutely essential. There is no code in this tutorial, just sit back and relax, and we will teach you:
- How to build and run GStreamer pipelines from the command line, without using C at all!
- How to find out what GStreamer elements you have available and their capabilities.
- How to discover the internal structure of media files.
These tools are available in the bin directory of the GStreamer
binaries. You need to move to this directory to execute them, because
it is not added to the system’s
PATH environment variable (to avoid
polluting it too much).
Just open a terminal (or console window) and go to the
of your GStreamer installation (Read again the Installing
GStreamer section to find our where this is),
and you are ready to start typing the commands given in this tutorial.
On Linux, you should use the GStreamer version installed with your distribution, the tools should be installed with a package named
gstreamer1on Fedora style distributions, or
gstreamer1.0-toolson Debian/Ubuntu style distributions.
In order to allow for multiple versions of GStreamer to coexists in the
same system, these tools are versioned, this is, a GStreamer version
number is appended to their name. This version is based on
GStreamer 1.0, so the tools are called
This tool accepts a textual description of a pipeline, instantiates it, and sets it to the PLAYING state. It allows you to quickly check if a given pipeline works, before going through the actual implementation using GStreamer API calls.
Bear in mind that it can only create simple pipelines. In particular, it can only simulate the interaction of the pipeline with the application up to a certain level. In any case, it is extremely handy to test pipelines quickly, and is used by GStreamer developers around the world on a daily basis.
Please note that
gst-launch-1.0 is primarily a debugging tool for
developers. You should not build applications on top of it. Instead, use
gst_parse_launch() function of the GStreamer API as an easy way to
construct pipelines from pipeline descriptions.
Although the rules to construct pipeline descriptions are very simple,
the concatenation of multiple elements can quickly make such
descriptions resemble black magic. Fear not, for everyone learns the
gst-launch-1.0 syntax, eventually.
The command line for gst-launch-1.0 consists of a list of options followed
by a PIPELINE-DESCRIPTION. Some simplified instructions are given next,
see the complete documentation at the reference page
In simple form, a PIPELINE-DESCRIPTION is a list of element types separated by exclamation marks (!). Go ahead and type in the following command:
gst-launch-1.0 videotestsrc ! videoconvert ! autovideosink
You should see a windows with an animated video pattern. Use CTRL+C on the terminal to stop the program.
This instantiates a new element of type
videotestsrc (an element which
generates a sample video pattern), an
videoconvert (an element
which does raw video format conversion, making sure other elements can
understand each other), and an
autovideosink (a window to which video
is rendered). Then, GStreamer tries to link the output of each element
to the input of the element appearing on its right in the description.
If more than one input or output Pad is available, the Pad Caps are used
to find two compatible Pads.
Properties may be appended to elements, in the form
*property=value *(multiple properties can be specified, separated by
spaces). Use the
gst-inspect-1.0 tool (explained next) to find out the
available properties for an
gst-launch-1.0 videotestsrc pattern=11 ! videoconvert ! autovideosink
You should see a static video pattern, made of circles.
Elements can be named using the
name property, in this way complex
pipelines involving branches can be created. Names allow linking to
elements created previously in the description, and are indispensable to
use elements with multiple output pads, like demuxers or tees, for
Named elements are referred to using their name followed by a dot.
gst-launch-1.0 videotestsrc ! videoconvert ! tee name=t ! queue ! autovideosink t. ! queue ! autovideosink
You should see two video windows, showing the same sample video pattern. If you see only one, try to move it, since it is probably on top of the second window.
This example instantiates a
videotestsrc, linked to a
videoconvert, linked to a
tee (Remember from Basic tutorial 7: Multithreading and Pad Availability that
tee copies to each of its output pads everything coming through its
input pad). The
tee is named simply ‘t’ (using the
and then linked to a
queue and an
autovideosink. The same
referred to using ‘t.’ (mind the dot) and then linked to a second
queue and a second
To learn why the queues are necessary read Basic tutorial 7: Multithreading and Pad Availability.
Instead of letting GStreamer choose which Pad to use when linking two
elements, you may want to specify the Pads directly. You can do this by
adding a dot plus the Pad name after the name of the element (it must be
a named element). Learn the names of the Pads of an element by using
This is useful, for example, when you want to retrieve one particular stream out of a demuxer:
gst-launch-1.0 souphttpsrc location=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! matroskademux name=d d.video_0 ! matroskamux ! filesink location=sintel_video.mkv
This fetches a media file from the internet using
is in webm format (a special kind of Matroska container, see Basic tutorial 2: GStreamer concepts). We
then open the container using
matroskademux. This media contains both
audio and video, so
matroskademux will create two output Pads, named
audio_0. We link
video_0 to a
to re-pack the video stream into a new container, and finally link it to
filesink, which will write the stream into a file named
location property specifies the name of the
All in all, we took a webm file, stripped it of audio, and generated a new matroska file with the video. If we wanted to keep only the audio:
gst-launch-1.0 souphttpsrc location=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! matroskademux name=d d.audio_0 ! vorbisparse ! matroskamux ! filesink location=sintel_audio.mka
vorbisparse element is required to extract some information from
the stream and put it in the Pad Caps, so the next element,
matroskamux, knows how to deal with the stream. In the case of video
this was not necessary, because
matroskademux already extracted this
information and added it to the Caps.
Note that in the above two examples no media has been decoded or played. We have just moved from one container to another (demultiplexing and re-multiplexing again).
When an element has more than one output pad, it might happen that the link to the next element is ambiguous: the next element may have more than one compatible input pad, or its input pad may be compatible with the Pad Caps of all the output pads. In these cases GStreamer will link using the first pad that is available, which pretty much amounts to saying that GStreamer will choose one output pad at random.
Consider the following pipeline:
gst-launch-1.0 souphttpsrc location=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! matroskademux ! filesink location=test
This is the same media file and demuxer as in the previous example. The
input Pad Caps of
ANY, meaning that it can accept any
kind of media. Which one of the two output pads of
be linked against the filesink?
audio_0? You cannot
You can remove this ambiguity, though, by using named pads, as in the previous sub-section, or by using Caps Filters:
gst-launch-1.0 souphttpsrc location=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! matroskademux ! video/x-vp8 ! matroskamux ! filesink location=sintel_video.mkv
A Caps Filter behaves like a pass-through element which does nothing and
only accepts media with the given Caps, effectively resolving the
ambiguity. In this example, between
video/x-vp8 Caps Filter to specify that we are interested in
the output pad of
matroskademux which can produce this kind of video.
To find out the Caps an element accepts and produces, use the
gst-inspect-1.0 tool. To find out the Caps contained in a particular file,
gst-discoverer-1.0 tool. To find out the Caps an element is
producing for a particular pipeline, run
gst-launch-1.0 as usual, with the
–v option to print Caps information.
Play a media file using
playbin (as in Basic tutorial 1: Hello world!):
gst-launch-1.0 playbin uri=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm
A fully operation playback pipeline, with audio and video (more or less
the same pipeline that
playbin will create
gst-launch-1.0 souphttpsrc location=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! matroskademux name=d ! queue ! vp8dec ! videoconvert ! autovideosink d. ! queue ! vorbisdec ! audioconvert ! audioresample ! autoaudiosink
A transcoding pipeline, which opens the webm container and decodes both streams (via uridecodebin), then re-encodes the audio and video branches with a different codec, and puts them back together in an Ogg container (just for the sake of it).
gst-launch-1.0 uridecodebin uri=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm name=d ! queue ! theoraenc ! oggmux name=m ! filesink location=sintel.ogg d. ! queue ! audioconvert ! audioresample ! flacenc ! m.
A rescaling pipeline. The
videoscale element performs a rescaling
operation whenever the frame size is different in the input and the
output caps. The output caps are set by the Caps Filter to
gst-launch-1.0 uridecodebin uri=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm ! queue ! videoscale ! video/x-raw-yuv,width=320,height=200 ! videoconvert ! autovideosink
This short description of
gst-launch-1.0 should be enough to get you
started. Remember that you have the complete documentation available
This tool has three modes of operation:
- Without arguments, it lists all available elements types, this is, the types you can use to instantiate new elements.
- With a file name as an argument, it treats the file as a GStreamer plugin, tries to open it, and lists all the elements described inside.
- With a GStreamer element name as an argument, it lists all information regarding that element.
Let's see an example of the third mode:
gst-inspect-1.0 vp8dec Factory Details: Rank primary (256) Long-name On2 VP8 Decoder Klass Codec/Decoder/Video Description Decode VP8 video streams Author David Schleef <firstname.lastname@example.org>, Sebastian Dröge <email@example.com> Plugin Details: Name vpx Description VP8 plugin Filename /usr/lib64/gstreamer-1.0/libgstvpx.so Version 1.6.4 License LGPL Source module gst-plugins-good Source release date 2016-04-14 Binary package Fedora GStreamer-plugins-good package Origin URL http://download.fedoraproject.org GObject +----GInitiallyUnowned +----GstObject +----GstElement +----GstVideoDecoder +----GstVP8Dec Pad Templates: SINK template: 'sink' Availability: Always Capabilities: video/x-vp8 SRC template: 'src' Availability: Always Capabilities: video/x-raw format: I420 width: [ 1, 2147483647 ] height: [ 1, 2147483647 ] framerate: [ 0/1, 2147483647/1 ] Element Flags: no flags set Element Implementation: Has change_state() function: gst_video_decoder_change_state Element has no clocking capabilities. Element has no URI handling capabilities. Pads: SINK: 'sink' Pad Template: 'sink' SRC: 'src' Pad Template: 'src' Element Properties: name : The name of the object flags: readable, writable String. Default: "vp8dec0" parent : The parent of the object flags: readable, writable Object of type "GstObject" post-processing : Enable post processing flags: readable, writable Boolean. Default: false post-processing-flags: Flags to control post processing flags: readable, writable Flags "GstVP8DecPostProcessingFlags" Default: 0x00000403, "mfqe+demacroblock+deblock" (0x00000001): deblock - Deblock (0x00000002): demacroblock - Demacroblock (0x00000004): addnoise - Add noise (0x00000400): mfqe - Multi-frame quality enhancement deblocking-level : Deblocking level flags: readable, writable Unsigned Integer. Range: 0 - 16 Default: 4 noise-level : Noise level flags: readable, writable Unsigned Integer. Range: 0 - 16 Default: 0 threads : Maximum number of decoding threads flags: readable, writable Unsigned Integer. Range: 1 - 16 Default: 0
The most relevant sections are:
- Pad Templates: This lists all the kinds of Pads this
element can have, along with their capabilities. This is where you
look to find out if an element can link with another one. In this
case, it has only one sink pad template, accepting only
video/x-vp8(encoded video data in VP8 format) and only one source pad template, producing
video/x-raw(decoded video data).
- Element Properties: This lists the properties of the element, along with their type and accepted values.
For more information, you can check the documentation
This tool is a wrapper around the
GstDiscoverer object shown in Basic tutorial 9: Media information gathering.
It accepts a URI from the command line and prints all information
regarding the media that GStreamer can extract. It is useful to find out
what container and codecs have been used to produce the media, and
therefore what elements you need to put in a pipeline to play it.
gst-discoverer-1.0 --help to obtain the list of available options,
which basically control the amount of verbosity of the output.
Let's see an example:
gst-discoverer-1.0 https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm -v Analyzing https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm Done discovering https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm Topology: container: video/webm audio: audio/x-vorbis, channels=(int)2, rate=(int)48000 Codec: audio/x-vorbis, channels=(int)2, rate=(int)48000 Additional info: None Language: en Channels: 2 Sample rate: 48000 Depth: 0 Bitrate: 80000 Max bitrate: 0 Tags: taglist, language-code=(string)en, container-format=(string)Matroska, audio-codec=(string)Vorbis, application-name=(string)ffmpeg2theora-0.24, encoder=(string)"Xiph.Org\ libVorbis\ I\ 20090709", encoder-version=(uint)0, nominal-bitrate=(uint)80000, bitrate=(uint)80000; video: video/x-vp8, width=(int)854, height=(int)480, framerate=(fraction)25/1 Codec: video/x-vp8, width=(int)854, height=(int)480, framerate=(fraction)25/1 Additional info: None Width: 854 Height: 480 Depth: 0 Frame rate: 25/1 Pixel aspect ratio: 1/1 Interlaced: false Bitrate: 0 Max bitrate: 0 Tags: taglist, video-codec=(string)"VP8\ video", container-format=(string)Matroska; Properties: Duration: 0:00:52.250000000 Seekable: yes Tags: video codec: VP8 video language code: en container format: Matroska application name: ffmpeg2theora-0.24 encoder: Xiph.Org libVorbis I 20090709 encoder version: 0 audio codec: Vorbis nominal bitrate: 80000 bitrate: 80000
This tutorial has shown:
- How to build and run GStreamer pipelines from the command line using
- How to find out what GStreamer elements you have available and their
capabilities, using the
- How to discover the internal structure of media files, using
It has been a pleasure having you here, and see you soon!
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