gstreamer/subprojects/gst-docs/markdown/additional/design/playbin.md

playbin

The purpose of this element is to decode and render the media contained in a given generic uri. The element extends GstPipeline and is typically used in playback situations.

Required features:

• accept and play any valid uri. This includes
• rendering video/audio
• overlaying subtitles on the video
• optionally read external subtitle files
• allow for hardware (non raw) sinks
• selection of audio/video/subtitle streams based on language.
• perform network buffering/incremental download
• gapless playback
• support for visualisations with configurable sizes
• ability to reject files that are too big, or of a format that would require too much CPU/memory usage.
• be very efficient with adding elements such as converters to reduce the amount of negotiation that has to happen.
• handle chained oggs. This includes having support for dynamic pad add and remove from a demuxer.

Components

decodebin

• performs the autoplugging of demuxers/decoders
• emits signals when for steering the autoplugging
• to decide if a non-raw media format is acceptable as output
• to sort the possible decoders for a non-raw format
• see also decodebin2 design doc

uridecodebin

• combination of a source to handle the given uri, an optional queueing element and one or more decodebin2 elements to decode the non-raw streams.

playsink

• handles display of audio/video/text.
• has request audio/video/text input pad. There is only one sinkpad per type. The requested pads define the configuration of the internal pipeline.
• allows for setting audio/video sinks or does automatic sink selection.
• allows for configuration of visualisation element.
• allows for enable/disable of visualisation, audio and video.

playbin

• combination of one or more uridecodebin elements to read the uri and subtitle uri.
• support for queuing new media to support gapless playback.
• handles stream selection.
• uses playsink to display.
• selection of sinks and configuration of uridecodebin with raw output formats.

Gapless playback feature

playbin has an about-to-finish signal. The application should configure a new uri (and optional suburi) in the callback. When the current media finishes, this new media will be played next.