# 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 out 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 `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, see 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://gstreamer.freedesktop.org/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 `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_0` and `audio_0`. We link `video_0` 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://gstreamer.freedesktop.org/data/media/sintel_trailer-480p.webm ! matroskademux name=d d.audio_0 ! 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://gstreamer.freedesktop.org/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_0` or `audio_0`? 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://gstreamer.freedesktop.org/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://gstreamer.freedesktop.org/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://gstreamer.freedesktop.org/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 different codecs (H.264 + AAC), and puts them back together into an MP4 container (just for the sake of it). Because of the way the x264enc encoder behaves by default (consuming multiple seconds of video input before outputtingi anything), we have to increase the size of the queue in the audio branch to make sure the pipeline can preroll and start up. Another solution would be to use `x264enc tune=zerolatency` but that results in lower quality and is more suitable for live streaming scenarios.

    gst-launch-1.0 uridecodebin uri=https://gstreamer.freedesktop.org/data/media/sintel_trailer-480p.webm name=d ! queue ! videoconvert ! x264enc ! video/x-h264,profile=high ! mp4mux name=m ! filesink location=sintel.mp4 d. ! queue max-size-time=5000000000 max-size-bytes=0 max-size-buffers=0 ! audioconvert ! audioresample ! voaacenc ! 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://gstreamer.freedesktop.org/data/media/sintel_trailer-480p.webm ! queue ! videoscale ! video/x-raw,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://gstreamer.freedesktop.org/data/media/sintel_trailer-480p.webm -v

    Analyzing https://gstreamer.freedesktop.org/data/media/sintel_trailer-480p.webm
    Done discovering https://gstreamer.freedesktop.org/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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