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Phonon backend
The phonon design is based around forming graphs using 3 basic components:
- a source component that generates raw audio/video/subtitle data, aka
- an effect component that applies effects to raw audio/video known as
AudioPath/VideoPath respectively. Subtitles are routed to a VideoPath
- output components that render audio or video called AudioOutput and
there is also a special input object that allows for feeding raw data in the
pipeline and specialized sinks to retrieve audio samples and video frames from
the pipeline.
A typical graph or a source that produces an audio and a video stream that
need to be played. The VideoPath and AudioPath typically contain no filters
in this case:
+----+ +---
| FX | | ...
+----+ +---
+-----------+ +-------------+
----->| VideoPath |----->| VideoOutput |
| +-----------+ +-------------+
| MediaObject |
| +-----------+ +-------------+
+---->| AudioPath |----->| AudioOutput |
+-----------+ +-------------+
^ ^
+----+ +---
| FX | | ...
+-+--+ +---
- This is very similar to a regular gstreamer playback pipeline.
A typical graph of playing and crosfading two sources:
| volume |
+-------------+ +-----------+
| MediaObject |--->| AudioPath |\
+-------------+ +-----------+ \ +-------------+
---->| AudioOutput |
+-------------+ +-----------+ / +-------------+
| MediaObject |--->| AudioPath |/
+-------------+ +-----------+
| volume |
- As soon as two audio paths are connected to one sink, the input signals are
mixed before sending them to the sink. The mixing is typically done in the
audio sink by an element such as adder.
Other types of graphs are possible too:
/| AudioPath |\
+-------------+ / +-----------+ \ +-------------+
| MediaObject |-- ---->| AudioOutput |
+-------------+ \ +-----------+ / +-------------+
\| AudioPath |/
- This graph sends the same out data to 2 effect filter graphs and then mixes
it to an audio output. The splitting of the graph typically happens with a
tee element after the media object.
1) do the following chains run
- synchronized with a shared clock?
+-------------+ +-----------+ +-------------+
| MediaObject |--->| AudioPath |--->| AudioOutput |
+-------------+ +-----------+ +-------------+
+-------------+ +-----------+ +-------------+
| MediaObject |--->| VideoPath |--->| VideoOutput |
+-------------+ +-----------+ +-------------+
- no API to set both MediaObjects atomically to play so it is assumed that
the playback starts and follows the rate of the global clock as soon as
the MediaObject is set to play. This makes unconnected chains run as if
they were in different GstPipelines.
2) Threading:
- Can signals be emited from any thread?
- what operations are permited from a signal handler?
3) Error reporting
- How does error reporting work?
* an audio/video device/port is busy.
* a fatal decoding error occured.
* a media type is not supported
- Setting up KDE and Phonon build environment
- Testing, identifying test applications, building test cases
- Asking questions to Phonon maintainers/designers
Essential classes
These classes are essential to implement a backend and should be implemented
Mostly exposes features in the registry like available decoders and effects.
Entry point for the GStreamer backend. Provides methods to create instances of
object from our backed.
Simple playback
The following classes need to be implemented in order to have simple playback
capabilities from the backend.
- Wrapper around audiosinks. Also needs provision for rate and format
- Mixing capabilities in the case when 2 audio paths are routed to it.
* is the volume related to the device or to the connection to the device.
- Wrapper around videosinks. Also needs provision for colorspace and size
conversions. Extends QWidget and probably needs to hook into the XOverlay
stuff to draw in the QT widget. Supports fullscreen mode with a switch.
- Needs mixing capabilities in the case when 2 video paths are routed to it.
- contains stream selection
- play/pause/stop
- seeking
- periodically performs tick callbacks.
- The object that decodes the media into raw audio/video/subtitle.
This object will use the GStreamer decodebin element to perform the
typefinding and decoding.
- Simple container for audio/video effect plugins.
- Handles adding/removing of effects, making sure that the streaming is not
interrupted and the formats are all compatible.
Effect support
Connects an AudioPath to a VideoWidget and allows for selection of a
visualisation plugin.
Base classes
Alows fade-in and fade-out with a configurable curve and time. Needs
Controls the brightness of video.
Playlist support
?? don't know yet where this fits in.
Synchronized audio and video capture.
Compress audio.
Advanced features
Feed raw data into the pipeline. Used for streaming network access.
Possibly a specialized source element connected to a decodebin.
* Phonon::ByteStream::writeData
- can it block?
* Phonon::ByteStream::setStreamSeekable
- If called before starting the ByteStream, decodebin might operate in pull
based mode when supported. Else the source is activated in push mode.
- If called after starting ByteStream, the Phonon::ByteStream::seekStream
signal can be called for push-based seekable streams.
* Can the signals be emited from a streaming thread?
Receive raw audio/video data from the pipeline. Used to allow applications to
deal with the raw data themselves.
Possibly a specialized sink element.
Notes :
* Phonon::AudioDataOutput::dataReady
- can this be emited from the streaming threads?
* Phonon::AudioDataOutput::endOfMedia
- can this be emited from the streaming threads?
- We need to grab this EOS message synchronously from the bus.
- should be emited _before_ sending the last dataReady. This means we need
to cache at least one dataReady.
* Phonon::AudioDataOutput::setDataSize
- can this be a _suggested_ data size or does every callback need to be of
this size?