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108 lines
5.2 KiB
Markdown
108 lines
5.2 KiB
Markdown
---
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title: Foundations
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...
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# Foundations
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This chapter of the guide introduces the basic concepts of GStreamer.
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Understanding these concepts will be important in reading any of the
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rest of this guide, all of them assume understanding of these basic
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concepts.
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## Elements
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An *element* is the most important class of objects in GStreamer. You
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will usually create a chain of elements linked together and let data
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flow through this chain of elements. An element has one specific
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function, which can be the reading of data from a file, decoding of this
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data or outputting this data to your sound card (or anything else). By
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chaining together several such elements, you create a *pipeline* that
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can do a specific task, for example media playback or capture. GStreamer
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ships with a large collection of elements by default, making the
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development of a large variety of media applications possible. If
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needed, you can also write new elements. That topic is explained in
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great deal in the *GStreamer Plugin Writer's Guide*.
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## Pads
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*Pads* are element's input and output, where you can connect other
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elements. They are used to negotiate links and data flow between
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elements in GStreamer. A pad can be viewed as a “plug” or “port” on an
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element where links may be made with other elements, and through which
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data can flow to or from those elements. Pads have specific data
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handling capabilities: a pad can restrict the type of data that flows
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through it. Links are only allowed between two pads when the allowed
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data types of the two pads are compatible. Data types are negotiated
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between pads using a process called *caps negotiation*. Data types are
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described as a `GstCaps`.
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An analogy may be helpful here. A pad is similar to a plug or jack on a
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physical device. Consider, for example, a home theater system consisting
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of an amplifier, a DVD player, and a (silent) video projector. Linking
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the DVD player to the amplifier is allowed because both devices have
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audio jacks, and linking the projector to the DVD player is allowed
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because both devices have compatible video jacks. Links between the
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projector and the amplifier may not be made because the projector and
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amplifier have different types of jacks. Pads in GStreamer serve the
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same purpose as the jacks in the home theater system.
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For the most part, all data in GStreamer flows one way through a link
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between elements. Data flows out of one element through one or more
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*source pads*, and elements accept incoming data through one or more
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*sink pads*. Source and sink elements have only source and sink pads,
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respectively. Data usually means buffers (described by the
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[`GstBuffer`](http://gstreamer.freedesktop.org/data/doc/gstreamer/stable/gstreamer/html/gstreamer-GstBuffer.html)
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object) and events (described by the
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[`GstEvent`](http://gstreamer.freedesktop.org/data/doc/gstreamer/stable/gstreamer/html/gstreamer-GstEvent.html)
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object).
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## Bins and pipelines
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A *bin* is a container for a collection of elements. Since bins are
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subclasses of elements themselves, you can mostly control a bin as if it
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were an element, thereby abstracting away a lot of complexity for your
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application. You can, for example change state on all elements in a bin
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by changing the state of that bin itself. Bins also forward bus messages
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from their contained children (such as error messages, tag messages or
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`EOS` messages).
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A *pipeline* is a top-level bin. It provides a bus for the application
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and manages the synchronization for its children. As you set it to
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`PAUSED` or `PLAYING` state, data flow will start and media processing will
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take place. Once started, pipelines will run in a separate thread until
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you stop them or the end of the data stream is reached.
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## Communication
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GStreamer provides several mechanisms for communication and data
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exchange between the *application* and the *pipeline*.
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- *buffers* are objects for passing streaming data between elements in
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the pipeline. Buffers always travel from sources to sinks
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(downstream).
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- *events* are objects sent between elements or from the application
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to elements. Events can travel upstream and downstream. Downstream
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events can be synchronised to the data flow.
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- *messages* are objects posted by elements on the pipeline's message
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bus, where they will be held for collection by the application.
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Messages can be intercepted synchronously from the streaming thread
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context of the element posting the message, but are usually handled
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asynchronously by the application from the application's main
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thread. Messages are used to transmit information such as errors,
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tags, state changes, buffering state, redirects etc. from elements
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to the application in a thread-safe way.
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- *queries* allow applications to request information such as duration
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or current playback position from the pipeline. Queries are always
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answered synchronously. Elements can also use queries to request
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information from their peer elements (such as the file size or
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duration). They can be used both ways within a pipeline, but
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upstream queries are more common.
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