Tag Archives: gstreamer

Tutorial for Python, GStreamer and GTK 3

I have wanted to write about programming with GStreamer and Python for a while. Jono Bacon wrote a nice introduction to GStreamer and Python a long time ago, but I want to share with you some specific tips.

At Collabora we work a lot with GStreamer including helping train developers at our customers to be better at GStreamer development. Being the lowly marketing guy at the company I don’t have the programming chops to teach the hard stuff, but I figured I should be able to put together a very simple article which explains some basics and shows of a little GStreamer development trick I have used to great success in Transmageddon.

Part of what triggered getting this little tutorial done was that I am looking into porting Transmageddon to GTK3 after its next release of Transmageddon. To understand how to write a GTK 3 Python application, using the introspection bindings, I decided a good learning tool for myself would be to try to port the 0.0.1 version of Transmageddon. This version was never released, in fact it was me trying to figure out the very basics of programming with GTK+ and GStreamer in Python.

The application litterally consists of a GTK+ UI with two buttons. One is a ‘transcode’ button which when pressed starts a GStreamer transcoding pipeline. The other is my little secret trick, called ‘Debug’. It will when pressed generate a png of the pipeline being run, or not being run for that matter. It has helped me solve a ton of bugs and issues in Transmageddon since I started the project and hopefully it can be a useful trick for you too.

You can find a tarball here with the code below, the .ui file from Glade and a which.py file (which.py is a python version of the Unix which tool, which I found online).

First let me give you the code of the application, I tried to annotate the code in detail to make it easy to follow, even if you haven’t played with either GTK3 or GStreamer before.

 
#!/usr/bin/env python
# Simple example GTK3 + GStreamer 0.10.x Application for transcoding 
# GTK3 using gobject introspection for bindings, GStreamer using manual bindings 
# Also includes how to set up dotfile generation

import sys
import os
import which

# Setting GST_DEBUG_DUMP_DOT_DIR environment variable enables us to have a dotfile generated
os.environ["GST_DEBUG_DUMP_DOT_DIR"] = "/tmp"
os.putenv('GST_DEBUG_DUMP_DIR_DIR', '/tmp')
try:
    import gi
except:
    pass
try:
    from gi.repository import Gtk
except:
    sys.exit(1)
try:
    import pygst
    pygst.require("0.10")
    import gst
except:
    pass

# creating a basic transcoder class
class Transcoder:
    def __init__(self):
        self.pipeline = gst.Pipeline("TranscodingPipeline") # creating overall pipeline object

        # Creating GStreamer filesrc element and sets it to read a specific mp3 file
        self.filesrc = gst.element_factory_make("filesrc", "filesrc")
        self.filesrc.set_property("location", """/home/cschalle/Music/tok.mp3""")
        self.pipeline.add(self.filesrc) # add this first plugin to the pipeline object

        # Use highlevel decodebin2 element to choose which GStreamer elments to use
        # for decoding automatically
        self.decoder = gst.element_factory_make("decodebin2", "decoder")

        # Connect to signal that will let us know that decodebin2 got a pad we can connect
        # to which has the decoded media file on it
        self.decoder.connect("new-decoded-pad", self.OnDynamicPad)
        self.pipeline.add(self.decoder)

        # create an audioconvert element to convert bitrate if needed
        self.audioconverter = gst.element_factory_make("audioconvert", "audioconverter")
        self.pipeline.add(self.audioconverter)

        # create audioencoder, in this case the Vorbis encoder
        self.audioencoder = gst.element_factory_make("vorbisenc", "audioencoder")
        self.pipeline.add(self.audioencoder)

        # create ogg muxer to hold vorbis audio
        self.oggmuxer = gst.element_factory_make("oggmux", "oggmuxer")
        self.pipeline.add(self.oggmuxer)

        # create file output element to write new file to disk
        self.filesink = gst.element_factory_make("filesink", "filesink")
        self.filesink.set_property("location", """/home/cschalle/Music/tok.ogg""")
        self.pipeline.add(self.filesink)

        # Now that all elements for the pipeline are create we link them together
        self.filesrc.link(self.decoder)
        self.audioconverter.link(self.audioencoder)
        self.audioencoder.link(self.oggmuxer)
        self.oggmuxer.link(self.filesink)

        # set pipeline to playing which means all the connected elements in the pipeline
        # starts pushing data to each other
        self.pipeline.set_state(gst.STATE_PLAYING)

    # create a simple function that is run when decodebin gives us the signal to let us 
    # know it got audio data for us. Use the get_pad call on the previously 
    #created audioconverter element asking to a "sink" pad.
    def OnDynamicPad(self, dbin, pad, islast):
        pad.link(self.audioconverter.get_pad("sink"))

# extremely simple UI using a GtkBuilder UI generated with Glade, just two buttons. 
# One to start transcode and one to run pipeline debug
class SuperSimpleUI:
    def __init__(self):
       self.builder = Gtk.Builder()
       self.uifile = "supersimple-gtk3.ui"
       self.builder.add_from_file(self.uifile)
        self.window = self.builder.get_object ("MainWindow")
       self.window.connect ("destroy", self.dialog_destroyed) # this allows the application
                                                              # to be cleanly killed
       # Call the two buttons in the UI
       self.transcodebutton = self.builder.get_object("transcodebutton")
       self.debugbutton = self.builder.get_object("debugbutton")

       # Connect to the clicked signal on both buttons
       self.transcodebutton.connect ("clicked", self.on_TranscodeButton_clicked)
       self.debugbutton.connect ("clicked", self.on_debug_activate)

      # set window size to avoid it being so small it gets lost on the desktop
      self.window.set_default_size (580, 435)
      self.window.show ()

    def on_TranscodeButton_clicked(self, widget):
        self._transcoder = Transcoder()
        print "transcoding"

    def dialog_destroyed (self, dialog):
        Gtk.main_quit ()

    # this function generates the dot file, checks that graphviz in installed and
    # then finally generates a png file, which it then displays
    def on_debug_activate(self, widget):
        dotfile = "/tmp/supersimple-debug-graph.dot"
        pngfile = "/tmp/supersimple-pipeline.png"
        if os.access(dotfile, os.F_OK):
            os.remove(dotfile)
        if os.access(pngfile, os.F_OK):
            os.remove(pngfile)
        gst.DEBUG_BIN_TO_DOT_FILE (self._transcoder.pipeline, \
        gst.DEBUG_GRAPH_SHOW_ALL, 'supersimple-debug-graph')
        # check if graphviz is installed with a simple test
        try:
            dot = which.which("dot")
            os.system(dot + " -Tpng -o " + pngfile + " " + dotfile)
            Gtk.show_uri(None, "file://"+pngfile, 0)
        except which.WhichError:
            print "The debug feature requires graphviz (dot) to be installed."
            print "Transmageddon can not find the (dot) binary."

if __name__ == "__main__":
hwg = SuperSimpleUI()
Gtk.main()

The first thing happening in the file after importing the basis system classes and the which.py tool, is that we set the ‘GST_DEBUG_DUMP_DOT_DIR’ environment variable. When you set this value, GStreamer will be able to at any time dump the pipeline and elements to a ‘dot’ file, which can be turned into a nice looking png by the graphviz command line tool (should be available in most distributions).

Next I import GTK and GStreamer, as you see I don’t yet use the gobject introspection version of GStreamer as that is not fully working yet, but I plan to try to port this simple application to GStreamer 1.0, in which gobject introspection will be the supported way of using Python.

Next is setting up the GStreamer pipeline. You always start by creating a pipeline object, consider this the canvas onto which you will paint the GStreamer streaming pipelines. The next step is to assemble all the GStreamer plugins we want to use in the application. First I create a filesrc object pointing to the file I want to transcode, be sure to point that to a file of your own if trying this application. Next is creating the decodebin2 element. Decodebin2 is one of a set of high level elements in GStreamer, called bins, which contains a wide range of plugins inside. These high level elements are there to make things a lot simpler, and in the case of decodebin2 it will automatically put together the plugins needed to convert your incoming file to raw audio and video (or just demux the file). This means your input doesn’t need to be a mp3 file, like I used, as decodebin2 will reconfigure itself to handle any file you throw at it. After this I create a series of elements to enable me to encode the data into a Ogg Vorbis file. I am doing that to help explain how elements are stringed together, but there is another high level element, encodebin, which I could have used instead. Transmageddon uses encodebin in its git version.

Once all the elements are created you can think of them as boxes spread around on your pipeline canvas, but in order for GStreamer to know how you want to connect them together you need to link them together, as you can see I do with statements like ‘self.filesrc.link(self.decoder)’, which connects the filesrc element I created with the decoder element.

The one special element here is decodebin, which being a dynamic element I need to link it once the pad found signal is fired. Also to link I need to request a compatible pad from the element I am linking with, in this case the audioconverter element.

The last part of the GStreamer setup is setting the pipeline to playing state, which is the state where the pipeline is running. While not a big concern in this very simple application, dealing with state changes in GStreamer is going to be one of the major items you look out for. The GStreamer plugin writers guide contains a chapter discussing the basics of the four states "NULL", "READY", "PAUSED" and "PLAYING". Your pipeline (and all elements) always start at Null state and will go through each of the other stanges to reach Playing. So while we only set state to PLAYING in this simple application, GStreamer will in the background go through READY and PAUSED. The reason the intermediary states matter is because certain things happen at each, so for instance if you want to do some analysis of a file before starting to run your pipeline fully you want to be in PAUSED state as GStreamer will then start pulling the initial data through the pipeline and thus allow you to get information from your elements about the stream or file. One important thing to keep in mind as you develop more advanced applications is that the individual elements can have a different state than the pipeline, but when the state of the pipeline changes it will change the state of the plugins along with it, so you never want your pipeline to be more than one level lower than any of your elements, as that will cause the element to jump down to that state and thus lose the negotiation and information it had assembled.

I am not going to go into a lot of detail about the GUI, it is a very simple GTK user interface built using Glade, and hooked up using the GTK3 gobject introspection bindings. If you got any questions about it post a comment and I be happy to talk about it. What I want to talk about instead is the on_debug_activate function. I wrote this for Transmageddon, but my hope is that it will be useful for anyone writing a Python application with GStreamer (and I guess it shouldn’t be to hard to port to another language). It will allow you to add a menu entry or button in your application that outputs a png file, like the one you see below, which gives you a nice full view of the pipeline used by GStreamer. Especially if you use things like decodebin2 and encodebin, or have a lot of code dynamically adding/removing elements, it can be really useful to see what pipeline ended up being used. And if you have elements that you created, but forgot to link inn, they will appear as orphaned boxes in the file, allowing you to detect such issues. The important thing to remember is that it needs the graphwiz application to be installed on your system and available in the executable path.

Image of GStreamer pipeline
Generated GStreamer pipeline png

Anyway, I hope this has been useful and I plan to post and updated version of this simple application, ported to use encodebin and GStreamer 1.0.

GStreamer Conference 2011 speakers announced

We put up the full schedule today for the GStreamer Conference 2011. I am very excited about the agenda this year and all the great talks we got lined up. I am especially happy that the effort to reach out to people outside the immediate GStreamer community has paid off, giving us exciting talks from John Luther and Matt Frost of WebM fame, the legendary Monty Montgomery of Xiph.org and Arun Raghavan speaking about Pulse Audio. This of course in addition to all our great GStreamer talks like Wim Taymans speaking about GStreamer 1.0.

Another talk I am especially excited about is the talk by Joshua Doe and Stephen Burks who works for the US Army RDECOM CERDEC NVESD unit(just rolls of the tongue doesn’t it :). Who will talk about their use of GStreamer. I know that there are quite a few cases in the military world where GStreamer is used, so I think this talk can be an interesting window into that world, which due to its high security nature often can be quite low profile even when things are not directly secret.

A big thanks you to our sponsors this year Collabora, Fluendo, Google and UbiCast. Without their support it would not have been possible to put this event together.

Make sure to register for this years GStreamer Conference before the 24th of September in order to get the early bird fee. Looking forward to seeing you all there!

And if you are still not 100% sure you will attend, make sure to look at the schedule and speakers overview, because I am sure they will convince you to come :)

First GStreamer 0.11 out, GStreamer 1.0 looming on the horizon

GStreamer maintainer Wim Taymans just released the first 0.11 development release of GStreamer. The 0.11 development series will lead up to the long awaited GStreamer 1.0 release!

The changes from 0.10.x to 0.11 might seem quite technical and obscure to most, with items such as reworked buffer memory management, arbitrary buffer metadata and integrated bufferpool management being among the advertised features, but all these changes are made to help GStreamer make significant leaps forward in terms of integration with hardware codecs like VAAPI and VDPAU and of course hardware codecs on embedded platforms like ARM for instance the OpenMax IL API. There are also a lot of important performance improvements, which will make applications like Totem and Banshee more snappy to use, but you probably will see the biggest improvements in applications like PiTiVi who relies on more complex pipelines and thus more complex pipeline negotiations. For devices which got more constrained CPU resources, like various embedded systems, these performance gains should also be very noticeable.

At Collabora we are putting a lot of effort towards GStreamer 1.0, most visible through letting Wim put most of his work hours into it, in collaboration with our partners at Texas Instruments. GStreamer is not just another open source project for us at Collabora, it is something we are truly passionate about. The open source software ecosystem can not compete with proprietary systems unless we have a top notch media framework and with GStreamer we are providing exactly that. Ever since the release of GStreamer 0.10 the project has gone from strength to strength, and when 1.0 gets released later this year it will be another major milestone towards world domination :)

For those interested to learn more about GStreamer 1.0 you have two good opportunities coming up, Collabora’s own Wim Taymans and Edward Hervey will be doing a talk about GStreamer 1.0 at the Desktop Summit on 8th of August. And Wim will also be doing a keynote speech about GStreamer 1.0 at the GStreamer Conference 2011 in October. So I hope to see you there.

I am also working on an interview with Wim Taymans about GStreamer 1.0 so if you have any questions you would like me to include, feel free to add them to the comments section of this blog post.

We will also be organizing some 0.11 hackfests online where people like Wim, Tim and Edward will be online to answer porting questions and the community can work together to port all important plugins to 0.11. There is some early stage porting documentation to be found here.

For now, go to the GStreamer website and grab the 0.11 tarballs and give them a spin, and if you have any questions, remember you are always welcome in #gstreamer on irc.freenode.net.

GStreamer Conference 2011 Update

I just sent out an email with the latest update on the GStreamer Conference 2011. Mostly to let people know there is an early bird registration fee for signing up to the conference before 24th of September.

I also updated the topics list to reflect some of the talks that will be held this year, even if we are not ready to release the full schedule yet, still some details that needs to be sorted for that.

Anyway, hoping to see as many of you as possible in Prague in October :)

Comments on Novacut

Been some discussion recently about Novacut, a open source non-linear video editor project which is trying to raise some money through kickstarter, a crowd sourcing system.

Personally I think that some of the criticism against Novacut and their effort is fair. It does seem extremely ambitious and it hard to reconcile the money requested and the ambitions of the project. That said I am not as sceptical to the endeavour as some, I even pledged some money to it in fact.

The thing is, a lot of open source projects starts out a bit naive about the challenges ahead of them, and frankly if they had known all the problems facing them when starting out, they would never had succeeded. For instance, if we had known up front it would take us 10 years to get GStreamer to where it is today I don’t think we would have ever started.

The other factor that plays on my mind is that even if they ‘fail’, they might be a great success. The thing is they plan on building Novacut on top of GStreamer and GNonlin, the same foundation as PiTiVi. As part of this process I am sure they will find and fix bugs in GStreamer and GNonlin and they will add new features to GStreamer and GNonlin. So even if ‘Novacut’ itself fails to take off, a lot of the work they will be doing will directly benefit GStreamer applications in general and probably PiTiVi especially. And who knows, even if ‘Novacut the application’ fails to ever reach a useable state, maybe ‘Novacut the idea’ will be alive and kicking inside PiTiVi in a year or two.

One could argue that Novacut should be done as a PiTiVi extension from the beginning, and I might agree, but to me, saying no to Novacut for not being a PiTiVi patch or extension is saying no to something good because you want to wait for something utopian.

Weekend hacking

Spent some time this weekend hacking on Transmageddon. Fixed various small bugs and UI issues that I had punted up until now for the UI. For instance with latest git when you create a pure audio file it doesn’t automatically get the suffix .mp3, which is nice in the cases when you are not creating a mp3 file :) And if you put aac into a quicktime container the file gets named .m4a instead of .mov.

Also started looking into the issue of how to handle multiple audio streams in the file being transcoded. Currently all streams gets transcoded to the same chosen format if the container format support its, if the container only supports 1 audio stream you get one by random. This is not ideal :)

Ended up filing this bug with a request for how we can improve the GStreamer API to make handling such things easier for application developers. Discoverer, uridecodebin and encodebin makes a lot of things a lot easier, but for handling files with multiple streams of the same type I think we still need some improvements.