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Element Klass definition
Applications should be able to retrieve elements from the registry of existing
elements based on specific capabilities or features of the element.
A playback application might want to retrieve all the elements that can be
used for visualisation, for example, or a video editor might want to select
all video effect filters.
The topic of defining the klass of elements should be based on use cases.
A list of classes that are used in a installation can be generated using:
gst-inspect-1.0 -a | grep -ho Class:.* | cut -c8- | sed "s/\//\\n/g" | sort | uniq
The GstElementDetails contains a field named klass that is a pointer to a
string describing the element type.
In this document we describe the format and contents of the string. Elements
should adhere to this specification although that is not enforced to allow
for wild (application specific) customisation.
1) string format
The string consists of an _unordered_ list of keywords separated with a '/'
character. While the / suggests a hierarchy, this is _not_ the case.
2) keyword categories
- functional
Categories are base on _intended usage_ of the element. Some elements
might have other side-effects (especially for filers/effects). The purpose
is to list enough keywords so that applications can do meaningful filtering,
not to completely describe the functionality, that is expressed in caps etc..
* Source : produces data
* Sink : consumes data
* Filter : filters/transforms data, no modification on the data is
intended (although it might be unavoidable). The
filter can decide on input and output caps independently
of the stream contents (GstBaseTransform).
* Effect : applies an effect to some data, changes to data are
intended. Examples are colorbalance, volume. These
elements can also be implemented with GstBaseTransform.
* Demuxer : splits audio, video, ... from a stream
* Muxer : interleave audio, video, ... into one stream, this is
like mixing but without losing or degrading each separate
input stream. The reverse operation is possible with a
Demuxer that reproduces the exact same input streams.
* Decoder : decodes encoded data into a raw format, there is typically
no relation between input caps and output caps. The output
caps are defined in the stream data. This separates the
Decoder from the Filter and Effect.
* Encoder : encodes raw data into an encoded format.
* Mixer : combine audio, video, .. this is like muxing but with
applying some algorithm so that the individual streams
are not extractable anymore, there is therefore no
reverse operation to mixing. (audio mixer, video mixer, ...)
* Converter : convert audio into video, text to audio, ... The converter
typically works on raw types only. The source media type
is listed first.
* Analyzer : reports about the stream contents.
* Control : controls some aspect of a hardware device
* Extracter : extracts tags/headers from a stream
* Formatter : adds tags/headers to a stream
* Connector : allows for new connections in the pipeline. (tee, ...)
* ...
- Based on media type
Purpose is to make a selection for elements operating on the different
types of media. An audio application must be able to filter out the
elements operating on audio, for example.
* Audio : operates on audio data
* Video : operates on video data
* Image : operates on image data. Usually this media type can also
be used to make a video stream in which case it is added
together with the Video media type.
* Text : operates on text data
* Metadata : operates on metadata
* ...
- Extra features
The purpose is to further specialize the element, mostly for
application specific needs.
* Network : element is used in networked situations
* Protocol : implements some protocol (RTSP, HTTP, ...)
* Payloader : encapsulate as payload (RTP, RDT,.. )
* Depayloader : strip a payload (RTP, RDT,.. )
* RTP : intended to be used in RTP applications
* Device : operates on some hardware device (disk, network,
audio card, video card, usb, ...)
* Visualisation : intended to be used for audio visualisation
* Debug : intended usage is more for debugging purposes.
- Categories found, but not yet in one of the above lists
* Bin : playbin, decodebin, bin, pipeline
* Codec : lots of decoders, encoder, demuxers
should be removed?
* Generic : should be removed?
* File : like network, should go to Extra?
* Editor : gnonlin, textoverlays
* DVD, GDP, LADSPA, Parser, Player, Subtitle, Testing, ...
3) suggested order:
<functional>[/<media type>]*[/<extra...>]*
4) examples:
apedemux : Extracter/Metadata
audiotestsrc : Source/Audio
autoaudiosink : Sink/Audio/Device
cairotimeoverlay : Mixer/Video/Text
dvdec : Decoder/Video
dvdemux : Demuxer
goom : Converter/Audio/Video
id3demux : Extracter/Metadata
udpsrc : Source/Network/Protocol/Device
videomixer : Mixer/Video
videoconvert : Filter/Video (intended use to convert video with as little
visible change as possible)
vertigotv : Effect/Video (intended use is to change the video)
volume : Effect/Audio (intended use is to change the audio data)
vorbisdec : Decoder/Audio
vorbisenc : Encoder/Audio
oggmux : Muxer
adder : Mixer/Audio
videobox : Effect/Video
alsamixer : Control/Audio/Device
audioconvert : Filter/Audio
audioresample : Filter/Audio
xvimagesink : Sink/Video/Device
navseek : Filter/Debug
decodebin : Decoder/Demuxer
level : Filter/Analyzer/Audio
tee : Connector/Debug
5) open issues:
- how to differentiate physical devices from logical ones?
autoaudiosink : Sink/Audio/Device
alsasink : Sink/Audio/Device
Use cases
- get a list of all elements implementing a video effect (pitivi):
klass.contains (Effect & Video)
- get list of muxers (pitivi):
klass.contains (Muxer)
- get list of video encoders (pitivi):
klass.contains (Encoder & video)
- Get a list of all audio/video visualisations (totem):
klass.contains (Visualisation)
- Get a list of all decoders/demuxer/metadata parsers/vis (playbin):
klass.contains (Visualisation | Demuxer | Decoder | (Extractor & Metadata))
- Get a list of elements that can capture from an audio device (gst-properties):
klass.contains (Source & Audio & Device)
* filters out audiotestsrc, since it is not a device