Element Klass definition
------------------------

Status:

  DRAFT.


Purpose:

  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-0.10 -a | grep -ho Class:.* | cut -c8- | sed "s/\//\\n/g" | sort | uniq

Proposal:

  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

      <keyword>['/'<keyword]*

    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 meaningfull 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
     * 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
     ffmpegcolorspace : 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 differencial 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