mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-06 01:19:38 +00:00
541 lines
23 KiB
Text
541 lines
23 KiB
Text
Overview
|
|
--------
|
|
|
|
This part gives an overview of the design of GStreamer with references to
|
|
the more detailed explanations of the different topics.
|
|
|
|
This document is intented for people that want to have a global overview of
|
|
the inner workings of GStreamer.
|
|
|
|
|
|
Introduction
|
|
~~~~~~~~~~~~
|
|
|
|
GStreamer is a set of libraries and plugins that can be used to implement various
|
|
multimedia applications ranging from desktop players, audio/video recorders,
|
|
multimedia servers, transcoders, etc.
|
|
|
|
Applications are built by constructing a pipeline composed of elements. An element
|
|
is an object that performs some action on a multimedia stream such as:
|
|
|
|
- read a file
|
|
- decode or encode between formats
|
|
- capture from a hardware device
|
|
- render to a hardware device
|
|
- mix or multiplex multiple streams
|
|
|
|
Elements have input and output pads called sink and source pads in GStreamer. An
|
|
application links elements together on pads to construct a pipeline. Below is
|
|
an example of an ogg/vorbis playback pipeline.
|
|
|
|
+-----------------------------------------------------------+
|
|
| ----------> downstream -------------------> |
|
|
| |
|
|
| pipeline |
|
|
| +---------+ +----------+ +-----------+ +----------+ |
|
|
| | filesrc | | oggdemux | | vorbisdec | | alsasink | |
|
|
| | src-sink src-sink src-sink | |
|
|
| +---------+ +----------+ +-----------+ +----------+ |
|
|
| |
|
|
| <---------< upstream <-------------------< |
|
|
+-----------------------------------------------------------+
|
|
|
|
The filesrc element reads data from a file on disk. The oggdemux element parses
|
|
the data and sends the compressed audio data to the vorbisdec element. The
|
|
vorbisdec element decodes the compressed data and sends it to the alsasink
|
|
element. The alsasink element sends the samples to the audio card for playback.
|
|
|
|
Downstream and upstream are the terms used to describe the direction in the
|
|
Pipeline. From source to sink is called "downstream" and "upstream" is
|
|
from sink to source. Dataflow always happens downstream.
|
|
|
|
The task of the application is to construct a pipeline as above using existing
|
|
elements. This is further explained in the pipeline building topic.
|
|
|
|
The application does not have to manage any of the complexities of the
|
|
actual dataflow/decoding/conversions/synchronisation etc. but only calls high
|
|
level functions on the pipeline object such as PLAY/PAUSE/STOP.
|
|
|
|
The application also receives messages and notifications from the pipeline such
|
|
as metadata, warning, error and EOS messages.
|
|
|
|
If the application needs more control over the graph it is possible to directly
|
|
access the elements and pads in the pipeline.
|
|
|
|
|
|
Design overview
|
|
~~~~~~~~~~~~~~~
|
|
|
|
GStreamer design goals include:
|
|
|
|
- Process large amounts of data quickly
|
|
- Allow fully multithreaded processing
|
|
- Ability to deal with multiple formats
|
|
- Synchronize different dataflows
|
|
- Ability to deal with multiple devices
|
|
|
|
The capabilities presented to the application depends on the number of elements
|
|
installed on the system and their functionality.
|
|
|
|
The GStreamer core is designed to be media agnostic but provides many features
|
|
to elements to describe media formats.
|
|
|
|
|
|
Elements
|
|
~~~~~~~~
|
|
|
|
The smallest building blocks in a pipeline are elements. An element provides a
|
|
number of pads which can be source or sinkpads. Sourcepads provide data and
|
|
sinkpads consume data. Below is an example of an ogg demuxer element that has
|
|
one pad that takes (sinks) data and two source pads that produce data.
|
|
|
|
+-----------+
|
|
| oggdemux |
|
|
| src0
|
|
sink src1
|
|
+-----------+
|
|
|
|
An element can be in four different states: NULL, READY, PAUSED, PLAYING. In the
|
|
NULL and READY state, the element is not processing any data. In the PLAYING state
|
|
it is processing data. The intermediate PAUSED state is used to preroll data in
|
|
the pipeline. A state change can be performed with gst_element_set_state().
|
|
|
|
An element always goes through all the intermediate state changes. This means that
|
|
when en element is in the READY state and is put to PLAYING, it will first go
|
|
through the intermediate PAUSED state.
|
|
|
|
An element state change to PAUSED will activate the pads of the element. First the
|
|
source pads are activated, then the sinkpads. When the pads are activated, the
|
|
pad activate function is called. Some pads will start a thread (GstTask) or some
|
|
other mechanism to start producing or consuming data.
|
|
|
|
The PAUSED state is special as it is used to preroll data in the pipeline. The purpose
|
|
is to fill all connected elements in the pipeline with data so that the subsequent
|
|
PLAYING state change happens very quickly. Some elements will therefore not complete
|
|
the state change to PAUSED before they have received enough data. Sink elements are
|
|
required to only complete the state change to PAUSED after receiving the first data.
|
|
|
|
Normally the state changes of elements are coordinated by the pipeline as explained
|
|
in [part-states.txt].
|
|
|
|
Different categories of elements exist:
|
|
|
|
- source elements, these are elements that do not consume data but only provide data
|
|
for the pipeline.
|
|
- sink elements, these are elements that do not produce data but renders data to
|
|
an output device.
|
|
- transform elements, these elements transform an input stream in a certain format
|
|
into a stream of another format. Encoder/decoder/converters are examples.
|
|
- demuxer elements, these elements parse a stream and produce several output streams.
|
|
- mixer/muxer elements, combine several input streams into one output stream.
|
|
|
|
Other categories of elements can be constructed (see part-klass.txt).
|
|
|
|
|
|
Bins
|
|
~~~~
|
|
|
|
A bin is an element subclass and acts as a container for other elements so that multiple
|
|
elements can be combined into one element.
|
|
|
|
A bin coordinates its children's state changes as explained later. It also distributes
|
|
events and various other functionality to elements.
|
|
|
|
A bin can have its own source and sinkpads by ghostpadding one or more of its children's
|
|
pads to itself.
|
|
|
|
Below is a picture of a bin with two elements. The sinkpad of one element is ghostpadded
|
|
to the bin.
|
|
|
|
+---------------------------+
|
|
| bin |
|
|
| +--------+ +-------+ |
|
|
| | | | | |
|
|
| /sink src-sink | |
|
|
sink +--------+ +-------+ |
|
|
+---------------------------+
|
|
|
|
|
|
Pipeline
|
|
~~~~~~~~
|
|
|
|
A pipeline is a special bin subclass that provides the following features to its
|
|
children:
|
|
|
|
- Select and manage a global clock for all its children.
|
|
- Manage running_time based on the selected clock. Running_time is the elapsed
|
|
time the pipeline spent in the PLAYING state and is used for
|
|
synchronisation.
|
|
- Manage latency in the pipeline.
|
|
- Provide means for elements to comunicate with the application by the GstBus.
|
|
- Manage the global state of the elements such as Errors and end-of-stream.
|
|
|
|
Normally the application creates one pipeline that will manage all the elements
|
|
in the application.
|
|
|
|
|
|
Dataflow and buffers
|
|
~~~~~~~~~~~~~~~~~~~~
|
|
|
|
GStreamer supports two possible types of dataflow, the push and pull model. In the
|
|
push model, an upstream element sends data to a downstream element by calling a
|
|
method on a sinkpad. In the pull model, a downstream element requests data from
|
|
an upstream element by calling a method on a source pad.
|
|
|
|
The most common dataflow is the push model. The pull model can be used in specific
|
|
circumstances by demuxer elements. The pull model can also be used by low latency
|
|
audio applications.
|
|
|
|
The data passed between pads is encapsulated in Buffers. The buffer contains
|
|
pointers to the actual memory and also metadata describing the memory. This metadata
|
|
includes:
|
|
|
|
- timestamp of the data, this is the time instance at which the data was captured
|
|
or the time at which the data should be played back.
|
|
- offset of the data: a media specific offset, this could be samples for audio or
|
|
frames for video.
|
|
- the duration of the data in time.
|
|
- additional flags describing special properties of the data such as
|
|
discontinuities or delta units.
|
|
- additional arbitrary metadata
|
|
|
|
When an element whishes to send a buffer to another element is does this using one
|
|
of the pads that is linked to a pad of the other element. In the push model, a
|
|
buffer is pushed to the peer pad with gst_pad_push(). In the pull model, a buffer
|
|
is pulled from the peer with the gst_pad_pull_range() function.
|
|
|
|
Before an element pushes out a buffer, it should make sure that the peer element
|
|
can understand the buffer contents. It does this by querying the peer element
|
|
for the supported formats and by selecting a suitable common format. The selected
|
|
format is then first sent to the peer element with a CAPS event before pushing
|
|
the buffer (see part-negotiation.txt).
|
|
|
|
When an element pad receives a CAPS event, it has to check if it understand the
|
|
media type. The element must refuse following buffers if the media type
|
|
preceding it was not accepted.
|
|
|
|
Both gst_pad_push() and gst_pad_pull_range() have a return value indicating whether
|
|
the operation succeeded. An error code means that no more data should be sent
|
|
to that pad. A source element that initiates the data flow in a thread typically
|
|
pauses the producing thread when this happens.
|
|
|
|
A buffer can be created with gst_buffer_new() or by requesting a usable buffer
|
|
from a buffer pool using gst_buffer_pool_acquire_buffer(). Using the second
|
|
method, it is possible for the peer element to implement a custom buffer
|
|
allocation algorithm.
|
|
|
|
The process of selecting a media type is called caps negotiation.
|
|
|
|
|
|
Caps
|
|
~~~~
|
|
|
|
A media type (Caps) is described using a generic list of key/value pairs. The key is
|
|
a string and the value can be a single/list/range of int/float/string.
|
|
|
|
Caps that have no ranges/list or other variable parts are said to be fixed and
|
|
can be used to put on a buffer.
|
|
|
|
Caps with variables in them are used to describe possible media types that can be
|
|
handled by a pad.
|
|
|
|
|
|
Dataflow and events
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
Parallel to the dataflow is a flow of events. Unlike the buffers, events can pass
|
|
both upstream and downstream. Some events only travel upstream others only downstream.
|
|
|
|
The events are used to denote special conditions in the dataflow such as EOS or
|
|
to inform plugins of special events such as flushing or seeking.
|
|
|
|
Some events must be serialized with the buffer flow, others don't. Serialized
|
|
events are inserted between the buffers. Non serialized events jump in front
|
|
of any buffers current being processed.
|
|
|
|
An example of a serialized event is a TAG event that is inserted between buffers
|
|
to mark metadata for those buffers.
|
|
|
|
An example of a non serialized event is the FLUSH event.
|
|
|
|
|
|
Pipeline construction
|
|
~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The application starts by creating a Pipeline element using gst_pipeline_new ().
|
|
Elements are added to and removed from the pipeline with gst_bin_add() and
|
|
gst_bin_remove().
|
|
|
|
After adding the elements, the pads of an element can be retrieved with
|
|
gst_element_get_pad(). Pads can then be linked together with gst_pad_link().
|
|
|
|
Some elements create new pads when actual dataflow is happening in the pipeline.
|
|
With g_signal_connect() one can receive a notification when an element has created
|
|
a pad. These new pads can then be linked to other unlinked pads.
|
|
|
|
Some elements cannot be linked together because they operate on different
|
|
incompatible data types. The possible datatypes a pad can provide or consume can
|
|
be retrieved with gst_pad_get_caps().
|
|
|
|
Below is a simple mp3 playback pipeline that we constructed. We will use this
|
|
pipeline in further examples.
|
|
|
|
+-------------------------------------------+
|
|
| pipeline |
|
|
| +---------+ +----------+ +----------+ |
|
|
| | filesrc | | mp3dec | | alsasink | |
|
|
| | src-sink src-sink | |
|
|
| +---------+ +----------+ +----------+ |
|
|
+-------------------------------------------+
|
|
|
|
|
|
Pipeline clock
|
|
~~~~~~~~~~~~~~
|
|
|
|
One of the important functions of the pipeline is to select a global clock
|
|
for all the elements in the pipeline.
|
|
|
|
The purpose of the clock is to provide a stricly increasing value at the rate
|
|
of one GST_SECOND per second. Clock values are expressed in nanoseconds.
|
|
Elements use the clock time to synchronize the playback of data.
|
|
|
|
Before the pipeline is set to PLAYING, the pipeline asks each element if they can
|
|
provide a clock. The clock is selected in the following order:
|
|
|
|
- If the application selected a clock, use that one.
|
|
- If a source element provides a clock, use that clock.
|
|
- Select a clock from any other element that provides a clock, start with the
|
|
sinks.
|
|
- If no element provides a clock a default system clock is used for the pipeline.
|
|
|
|
In a typical playback pipeline this algorithm will select the clock provided by
|
|
a sink element such as an audio sink.
|
|
|
|
In capture pipelines, this will typically select the clock of the data producer, which
|
|
in most cases can not control the rate at which it produces data.
|
|
|
|
|
|
Pipeline states
|
|
~~~~~~~~~~~~~~~
|
|
|
|
When all the pads are linked and signals have been connected, the pipeline can
|
|
be put in the PAUSED state to start dataflow.
|
|
|
|
When a bin (and hence a pipeline) performs a state change, it will change the state
|
|
of all its children. The pipeline will change the state of its children from the
|
|
sink elements to the source elements, this to make sure that no upstream element
|
|
produces data to an element that is not yet ready to accept it.
|
|
|
|
In the mp3 playback pipeline, the state of the elements is changed in the order
|
|
alsasink, mp3dec, filesrc.
|
|
|
|
All intermediate states are traversed for each element resulting in the following
|
|
chain of state changes:
|
|
|
|
alsasink to READY: the audio device is probed
|
|
mp3dec to READY: nothing happens.
|
|
filesrc to READY: the file is probed
|
|
alsasink to PAUSED: the audio device is opened. alsasink is a sink and returns
|
|
ASYNC because it did not receive data yet.
|
|
mp3dec to PAUSED: the decoding library is initialized
|
|
filesrc to PAUSED: the file is opened and a thread is started to push data to
|
|
mp3dec
|
|
|
|
At this point data flows from filesrc to mp3dec and alsasink. Since mp3dec is PAUSED,
|
|
it accepts the data from filesrc on the sinkpad and starts decoding the compressed
|
|
data to raw audio samples.
|
|
|
|
The mp3 decoder figures out the samplerate, the number of channels and other audio
|
|
properties of the raw audio samples and sends out a caps event with the media type.
|
|
|
|
Alsasink then receives the caps event, inspects the caps and reconfigures
|
|
itself to process the media type.
|
|
|
|
mp3dec then puts the decoded samples into a Buffer and pushes this buffer to the next
|
|
element.
|
|
|
|
Alsasink receives the buffer with samples. Since it received the first buffer of
|
|
samples, it completes the state change to the PAUSED state. At this point the
|
|
pipeline is prerolled and all elements have samples. Alsasink is now also
|
|
capable of providing a clock to the pipeline.
|
|
|
|
Since alsasink is now in the PAUSED state it blocks while receiving the first buffer. This
|
|
effectively blocks both mp3dec and filesrc in their gst_pad_push().
|
|
|
|
Since all elements now return SUCCESS from the gst_element_get_state() function,
|
|
the pipeline can be put in the PLAYING state.
|
|
|
|
Before going to PLAYING, the pipeline select a clock and samples the current time of
|
|
the clock. This is the base_time. It then distributes this time to all elements.
|
|
Elements can then synchronize against the clock using the buffer running_time +
|
|
base_time (See also part-synchronisation.txt).
|
|
|
|
The following chain of state changes then takes place:
|
|
|
|
alsasink to PLAYING: the samples are played to the audio device
|
|
mp3dec to PLAYING: nothing happens
|
|
filesrc to PLAYING: nothing happens
|
|
|
|
|
|
Pipeline status
|
|
~~~~~~~~~~~~~~~
|
|
|
|
The pipeline informs the application of any special events that occur in the
|
|
pipeline with the bus. The bus is an object that the pipeline provides and that
|
|
can be retrieved with gst_pipeline_get_bus().
|
|
|
|
The bus can be polled or added to the glib mainloop.
|
|
|
|
The bus is distributed to all elements added to the pipeline. The elements use the bus
|
|
to post messages on. Various message types exist such as ERRORS, WARNINGS, EOS,
|
|
STATE_CHANGED, etc..
|
|
|
|
The pipeline handles EOS messages received from elements in a special way. It will
|
|
only forward the message to the application when all sink elements have posted an
|
|
EOS message.
|
|
|
|
Other methods for obtaining the pipeline status include the Query functionality that
|
|
can be performed with gst_element_query() on the pipeline. This type of query
|
|
is useful for obtaining information about the current position and total time of
|
|
the pipeline. It can also be used to query for the supported seeking formats and
|
|
ranges.
|
|
|
|
|
|
Pipeline EOS
|
|
~~~~~~~~~~~~
|
|
|
|
When the source filter encounters the end of the stream, it sends an EOS event to
|
|
the peer element. This event will then travel downstream to all of the connected
|
|
elements to inform them of the EOS. The element is not supposed to accept any more
|
|
data after receiving an EOS event on a sinkpad.
|
|
|
|
The element providing the streaming thread stops sending data after sending the
|
|
EOS event.
|
|
|
|
The EOS event will eventually arrive in the sink element. The sink will then post
|
|
an EOS message on the bus to inform the pipeline that a particular stream has
|
|
finished. When all sinks have reported EOS, the pipeline forwards the EOS message
|
|
to the application. The EOS message is only forwarded to the application in the
|
|
PLAYING state.
|
|
|
|
When in EOS, the pipeline remains in the PLAYING state, it is the applications
|
|
responsability to PAUSE or READY the pipeline. The application can also issue
|
|
a seek, for example.
|
|
|
|
|
|
Pipeline READY
|
|
~~~~~~~~~~~~~~
|
|
|
|
When a running pipeline is set from the PLAYING to READY state, the following
|
|
actions occur in the pipeline:
|
|
|
|
alsasink to PAUSED: alsasink blocks and completes the state change on the
|
|
next sample. If the element was EOS, it does not wait for
|
|
a sample to complete the state change.
|
|
mp3dec to PAUSED: nothing
|
|
filesrc to PAUSED: nothing
|
|
|
|
Going to the intermediate PAUSED state will block all elements in the _push()
|
|
functions. This happens because the sink element blocks on the first buffer
|
|
it receives.
|
|
|
|
Some elements might be performing blocking operations in the PLAYING state that
|
|
must be unblocked when they go into the PAUSED state. This makes sure that the
|
|
state change happens very fast.
|
|
|
|
In the next PAUSED to READY state change the pipeline has to shut down and all
|
|
streaming threads must stop sending data. This happens in the following sequence:
|
|
|
|
alsasink to READY: alsasink unblocks from the _chain() function and returns a
|
|
FLUSHING return value to the peer element. The sinkpad is
|
|
deactivated and becomes unusable for sending more data.
|
|
mp3dec to READY: the pads are deactivated and the state change completes when
|
|
mp3dec leaves its _chain() function.
|
|
filesrc to READY: the pads are deactivated and the thread is paused.
|
|
|
|
The upstream elements finish their chain() function because the downstream element
|
|
returned an error code (FLUSHING) from the _push() functions. These error codes
|
|
are eventually returned to the element that started the streaming thread (filesrc),
|
|
which pauses the thread and completes the state change.
|
|
|
|
This sequence of events ensure that all elements are unblocked and all streaming
|
|
threads stopped.
|
|
|
|
|
|
Pipeline seeking
|
|
~~~~~~~~~~~~~~~~
|
|
|
|
Seeking in the pipeline requires a very specific order of operations to make
|
|
sure that the elements remain synchronized and that the seek is performed with
|
|
a minimal amount of latency.
|
|
|
|
An application issues a seek event on the pipeline using gst_element_send_event()
|
|
on the pipeline element. The event can be a seek event in any of the formats
|
|
supported by the elements.
|
|
|
|
The pipeline first pauses the pipeline to speed up the seek operations.
|
|
|
|
The pipeline then issues the seek event to all sink elements. The sink then forwards
|
|
the seek event upstream until some element can perform the seek operation, which is
|
|
typically the source or demuxer element. All intermediate elements can transform the
|
|
requested seek offset to another format, this way a decoder element can transform a
|
|
seek to a frame number to a timestamp, for example.
|
|
|
|
When the seek event reaches an element that will perform the seek operation, that
|
|
element performs the following steps.
|
|
|
|
1) send a FLUSH_START event to all downstream and upstream peer elements.
|
|
2) make sure the streaming thread is not running. The streaming thread will
|
|
always stop because of step 1).
|
|
3) perform the seek operation
|
|
4) send a FLUSH done event to all downstream and upstream peer elements.
|
|
5) send SEGMENT event to inform all elements of the new position and to complete
|
|
the seek.
|
|
|
|
In step 1) all downstream elements have to return from any blocking operations
|
|
and have to refuse any further buffers or events different from a FLUSH done.
|
|
|
|
The first step ensures that the streaming thread eventually unblocks and that
|
|
step 2) can be performed. At this point, dataflow is completely stopped in the
|
|
pipeline.
|
|
|
|
In step 3) the element performs the seek to the requested position.
|
|
|
|
In step 4) all peer elements are allowed to accept data again and streaming
|
|
can continue from the new position. A FLUSH done event is sent to all the peer
|
|
elements so that they accept new data again and restart their streaming threads.
|
|
|
|
Step 5) informs all elements of the new position in the stream. After that the
|
|
event function returns back to the application. and the streaming threads start
|
|
to produce new data.
|
|
|
|
Since the pipeline is still PAUSED, this will preroll the next media sample in the
|
|
sinks. The application can wait for this preroll to complete by performing a
|
|
_get_state() on the pipeline.
|
|
|
|
The last step in the seek operation is then to adjust the stream running_time of
|
|
the pipeline to 0 and to set the pipeline back to PLAYING.
|
|
|
|
The sequence of events in our mp3 playback example.
|
|
|
|
| a) seek on pipeline
|
|
| b) PAUSE pipeline
|
|
+----------------------------------V--------+
|
|
| pipeline | c) seek on sink
|
|
| +---------+ +----------+ +---V------+ |
|
|
| | filesrc | | mp3dec | | alsasink | |
|
|
| | src-sink src-sink | |
|
|
| +---------+ +----------+ +----|-----+ |
|
|
+-----------------------------------|-------+
|
|
<------------------------+
|
|
d) seek travels upstream
|
|
|
|
--------------------------> 1) FLUSH event
|
|
| 2) stop streaming
|
|
| 3) perform seek
|
|
--------------------------> 4) FLUSH done event
|
|
--------------------------> 5) SEGMENT event
|
|
|
|
| e) update running_time to 0
|
|
| f) PLAY pipeline
|
|
|