This chapter of the guide introduces the basic concepts of GStreamer. Understanding these concepts will help you grok the issues involved in extending GStreamer. Many of these concepts are explained in greater detail in the GStreamer Application Development Manual; the basic concepts presented here serve mainly to refresh your memory.
Elements and Plugins
Elements are at the core of GStreamer. In the context of plugin
development, an element is an object derived from the
GstElement class. Elements
provide some sort of functionality when linked with other elements: For
example, a source element provides data to a stream, and a filter
element acts on the data in a stream. Without elements, GStreamer is
just a bunch of conceptual pipe fittings with nothing to link. A large
number of elements ship with GStreamer, but extra elements can also be
Just writing a new element is not entirely enough, however: You will need to encapsulate your element in a plugin to enable GStreamer to use it. A plugin is essentially a loadable block of code, usually called a shared object file or a dynamically linked library. A single plugin may contain the implementation of several elements, or just a single one. For simplicity, this guide concentrates primarily on plugins containing one element.
A filter is an important type of element that processes a stream of data. Producers and consumers of data are called source and sink elements, respectively. Bin elements contain other elements. One type of bin is responsible for synchronization of the elements that they contain so that data flows smoothly. Another type of bin, called autoplugger elements, automatically add other elements to the bin and links them together so that they act as a filter between two arbitrary stream types.
The plugin mechanism is used everywhere in GStreamer, even if only the standard packages are being used. A few very basic functions reside in the core library, and all others are implemented in plugins. A plugin registry is used to store the details of the plugins in a binary registry file. This way, a program using GStreamer does not have to load all plugins to determine which are needed. Plugins are only loaded when their provided elements are requested.
Pads are used to negotiate links and data flow between elements in GStreamer. A pad can be viewed as a “place” or “port” on an element where links may be made with other elements, and through which data can flow to or from those elements. Pads have specific data handling capabilities: A pad can restrict the type of data that flows through it. Links are only allowed between two pads when the allowed data types of the two pads are compatible.
An analogy may be helpful here. A pad is similar to a plug or jack on a physical device. Consider, for example, a home theater system consisting of an amplifier, a DVD player, and a (silent) video projector. Linking the DVD player to the amplifier is allowed because both devices have audio jacks, and linking the projector to the DVD player is allowed because both devices have compatible video jacks. Links between the projector and the amplifier may not be made because the projector and amplifier have different types of jacks. Pads in GStreamer serve the same purpose as the jacks in the home theater system.
For the most part, all data in GStreamer flows one way through a link between elements. Data flows out of one element through one or more source pads, and elements accept incoming data through one or more sink pads. Source and sink elements have only source and sink pads, respectively.
See the GStreamer Library Reference for the current implementation
details of a
GstMiniObject, Buffers and Events
All streams of data in GStreamer are chopped up into chunks that are passed from a source pad on one element to a sink pad on another element. GstMiniObject is the structure used to hold these chunks of data.
GstMiniObject contains the following important types:
An exact type indicating what type of data (event, buffer, ...) this GstMiniObject is.
A reference count indicating the number of elements currently holding a reference to the miniobject. When the reference count falls to zero, the miniobject will be disposed, and its memory will be freed in some sense (see below for more details).
For data transport, there are two types of GstMiniObject defined: events (control) and buffers (content).
Buffers may contain any sort of data that the two linked pads know how to handle. Normally, a buffer contains a chunk of some sort of audio or video data that flows from one element to another.
Buffers also contain metadata describing the buffer's contents. Some of the important types of metadata are:
Pointers to one or more GstMemory objects. GstMemory objects are refcounted objects that encapsulate a region of memory.
A timestamp indicating the preferred display timestamp of the content in the buffer.
Events contain information on the state of the stream flowing between the two linked pads. Events will only be sent if the element explicitly supports them, else the core will (try to) handle the events automatically. Events are used to indicate, for example, a media type, the end of a media stream or that the cache should be flushed.
Events may contain several of the following items:
A subtype indicating the type of the contained event.
The other contents of the event depend on the specific event type.
Events will be discussed extensively in Events: Seeking, Navigation and More. Until then, the only event that will be used is the EOS event, which is used to indicate the end-of-stream (usually end-of-file).
Buffers are able to store chunks of memory of several different types. The most generic type of buffer contains memory allocated by malloc(). Such buffers, although convenient, are not always very fast, since data often needs to be specifically copied into the buffer.
Many specialized elements create buffers that point to special memory. For example, the filesrc element usually maps a file into the address space of the application (using mmap()), and creates buffers that point into that address range. These buffers created by filesrc act exactly like generic buffers, except that they are read-only. The buffer freeing code automatically determines the correct method of freeing the underlying memory. Downstream elements that receive these kinds of buffers do not need to do anything special to handle or unreference it.
Another way an element might get specialized buffers is to request them from a downstream peer through a GstBufferPool or GstAllocator. Elements can ask a GstBufferPool or GstAllocator from the downstream peer element. If downstream is able to provide these objects, upstream can use them to allocate buffers. See more in Memory allocation.
Many sink elements have accelerated methods for copying data to hardware, or have direct access to hardware. It is common for these elements to be able to create a GstBufferPool or GstAllocator for their upstream peers. One such example is ximagesink. It creates buffers that contain XImages. Thus, when an upstream peer copies data into the buffer, it is copying directly into the XImage, enabling ximagesink to draw the image directly to the screen instead of having to copy data into an XImage first.
Filter elements often have the opportunity to either work on a buffer in-place, or work while copying from a source buffer to a destination buffer. It is optimal to implement both algorithms, since the GStreamer framework can choose the fastest algorithm as appropriate. Naturally, this only makes sense for strict filters -- elements that have exactly the same format on source and sink pads.
Media types and Properties
GStreamer uses a type system to ensure that the data passed between elements is in a recognized format. The type system is also important for ensuring that the parameters required to fully specify a format match up correctly when linking pads between elements. Each link that is made between elements has a specified type and optionally a set of properties. See more about caps negotiation in Caps negotiation.
The Basic Types
GStreamer already supports many basic media types. Following is a table of a few of the basic types used for buffers in GStreamer. The table contains the name ("media type") and a description of the type, the properties associated with the type, and the meaning of each property. A full list of supported types is included in List of Defined Types.
|Media Type||Description||Property||Property Type||Property Values||Property Description|
|audio/*||All audio types||rate||integer||greater than 0||The sample rate of the data, in samples (per channel) per second.|
|channels||integer||greater than 0||The number of channels of audio data.|
|audio/x-raw||Unstructured and uncompressed raw integer audio data.||format||string||S8 U8 S16LE S16BE U16LE U16BE S24_32LE S24_32BE U24_32LE U24_32BE S32LE S32BE U32LE U32BE S24LE S24BE U24LE U24BE S20LE S20BE U20LE U20BE S18LE S18BE U18LE U18BE F32LE F32BE F64LE F64BE||The format of the sample data.|
|audio/mpeg||Audio data compressed using the MPEG audio encoding scheme.||mpegversion||integer||1, 2 or 4||The MPEG-version used for encoding the data. The value 1 refers to MPEG-1, -2 and -2.5 layer 1, 2 or 3. The values 2 and 4 refer to the MPEG-AAC audio encoding schemes.|
|framed||boolean||0 or 1||A true value indicates that each buffer contains exactly one frame. A false value indicates that frames and buffers do not necessarily match up.|
|layer||integer||1, 2, or 3||The compression scheme layer used to compress the data (only if mpegversion=1).|
|bitrate||integer||greater than 0||The bitrate, in bits per second. For VBR (variable bitrate) MPEG data, this is the average bitrate.|
|audio/x-vorbis||Vorbis audio data||There are currently no specific properties defined for this type.|
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