The small application we created in the previous chapter used the
concept of a factory to create the elements. In this chapter we will
show you how to use the factory concepts to create elements based
- on what they do instead of how they are called.
+ on what they do instead of what they are called.
</para>
<para>
<para>
While this mechanism is quite effective it also has some big problems:
The elements are created based on their name. Indeed, we create an
- element mad by explicitly stating the mad element's name. Our little
+ element, mad, by explicitly stating the mad element's name. Our little
program therefore always uses the mad decoder element to decode
- the MP3 audio stream, even if there are 3 other MP3 decoders in the
+ the MP3 audio stream, even if there are three other MP3 decoders in the
system. We will see how we can use a more general way to create an
MP3 decoder element.
</para>
<sect1>
<title>More on MIME Types</title>
<para>
- GStreamer uses MIME types to indentify the different types of data
+ GStreamer uses MIME types to identify the different types of data
that can be handled by the elements. They are the high level
mechanisms to make sure that everyone is talking about the right
kind of data.
</para>
<para>
- A MIME (Multipurpose Internet Mail Extension) types are a set of
- string that denote a certain type of data. examples include:
+ A MIME (Multipurpose Internet Mail Extension) type is a pair of
+ strings that denote a certain type of data. Examples include:
<itemizedlist>
<listitem>
<para>
</listitem>
<listitem>
<para>
- audio/mpeg : mpeg audio
+ audio/mpeg : MPEG audio
</para>
</listitem>
<listitem>
<para>
- video/mpeg : mpeg video
+ video/mpeg : MPEG video
</para>
</listitem>
</itemizedlist>
will see.
</para>
<para>
- As we have seen in the previous chapter, the MIME types are added
+ As we have seen in the previous chapter, MIME types are added
to the Capability structure of a pad.
</para>
</para>
<para>
There is also an association between a MIME type and a file extension,
- but the use of typefind functions (similar to file(1)) is preferred..
+ but the use of typefind functions (similar to file(1)) is preferred.
</para>
<para>
The type information is maintained in a list of
<para>
All operations on <classname>GstType</classname> occur
via their <classname>guint16 id</classname> numbers, with
- <classname>GstType</classname> structure private to the GStreamer
+ the <classname>GstType</classname> structure private to the GStreamer
library.
</para>
type = gst_type_find_by_id (id);
</programlisting>
<para>
- This function will return NULL if the id was associated with
+ This function will return NULL if the id was not associated with
any known <classname>GstType</classname>
</para>
</sect2>
</sect1>
<sect1>
- <title>creating elements with the factory</title>
+ <title>Creating elements with the factory</title>
<para>
In the previous section we described how you could obtain
an element factory using MIME types. One the factory has been
<sect1>
<title>Getting Started</title>
<para>
- The dparams subsystem is contained within the
+ The Dynamic Parameters subsystem is contained within the
<filename>gstcontrol</filename> library.
- You need to include the header in your applications's source file:
+ You need to include the header in your application's source file:
</para>
<programlisting>
...
Your application should link to the shared library <filename>gstcontrol</filename>.
</para>
<para>
- The <filename>gstcontrol</filename> library needs to be initialised
+ The <filename>gstcontrol</filename> library needs to be initialized
when your application is run. This can be done after the the GStreamer
- library has been initialised.
+ library has been initialized.
</para>
<programlisting>
...
<chapter id="cha-scheduler">
<title>Understanding schedulers</title>
<para>
- The scheduler is responsible for managing the plugins at runtime. The
- main responsabilities are:
+ The scheduler is responsible for managing the plugins at runtime. Its
+ main responsibilities are:
<itemizedlist>
<listitem>
<para>
</itemizedlist>
</para>
<para>
- The scheduler is a plugable component, this means that alternative
+ The scheduler is a plugable component; this means that alternative
schedulers can be written and plugged into GStreamer. The default scheduler
uses cothreads to schedule the plugins in a pipeline. Cothreads are fast
and lightweight user-space threads.
</para>
<para>
There is usually no need to interact with the scheduler directly, however
- it some cases it is feasable to set a specific clock or force a specific
+ in some cases it is feasible to set a specific clock or force a specific
plugin as the entry point in the pipeline.
</para>
<para>
The above program will create a thread with two elements in it. As soon
as it is set to the PLAYING state, the thread will start to iterate
- itself. You never need to manually iterate a thread.
+ itself. You never need to explicitly iterate a thread.
</para>
<sect2>
between the threads, in a thread-safe fashion. This element is the
queue, described more fully in <xref linkend="cha-queues"/>. It doesn't
matter if the queue is placed in the containing bin or in the thread
- itself, but it needs to be present on one side of the other to enable
+ itself, but it needs to be present on one side or the other to enable
inter-thread communication.
</para>
</sect2>
</para>
<para>
- You can also use the the parser in you own
+ You can also use the parser in you own
code. <application>GStreamer</application> provides a function
- gst_parse_launch () that you can use to construt a pipeline.
- The following programs lets you create an mp3 pipeline using the
+ gst_parse_launch () that you can use to construct a pipeline.
+ The following program lets you create an MP3 pipeline using the
gst_parse_launch () function:
</para>
<programlisting>
<para>
A bare identifier (a string beginning with a letter and containing
only letters, numbers, dashes, underscores, percent signs, or colons)
- will create an element from a given elementfactory. In this example,
- an instance of the "mad" mp3 decoding plugin will be created.
+ will create an element from a given element factory. In this example,
+ an instance of the "mad" MP3 decoding plugin will be created.
</para>
</sect3>
<sect3>
The autoplugger API is implemented in an abstract class. Autoplugger
implementations reside in plugins and are therefore optional and can be
optimized for a specific task. Two types of autopluggers exist: renderer
- ones and non renderer ones. the renderer autopluggers will not have any
- src pads while the non renderer ones do. The renderer autopluggers are
+ ones and non-renderer ones. The renderer autopluggers will not have any
+ source pads while the non-renderer ones do. The renderer autopluggers are
mainly used for media playback while the non renderer ones are used for
arbitrary format conversion.
</para>
A list of all available autopluggers can be obtained with gst_autoplug_factory_get_list().
</para>
<para>
- If the autoplugger supports the RENDERER API, use
- gst_autoplug_to_renderers() call to create a bin that connects the
- src caps to the specified render elements. You can then add the bin
- to a pipeline and run it.
+ If the autoplugger supports the RENDERER API, use the
+ gst_autoplug_to_renderers() function to create a bin that connects
+ the source caps to the specified render elements. You can then add
+ the bin to a pipeline and run it.
<programlisting>
</para>
<para>
If the autoplugger supports the CAPS API, use the gst_autoplug_to_caps()
- function to connect the src caps to the destination caps. The created
- bin will have src and sink pads compatible with the provided caps.
+ function to connect the source caps to the destination caps. The created
+ bin will have source and sink pads compatible with the provided caps.
<programlisting>
<listitem>
<para>
Add the autoplugcache element to a bin and connect the sink pad
- to the src pad of an element with unknown caps.
+ to the source pad of an element with unknown caps.
</para>
</listitem>
<listitem>
<para>
- Connect the src pad of the autoplugcache to the sink pad of the
- typefind element.
+ Connect the source pad of the autoplugcache to the sink pad of
+ the typefind element.
</para>
</listitem>
<listitem>
<para>
- Loop the pipeline until the typefind element has found a type.
+ Iterate the pipeline until the typefind element has found a type.
</para>
</listitem>
<listitem>
<para>
Remove the typefind element and add the plugins needed to play
- back the discovered media type to the autoplugcache src pad.
+ back the discovered media type to the autoplugcache source pad.
</para>
</listitem>
<listitem>
<title>Another approach to autoplugging</title>
<para>
The autoplug API is interesting, but often impractical. It is static;
- it cannot deal with dynamic pipelines (insert ref here). What one
- often wants is just an element to stick into a pipeline that will DWIM
- (ref). Enter the spider.
+ it cannot deal with dynamic pipelines (insert ref here). What you
+ often want is just an element to stick into a pipeline that will DWIM
+ (Do What I Mean)(ref). Enter the spider.
</para>
<sect2>
<title>The spider element</title>
</listitem>
<listitem>
<para>
- Has request pads on the src side. This means that it can autoplug
- one source stream into many sink streams. For example, a MPEG1
- system stream can have audio as well as video; that pipeline
- would be represented in gst-launch syntax as
+ Has request pads on the source side. This means that it can
+ autoplug one source stream into many sink streams. For example,
+ an MPEG1 system stream can have audio as well as video; that
+ pipeline would be represented in gst-launch syntax as
<programlisting>
$ gst-launch filesrc location=my.mpeg1 ! spider ! { queue ! osssink } spider.src_%d!
<sect1 id="sec-bin-create">
<title>Creating a bin</title>
<para>
- Bins register themselves in the GStreamer registry, so they can be
- created in the normal way:
+ Bins are created in the same way that other elements are created. ie.
+ using an element factory, or any of the associated convenience functions:
</para>
<programlisting>
GstElement *bin, *thread, *pipeline;
</programlisting>
<para>
You can see that the name of the element becomes very handy
- for retrieving the element from an bin by using the element's
+ for retrieving the element from a bin by using the element's
name. gst_bin_get_by_name () will recursively search nested bins.
</para>
<para>
...
</programlisting>
<para>
- To remove an element from a bin use:
+ To remove an element from a bin, use:
</para>
<programlisting>
GstElement *element;
...
</programlisting>
<para>
- To add many elements to a bin at the same time, try the gst_bin_add_many
- () API. Remember to pass NULL as the last argument.
+ To add many elements to a bin at the same time, use the gst_bin_add_many
+ () function. Remember to pass NULL as the last argument.
</para>
<programlisting>
GstElement *filesrc, *decoder, *audiosink;
<title>Custom bins</title>
<para>
The application programmer can create custom bins packed with elements
- to perform a specific task. This allow you to write an MPEG audio
- decoder with just the follwing lines of code:
+ to perform a specific task. This allows you to write an MPEG audio
+ decoder with just the following lines of code:
</para>
<programlisting>
from a <classname>GstThread</classname>, which begins to play as soon
as its state is set to PLAYING. Other bin types may need explicit
iteration. For more information, see <xref linkend="cha-threads"/>.
-
+ </para>
+ <para>
Custom bins can be created with a plugin or an XML description. You
will find more information about creating custom bin in the Plugin
Writers Guide (FIXME ref).
</programlisting>
<para>
In the above example, the bin now also has a pad: the pad called 'sink'
- of the given element. We can now, for example, connect the srcpad of
- a filesrc to the bin with:
+ of the given element.
+ </para>
+ <para>
+ We can now, for example, connect the source pad of a filesrc element
+ to the bin with:
</para>
<programlisting>
GstElement *filesrc;
<para>
Buffers contain the data that will flow through the pipeline you have
created. A source element will typically create a new buffer and pass
- it through the pad to the next element in the chain. When using the
+ it through a pad to the next element in the chain. When using the
GStreamer infrastructure to create a media pipeline you will not have
to deal with buffers yourself; the elements will do that for you.
</para>
<para>
The simple case is that a buffer is created, memory allocated, data put
- in it, and passed to the next filter. That filter reads the data, does
+ in it, and passed to the next element. That element reads the data, does
something (like creating a new buffer and decoding into it), and
unreferences the buffer. This causes the data to be freed and the buffer
to be destroyed. A typical MPEG audio decoder works like this.
</para>
<para>
Below you see how we will visualize the element.
- We always draw a src pad to the right of the element.
+ We always draw a source pad to the right of the element.
</para>
<figure float="1" id="sec-element-srcimg">
<title>Visualisation of a source element</title>
</figure>
<para>
Source elements do not accept data, they only generate data. You can
- see this in the figure because it only has a src pad. A src pad can
- only generate data.
+ see this in the figure because it only has a source pad. A source
+ pad can only generate data.
</para>
</sect2>
<title>Filters and codecs</title>
<para>
Filter elements both have input and output pads. They operate on
- data they receive in their sink pads and produce data on their src
+ data they receive in their sink pads and produce data on their source
pads. For example, MPEG decoders and volume filters would fall into
this category.
</para>
</figure>
<para>
The above figure shows the visualisation of a filter element.
- This element has one sink (input) pad and one src (output) pad.
+ This element has one sink (input) pad and one source (output) pad.
Sink pads are drawn on the left of the element.
</para>
<figure float="1" id="sec-element-multifilterimg">
This chapter describes the most rudimentary aspects of a
<application>GStreamer</application> application, including initializing
the libraries, creating elements, packing them into a pipeline and playing,
- pause and stop the pipeline.
+ pausing and stopping the pipeline.
</para>
<sect1>
<para>
We will create a simple first application, a complete MP3 player, using
standard <application>GStreamer</application> components. The player
- will read from a file that is given as the first argument of the program.
+ will read from a file that is given as the first argument to the program.
</para>
<programlisting>
</programlisting>
<para>
- We are going to create 3 elements and one pipeline. Since all elements
- share the same base type, <classname>GstElement</classname>, we can
- define them as:
+ We are going to create three elements and one pipeline. Since all
+ elements share the same base type, <classname>GstElement</classname>,
+ we can define them as:
</para>
<programlisting>
...
<para>
Next, we are going to create an empty pipeline. As you have seen in
the basic introduction, this pipeline will hold and manage all the
- elements we are going to stuff into it.
+ elements we are going to pack into it.
</para>
<programlisting>
/* create a new pipeline to hold the elements */
pipeline = gst_pipeline_new ("pipeline");
</programlisting>
<para>
- We use the standard constructor for a pipeline: gst_pipeline_new ("name").
+ We use the standard constructor for a pipeline: gst_pipeline_new ().
</para>
<para>
<para>
Finally we create our audio sink element. This element will be able
- to playback the audio using OSS.
+ to play back the audio using OSS.
</para>
<programlisting>
/* and an audio sink */
</note>
<para>
- Since we do not use threads, nothing will happen yet. We manually have to
+ Since we do not use threads, nothing will happen yet. We have to
call gst_bin_iterate() to execute one iteration of the pipeline.
</para>
<programlisting>
</programlisting>
<para>
The gst_bin_iterate() function will return TRUE as long as something
- interesting happended inside the pipeline. When the end-of-file has been
+ interesting happened inside the pipeline. When the end-of-file has been
reached the _iterate function will return FALSE and we can end the loop.
</para>
<programlisting>
<title>Conclusion</title>
<para>
This concludes our first example. As you see, setting up a pipeline
- is very lowlevel but powerful. You will later in this manual how
- you can create a custom MP3 element with a more high level API.
+ is very low-level but powerful. You will see later in this manual how
+ you can create a custom MP3 element with a higher-level API.
</para>
<para>
It should be clear from the example that we can very easily replace the
- filesrc element with an httpsrc, giving you instant network streaming.
- An element could be build to handle icecast connections, for example.
+ filesrc element with an httpsrc element, giving you instant network
+ streaming. An element could be built to handle icecast connections,
+ for example.
</para>
<para>
We can also choose to use another type of sink instead of the audiosink.
This function will get the pad named "src" from the given element.
</para>
<para>
- Alternatively, you can also request a GList of pads from the element. The
+ Alternatively, you can request a GList of pads from the element. The
following code example will print the names of all the pads of an
element.
</para>
</para>
<para>
gst_pad_get_direction (GstPad *pad) can be used to query if the pad
- is a sink or a src pad. Remember a src pad is a pad that can output
- data and a sink pad is one that accepts data.
+ is a sink or a source pad. Remember that a source pad is a pad that
+ can output data and a sink pad is one that accepts data.
</para>
<para>
You can get the parent of the pad, this is the element that this pad belongs to,
<title>Dynamic pads</title>
<para>
Some elements might not have their pads when they are created. This
- can, for example, happen with an MPEG2 system demuxer. The demuxer will
- create its pads at runtime when it detects the different elementary
- streams in the MPEG2 system stream.
+ can happen, for example, with an MPEG2 system demultiplexer. The
+ demultiplexer will create its pads at runtime when it detects the
+ different elementary streams in the MPEG2 system stream.
</para>
<para>
Running <application>gst-inspect mpegdemux</application> will show that
GstElement *pipeline;
GstElement *mpeg2parser;
- // create pipeline and do something usefull
+ // create pipeline and do something useful
...
mpeg2parser = gst_element_factory_make ("mpegdemux", "mpegdemux");
<title>Request pads</title>
<para>
An element can also have request pads. These pads are not created
- automatically but are only created on demand. This is very usefull
- for muxers, aggregators and tee elements.
+ automatically but are only created on demand. This is very useful
+ for multiplexers, aggregators and tee elements.
</para>
<para>
The tee element, for example, has one input pad and a request padtemplate for the
</para>
<para>
It is also possible to request a pad that is compatible with another
- padtemplate. This is very usefull if you want to connect an element to
- a muxer element and you need to request a pad that is compatible. The
- gst_element_get_compatible_pad is used to request a compatible pad,
- as is shown in the next example.
+ pad template. This is very useful if you want to connect an element
+ to a multiplexer element and you need to request a pad that is
+ compatible. The gst_element_get_compatible_pad is used to request
+ a compatible pad, as is shown in the next example.
</para>
<programlisting>
...
three properties: layer, bitrate and framed.
</para>
<para>
- The src pad (output pad) is called 'src' and outputs data of MIME
- type 'audio/raw'. It also has four properties: format, depth, rate
- and channels.
+ The source pad (output pad) is called 'src' and outputs data of
+ MIME type 'audio/raw'. It also has four properties: format, depth,
+ rate and channels.
</para>
<programlisting>
Pads:
<sect2 id="sec-pads-props">
<title>What are properties</title>
<para>
- Properties are used to describe extra information for the
- capabilities. The properties basically exist of a key (a string) and
+ Properties are used to describe extra information for
+ capabilities. The properties basically consist of a key (a string) and
a value. There are different possibile value types that can be used:
</para>
<listitem>
<para>
An integer range value. The property denotes a range of possible
- values. In the case of the mad element: the src pad has a property
- rate that can go from 11025 to 48000.
+ values. In the case of the mad element, the source pad has a
+ property rate that can go from 11025 to 48000.
</para>
</listitem>
<listitem>
<listitem>
<para>
a fourcc value: this is a value that is commonly used to describe an encoding for video,
- as used be the AVI specification.
+ as used by the AVI specification.
</para>
</listitem>
<listitem>
</sect2>
<sect2 id="sec-pads-caps-use">
- <title>What are the capabilities used for?</title>
+ <title>What capabilities are used for</title>
<para>
Capabilities describe in great detail the type of media that is handled by the pads.
They are mostly used for:
</programlisting>
</sect2>
<sect2 id="sec-pads-caps-create">
- <title>Creating capabilities structures</title>
+ <title>Creating capability structures</title>
<para>
- While the capabilities are mainly used inside the plugin to describe
- the media type of the pads, the application programmer also has
- to have basic understanding of caps in order to interface with the
+ While capabilities are mainly used inside a plugin to describe the
+ media type of the pads, the application programmer also has to have
+ basic understanding of capabilities in order to interface with the
plugins, specially when using the autopluggers.
</para>
<para>
As we said, a capability has a name, a mime-type and some
properties. The signature of the function to create a new
- <classname>GstCaps</classname> structure is like:
+ <classname>GstCaps</classname> structure is:
<programlisting>
GstCaps* gst_caps_new (const gchar *name, const gchar *mime, GstProps *props);
NULL
);
</programlisting>
- Optionally the convenient shortcut macro can be used. The above complex
+ Optionally, the convenient shortcut macro can be used. The above complex
capability can be created with:
<programlisting>
GstCaps *newcaps;
<sect1 id="sec-bin-create">
<title>Creating a bin</title>
<para>
- Bins register themselves in the GStreamer registry, so they can be
- created in the normal way:
+ Bins are created in the same way that other elements are created. ie.
+ using an element factory, or any of the associated convenience functions:
</para>
<programlisting>
GstElement *bin, *thread, *pipeline;
</programlisting>
<para>
You can see that the name of the element becomes very handy
- for retrieving the element from an bin by using the element's
+ for retrieving the element from a bin by using the element's
name. gst_bin_get_by_name () will recursively search nested bins.
</para>
<para>
...
</programlisting>
<para>
- To remove an element from a bin use:
+ To remove an element from a bin, use:
</para>
<programlisting>
GstElement *element;
...
</programlisting>
<para>
- To add many elements to a bin at the same time, try the gst_bin_add_many
- () API. Remember to pass NULL as the last argument.
+ To add many elements to a bin at the same time, use the gst_bin_add_many
+ () function. Remember to pass NULL as the last argument.
</para>
<programlisting>
GstElement *filesrc, *decoder, *audiosink;
<title>Custom bins</title>
<para>
The application programmer can create custom bins packed with elements
- to perform a specific task. This allow you to write an MPEG audio
- decoder with just the follwing lines of code:
+ to perform a specific task. This allows you to write an MPEG audio
+ decoder with just the following lines of code:
</para>
<programlisting>
from a <classname>GstThread</classname>, which begins to play as soon
as its state is set to PLAYING. Other bin types may need explicit
iteration. For more information, see <xref linkend="cha-threads"/>.
-
+ </para>
+ <para>
Custom bins can be created with a plugin or an XML description. You
will find more information about creating custom bin in the Plugin
Writers Guide (FIXME ref).
</programlisting>
<para>
In the above example, the bin now also has a pad: the pad called 'sink'
- of the given element. We can now, for example, connect the srcpad of
- a filesrc to the bin with:
+ of the given element.
+ </para>
+ <para>
+ We can now, for example, connect the source pad of a filesrc element
+ to the bin with:
</para>
<programlisting>
GstElement *filesrc;
<para>
Buffers contain the data that will flow through the pipeline you have
created. A source element will typically create a new buffer and pass
- it through the pad to the next element in the chain. When using the
+ it through a pad to the next element in the chain. When using the
GStreamer infrastructure to create a media pipeline you will not have
to deal with buffers yourself; the elements will do that for you.
</para>
<para>
The simple case is that a buffer is created, memory allocated, data put
- in it, and passed to the next filter. That filter reads the data, does
+ in it, and passed to the next element. That element reads the data, does
something (like creating a new buffer and decoding into it), and
unreferences the buffer. This causes the data to be freed and the buffer
to be destroyed. A typical MPEG audio decoder works like this.
</figure>
<para>
By connecting these three elements, we have created a very simple
- pipeline. The effect of this will be that the output of the source element
+ chain. The effect of this will be that the output of the source element
(element1) will be used as input for the filter element (element2). The
filter element will do something with the data and send the result to
the final sink element (element3).
</para>
<para>
- Imagine the above graph as a simple mpeg audio decoder. The source
- element is a disk source, the filter element is the mpeg decoder and
+ Imagine the above graph as a simple MPEG audio decoder. The source
+ element is a disk source, the filter element is the MPEG decoder and
the sink element is your audiocard. We will use this simple graph to
- construct an mpeg player later in this manual.
+ construct an MPEG player later in this manual.
</para>
<sect1 id="sec-conn-basic">
You can query if a pad is connected with GST_PAD_IS_CONNECTED (pad).
</para>
<para>
- To query for the <classname>GstPad</classname> this srcpad is connected
- to, use gst_pad_get_peer (srcpad).
+ To query for the <classname>GstPad</classname> a pad is connected to, use
+ gst_pad_get_peer (pad).
</para>
</sect1>
</para>
<para>
Before proceding to the concept of loop-based elements we will first
- explain the chain-based elements
+ explain the chain-based elements.
</para>
<sect1 id="sec-chain-based">
When the request for a buffer cannot immediatly satisfied, the control
will be given to the source element of the loop-based element until it
performs a push on its source pad. At that time the control is handed
- back to the loop-based element, etc... The the execution trace can get
+ back to the loop-based element, etc... The execution trace can get
fairly complex using cothreads when there are multiple input/output
pads for the loop-based element. Cothread switches are performed within
the call to gst_pad_pull and gst_pad_push; from the perspective of
<para>
Loop based elements are mainly used for the more complex elements
that need a specific amount of data before they can start to produce
- output. An example of such an element is the mpeg video decoder. the
- element will pull a buffer, performs some decoding on it and optionally
- requests more buffers to decode, when a complete video frame has
- been decoded, a buffer is send out. For example, any plugin using the
+ output. An example of such an element is the MPEG video decoder. The
+ element will pull a buffer, perform some decoding on it and optionally
+ request more buffers to decode, and when a complete video frame has
+ been decoded, a buffer is sent out. For example, any plugin using the
bytestream library will need to be loop-based.
</para>
<para>
<sect1>
<title>Getting Started</title>
<para>
- The dparams subsystem is contained within the
+ The Dynamic Parameters subsystem is contained within the
<filename>gstcontrol</filename> library.
- You need to include the header in your applications's source file:
+ You need to include the header in your application's source file:
</para>
<programlisting>
...
Your application should link to the shared library <filename>gstcontrol</filename>.
</para>
<para>
- The <filename>gstcontrol</filename> library needs to be initialised
+ The <filename>gstcontrol</filename> library needs to be initialized
when your application is run. This can be done after the the GStreamer
- library has been initialised.
+ library has been initialized.
</para>
<programlisting>
...
<title>Dynamic pipelines</title>
<para>
In this chapter we will see how you can create a dynamic pipeline. A
- dynamic pipeline is a pipeline that is updated or created while media
+ dynamic pipeline is a pipeline that is updated or created while data
is flowing through it. We will create a partial pipeline first and add
more elements while the pipeline is playing. Dynamic pipelines cause
all sorts of scheduling issues and will remain a topic of research for
a long time in GStreamer.
</para>
<para>
- We will show how to create an mpeg1 video player using dynamic pipelines.
+ We will show how to create an MPEG1 video player using dynamic pipelines.
As you have seen in the pad section, we can attach a signal to an element
when a pad is created. We will use this to create our MPEG1 player.
</para>
</para>
<para>
Below you see how we will visualize the element.
- We always draw a src pad to the right of the element.
+ We always draw a source pad to the right of the element.
</para>
<figure float="1" id="sec-element-srcimg">
<title>Visualisation of a source element</title>
</figure>
<para>
Source elements do not accept data, they only generate data. You can
- see this in the figure because it only has a src pad. A src pad can
- only generate data.
+ see this in the figure because it only has a source pad. A source
+ pad can only generate data.
</para>
</sect2>
<title>Filters and codecs</title>
<para>
Filter elements both have input and output pads. They operate on
- data they receive in their sink pads and produce data on their src
+ data they receive in their sink pads and produce data on their source
pads. For example, MPEG decoders and volume filters would fall into
this category.
</para>
</figure>
<para>
The above figure shows the visualisation of a filter element.
- This element has one sink (input) pad and one src (output) pad.
+ This element has one sink (input) pad and one source (output) pad.
Sink pads are drawn on the left of the element.
</para>
<figure float="1" id="sec-element-multifilterimg">
The small application we created in the previous chapter used the
concept of a factory to create the elements. In this chapter we will
show you how to use the factory concepts to create elements based
- on what they do instead of how they are called.
+ on what they do instead of what they are called.
</para>
<para>
<para>
While this mechanism is quite effective it also has some big problems:
The elements are created based on their name. Indeed, we create an
- element mad by explicitly stating the mad element's name. Our little
+ element, mad, by explicitly stating the mad element's name. Our little
program therefore always uses the mad decoder element to decode
- the MP3 audio stream, even if there are 3 other MP3 decoders in the
+ the MP3 audio stream, even if there are three other MP3 decoders in the
system. We will see how we can use a more general way to create an
MP3 decoder element.
</para>
<sect1>
<title>More on MIME Types</title>
<para>
- GStreamer uses MIME types to indentify the different types of data
+ GStreamer uses MIME types to identify the different types of data
that can be handled by the elements. They are the high level
mechanisms to make sure that everyone is talking about the right
kind of data.
</para>
<para>
- A MIME (Multipurpose Internet Mail Extension) types are a set of
- string that denote a certain type of data. examples include:
+ A MIME (Multipurpose Internet Mail Extension) type is a pair of
+ strings that denote a certain type of data. Examples include:
<itemizedlist>
<listitem>
<para>
</listitem>
<listitem>
<para>
- audio/mpeg : mpeg audio
+ audio/mpeg : MPEG audio
</para>
</listitem>
<listitem>
<para>
- video/mpeg : mpeg video
+ video/mpeg : MPEG video
</para>
</listitem>
</itemizedlist>
will see.
</para>
<para>
- As we have seen in the previous chapter, the MIME types are added
+ As we have seen in the previous chapter, MIME types are added
to the Capability structure of a pad.
</para>
</para>
<para>
There is also an association between a MIME type and a file extension,
- but the use of typefind functions (similar to file(1)) is preferred..
+ but the use of typefind functions (similar to file(1)) is preferred.
</para>
<para>
The type information is maintained in a list of
<para>
All operations on <classname>GstType</classname> occur
via their <classname>guint16 id</classname> numbers, with
- <classname>GstType</classname> structure private to the GStreamer
+ the <classname>GstType</classname> structure private to the GStreamer
library.
</para>
type = gst_type_find_by_id (id);
</programlisting>
<para>
- This function will return NULL if the id was associated with
+ This function will return NULL if the id was not associated with
any known <classname>GstType</classname>
</para>
</sect2>
</sect1>
<sect1>
- <title>creating elements with the factory</title>
+ <title>Creating elements with the factory</title>
<para>
In the previous section we described how you could obtain
an element factory using MIME types. One the factory has been
<partintro>
<para>
<application>GStreamer</application> comes prepackaged with a few
- programs. and some usefull debugging options.
+ programs, and some useful debugging options.
</para>
</partintro>
This chapter describes the most rudimentary aspects of a
<application>GStreamer</application> application, including initializing
the libraries, creating elements, packing them into a pipeline and playing,
- pause and stop the pipeline.
+ pausing and stopping the pipeline.
</para>
<sect1>
<para>
We will create a simple first application, a complete MP3 player, using
standard <application>GStreamer</application> components. The player
- will read from a file that is given as the first argument of the program.
+ will read from a file that is given as the first argument to the program.
</para>
<programlisting>
</programlisting>
<para>
- We are going to create 3 elements and one pipeline. Since all elements
- share the same base type, <classname>GstElement</classname>, we can
- define them as:
+ We are going to create three elements and one pipeline. Since all
+ elements share the same base type, <classname>GstElement</classname>,
+ we can define them as:
</para>
<programlisting>
...
<para>
Next, we are going to create an empty pipeline. As you have seen in
the basic introduction, this pipeline will hold and manage all the
- elements we are going to stuff into it.
+ elements we are going to pack into it.
</para>
<programlisting>
/* create a new pipeline to hold the elements */
pipeline = gst_pipeline_new ("pipeline");
</programlisting>
<para>
- We use the standard constructor for a pipeline: gst_pipeline_new ("name").
+ We use the standard constructor for a pipeline: gst_pipeline_new ().
</para>
<para>
<para>
Finally we create our audio sink element. This element will be able
- to playback the audio using OSS.
+ to play back the audio using OSS.
</para>
<programlisting>
/* and an audio sink */
</note>
<para>
- Since we do not use threads, nothing will happen yet. We manually have to
+ Since we do not use threads, nothing will happen yet. We have to
call gst_bin_iterate() to execute one iteration of the pipeline.
</para>
<programlisting>
</programlisting>
<para>
The gst_bin_iterate() function will return TRUE as long as something
- interesting happended inside the pipeline. When the end-of-file has been
+ interesting happened inside the pipeline. When the end-of-file has been
reached the _iterate function will return FALSE and we can end the loop.
</para>
<programlisting>
<title>Conclusion</title>
<para>
This concludes our first example. As you see, setting up a pipeline
- is very lowlevel but powerful. You will later in this manual how
- you can create a custom MP3 element with a more high level API.
+ is very low-level but powerful. You will see later in this manual how
+ you can create a custom MP3 element with a higher-level API.
</para>
<para>
It should be clear from the example that we can very easily replace the
- filesrc element with an httpsrc, giving you instant network streaming.
- An element could be build to handle icecast connections, for example.
+ filesrc element with an httpsrc element, giving you instant network
+ streaming. An element could be built to handle icecast connections,
+ for example.
</para>
<para>
We can also choose to use another type of sink instead of the audiosink.
<para>
We create a simple pipeline and write it to stdout with
- gst_xml_write_file (). The following code constructs an mp3 player
+ gst_xml_write_file (). The following code constructs an MP3 player
pipeline with two threads and then writes out the XML both to stdout
- and to a file. Use this program with one argument: the mp3 file on disk.
+ and to a file. Use this program with one argument: the MP3 file on disk.
</para>
<programlisting>
<para>
In addition to loading a file, you can also load a from a xmlDocPtr and
an in memory buffer using gst_xml_parse_doc and gst_xml_parse_memory
- respectivily. both of these methods return a gboolean indicating
+ respectively. Both of these methods return a gboolean indicating
success or failure of the requested action.
</para>
</sect1>
<para>
It is possible to add custom XML tags to the core XML created with
gst_xml_write. This feature can be used by an application to add more
- information to the save plugins. the editor will for example insert
+ information to the save plugins. The editor will for example insert
the position of the elements on the screen using the custom XML tags.
</para>
<para>
<para>
Whenever a new object has been loaded, the xml_loaded function will
- be called. this function looks like:
+ be called. This function looks like:
</para>
<programlisting>
static void
One of the the most obvious uses of GStreamer is using it to build
a media player. GStreamer already includes components for building a
media player that can support a very wide variety of formats, including
- mp3, Ogg Vorbis, MPEG1, MPEG2, AVI, Quicktime, mod and so on. GStreamer,
+ MP3, Ogg Vorbis, MPEG1, MPEG2, AVI, Quicktime, mod and so on. GStreamer,
however, is much more than just another media player. Its main advantages
are that the pluggable components can be mixed and matched into arbitrary
pipelines so that it's possible to write a full-fledged video or audio
One of the the most obvious uses of GStreamer is using it to build
a media player. GStreamer already includes components for building a
media player that can support a very wide variety of formats, including
- mp3, Ogg Vorbis, MPEG1, MPEG2, AVI, Quicktime, mod and so on. GStreamer,
+ MP3, Ogg Vorbis, MPEG1, MPEG2, AVI, Quicktime, mod and so on. GStreamer,
however, is much more than just another media player. Its main advantages
are that the pluggable components can be mixed and matched into arbitrary
pipelines so that it's possible to write a full-fledged video or audio
This function will get the pad named "src" from the given element.
</para>
<para>
- Alternatively, you can also request a GList of pads from the element. The
+ Alternatively, you can request a GList of pads from the element. The
following code example will print the names of all the pads of an
element.
</para>
</para>
<para>
gst_pad_get_direction (GstPad *pad) can be used to query if the pad
- is a sink or a src pad. Remember a src pad is a pad that can output
- data and a sink pad is one that accepts data.
+ is a sink or a source pad. Remember that a source pad is a pad that
+ can output data and a sink pad is one that accepts data.
</para>
<para>
You can get the parent of the pad, this is the element that this pad belongs to,
<title>Dynamic pads</title>
<para>
Some elements might not have their pads when they are created. This
- can, for example, happen with an MPEG2 system demuxer. The demuxer will
- create its pads at runtime when it detects the different elementary
- streams in the MPEG2 system stream.
+ can happen, for example, with an MPEG2 system demultiplexer. The
+ demultiplexer will create its pads at runtime when it detects the
+ different elementary streams in the MPEG2 system stream.
</para>
<para>
Running <application>gst-inspect mpegdemux</application> will show that
GstElement *pipeline;
GstElement *mpeg2parser;
- // create pipeline and do something usefull
+ // create pipeline and do something useful
...
mpeg2parser = gst_element_factory_make ("mpegdemux", "mpegdemux");
<title>Request pads</title>
<para>
An element can also have request pads. These pads are not created
- automatically but are only created on demand. This is very usefull
- for muxers, aggregators and tee elements.
+ automatically but are only created on demand. This is very useful
+ for multiplexers, aggregators and tee elements.
</para>
<para>
The tee element, for example, has one input pad and a request padtemplate for the
</para>
<para>
It is also possible to request a pad that is compatible with another
- padtemplate. This is very usefull if you want to connect an element to
- a muxer element and you need to request a pad that is compatible. The
- gst_element_get_compatible_pad is used to request a compatible pad,
- as is shown in the next example.
+ pad template. This is very useful if you want to connect an element
+ to a multiplexer element and you need to request a pad that is
+ compatible. The gst_element_get_compatible_pad is used to request
+ a compatible pad, as is shown in the next example.
</para>
<programlisting>
...
three properties: layer, bitrate and framed.
</para>
<para>
- The src pad (output pad) is called 'src' and outputs data of MIME
- type 'audio/raw'. It also has four properties: format, depth, rate
- and channels.
+ The source pad (output pad) is called 'src' and outputs data of
+ MIME type 'audio/raw'. It also has four properties: format, depth,
+ rate and channels.
</para>
<programlisting>
Pads:
<sect2 id="sec-pads-props">
<title>What are properties</title>
<para>
- Properties are used to describe extra information for the
- capabilities. The properties basically exist of a key (a string) and
+ Properties are used to describe extra information for
+ capabilities. The properties basically consist of a key (a string) and
a value. There are different possibile value types that can be used:
</para>
<listitem>
<para>
An integer range value. The property denotes a range of possible
- values. In the case of the mad element: the src pad has a property
- rate that can go from 11025 to 48000.
+ values. In the case of the mad element, the source pad has a
+ property rate that can go from 11025 to 48000.
</para>
</listitem>
<listitem>
<listitem>
<para>
a fourcc value: this is a value that is commonly used to describe an encoding for video,
- as used be the AVI specification.
+ as used by the AVI specification.
</para>
</listitem>
<listitem>
</sect2>
<sect2 id="sec-pads-caps-use">
- <title>What are the capabilities used for?</title>
+ <title>What capabilities are used for</title>
<para>
Capabilities describe in great detail the type of media that is handled by the pads.
They are mostly used for:
</programlisting>
</sect2>
<sect2 id="sec-pads-caps-create">
- <title>Creating capabilities structures</title>
+ <title>Creating capability structures</title>
<para>
- While the capabilities are mainly used inside the plugin to describe
- the media type of the pads, the application programmer also has
- to have basic understanding of caps in order to interface with the
+ While capabilities are mainly used inside a plugin to describe the
+ media type of the pads, the application programmer also has to have
+ basic understanding of capabilities in order to interface with the
plugins, specially when using the autopluggers.
</para>
<para>
As we said, a capability has a name, a mime-type and some
properties. The signature of the function to create a new
- <classname>GstCaps</classname> structure is like:
+ <classname>GstCaps</classname> structure is:
<programlisting>
GstCaps* gst_caps_new (const gchar *name, const gchar *mime, GstProps *props);
NULL
);
</programlisting>
- Optionally the convenient shortcut macro can be used. The above complex
+ Optionally, the convenient shortcut macro can be used. The above complex
capability can be created with:
<programlisting>
GstCaps *newcaps;
</para>
<para>
- You can also use the the parser in you own
+ You can also use the parser in you own
code. <application>GStreamer</application> provides a function
- gst_parse_launch () that you can use to construt a pipeline.
- The following programs lets you create an mp3 pipeline using the
+ gst_parse_launch () that you can use to construct a pipeline.
+ The following program lets you create an MP3 pipeline using the
gst_parse_launch () function:
</para>
<programlisting>
<para>
A bare identifier (a string beginning with a letter and containing
only letters, numbers, dashes, underscores, percent signs, or colons)
- will create an element from a given elementfactory. In this example,
- an instance of the "mad" mp3 decoding plugin will be created.
+ will create an element from a given element factory. In this example,
+ an instance of the "mad" MP3 decoding plugin will be created.
</para>
</sect3>
<sect3>
<para>
A buffer that is sinked to a Queue will not automatically be pushed to the
next connected element but will be buffered. It will be pushed to the next
- element as soon as a gst_pad_pull () is called on the queues srcpad.
+ element as soon as a gst_pad_pull () is called on the queue's source pad.
</para>
<para>
Queues are mostly used in conjunction with a <classname>GstThread</classname> to
</programlisting>
<para>
- The following mp3 player shows you how to create the above pipeline
+ The following MP3 player shows you how to create the above pipeline
using a thread and a queue.
</para>
<chapter id="cha-scheduler">
<title>Understanding schedulers</title>
<para>
- The scheduler is responsible for managing the plugins at runtime. The
- main responsabilities are:
+ The scheduler is responsible for managing the plugins at runtime. Its
+ main responsibilities are:
<itemizedlist>
<listitem>
<para>
</itemizedlist>
</para>
<para>
- The scheduler is a plugable component, this means that alternative
+ The scheduler is a plugable component; this means that alternative
schedulers can be written and plugged into GStreamer. The default scheduler
uses cothreads to schedule the plugins in a pipeline. Cothreads are fast
and lightweight user-space threads.
</para>
<para>
There is usually no need to interact with the scheduler directly, however
- it some cases it is feasable to set a specific clock or force a specific
+ in some cases it is feasible to set a specific clock or force a specific
plugin as the entry point in the pipeline.
</para>
<chapter id="cha-states">
<title>Element states</title>
<para>
- One you have created a pipeline packed with elements, nothing will
- happen yet. This is where the different states come into play.
+ Once you have created a pipeline packed with elements, nothing will happen
+ right away. This is where the different states come into play.
</para>
<sect1 id="sec-states">
<para>
All elements start with the NULL state. The elements will go throught
the following state changes: NULL -> READY -> PAUSED ->
- PLAYING. Remember when going from PLAYING to READY GStreamer will
+ PLAYING. Remember when going from PLAYING to READY, GStreamer will
internally go throught the intermediate states.
</para>
<para>
<note>
<para>
You can also go from the NULL to PLAYING state directly without
- going through the READY state. this is a shortcut, the framework
+ going through the READY state. This is a shortcut; the framework
will internally go through the READY and the PAUSED state for you.
</para>
</note>
<para>
The PAUSED state is available for temporarily freezing the pipeline.
Elements will typically not free their resources in the PAUSED state.
- Use the NULL state if you want to stop the data flow permanantly.
+ Use the NULL state if you want to stop the data flow permanently.
</para>
</note>
<para>
<para>
The above program will create a thread with two elements in it. As soon
as it is set to the PLAYING state, the thread will start to iterate
- itself. You never need to manually iterate a thread.
+ itself. You never need to explicitly iterate a thread.
</para>
<sect2>
between the threads, in a thread-safe fashion. This element is the
queue, described more fully in <xref linkend="cha-queues"/>. It doesn't
matter if the queue is placed in the containing bin or in the thread
- itself, but it needs to be present on one side of the other to enable
+ itself, but it needs to be present on one side or the other to enable
inter-thread communication.
</para>
</sect2>
<chapter id="cha-typedetection">
<title>Typedetection</title>
<para>
- Sometimes the capabilities of a pad are not specificied. The filesrc,
- for example, does not know what type of file it is reading. Before you
- can attach an element to the pad of the filesrc, you need to determine
+ Sometimes the capabilities of a pad are not specificied. The filesrc
+ element, for example, does not know what type of file it is reading. Before
+ you can attach an element to the pad of the filesrc, you need to determine
the media type in order to be able to choose a compatible element.
</para>
<para>
<para>
We create a very simple pipeline with only a filesrc and the typefind
element in it. The sinkpad of the typefind element has been connected
- to the src pad of the filesrc.
+ to the source pad of the filesrc.
</para>
<para>
We attached a signal 'have_type' to the typefind element which will be called
when the type of the media stream as been detected.
</para>
<para>
- the typefind function will loop over all the registered types and will
+ The typefind function will loop over all the registered types and will
execute each of the typefind functions. As soon as a function returns
a GstCaps pointer, the type_found function will be called:
</para>
XML representation of the caps structure to stdout.
</para>
<para>
- A more usefull option would be to use the registry to look up an element
+ A more useful option would be to use the registry to look up an element
that can handle this particular caps structure, or we can also use the
autoplugger to connect this caps structure to, for example, a videosink.
</para>
<para>
We create a simple pipeline and write it to stdout with
- gst_xml_write_file (). The following code constructs an mp3 player
+ gst_xml_write_file (). The following code constructs an MP3 player
pipeline with two threads and then writes out the XML both to stdout
- and to a file. Use this program with one argument: the mp3 file on disk.
+ and to a file. Use this program with one argument: the MP3 file on disk.
</para>
<programlisting>
<para>
In addition to loading a file, you can also load a from a xmlDocPtr and
an in memory buffer using gst_xml_parse_doc and gst_xml_parse_memory
- respectivily. both of these methods return a gboolean indicating
+ respectively. Both of these methods return a gboolean indicating
success or failure of the requested action.
</para>
</sect1>
<para>
It is possible to add custom XML tags to the core XML created with
gst_xml_write. This feature can be used by an application to add more
- information to the save plugins. the editor will for example insert
+ information to the save plugins. The editor will for example insert
the position of the elements on the screen using the custom XML tags.
</para>
<para>
<para>
Whenever a new object has been loaded, the xml_loaded function will
- be called. this function looks like:
+ be called. This function looks like:
</para>
<programlisting>
static void
projects / platform / upstream / gstreamer.git / commitdiff