Gstreamer: brief introduction

Gstreamer is a free framework for media applications; it provides a set of plugins that let the user to build applications by connecting them as in a pipeline. It has been ported to a wide range of operating systems, processors and compilers. Also Nvidia developed plugins for its platforms and we employ its h264 encode in order to take advantage from its hardware codec.

A Gstreamer application is composed by a chain of elements, the base construction block of a Gstreamer application. An element takes an input stream from previous element in the chain, carries out its function, like encode, and passes the modified stream to the next element. Usually each element is a plugin.

The user can develop application in two way: the first consists in write an application in c/c++, where the elements are connected using API, while the second uses the gst-launch command-line tool. In the following an example of how to use gst-launch command:

gst-launch-1.0 -v videotestsrc ! ‘video/x-raw, format=(string)I420, width=(int)640, height=(int)480’ ! x264enc ! h264parse ! avdec_h264 ! autovideosink

This command creates a source video test with the properties specified in this string “video/x-raw, format=(string)I420, width=(int)640, height=(int)480”; after it is encoded in h264, then decoded and shown. Each element of this pipeline, except the property element, is plugins dynamically loaded. The videotestsrc element lets the user to see a stream without using camera.

The previous command works on Linux, but since Gstreamer is platform independent, we can launch the same command on Windows taking care to change only hardware dependent plugin. So the same command on Window is:

gst-launch-1.0 -v videotestsrc ! “video/x-raw, format=(string)I420, width=(int)640, height=(int)480” !
                openh264enc ! h264parse ! avdec_h264 ! autovideosink

It’s important to notice that the changed element is the encoder (openh264enc), while the decoder is the same. This because the decoder belongs to the plugin that wraps libav library, a cross-platform library to convert stream in a wide range of multimedia formats. [see References chapter]

Please see Notes section about commands in this tutorial.

How to stream in h264

The server grabs images from cameras, so it needs to run on where cameras are connected. The server is a Gstreamer command pipeline, while the client could be a yarp or a Gstreamer application connected to the robot’s network.

Since Gstreamer is wide spread framework for media application, wrap it into yarp it is not interesting and is worthless. Instead is more intriguing that a yarp application can read “standard” streams using the h264 carrier. For these reasons, the server streamer is a native Gstreamer pipeline, while the client side has been developed in yarp like a carrier.

Server side:

The server application consists in the following Gstreamer command:

gst-launch-1.0 -v v4l2src device="/dev/video1" ! ‘video/x-raw, width=1280, height=480, format=(string)I420’ !
                omxh264enc ! h264parse ! rtph264pay pt=96 config-interval=5 ! udpsink host=224.0.0.1 auto-multicast=true  port=33000

v4l2src device="/dev/video1": reads the cameras using v4l (video for linux) driver
‘video/x-raw, width=640, height=480, format=(string)I420’ : this is a property-element that specifies image size and format. In order to know which sizes and format are available you can use “v4l2-ctl” utility.
omxh264enc: encodes in h264: plugin developed by Nvidia
h264parse: signals downstream the format of the stream
rtph264pay: puts h264 stream in rtp packets
udpsink: this is the last element and sends out the stream. In this case we use multicast, but it is possible to send the stream using unicast in this way: udpsink host=IP_ADDRESS_OF_CLIENT port=NOT_WELL_KNOWN_PORT_NUMBER

Currently is not available a Gstreamer application that implements the pipeline. In the near future, it could be developed in order to make easier configure the server.

Client side

The client can read the stream using Gstreamer native command or yarp.

In the first case the Gstreamer command is:

gst-launch-1.0 -v udpsrc multicast-group=224.0.0.1 auto-multicast=true port=3000 caps="application/x-rtp, media=(string)video, encoding-name=(string)H264, payload=(int)96" !
                rtph264depay ! h264parse ! avdec_h264! autovideosink

If you want use a yarp application to read the stream you need to:

1)  install Gstreamer (see \ref how_to_install_gstreamer )
2)  compile yarp with “ENABLE_yarpcar_h264” option enabled.
3)  register the stream server to the yarp server: “yarp name register <SERVER_NAME_PORT>  h264  <SERVER_IP_ADRESS> <SERVER_IP_PORT>”
4)  run your application
5)  connect client and server port using h264 carrier: “yarp connect <SERVER_NAME_PORT> <CLIENT_PORT> h264”

Some options

set the frame rate : in server side exist the parameter framerate=30/1, that configures the framerate to which grab images. Insert in in property element: 'video/x-raw, format=(string)I420, width=(int)640, height=(int)480, framerate=30/1'
In some cases could be useful that server streams video at constant rate. You can achieve this adding this parameter to the encoder plugin: control-rate=2 bitrate=5000000
remove the jitter: in the client it is possible adding the rtpjitterbuffer plugin in this way: If you want to remove the jitter in h264 yarp carrier, please add parameter “+removeJitter.1” in connect command. (Note that the syntax is the usual used to specify parameters to yarp carriers). Therefore the connect command to your application could be:
```
    yarp connect <SERVER_NAME_PORT> <CLIENT_PORT> h264+removeJitter.1
```
The yarp carrier lets you to crop each frame specifying the number of pixel to crop on each side of image in carrier parameter in connection command. For example if you want to crop 200 pixel on top the command appears like: \verbatimyarp connect <SERVER_NAME_PORT> <CLIENT_PORT> h264+cropTop.200. Instead, if you want to use native Gstreamer client the plugin “videocrop” performs this operation.
```
    gst-launch-1.0 -v udpsrc port=33000 caps="application/x-rtp, media=(string)video,  encoding-name=(string)H264, payload=(int)96" !
                    rtph264depay ! h264parse ! avdec_h264 ! videocrop left=10, right=30, top=50, bottom=50 ! autovideosink
```

How to install Gstreamer

Currently we are using 1.8.3 version

On Ubuntu

Packages required to build
- libgstreamer1.0-dev
- libgstreamer-plugins-base1.0-dev
Packages required to run
- gstreamer1.0-plugins-base (for videoconvert and appsink)
- gstreamer1.0-plugins-good (for udpsrc and rtph264depay)
- gstreamer1.0-plugins-bad (for h264parse)
- gstreamer1.0-libav (for avdec_h264)
Useful packages but not required
- gstreamer1.0-tools

On windows

You need to download both the main package and the devel package from here: https://gstreamer.freedesktop.org/data/pkg/windows/

Installation of package Gstreamer:

choose typical installation
redo installation adding:
- Gstreamer 1.0 devtools
- Gstreamer 1.0 Libav wrapper

Installation of grstreamer devel package:

choose typical installation
redo installation adding:
- Gstreamer 1.0 devtools
- Gstreamer 1.0 Libav wrapper
Add in path environment variable the path to executable (Usually is C:\gstreamer\1.0\x86_64\bin)

Verify your installation

First of all, you need to verify if Gstreamer has been installed successfully by using these commands:

Server side (example on Windows)

gst-launch-1.0 -v videotestsrc ! "video/x-raw, format=(string)I420, width=(int)640, height=(int)480" !
                openh264enc ! h264parse ! rtph264pay pt=96 config-interval=5 ! udpsink host=<YOUR_IP_ADDRESS> port=<A_PORT_NUMBER>

Client side

gst-launch-1.0 -v udpsrc port=<A_PORT_NUMBER> caps="application/x-rtp, media=(string)video, encoding-name=(string)H264, payload=(int)96" !
                rtph264depay ! h264parse ! avdec_h264! autovideosink

After you can substitute the client side with a yarp application, for example yarpview. So, after yarp server had been launched, please run: