Basic tutorial 10: GStreamer tools

Goal

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.

Introduction

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 bin directory 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.

Information

On Linux, you should use the GStreamer version installed with your distribution, the tools should be installed with a package named gstreamer1 on Fedora style distributions, or gstreamer1.0-tools on 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 gst-launch-1.0, gst-inspect-1.0 and gst-discoverer-1.0

gst-launch-1.0

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 the 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, se the complete documentation at the reference page for gst-launch-1.0.

Elements

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

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 element.

gst-launch-1.0 videotestsrc pattern=11 ! videoconvert ! autovideosink

You should see a static video pattern, made of circles.

Named elements

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 example.

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 a tee copies to each of its output pads everything coming through its input pad). The tee is named simply ‘t’ (using the name property) and then linked to a queue and an autovideosink. The same tee is referred to using ‘t.’ (mind the dot) and then linked to a second queue and a second autovideosink.

To learn why the queues are necessary read Basic tutorial 7: Multithreading and Pad Availability.

Pads

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 the gst-inspect-1.0 tool.

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_00 ! matroskamux ! filesink location=sintel_video.mkv

This fetches a media file from the internet using souphttpsrc, which 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 video_00 and audio_00. We link video_00 to a matroskamux element to re-pack the video stream into a new container, and finally link it to a filesink, which will write the stream into a file named "sintel_video.mkv" (the location property specifies the name of the file).

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_00 ! vorbisparse ! matroskamux ! filesink location=sintel_audio.mka

The 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).

Caps filters

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 filesink are ANY, meaning that it can accept any kind of media. Which one of the two output pads of matroskademux will be linked against the filesink? video_00 or audio_00? You cannot know.

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 matroskademux and matroskamux we added a 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, use the 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.

Examples

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 internally):

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 320x200.

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 here.

gst-inspect-1.0

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 <ds@entropywave.com>, Sebastian Dröge <sebastian.droege@collabora.co.uk>

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 page of gst-inspect-1.0.

gst-discoverer-1.0

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.

Use 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

Conclusion

This tutorial has shown:

  • How to build and run GStreamer pipelines from the command line using the gst-launch-1.0 tool.
  • How to find out what GStreamer elements you have available and their capabilities, using the gst-inspect-1.0 tool.
  • How to discover the internal structure of media files, using gst-discoverer-1.0.

It has been a pleasure having you here, and see you soon!

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