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166 lines
8 KiB
Text
166 lines
8 KiB
Text
GstParse
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========
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This document describes how GstParse works. GstParse is the infrastructure used
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to describe pipelines \(or parts thereof) as a string. This powerful yet easy
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language is for example by the gst-launch command-line utility or in the GConf
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keys that are used by various applications, such as Rhythmbox or Gst-Player.
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\(You can find these keys in /system/gstreamer/default in GConf)
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The following examples will show commands for gst-launch. Please note that some
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characters need to be escaped when the pipeline is written in the shell as
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gst-launch command.
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If you want to debug the GstParse part of an application, you can start it with
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the --gst-mask=0x10000 command line option.
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Basics
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======
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Elements
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--------
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The most basic thing you specify in a pipeline are elements. You add an element
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by the name of its type, so "filesrc" will add a file source and "identity" will
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add an identity element.
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Gstparse will put all these elements into a toplevel pipeline element.
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example:#> gst-launch filesrc identity
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This adds a filesrc and an identity element to a pipeline. Of course this
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pipeline will not run, because the elements are not yet connected.
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Properties
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----------
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You can set element properties by specifying them directly after the element in
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the form name=value \(you need to escape here) or name="value" \(you only need to
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escape the " sign.
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example:#> gst-launch filesrc name=source location="music.mp3"
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This adds a filesrc element with the name "source" pointing to the location
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music.mp3 to your pipeline. \(This pipeline doesn't work either.)
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It's a good idea to set the name property on important elements, because if they
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are referenced \(see below), they will be referenced by name and the name of an
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element is unspecified if it wasn't set before.
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Please note that not all properties can be set yet. You might get a warning like
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property "myproperty" in element myelement cannot be set
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In that case, please file a feature request. We can't guess all properties in
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advance.
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Bins
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----
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If you want to add bins to your pipeline, you can do so by specifying
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<bintype> . ( <properties> <elements> )
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in your pipeline. This adds a bin to your pipeline and puts all elements that
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are specified between the brackets inside this bin. You can specify properties
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of the bin directly after the opening bracket.
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There are to special bins: if you don't specify a bintype and no dot either, the
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type of the bin defaults to "bin". And you can use curly brackets { } to get a
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bin of type "thread".
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example:#> gst-launch \( { fakesrc pipeline . \( fakesink \) } \)
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This will put a fakesrc element inside a thread inside a bin and a fakesink into
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a pipeline element inside the thread inside the bin.
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Please note that this pipeline would not work, even if the elements were
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connected properly, because the pipeline only specifies one top level element,
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the element is not put inside a pipeline but returned directly. So if you don't
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want your elements be put into a pipeline, just add a bin of whatever type you
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wish around the pipeline.
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Links
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-----
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What would a pipeline be without the ability to link the elements?
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Well it's very easy. Just put the link sign, an exclamation mark, between two
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elements and they will be linked.
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example:#> gst-launch fakesrc ! fakesink
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This will put the two elements into a pipeline and connect the fakesrc to the
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fakesink. This is the first pipeline that should run - and run it will until you
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terminate it.
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You can't however control which pads are linked, you can reference the pad\(s)
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you want to connect by name with a dot before the name.
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example:#> gst-launch fakesrc .src ! fakesink
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example:#> gst-launch fakesrc ! .sink fakesink
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example:#> gst-launch fakesrc .src ! .sink fakesink
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These examples will all do the exact same thing as the example before. You have
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however referenced the pads you wanted to connect in different ways.
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Another way to reference is to reference the element you want to connect, too.
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In this case you have to add the element name before the dot. You can even not
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reference the pad and only the element name, to connect any pad.
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example:#> gst-launch fakesrc name=src fakesink name=sink src.src ! sink.sink
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example:#> gst-launch fakesrc name=src ! sink.sink fakesink name=sink
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example:#> gst-launch fakesrc name=src src. ! fakesink name=sink
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example:#> gst-launch fakesrc name=src fakesink name=sink src. ! sink.
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These examples all do the same as the other ones. Please note that we specified
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the element name property to be sure to reference the right element.
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In these examples you may have noticed, that an omitted element name on a link
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automatically means the element directly in front of / after the link is used
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to create the link to save you some typing work and that not specifying a pad
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name will automatically create 1 connection between the two elements. If more
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than one connection was possible you might end up surprised.
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You may even specify pad lists to connect elements. They are separated by
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commas.
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example:#> gst-launch fakesrc ! tee name=tee1 .src0,src1 ! .sink0, sink1 aggregator ! fakesink
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example:#> gst-launch fakesrc ! tee name=tee aggregator name=aggregator ! fakesink tee.src0,src1 ! aggregator.sink0, sink1
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example:#> gst-launch fakesrc ! tee name=tee .src0 ! .sink0 aggregator name=aggregator ! fakesink tee.src1 ! aggregator.sink1
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These will all do the same. They will connect the tee twice to the aggregator.
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Examples
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========
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Audio playback
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--------------
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The most common audio pipeline for playing back an mp3 file is this:
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example:#> gst-launch filesrc location=/path/to/file.mp3 ! mad ! osssink
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You can however improve on this by adding visualization:
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example:#> gst-launch filesrc location=/path/to/file.mp3 ! mad ! tee name=tee ! osssink tee.src%d ! goom ! colorspace ! xvideosink
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Note that the reference of the tee element has a pad named "src%d". This is as
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used by printf and is a useful way to specify request pads where you don't know
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how this pad is named. Yes, the pad name could have been omitted anyway.
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If you have jerky playback you might want to use this pipeline threaded so audio
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and video have less chance of problems and you have a little buffer:
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example:#> gst-launch filesrc location=/path/to/file.mp3 ! mad ! tee name=tee ! { queue ! osssink } tee name=tee { ! queue ! osssink } { tee. ! queue ! goom ! colorspace ! xvideosink }
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Remember to put queues at thread boundaries or it will not work.
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Video playback
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--------------
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The basic pipeline to playback a standard mpeg video would be this:
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example:#> gst-launch filesrc location=/path/to/file.mpeg ! mpegdemux name=demux ! mad ! osssink demux. ! mpeg2dec ! xvideosink
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Or using threads for better playback:
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example:#> gst-launch filesrc location=/path/to/file.mpeg ! mpegdemux name=demux ! { queue ! mad ! osssink } demux. ! { queue ! mpeg2dec ! xvideosink }
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For a DivX avi, this might work:
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example:#> gst-launch filesrc location=/path/to/file.avi ! avidemux name=demux ! { queue ! mad ! osssink } { demux. ! ffdec_msmpeg4 ! xvideosink }
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If these don't work the video might need different treatment. You should skip to
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the autoplugger part.
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Please be aware of the fact that the mpegdemux element uses pads that are not
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always available. GstParse will wait for them to become available and disable
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the elements that need to be connected by disabling their state until the link
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can be established.
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Autoplugging
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------------
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You can also use the spider element for autoplugging. This will take care of
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selecting the right elements for you. Just use this pipeline for all your needs:
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example:#> gst-launch filesrc location=/path/to/anything ! spider name=spider ! { queue ! osssink } { spider. ! queue ! xvideosink }
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To throw you out
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----------------
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Finally the current experimental pipeline that powers the gst-player specified
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as a simple (ahem...) command line:
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example:#> gst-launch pipeline. \( { filesrc location=/path/to/anything ! spider name=spider ! { queue ! volume ! \( tee name=tee ! { queue ! \( goom \) ! colorspace ! \( xvideosink \) } tee. ! { queue ! \( osssink \) } \) } spider. ! { queue ! colorspace \( xvideosink \) } } \)
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