ruby-****@sourc*****
ruby-****@sourc*****
2004年 4月 1日 (木) 21:02:28 JST
-------------------------
REMOTE_ADDR = 195.207.101.112
REMOTE_HOST =
URL = http://ruby-gnome2.sourceforge.jp/en/?tut-gst-helloworld
-------------------------
= Hello World
+ {{link("tut-gst-elements-state", nil, "tut-gst", "tut-gst-helloworld-conclusion")}}
We will create a simple first application, a complete MP3 player, using standard GStreamer components. The player will read from a file that is given as the first argument to the program.
== The code
require 'gst'
Gst.init
unless ARGV.length == 1
$stderr.puts "Usage: #{__FILE__} <mp3 filename>"
exit 1
end
# create a new pipeline to hold the elements
pipeline = Gst::Pipeline.new
# create a disk reader
filesrc = Gst::ElementFactory.make("filesrc")
filesrc.location = ARGV.first
# now it's time to get the decoder
decoder = Gst::ElementFactory.make("mad")
# and an audio sink
audiosink = Gst::ElementFactory.make("osssink")
# add objects to the main pipeline
pipeline.add(filesrc, decoder, audiosink)
# link elements
filesrc >> decoder >> audiosink
# start playing
pipeline.play
while pipeline.iterate do end
# stop the pipeline
pipeline.stop
== The Code Explained
Let's go through this example step by step.
The first thing you have to do is to require the Ruby/GStreamer library and initialize the framework.
require 'gst'
Gst.init
...
We are going to create an empty pipeline. As you have seen in the basic introduction, this pipeline will hold and manage all the elements we are going to pack into it.
...
# create a new pipeline to hold the elements
pipeline = Gst::Pipeline.new
...
We use the standard constructor for a pipeline: Gst::Pipeline.new.
We then create a disk source element. The disk source element is able to read from a file. We use the standard GLib::Object property mechanism to set a property of the element: the file to read from.
...
# create a disk reader
filesrc = Gst::ElementFactory.make("filesrc")
filesrc.location = ARGV.first
...
((*Note*)) You can check if filesrc.nil? to verify the creation of the disk source element.
We now create the MP3 decoder element. This assumes that the 'mad' plugin is installed on the system where this application is executed.
...
# now it's time to get the decoder
decoder = Gst::ElementFactory.make("mad")
...
Gst::ElementFactory.make may take two arguments: a string that will identify the element you need and a second argument: how you want to name the element. The name of the element is something you can choose yourself and might be used to retrieve the element from a bin/pipeline. Here we choose to not provide a name for the element, it means that GStreamer will automatically generate a name for us.
Finally we create our audio sink element. This element will be able to play back the audio using OSS.
...
# and an audio sink
audiosink = Gst::ElementFactory.make("osssink")
...
We then add the elements to the pipeline.
...
# add objects to the main pipeline
pipeline.add(filesrc, decoder, audiosink)
...
We link the different pads of the elements together like this:
...
# link elements
filesrc >> decoder >> audiosink
...
We now have a created a complete pipeline. We can visualise the pipeline as follows:
{{image_left("hello-world.png")}}
{{br}}
Everything is now set up to start streaming. We use the following statements to change the state of the pipeline:
...
# start playing
pipeline.play
...
((*Note*)) GStreamer will take care of the READY and PAUSED state for you when going from NULL to PLAYING.
Since we do not use threads, nothing will happen yet. We have to call Gst::Bin#iterate to execute one iteration of the pipeline.
...
while pipeline.iterate do end
...
The Gst::Bin#iterate method will return true as long as something interesting happened inside the pipeline. When the end-of-file has been reached the Gst::Bin#iterate method will return false and we can end the loop.
...
# stop the pipeline
pipeline.stop
((*Note*)) Don't forget to stop the pipeline after use. This will free all of the resources held by the elements.
== Running Hello World
Just save the code in a file named (({helloworld.rb})) and do:
ruby helloworld.rb foo.mp3