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163 lines
7.1 KiB
XML
163 lines
7.1 KiB
XML
<chapter id="chapter-checklist-element">
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<title>Things to check when writing an application</title>
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<para>
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This chapter contains a fairly random selection of things that can be
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useful to keep in mind when writing &GStreamer;-based applications. It's
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up to you how much you're going to use the information provided here.
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We will shortly discuss how to debug pipeline problems using &GStreamer;
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applications. Also, we will touch upon how to acquire knowledge about
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plugins and elements and how to test simple pipelines before building
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applications around them.
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</para>
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<sect1 id="section-checklist-programming">
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<title>Good programming habits</title>
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<itemizedlist>
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<listitem>
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<para>
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Always connect to the <quote>error</quote> signal of your topmost
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pipeline to be notified of errors in your pipeline.
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</para>
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</listitem>
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<listitem>
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<para>
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Always check return values of &GStreamer; functions. Especially,
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check return values of <function>gst_element_link ()</function>
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and <function>gst_element_set_state ()</function>.
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</para>
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</listitem>
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<listitem>
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<para>
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Always use your pipeline object to keep track of the current state
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of your pipeline. Don't keep private variables in your application.
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Also, don't update your user interface if a user presses the
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<quote>play</quote> button. Instead, connect to the
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<quote>state-changed</quote> signal of your topmost pipeline and
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update the user interface whenever this signal is triggered.
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</para>
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</listitem>
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</itemizedlist>
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</sect1>
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<sect1 id="section-checklist-debug">
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<title>Debugging</title>
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<para>
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Applications can make use of the extensive &GStreamer; debugging system
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to debug pipeline problems. Elements will write output to this system
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to log what they're doing. It's not used for error reporting, but it
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is very useful for tracking what an element is doing exactly, which
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can come in handy when debugging application issues (such as failing
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seeks, out-of-sync media, etc.).
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</para>
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<para>
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Most &GStreamer;-based applications accept the commandline option
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<option>--gst-debug=LIST</option> and related family members. The
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list consists of a comma-separated list of category/level pairs,
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which can set the debugging level for a specific debugging category.
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For example, <option>--gst-debug=oggdemux:5</option> would turn
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on debugging for the Ogg demuxer element. You can use wildcards as
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well. A debugging level of 0 will turn off all debugging, and a level
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of 5 will turn on all debugging. Intermediate values only turn on
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some debugging (based on message severity; 2, for example, will only
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display errors and warnings). Here's a list of all available options:
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</para>
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<para>
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<itemizedlist>
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<listitem>
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<para>
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<option>--gst-debug-help</option> will print available debug
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categories and exit.
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</para>
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</listitem>
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<listitem>
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<para>
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<option>--gst-debug-level=<replaceable>LEVEL</replaceable></option>
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will set the default debug level (which can range from 0 (no
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output) to 5 (everything)).
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</para>
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</listitem>
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<listitem>
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<para>
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<option>--gst-debug=<replaceable>LIST</replaceable></option>
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takes a comma-separated list of category_name:level pairs to
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set specific levels for the individual categories. Example:
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<option>GST_AUTOPLUG:5,avidemux:3</option>.
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</para>
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</listitem>
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<listitem>
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<para>
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<option>--gst-debug-no-color</option> will disable color debugging.
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</para>
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</listitem>
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<listitem>
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<para>
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<option>--gst-debug-disable</option> disables debugging alltogether.
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</para>
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</listitem>
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<listitem>
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<para>
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<option>--gst-plugin-spew</option> enables printout of errors while
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loading &GStreamer; plugins.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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</sect1>
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<sect1 id="section-checklist-conversion">
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<title>Conversion plugins</title>
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<para>
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&GStreamer; contains a bunch of conversion plugins that most
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applications will find useful. Specifically, those are videoscalers
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(videoscale), colorspace convertors (ffmpegcolorspace), audio format
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convertors and channel resamplers (audioconvert) and audio samplerate
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convertors (audioscale). Those convertors don't do anything when not
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required, they will act in passthrough mode. They will activate when
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the hardware doesn't support a specific request, though. All
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applications are recommended to use those elements.
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</para>
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</sect1>
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<sect1 id="section-checklist-applications">
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<title>Utility applications provided with &GStreamer;</title>
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<para>
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&GStreamer; comes with a default set of command-line utilities that
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can help in application development. We will discuss only
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<command>gst-launch</command> and <command>gst-inspect</command> here.
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</para>
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<sect2 id="section-applications-launch">
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<title><command>gst-launch</command></title>
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<para>
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<command>gst-launch</command> is a simple script-like commandline
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application that can be used to test pipelines. For example, the
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command <command>gst-launch sinesrc ! alsasink</command> will run
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a pipeline which generates a sine-wave audio stream and plays it
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to your ALSA audio card. <command>gst-launch</command> also allows
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the use of threads (using curly brackets, so <quote>{</quote>
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and <quote>}</quote>) and bins (using brackets, so <quote>(</quote>
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and <quote>)</quote>). You can use dots to imply padnames on elements,
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or even omit the padname to automatically select a pad. Using
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all this, the pipeline <command>gst-launch filesrc location=file.ogg
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! oggdemux name=d { d. ! theoradec ! ffmpegcolorspace ! xvimagesink
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} { d. ! vorbisdec ! alsasink }</command> will play an Ogg file
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containing a Theora video-stream and a Vorbis audio-stream. You can
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also use autopluggers such as decodebin on the commandline. See the
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manual page of <command>gst-launch</command> for more information.
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</para>
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</sect2>
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<sect2 id="section-applications-inspect">
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<title><command>gst-inspect</command></title>
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<para>
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<command>gst-inspect</command> can be used to inspect all properties,
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signals, dynamic parameters and the object hierarchy of an element.
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This can be very useful to see which <classname>GObject</classname>
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properties or which signals (and using what arguments) an element
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supports. Run <command>gst-inspect fakesrc</command> to get an idea
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of what it does. See the manual page of <command>gst-inspect</command>
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for more information.
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</para>
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</sect2>
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</sect1>
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</chapter>
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