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244 lines
8.5 KiB
XML
244 lines
8.5 KiB
XML
<chapter id="chapter-pads">
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<title>Pads</title>
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<para>
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As we have seen in <xref linkend="chapter-elements"/>, the pads are the element's
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interface to the outside world.
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</para>
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<para>
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The specific type of media that the element can handle will be exposed by the pads.
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The description of this media type is done with capabilities(see
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<xref linkend="section-caps"/>)
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</para>
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<para>
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Pads are either source or sink pads. The terminology is defined from the
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view of the element itself: elements accept data on their sink pads, and
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send data out on their source pads. Sink pads are drawn on the left,
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while source pads are drawn on the right of an element. In general,
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data flows from left to right in the graph.<footnote>
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<para>
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In reality, there is no objection to data flowing from a
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source pad to the sink pad of an element upstream. Data will, however,
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always flow from a source pad of one element to the sink pad of
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another.
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</para></footnote>
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</para>
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<sect1 id="section-pads-type">
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<title>Types of pads</title>
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<sect2 id="section-pads-dynamic">
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<title>Dynamic pads</title>
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<para>
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Some elements might not have all of their pads when the element is
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created. This
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can happen, for example, with an MPEG system demultiplexer. The
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demultiplexer will create its pads at runtime when it detects the
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different elementary streams in the MPEG system stream.
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</para>
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<para>
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Running <application>gst-inspect mpegdemux</application> will show that
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the element has only one pad: a sink pad called 'sink'. The other pads are
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"dormant". You can see this in the pad template because there is
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an 'Exists: Sometimes'
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property. Depending on the type of MPEG file you play, the pads will
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be created. We
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will see that this is very important when you are going to create dynamic
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pipelines later on in this manual.
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</para>
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</sect2>
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<sect2 id="section-pads-request">
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<title>Request pads</title>
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<para>
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An element can also have request pads. These pads are not created
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automatically but are only created on demand. This is very useful
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for multiplexers, aggregators and tee elements.
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</para>
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<para>
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The tee element, for example, has one input pad and a request padtemplate for the
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output pads. Whenever an element wants to get an output pad from the tee element, it
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has to request the pad.
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</para>
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</sect2>
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</sect1>
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<sect1 id="section-caps">
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<title>Capabilities of a pad</title>
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<para>
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Since the pads play a very important role in how the element is viewed by the
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outside world, a mechanism is implemented to describe the data that can
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flow through the pad by using capabilities.
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</para>
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<para>
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We will briefly describe what capabilities are, enough for you to get a basic understanding
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of the concepts. You will find more information on how to create capabilities in the
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Plugin Writer's Guide.
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</para>
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<sect2 id="section-pads-caps">
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<title>Capabilities</title>
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<para>
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Capabilities are attached to a pad in order to describe
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what type of media the pad can handle.
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</para>
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<para>
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Capabilities is shorthand for "capability chain". A capability chain
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is a chain of one capability or more.
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</para>
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<para>
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The basic entity is a capability, and is defined by a name, a MIME
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type and a set of properties. A capability can be chained to
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another capability, which is why we commonly refer to a chain of
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capability entities as "capabilities".
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<footnote>
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<para>
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It is important to understand that the term "capabilities" refers
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to a chain of one capability or more. This will be clearer when
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you see the structure definition of a <classname>GstCaps</classname>
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element.
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</para>
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</footnote>
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</para>
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<para>
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Below is a dump of the capabilities of the element mad, as shown by
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<command>gst-inspect</command>.
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You can see two pads: sink and src. Both pads have capability information attached to them.
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</para>
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<para>
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The sink pad (input pad) is called 'sink' and takes data of MIME type 'audio/mp3'. It also has
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three properties: layer, bitrate and framed.
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</para>
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<para>
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The source pad (output pad) is called 'src' and outputs data of
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MIME type 'audio/raw'. It also has four properties: format, depth,
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rate and channels.
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</para>
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<programlisting>
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Pads:
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SINK template: 'sink'
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Availability: Always
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Capabilities:
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'mad_sink':
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MIME type: 'audio/mp3':
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SRC template: 'src'
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Availability: Always
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Capabilities:
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'mad_src':
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MIME type: 'audio/raw':
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format: String: int
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endianness: Integer: 1234
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width: Integer: 16
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depth: Integer: 16
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channels: Integer range: 1 - 2
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law: Integer: 0
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signed: Boolean: TRUE
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rate: Integer range: 11025 - 48000
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</programlisting>
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</sect2>
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<sect2 id="section-pads-props">
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<title>What are properties ?</title>
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<para>
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Properties are used to describe extra information for
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capabilities. A property consists of a key (a string) and
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a value. There are different possible value types that can be used:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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basic types:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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an integer value: the property has this exact value.
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</para>
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</listitem>
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<listitem>
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<para>
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a boolean value: the property is either TRUE or FALSE.
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</para>
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</listitem>
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<listitem>
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<para>
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a fourcc value: this is a value that is commonly used to
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describe an encoding for video,
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as used for example by the AVI specification.
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<footnote><para>
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fourcc values consist of four bytes.
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<ulink url="http://www.fourcc.org" type="http">The FOURCC
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Definition List</ulink> is the most complete resource
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on the allowed fourcc values.
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</para></footnote>
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</para>
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</listitem>
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<listitem>
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<para>
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a float value: the property has this exact floating point value.
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</para>
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</listitem>
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<listitem>
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<para>
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a string value.
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</para>
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</listitem>
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</itemizedlist>
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</listitem>
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<listitem>
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<para>
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range types:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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an integer range value: the property denotes a range of
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possible integer. For example, the wavparse element has
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a source pad where the "rate" property can go from 8000 to
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48000.
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</para>
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</listitem>
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<listitem>
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<para>
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a float range value: the property denotes a range of possible
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floating point values.
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</para>
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</listitem>
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</itemizedlist>
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</listitem>
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<listitem>
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<para>
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a list value: the property can take any value from a list of
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basic value types or range types.
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</para>
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</listitem>
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</itemizedlist>
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</sect2>
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<sect2 id="section-pads-caps-use">
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<title>What capabilities are used for</title>
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<para>
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Capabilities describe in great detail the type of media that is handled by the pads.
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They are mostly used for:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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Autoplugging: automatically finding plugins for a set of capabilities
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</para>
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</listitem>
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<listitem>
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<para>
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Compatibility detection: when two pads are linked, <application>GStreamer</application>
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can verify if the two pads are talking about the same media types.
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The process of linking two pads and checking if they are compatible
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is called "caps negotiation".
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</para>
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</listitem>
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</itemizedlist>
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</sect2>
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</sect1>
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</chapter>
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