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