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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii" /><title>Chapter 1. Howto</title><link rel="stylesheet" href="manual.css" type="text/css" /><link rev="made" href="mailto:mnauw@users.sourceforge.net" /><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /><link rel="start" href="index.html" title="GStreamer Transcoding" /><link rel="up" href="index.html" title="GStreamer Transcoding" /><link rel="prev" href="index.html" title="GStreamer Transcoding" /><link rel="next" href="package.html" title="Chapter 2. Package Documentation" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 1. Howto</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="index.html"><< Previous</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="package.html">Next >></a></td></tr></table><hr /></div><div class="chapter" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="howto"></a>Chapter 1. Howto</h2></div></div></div><div class="toc"><dl><dt><span class="sect1"><a href="howto.html#sect-introduction">Introduction</a></span></dt><dt><span class="sect1"><a href="howto.html#id522649">GStreamer Pipelines</a></span></dt><dt><span class="sect1"><a href="howto.html#id523110">Media components</a></span></dt><dd><dl><dt><span class="sect2"><a href="howto.html#sect-mpeg-encoding">MPEG encoding</a></span></dt><dt><span class="sect2"><a href="howto.html#sect-ffmpeg-encoders">FFMPEG encoders</a></span></dt><dt><span class="sect2"><a href="howto.html#sect-avi-format">AVI format</a></span></dt><dt><span class="sect2"><a href="howto.html#id523308">OGG/OGM format</a></span></dt><dt><span class="sect2"><a href="howto.html#sect-matroska-format">Matroska format</a></span></dt><dt><span class="sect2"><a href="howto.html#sect-quicktime-format">QuickTime format</a></span></dt><dt><span class="sect2"><a href="howto.html#sect-mpeg-muxing">MPEG PS muxing</a></span></dt><dt><span class="sect2"><a href="howto.html#sect-ffmpeg-muxers">FFMPEG muxers</a></span></dt><dt><span class="sect2"><a href="howto.html#sect-subtitles">
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</a></span></dt><dt><span class="sect2"><a href="howto.html#sect-recording">Recording</a></span></dt><dt><span class="sect2"><a href="howto.html#id524281">Multiple Images</a></span></dt></dl></dd><dt><span class="sect1"><a href="howto.html#sect-processing">Media processing</a></span></dt><dt><span class="sect1"><a href="howto.html#sect-motivation">Comparison and Motivation</a></span></dt></dl></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="sect-introduction"></a>Introduction</h2></div></div></div><p>
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In this context, <em class="firstterm">transcoding</em> can be loosely described
6
as decoding, optionally processing and re-encoding of media data (video, audio,
7
…) possibly from one container format to another.
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<em class="firstterm">Recording</em> is very related to this, but only differs in
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that the source material is taken from some live source, rather than an
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existing medium. In particular, the sequel applies equally to either of these.
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<a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> is a multimedia (processing) framework that allows the construction of
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graphs of media-handling components ranging from simple playback to
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complex audio (mixing) and video processing, more explanation on
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which can be found on its <a class="ulink" href="http://www.gstreamer.net/" target="_top">homepage</a>.
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It is by no means the only (Un*x) software dealing with
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multimedia; e.g. other players include <a class="ulink" href="http://www.mplayerhq.hu/" target="_top">MPlayer</a>,
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<a class="ulink" href="http://www.xine.org/" target="_top">xine</a>,
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<a class="ulink" href="http://www.videolan.org/" target="_top">VideoLAN</a>,
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…, and the list of codec processing libraries is equally extensive.
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It is, however, more unique in that it can actually be considered a
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<span class="emphasis"><em>framework</em></span>, and the sequel will (not surprisingly)
23
focus on (a particular approach to) transcoding using <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a>.
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The primary reason being that there are not so many alternatives
25
(including the above) that allow for
26
(comfortably) processing and particularly <span class="emphasis"><em>creating</em></span> a
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(fairly wide range of) media files. Some more considerations can be found in
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<a class="xref" href="howto.html#sect-motivation" title="Comparison and Motivation">“Comparison and Motivation”</a>.
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Of course, there are many more expositions on multimedia (processing); who
31
knows (?) what is out there for Google to find, or explanations that can be
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found on Wikipedia … Besides all that, there is some
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consolidated information in e.g. the format and codecs sections of <a class="ulink" href="http://www.mplayerhq.hu/design7/info.html#docs" target="_top">MPlayer
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documentation</a>. All of this need not be recited here, so it
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now “<span class="quote">only</span>”remains to consider a <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> approach and point of
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</p></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="id522649"></a>GStreamer Pipelines</h2></div></div></div><p>
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A <em class="firstterm">pipeline</em> is a central concept in <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a>.
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It is a (usually simple) directed graph connecting media handling components,
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typically demuxer, decoder, encoder and muxer or display elements (in this
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order). For the later purposes here, it should suffice to know and
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</p><div class="itemizedlist"><ul><li><p>
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a "media handling component" is usually referred to as
45
<em class="firstterm">element</em>. There are many kinds of elements, for
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instance (but not exhaustive); demuxer (splits a container file into its
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constituent streams), decoder (decodes compressed data into raw format), and
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symmetrically encoder and muxer …. Do note that each type of container
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or compressed data has its own element, e.g. <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-avidemux.html" target="_top"><span class="emphasis"><em>avidemux</em></span></a>,
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<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-vorbisenc.html" target="_top"><span class="emphasis"><em>vorbisenc</em></span></a>, … Elements are typically made available
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through installed <em class="firstterm">plugins</em>, which are basically
52
dynamically loaded libraries.
54
elements have <em class="firstterm">pads</em>, which are like
55
“<span class="quote">ports</span>” or “<span class="quote">plugs</span>” that connect elements
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and through which data flows. For example, an encoder/decoder will basically
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have 1 input (sink) and output (source) pad each, whereas typically a demuxer
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has 1 sink pad and several source pads, and a muxer has several sink pads and
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1 source pad. Evidently, an element's source pad connects to a downstream
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(succeeding) element's sink pad.
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data that flows through pads is described by <em class="firstterm">caps</em>
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(short for capabilities),
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which is roughly a mime-type (e.g. <code class="literal">audio/x-raw-int</code>,
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<code class="literal">video/x-raw-yuv</code>) along with a set of descriptive/defining
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properties <span class="emphasis"><em>depending</em></span> on the mime-type (e.g.
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<code class="literal">width</code>, <code class="literal">height</code>,
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<code class="literal">depth</code>).
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Each block/buffer of data typically holds an (un)compressed video frame, or
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some number of (un)compressed audio samples, or even any part of a
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bitstream taken from file or network, … typically along with some
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metadata (if applicable), e.g. indicating whether or not keyframe, (media)time
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and (media)duration of contained data, …
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there are elements that “<span class="quote">interface with the world outside a
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pipeline</span>”. Such elements have only 1 source pad (source element) or
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only 1 sink pad (sink elements), and are consequently also simply
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collectively called <em class="firstterm">sources</em> or
79
<em class="firstterm">sinks</em> respectively. For example,
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<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gstreamer-plugins/html/gstreamer-plugins-filesrc.html" target="_top"><span class="emphasis"><em>filesrc</em></span></a> takes care of reading data from a
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file and feeding this into a pipeline, and <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gstreamer-plugins/html/gstreamer-plugins-filesink.html" target="_top"><span class="emphasis"><em>filesink</em></span></a>
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will happily save data a pipeline has been labouring to produce. The
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“<span class="quote">outside</span>” can be much more live as well;
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<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-v4l2src.html" target="_top"><span class="emphasis"><em>v4l2src</em></span></a>
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will capture data from a video4linux compliant device (such as a TV capture
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card), whereas e.g. <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-alsasink.html" target="_top"><span class="emphasis"><em>alsasink</em></span></a> is equally instrumental
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on the downstream side.
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<span class="citerefentry"><span class="refentrytitle"><span class="command"><strong>gst-inspect</strong></span></span>(1)</span> can be used to “<span class="quote">bring the above to life</span>” by
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providing concrete information on elements. Specifically, just running
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<code class="literal">gst-inspect</code> yields a listing of (a.o.)
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all elements that are available along with some terse description. Executing
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<code class="literal">gst-inspect <em class="replaceable"><code>element</code></em></code>
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for some <em class="replaceable"><code>element</code></em> shows (again a.o.):
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</p><div class="itemizedlist"><ul compact="compact"><li><p>
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in the <code class="computeroutput">Pad Templates</code> section; the
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pads the elements has and the type of data to flow through them
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in the <code class="computeroutput">Element Properties</code> section;
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various settings determining the operation of the element, e.g. a bitrate for
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encoding, number of (top)lines to crop, volume scale factor to apply, …
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</p></li></ul></div><p>
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many elements provide some additional documentation, which should normally
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be installed along with the plugins themselves (see your distribution system)
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and accessible using Devhelp. Alternatively, it can also be found on-line in
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<a class="ulink" href="http://www.gstreamer.net/documentation" target="_top">GStreamer's plugin documentation</a>
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intermediate conversion between various (raw)
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<a class="ulink" href="http://www.fourcc.org/" target="_top">(YUV or RGB) video</a> and audio formats may be
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necessary in order to connect elements whose caps differ (only in raw format).
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<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-ffmpegcolorspace.html" target="_top"><span class="emphasis"><em>ffmpegcolorspace</em></span></a> and
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<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-audioconvert.html" target="_top"><span class="emphasis"><em>audioconvert</em></span></a>
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can take care of this, and may have to be inserted one or more times, depending on
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various formats supported by elements in the pipeline. As any element, these
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can detect the caps supported by their neighbours and will conduct best effort
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to have linking and subsequent negotiation and dataflow succeed, in this case by
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performing conversion as needed.
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having a connected pipeline (graph) is one (static) thing, making it do
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something is another. This can be loosely called running or activating the
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pipeline, which typically means that sources really get going (reading,
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capturing, …, and processing/playing takes off.
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In any case, this is a matter for an application to take care of, and similarly
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so for keeping an eye on the pipeline as it runs, providing notifications when
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appropriate, dealing with errors, and finally allowing for clean shutdown when
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the pipeline informs it has had enough.
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</p></li></ul></div><p>
129
By now, it should be quite conceivable that all of the above (concepts) can
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nicely cooperate and be linked together to form a pipeline, for instance using a
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syntax as described in <span class="citerefentry"><span class="refentrytitle"><span class="command"><strong>gst-launch</strong></span></span>(1)</span>. For even more details, one might boldly
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go into e.g. the introductory part(s) of
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<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/manual/html/index.html" target="_top">
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GStreamer Application Development Manual</a>, although this
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actually targets a bit lower technical level.
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When dealing with <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> applications, one usually need not be aware of or
138
concerned with all these things, as it is typically the very goal and merit of an
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application to construct and manage such a pipeline behind the scenes.
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Such is the case for instance with media player <a class="ulink" href="http://www.gnome.org/projects/totem/" target="_top">totem</a>, for which <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> can serve as a backend.
141
However, in the goal we are pursuing here, it shall be up to the reader/user to
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conduct transcoding by orchestrating a pipeline, by means of <span class="citerefentry"><span class="refentrytitle"><span class="command"><strong>gst-launch</strong></span></span>(1)</span> or (much)
143
more comfortably <a class="xref" href="entrans.html" title="entrans"><span class="command"><strong>entrans</strong></span></a>. After all, the latter provides (a.o.) a small
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application convenience layer, which alleviates part of the burden, boilerplate,
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minutia and intricacy of pipeline building (hm, what fun is still left
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To emphasize again, the above (possibly daunting) level of detail is not meant
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to add to the rumours that <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> is “<span class="quote">(way too) complicated</span>”.
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Rather, on the one hand, it intends to dispel these by unveiling some of the
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mystery by plain and simple overview. On the other hand, as mentioned above,
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any <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> application can choose to go about its business (including
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transcoding) taking care to (more or less) hide the above details.
154
That is, however, simply not the
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approach that has been chosen here (see also e.g. <a class="xref" href="howto.html#sect-motivation" title="Comparison and Motivation">“Comparison and Motivation”</a>).
157
The general idea presented here, along with additional explanations and
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specific and concrete examples in <span class="citerefentry"><span class="refentrytitle"><span class="command"><strong>gst-launch</strong></span></span>(1)</span> and <a class="xref" href="entrans.html" title="entrans"><span class="command"><strong>entrans</strong></span></a> should suffice
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to cover quite some transcoding cases.
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Next section(s) will provide some specific information and gotchas to
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consider in pipeline assembling and media handling, depending on the particular
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</p></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="id523110"></a>Media components</h2></div></div></div><p>
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The purpose here is neither to re-iterate specific <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> related information
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on pads, caps, … of elements that can be obtained by means of
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<span class="citerefentry"><span class="refentrytitle"><span class="command"><strong>gst-inspect</strong></span></span>(1)</span>, nor to lecture (at least not in great detail) on what is already
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ubiquitously available “<span class="quote">out there</span>” (see e.g. references in <a class="xref" href="howto.html#sect-introduction" title="Introduction">“Introduction”</a>).
168
Similarly, <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> (transcoding) is by no means limited to types
169
or formats that are mentioned below (use again <span class="citerefentry"><span class="refentrytitle"><span class="command"><strong>gst-inspect</strong></span></span>(1)</span> to see all that
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Rather, it is endeavoured here to provide some minimal (technical) background
172
that is pertinent in explaining or avoiding phenomena involving certain
173
components turning up as FAQs (on mailinglist, for example).
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</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="sect-mpeg-encoding"></a>MPEG encoding</h3></div></div></div><p>
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MPEG-1/2 compressed video can be obtained by means of
176
<span class="emphasis"><em>ffenc_mpeg1video</em></span>,
177
<span class="emphasis"><em>ffenc_mpeg2video</em></span> or
178
<span class="emphasis"><em>mpeg2enc</em></span>, the formers based on <a class="ulink" href="http://ffmpeg.sourceforge.net/" target="_top">ffmpeg</a>'s avcodec library,
179
and the latter on <a class="ulink" href="http://mjpeg.sourceforge.net/" target="_top">mjpegtools</a>' mpeg2enc library.
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Further documentation or usage shall not be discussed here.
182
It is noted here, however, that the nature of
183
<span class="emphasis"><em>mpeg2enc</em></span>
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leads to it output not having metadata on timestamps
185
(at least, for not so recent versions). As this output is
186
typically muxed into <span class="emphasis"><em>mplex</em></span> (see also later in <a class="xref" href="howto.html#sect-mpeg-muxing" title="MPEG PS muxing">“MPEG PS muxing”</a>), this is usually not a concern. If one were to
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put this content into e.g. AVI (which is possible, although why doing so is
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another), then <a class="ulink" href="../../gst-entrans-plugins/html/gst-entrans-plugins-stamp.html" target="_top"><span class="emphasis"><em>stamp</em></span></a> can easily help one out if needed, as in the fragment
189
</p><div class="informalexample"><pre class="programlisting">
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mpeg2enc format=3 ! stamp ! avimux
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</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="sect-ffmpeg-encoders"></a>FFMPEG encoders</h3></div></div></div><p><span class="citerefentry"><span class="refentrytitle"><span class="command"><strong>gst-inspect</strong></span></span>(1)</span> shows quite a list of ffmpeg based encoders
193
<code class="literal">ffenc_<em class="replaceable"><code>name</code></em></code>, most of which
194
are pretty straightforward to use. They may have limitations in supported
195
video sizes or formats, most common of which are exposed in recent releases
196
<sup>[<a id="id523269" href="#ftn.id523269" class="footnote">1</a>]</sup>,
197
in which case following pipeline fragment illustrates how to have this
198
automatically dealt with:
199
</p><div class="informalexample"><pre class="programlisting">
200
videoscale ! ffmpegcolorspace ! ffenc_huffyuv
202
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="sect-avi-format"></a>AVI format</h3></div></div></div><p>
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AVI (Audio Video Interleaved) files, designed by Microsoft, have a plain and
204
simple format. Though it has various limitations (for example in streaming),
205
it handles many basic purposes well.
207
One of the limitations is that it does not store any timedata, it only
208
“<span class="quote">knows</span>” of framerate and bitrate. This is then used to
209
deduce what belongs when, which is possible at least with CBR audio and is why
210
e.g. VBR is not supported (other than by some esoteric not well-supported
212
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id523308"></a>OGG/OGM format</h3></div></div></div><p>
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This is a fileformat from <a class="ulink" href="http://www.xiph.org/" target="_top">Xiphophorus</a>
214
typically (but not necessarily) containing <a class="ulink" href="http://www.xiph.org/vorbis/" target="_top">Vorbis</a>
215
and/or <a class="ulink" href="http://theora.org/" target="_top">Theora</a> streams.
216
It can be handled by means of <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-oggdemux.html" target="_top"><span class="emphasis"><em>oggdemux</em></span></a>
217
and <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-oggmux.html" target="_top"><span class="emphasis"><em>oggmux</em></span></a>, and e.g.
218
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-vorbisdec.html" target="_top"><span class="emphasis"><em>vorbisdec</em></span></a>,
219
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-vorbisenc.html" target="_top"><span class="emphasis"><em>vorbisenc</em></span></a>,
220
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-theoradec.html" target="_top"><span class="emphasis"><em>theoradec</em></span></a>
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and <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-theoraenc.html" target="_top"><span class="emphasis"><em>theoraenc</em></span></a>
222
will probably come along to play as well.
224
Although it is not quite subject to limitations mentioned above
225
(e.g. does handle VBR audio), the format typically expects a
226
particular (video) framerate (as much as a specific audio samplerate also
227
always applies), and therefore corresponding (time-)regular streams.
229
Some stand-alone tools that may help in manipulating Ogg include the
230
<a class="ulink" href="http://www.xiph.org/" target="_top">vorbis-tools</a> and the
231
<a class="ulink" href="http://www.bunkus.org/videotools/ogmtools/" target="_top">OGMtools</a>
232
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="sect-matroska-format"></a>Matroska format</h3></div></div></div><p>
233
Although <a class="ulink" href="http://www.matroska.org/" target="_top">Matroska</a> is not related to
234
AVI in either in its origin or its bitstream structure, it is similar
235
in that it basically stores chunks/blocks of data, preceded by a header and
236
(optionally) followed by index-like data. A simple
237
<span class="emphasis"><em>gst-inspect</em></span>ion
238
(of <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-matroskamux.html" target="_top"><span class="emphasis"><em>matroskamux</em></span></a> or
239
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-matroskademux.html" target="_top"><span class="emphasis"><em>matroskademux</em></span></a>) easily shows that it can contain quite a range
240
of material, forming a superset of AVI capabilities in this regard.
241
There are some more differences not evident at this first sight:
242
</p><div class="itemizedlist"><ul><li><p>
243
it can hold (any number of) video, audio or subtitle streams, although the
244
latter are just not yet supported by <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-matroskamux.html" target="_top"><span class="emphasis"><em>matroskamux</em></span></a>
245
(however, see also <a class="xref" href="howto.html#sect-subtitles" title="Subtitles">“
248
[whereas an AVI contains at most 1 video stream and a number of audio ones]
250
each datablock is recorded along with a (media)timestamp and (media)duration;
251
in particular this caters for (audio) VBR [whereas AVI does not record such
252
information and typically only relies on framerate or constant bitrate]
254
a table of references (index) is included which could (theoretically) refer to
255
every datablock, but usually only does so to a (fairly dense) subset of
256
keyframes [whereas an AVI holds an index to all datachunks]
258
only v2 Matroska files may hold detailed keyframe information
259
(as only <code class="literal">SimpleBlock</code> indicates whether or not a block
260
holds a keyframe; but this is not used in Matroska v1 files)
261
[whereas keyframe info is fully available in any decent non-corrupt AVI]
262
</p></li></ul></div><p>
264
Hence, some practical considerations follow
265
</p><div class="itemizedlist"><ul><li><p>
266
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-matroskamux.html" target="_top"><span class="emphasis"><em>matroskamux</em></span></a> can basically be freely used
267
in stead of <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-avimux.html" target="_top"><span class="emphasis"><em>avimux</em></span></a>.
268
One need only (additionally) make sure that the
269
timestamps going into the former make up a nice consecutive (perfect) stream, as
270
they will be recorded for posterity and used in later playback. This could be
271
achieved e.g. by means of <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gstreamer-plugins/html/gstreamer-plugins-identity.html" target="_top"><span class="emphasis"><em>identity</em></span></a> (with
272
<span class="property">single-segment</span> <code class="literal">true</code>) or <a class="ulink" href="../../gst-entrans-plugins/html/gst-entrans-plugins-stamp.html" target="_top"><span class="emphasis"><em>stamp</em></span></a>
273
inserted before an encoder element or muxer (some encoders like competing with
274
muxer on the notion of time, and also keep an eye on this).
276
Pass-through transcoding is possible with Matroska input, but may likely need
277
an element such as <a class="ulink" href="../../gst-entrans-plugins/html/gst-entrans-plugins-divxkey.html" target="_top"><span class="emphasis"><em>divxkey</em></span></a> to ensure that the
278
meta-information on keyframe-ness is properly marked. If not, lacking this
279
metadata in the input, all data will be assumed a keyframe by default.
281
If interested in a bit of history and some inter-operability; some (not so
282
much) older versions of <a class="ulink" href="http://www.mplayerhq.hu/" target="_top">MPlayer</a> do not recognize the full set of possible
283
bytetags in a Matroska file, and may consequently fail to find a valid
285
This could be prevented by nudging tags into another direction by means of a
286
capsfilter, an example fragment being
287
</p><div class="informalexample"><pre class="programlisting">
288
stamp ! ffenc_mpeg4 ! video/x-divx,divxversion=5 ! matroskamux
290
Similarly, older <a class="ulink" href="http://www.mplayerhq.hu/" target="_top">MPlayer</a> versions cannot handle v2 files.
291
</p></li></ul></div><p>
292
If needed or desired, the stand-alone
293
<a class="ulink" href="http://www.bunkus.org/videotools/mkvtoolnix/" target="_top">MKVToolnix</a> tools can aid in managing
295
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="sect-quicktime-format"></a>QuickTime format</h3></div></div></div><p>
296
Likewise, <a class="ulink" href="http://developer.apple.com/quicktime/" target="_top">QuickTime</a>, designed by Apple,
297
is quite different in origin and bitstream, but alike in that it also basically
298
stores variably sized blocks/atoms of data, along with metadata describing
299
the streams/tracks contained in the file and providing index-like data.
300
The very same syntactical format is also used as an ISO MPEG-4 container format
301
(<code class="literal">.m4a</code>, <code class="literal">.mp4</code>),
302
albeit with some variation in types of boxes considered
303
(where ISO specs refer to the syntactical entity “<span class="quote">atom</span>” as a
304
“<span class="quote">box</span>”, see e.g. the concise ISO MPEG-4 14496-12 base media specification
305
in the <a class="ulink" href="http://standards.iso.org/ittf/PubliclyAvailableStandards/index.html" target="_top">publicly available ISO standards</a>).
306
Though hardly a practical concern, it might be noted that the metadata is
307
typically stored at the end of a file following the actual media contents,
308
unlike other formats considered (and somewhat contrary to colloquially
309
referring to metadata as a “<span class="quote">header</span>”).
311
The comments and practical considerations of the previous section apply
312
just as well in this case, except for the following replacement or additional
314
</p><div class="itemizedlist"><ul><li><p>
315
QuickTime metadata indexes all datablocks (samples) and also holds full keyframe
316
info (synchronization sample table).
318
<span class="emphasis"><em>ffmux_mov</em></span> and
319
<span class="emphasis"><em>ffmux_mp4</em></span> provide a working QuickTime
320
and MP4 muxer respectively, with only some moderate variation in the resulting
321
output and types of accepted input stream (depending on the specification
322
involved). However, the similarly related <span class="emphasis"><em>qtmux</em></span>,
323
<span class="emphasis"><em>mp4mux</em></span>, <span class="emphasis"><em>gppmux</em></span>
324
and <span class="emphasis"><em>mj2mux</em></span> should be considered more reliable.
325
Recall that similar observations with respect to timestamps
326
as in the foregoing section apply.
327
</p></li></ul></div><p>
328
Some stand-alone helper and/or debug management utilities are provided by
329
<a class="ulink" href="http://mpeg4ip.sourceforge.net/" target="_top">MPEG4IP</a> or
330
<a class="ulink" href="http://libquicktime.sourceforge.net/" target="_top">libquicktime</a>.
331
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="sect-mpeg-muxing"></a>MPEG PS muxing</h3></div></div></div><p>
332
Either <span class="emphasis"><em>ffmux_mpeg</em></span>
333
or <span class="emphasis"><em>mplex</em></span><sup>[<a id="id523746" href="#ftn.id523746" class="footnote">2</a>]</sup>
334
produce an MPEG program stream,
335
the former based on <a class="ulink" href="http://ffmpeg.sourceforge.net/" target="_top">ffmpeg</a>'s <span class="package">avformat</span> library, and the
336
latter on <a class="ulink" href="http://mjpeg.sourceforge.net/" target="_top">mjpegtools</a>' mplex. Though neither confirmed nor explained, it
337
appears that the output of the former element may only (??)
338
be handled (well ??) by ffmpeg aware/related (software) players (notably
339
<a class="ulink" href="http://www.mplayerhq.hu/" target="_top">MPlayer</a>), a (possible ??) limitation not so affecting the latter's results.
341
Unlike any other (typical) muxer element, <span class="emphasis"><em>mplex</em></span>
342
totally disregards any provided time metadata in input streams.
343
This is (implicitly) determined from other information extracted from
344
the (compressed) bitstream itself, such as framerate, samplerate, …
345
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="sect-ffmpeg-muxers"></a>FFMPEG muxers</h3></div></div></div><p>
346
As already indicated by example in the previous section,
347
various muxers of <a class="ulink" href="http://ffmpeg.sourceforge.net/" target="_top">ffmpeg</a>'s avformat library are wrapped and provided
348
by the ffmux plugin. They should<sup>[<a id="id523807" href="#ftn.id523807" class="footnote">3</a>]</sup> produce results similar to those
349
of the standalone <span class="command"><strong>ffmpeg</strong></span> binary, possibly with varying
351
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="sect-subtitles"></a>
353
</h3></div></div></div><p>
354
Subtitles can come in a variety of <a class="ulink" href="http://www.matroska.org/technical/specs/subtitles/index.html" target="_top">formats</a>, be it
355
containing the actual text to be displayed (optionally with some sort of style
356
information) or simply a direct bitmap (encoded) form of what the end-result
357
text should be on screen (as for <a class="ulink" href="http://sam.zoy.org/writings/dvd/subtitles/" target="_top">DVD
359
In either case, they can also be supplied in a separate file.
361
Although all of the above are internally basically well supported,
362
most <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> (<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-playbin.html" target="_top"><span class="emphasis"><em>playbin</em></span></a> based) players are
363
limited to text-based subtitles (whether or not contained in a separate file).
364
In particular, that leaves a few gaps when it comes to DVD subtitle playback
365
or muxing subtitle streams into a (supporting) container format, e.g. Matroska.
366
However, there are some patches in <a class="ulink" href="http://bugzilla.gnome.org/show_bug.cgi?id=415754" target="_top">#415754</a> that
367
take care of this. For instance, having applied these, a DVD
368
can be transcoded (including subtitles) into a Matroska container by means of
370
</p><div class="informalexample"><pre class="programlisting">
371
dvdreadsrc title=2 ! dvddemux name=demux \
372
matroskamux name=mux ! filesink location=dvd.mkv \
373
demux. ! mpeg2dec ! queue ! ffenc_mpeg4 ! mux. \
374
demux.audio_00 ! a52dec ! queue ! audioconvert ! lame ! mux. \
375
demux.subpicture_05 ! queue ! dvdsubparse ! mux.subtitle_0
377
Alternatively, <span class="command"><strong>entrans</strong></span> could be used in dynamic mode, with a
378
<span class="emphasis"><em>decodebin2</em></span> based pipeline,
379
which is much more convenient when it comes to selecting (several) desired streams:
380
</p><div class="informalexample"><pre class="programlisting">
381
entrans.py --decoder decodebin2 \
382
--set-prop 'decodebin2:caps:video/x-raw-yuv;audio/x-raw-int;
383
audio/x-raw-float;video/x-dvd-subpicture' \
384
-i dvd://2 -o dvd.mkv --an 1,3 --on 6,16,19,21 -- \
385
--video ffenc_mpeg4 \
386
--audio audioconvert ! lame \
389
Of course, in either case,
390
some (source and stream selecting) numbers may be different, and it
391
can also be extended to several audio and subtitle streams with the
392
corresponding evident modifications. Similarly, other codecs could
393
easily be chosen and configured as desired.
394
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="sect-recording"></a>Recording</h3></div></div></div><p>
395
In this case, the “<span class="quote">input</span>” is some live real-time source, data
396
from which is to be recorded and kept for future use by means of (a.o.) one of
397
the above muxers. Although this source can be a great many things, e.g.
398
network, or nowadays often some type of DVB source, the intention here is to
399
focus on a “<span class="quote">raw</span>” source, i.e. typically a <a class="ulink" href="url-v4l" target="_top">video4linux</a> device
400
(such as an analogue tv capture card or webcam).
402
In many ways, there is nothing special about this, and one can record
403
using a pipeline built according to basic principles as
405
</p><div class="informalexample"><pre class="programlisting">
406
v4l2src queue-size=15 ! video/x-raw-yuv,framerate=25/1,width=384,height=576 ! \
407
ffenc_mpeg4 name=venc \
408
alsasrc ! audio/x-raw-int,rate=48000,channels=1 ! audioconvert ! lame name=aenc \
409
avimux name=mux ! filesink location=rec.avi venc. ! mux. aenc. ! mux.
411
Of course, the particular image and audio capture settings may vary,
412
albeit within some limitations typically inherent to the device.
413
Also, this pipeline may elicit some complaints from the encoder regarding
414
timestamps; see also further on.
416
Although the above will (how?) typically work, it is nevertheless reasonable to
417
be concerned or have questions about:
418
</p><div class="itemizedlist"><ul><li><p><b>Buffering. </b>
419
A lot of things may be going on in a multi-tasking system, which can keep
420
(mainly) CPU and other resources temporarily occupied elsewhere, although the
421
<span class="emphasis"><em>real-time</em></span> data keeps coming in, and we don't want to miss
422
any of this (or as little as possible at least). Is there some
423
buffering going on to (shortly) hold on to data for it to be picked up a bit
424
later, as opportunity permits ?
425
</p></li><li><p><b>Synchronization. </b>
426
Is there any system in place that (at least) tries to ensure that
427
videoframes end up “<span class="quote">along-side</span>” the proper audio data
429
</p></li></ul></div><p>
431
Buffering here occurs on a low (device) level; the driver will arrange
432
for a number of buffers and configure the device to capture data into these
433
buffers. As such, the device can always continue capturing in real-time for at
434
least a while, even if there are some delays elsewhere. In case of e.g.
435
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-v4l2src.html" target="_top"><span class="emphasis"><em>v4l2src</em></span></a>, the number of framebuffers is
436
controlled by the <span class="property">queue-size</span> property, although the
437
actual resulting queue still depends on the particular device's capability.
438
Similarly, <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-alsasrc.html" target="_top"><span class="emphasis"><em>alsasrc</em></span></a>'
439
<span class="property">buffer-time</span> and <span class="property">latency-time</span>
440
control the total capacity of the buffer and the size of each individual buffer
441
(respectively), again up to best effort approximation by the device(driver).
443
A live audio source such as <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-alsasrc.html" target="_top"><span class="emphasis"><em>alsasrc</em></span></a> will
444
provide the pipeline with an (audio) clock, which (basically) advances
445
according to the number of captured samples. All captured data is then
446
“<span class="quote">stamped</span>” with the time (of capture) as perceived by this
447
clock. Evidently, this already ensures that the captured audio stream makes up
448
a very nice, time-perfect stream. As videoframes are also stamped this way,
449
this aligns video data with audio data, for the most part …
450
As indicated in previous sections, most formats (and muxers) expect video data
451
to have a particular framerate, and the incoming data to abide by this.
452
If not, strange things (notably loss of synchronization) will happen.
453
Data may fail to abide due to:
454
</p><div class="itemizedlist"><ul><li><p>
455
device (mal)performance; a tv capture card usually manages to deliver frames
456
in a regular pace at the proper framerate, but this need not be the case for all
457
devices, e.g. webcams
459
delay effects introduced by buffering, that is, when a (video) buffer is taken
460
out of a (device) queue and “<span class="quote">timestamped</span>”, there is a delay with
461
respect to the “<span class="quote">real</span>” capture time. In particular, this can lead
462
to irregular clusters of timestamps (even if actual capturing goes by the book).
463
In fact, this type of “<span class="quote">clustering</span>” prompts the encoder
464
to complain about timestamps, as indicated earlier.
465
</p></li></ul></div><p>
466
So, the video stream needs to be tamed and regularized before encoding and
467
muxing it, as a muxer may actually disregard the (nicely accurately computed)
468
synchronized timestamp and rely on expected framerate only.
469
Such taming could be done by inserting <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-videorate.html" target="_top"><span class="emphasis"><em>videorate</em></span></a> (somewhere before the encoder), which would then
470
drop and/or duplicate frames as needed to obtain a perfect and (still) synchronized
472
However, due to <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-videorate.html" target="_top"><span class="emphasis"><em>videorate</em></span></a>'s current technique,
473
this may lead to more frame drops or duplicates than really needed,
474
particularly due to the above timestamp clustering; a concentration of frames
475
would call for dropping, a subsequent lack for duplication, …
476
An alternative is to use the following fragment
477
</p><div class="informalexample"><pre class="programlisting">
478
v4l2src ! stamp sync-margin=2
480
<a class="ulink" href="../../gst-entrans-plugins/html/gst-entrans-plugins-stamp.html" target="_top"><span class="emphasis"><em>stamp</em></span></a> is aware of desired framerate and also counts incoming frames. At
481
regular intervals (set by <span class="property">sync-interval</span>), it verifies
482
whether the incoming videostream timestamp is off by more than
483
<span class="property">sync-margin</span> frames with respect to expected timestamp.
484
If so, proper dropping or duplication is performed to resync. Periodic checking
485
against “<span class="quote">real-time</span>” makes the process less sensitive to
486
intermittent variations.
488
In conclusion, the following pipeline should cater for recording in most
489
conditions and circumstances:
490
</p><div class="informalexample"><pre class="programlisting">
491
v4l2src queue-size=16 ! video/x-raw-yuv,framerate=25/1,width=384,height=576 ! \
492
stamp sync-margin=2 sync-interval=5 ! queue ! ffenc_mpeg4 name=venc \
493
alsasrc buffer-time=1000000 ! audio/x-raw-int,rate=48000,channels=1 ! \
494
queue ! audioconvert ! lame name=aenc \
495
avimux name=mux ! filesink location=test.avi aenc. ! mux. venc. ! mux.
497
In particular, it features resync'ing at a higher frequency (in the same thread
498
as and immediately following capturing), and provides additional buffering
499
(beyond that on driver level).
501
Alternatively, one might turn to a container format and muxer that does
502
<span class="emphasis"><em>not</em></span> expect its (video) input to have a particular framerate,
503
such as notably the <a class="link" href="howto.html#sect-matroska-format" title="Matroska format">Matroska format</a>.
504
Such format caters for faithful snapshot recording and subsequent playback
505
(including original timestamps) without needing to regularize input.
506
In principle, there is then no need for <a class="ulink" href="../../gst-entrans-plugins/html/gst-entrans-plugins-stamp.html" target="_top"><span class="emphasis"><em>stamp</em></span></a> or
507
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-videorate.html" target="_top"><span class="emphasis"><em>videorate</em></span></a>, though an encoder might still complain
508
about the quality/regularity of timestamps, as mentioned earlier.
509
</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
510
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-v4l2src.html" target="_top"><span class="emphasis"><em>v4l2src</em></span></a> may produce buffers that contain
511
mmap'ed driver buffers (only if <span class="property">always-copy</span>
512
is <code class="literal">false</code> in latest version),
513
and these may (depending on your mileage) wreac havoc
514
with (separate thread) encoding performance. Presumably, the recycling of
515
mmap'ed v4l2src buffers leads to additional (lock or driver)
516
contention (particularly when such releasing occurs in a separate thread).
517
The above example <span class="emphasis"><em>does not</em></span> suffer from this,
518
because <a class="ulink" href="../../gst-entrans-plugins/html/gst-entrans-plugins-stamp.html" target="_top"><span class="emphasis"><em>stamp</em></span></a> copies the data into a
519
“<span class="quote">normal</span>” buffer. Alternatively, <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-efence.html" target="_top"><span class="emphasis"><em>efence</em></span></a> or a combination of 2 counter-acting
520
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-videobox.html" target="_top"><span class="emphasis"><em>videobox</em></span></a>es could also be (ab)used for this purpose.
522
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="id524281"></a>Multiple Images</h3></div></div></div><p>
523
In this case, the “<span class="quote">input</span>” or “<span class="quote">output</span>” consists of a
524
collection of (image)files.
526
The easiest native <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> way of handling these is for example
527
</p><div class="informalexample"><pre class="programlisting">
528
multifilesrc location=image-%05d.jpg num-buffers=25 ! image/jpeg,framerate=25/1 ! \
529
jpegdec ! videoflip method=vertical-flip ! jpegenc ! \
530
multifilesink location=image-out-%05d.jpg
532
(assuming that it concerns 25 input files <code class="literal">image-00000.jpg</code>,
533
<code class="literal">image-00001.jpg</code>, …)
535
Another approach is to use external tools to merge/split the set of files
536
to/from one file and deal with this file. One candidate is the multipart
537
format used by <a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-multipartmux.html" target="_top"><span class="emphasis"><em>multipartmux</em></span></a> and
538
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-multipartdemux.html" target="_top"><span class="emphasis"><em>multipartdemux</em></span></a>. Each block of data in multipart
539
files is preceded by (in <code class="function">printf</code> format)
540
</p><pre class="programlisting">
541
"\r\n--%s\r\nContent-Type: %s\r\nContent-Length: %u\r\n\r\n",
542
<em class="lineannotation"><span class="lineannotation">boundary, mime-type, size</span></em>
544
where boundary is configurable (<code class="literal">ThisRandomString</code> by
545
default), and the <code class="literal">Content-Length</code> line is optional.
546
Clearly, a fairly simple (shell) script or pipeline
547
suffices to transform such a file from/to a set a files. Having such at hand,
548
the above example then becomes:
549
</p><div class="informalexample"><pre class="programlisting">
550
filesrc location=image.multipart ! multipartdemux ! \
551
jpegdec ! videoflip method=vertical-flip ! jpegenc ! \
552
multipartmux ! filesink location=image-out.multipart
555
A similar approach could use an intermediate single file in <a class="ulink" href="http://mjpeg.sourceforge.net/" target="_top">mjpegtools</a>'
556
YUV4MPEG2 format, which is really a sequence of uncompressed images,
557
see <a class="ulink" href="../../gst-entrans-plugins/html/ch02.html" target="_top">entrans plugin collection</a>
558
documentation for more details on this.
559
</p></div></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="sect-processing"></a>Media processing</h2></div></div></div><p>
560
Of course, transcoding is typically not limited to a transformation from one
561
(codec or muxer) format to another, but usually also involves applying some
562
(filter) operations. These can range from basic operations such as scaling
563
(<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-videoscale.html" target="_top"><span class="emphasis"><em>videoscale</em></span></a>) or
564
clipping (<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-videobox.html" target="_top"><span class="emphasis"><em>videobox</em></span></a>) to
565
various effects (e.g. effectv plugin) or any transformation one can envision.
566
A <span class="citerefentry"><span class="refentrytitle"><span class="command"><strong>gst-inspect</strong></span></span>(1)</span> browse is bound to reveal many more examples.
568
Although the details of the operation performed by such filters is typically
569
governed by their properties, some of these elements
570
(<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-videoscale.html" target="_top"><span class="emphasis"><em>videoscale</em></span></a>,
571
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-videorate.html" target="_top"><span class="emphasis"><em>videorate</em></span></a>,
572
<a class="ulink" href="http://www.gstreamer.net/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-audiorate.html" target="_top"><span class="emphasis"><em>audiorate</em></span></a>, …) must necessarily also consider
573
the caps of connected elements. Conversely, their operation can be affected by applying
574
a specific capsfilter, see e.g. examples in <a class="xref" href="howto.html#sect-ffmpeg-encoders" title="FFMPEG encoders">“FFMPEG encoders”</a>.
576
Last but not least, some additional filters are available in the
577
<a class="ulink" href="../../gst-entrans-plugins/html/ch02.html" target="_top">entrans plugin collection</a>.
578
</p></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="sect-motivation"></a>Comparison and Motivation</h2></div></div></div><p>
579
As mentioned earlier, there is a great many software that handles
580
multimedia playback. <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a>, as a framework, makes it (relatively) easy
581
to build a player; e.g. GNOME player <a class="ulink" href="http://www.gnome.org/projects/totem/" target="_top">totem</a> has a <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a>
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backend, and <a class="ulink" href="http://www.fluendo.com/elisa" target="_top">elisa</a>, to name a
583
few among <a class="ulink" href="http://www.gstreamer.net//apps" target="_top">GStreamer
586
When it comes to <span class="emphasis"><em>creating</em></span> media files, e.g. by transcoding
587
existing media, there is not such an abundance of choice.
588
The following table compares some media processing and <span class="emphasis"><em>creation</em></span>
590
</p><div class="informaltable"><table border="1"><colgroup><col /><col /><col /><col /><col /><col /><col /></colgroup><thead><tr><th> </th><th>Ogg</th><th>Matroska</th><th>AVI</th><th>MPG</th><th>Filters</th><th>Cutting</th></tr></thead><tbody><tr><td><a class="ulink" href="http://www.mplayerhq.hu/" target="_top">mencoder</a></td><td>no</td><td>no</td><td>yes</td><td>yes</td><td>yes</td><td>yes</td></tr><tr><td><a class="ulink" href="http://ffmpeg.sourceforge.net/" target="_top">ffmpeg</a></td><td>yes</td><td>yes</td><td>yes</td><td>yes</td><td>no</td><td>no</td></tr><tr><td><a class="ulink" href="http://www.transcoding.org/" target="_top">transcode</a></td><td>no</td><td>no</td><td>yes</td><td>no</td><td>yes</td><td>yes</td></tr><tr><td><a class="ulink" href="http://avidemux.sourceforge.net/" target="_top">avidemux</a></td><td>yes</td><td>no</td><td>yes</td><td>yes</td><td>yes</td><td>yes</td></tr><tr><td><a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a></td><td>yes</td><td>yes</td><td>yes</td><td>yes</td><td>yes</td><td>yes</td></tr></tbody></table></div><p>
591
Quite possibly, other (demuxing, decoding) features that may be of interest have
592
been disregarded here, but as it stands above, a (tentative) conclusion follows.
593
Fortunately, some applications do actually make use of the above <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a>
594
capabilities, e.g. <a class="ulink" href="http://www.pitivi.org/" target="_top">PiTiVi</a> (a
595
video editor), <a class="ulink" href="http://thoggen.net/" target="_top">Thoggen</a> (a DVD ripper), and of course <span class="command"><strong>entrans</strong></span>.
596
So, a non-exhaustive overview of the application spectrum is:
597
</p><div class="informaltable"><table border="1"><colgroup><col /><col /><col /><col /></colgroup><thead><tr><th> </th><th>Non-GStreamer</th><th>GStreamer</th><th>Comments</th></tr></thead><tbody><tr><td>player</td><td>many</td><td>many</td><td class="auto-generated"> </td></tr><tr><td>cmd-line transcoding</td><td><a class="ulink" href="http://www.mplayerhq.hu/" target="_top">mencoder</a>, <a class="ulink" href="http://www.transcoding.org/" target="_top">transcode</a></td><td><span class="command"><strong>entrans</strong></span></td><td class="auto-generated"> </td></tr><tr><td>GUI editing/transcoding</td><td><a class="ulink" href="http://avidemux.sourceforge.net/" target="_top">avidemux</a></td><td><a class="ulink" href="http://www.pitivi.org/" target="_top">PiTiVi</a>, <a class="ulink" href="http://thoggen.net/" target="_top">Thoggen</a></td><td class="auto-generated"> </td></tr></tbody></table></div><p>
599
It should again be noted that all these (<a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a>) applications make use of a
600
common API and codebase, and as such can benefit from a “<span class="quote">single</span>”
601
collective developer build and maintenance effort. Furthermore, due to its
602
plugin system, <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> is not only free software (as many others), but also
603
offers the freedom to extend it (comfortably) at will to include more formats,
604
filters, …, which then become available at once to all the above
607
Finally, let's demonstrate the “<span class="quote">many players</span>” mentioned above,
608
although not by lengthy list of examples.
609
A <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> player can indeed arise quite easily. In fact,
610
simply running the following pipeline already yields an extremely basic
611
“<span class="quote">player</span>” with some minimal navigation<sup>[<a id="id524944" href="#ftn.id524944" class="footnote">4</a>]</sup>.
612
It requires only trivial
613
modification to control which (if any) effects and filters to apply on-the-fly.
614
</p><pre class="programlisting">
615
filesrc location=example-in.ogm ! decodebin name=bin \
616
bin. ! progressreport ! navseek seek-offset=10 ! timeoverlay ! xvimagesink \
617
bin. ! audioconvert ! alsasink
619
</p></div><div class="footnotes"><br /><hr width="100" align="left" /><div class="footnote"><p><sup>[<a id="ftn.id523269" href="#id523269" class="para">1</a>] </sup>As of release 0.10.5 of gst-ffmpeg.</p></div><div class="footnote"><p><sup>[<a id="ftn.id523746" href="#id523746" class="para">2</a>] </sup>As of release 0.10.8
620
of gst-plugins-bad, and only operates with latest <a class="ulink" href="http://mjpeg.sourceforge.net/" target="_top">mjpegtools</a> release candidates.
621
</p></div><div class="footnote"><p><sup>[<a id="ftn.id523807" href="#id523807" class="para">3</a>] </sup>
622
The standalone <span class="command"><strong>ffmpeg</strong></span>'s architecture provides the avformat code
623
with full (intimate) access to a corresponding encoder
624
<span class="structname">AVCodecContext</span> structure,
625
which in ffmux can only be built approximately by means of caps information.
626
Results may or may not vary depending on particular format used and the way
628
</p></div><div class="footnote"><p><sup>[<a id="ftn.id524944" href="#id524944" class="para">4</a>] </sup>
629
Requires at least <a class="ulink" href="http://www.gstreamer.net/" target="_top"><span class="application">GStreamer</span></a> core 0.10.13.</p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="index.html"><< Previous</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="package.html">Next >></a></td></tr><tr><td width="40%" align="left" valign="top">GStreamer Transcoding </td><td width="20%" align="center"><a accesskey="h" href="index.html">Table of Contents</a></td><td width="40%" align="right" valign="top"> Chapter 2. Package Documentation</td></tr></table></div><div class="footer-homepage"><table width="100%"><tr><td width="40%" align="left">Last created:
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2010-03-03 22:13:23+01:00</td><td width="20%" align="center"><a href="http://gentrans.sourceforge.net/" title="GEntrans: Home page">GEntrans home page</a></td><td width="40%" align="right"></td></tr></table></div></body></html>
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