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257 lines
10 KiB
Markdown
257 lines
10 KiB
Markdown
# What to do when a plugin is missing
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The mechanism and API described in this document requires GStreamer core
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and gst-plugins-base versions \>= 0.10.12. Further information on some
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aspects of this document can be found in the libgstbaseutils API
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reference.
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We only discuss playback pipelines for now.
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A three step process:
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# GStreamer level
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Elements will use a "missing-plugin" element message to report
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missing plugins, with the following fields set:
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* **`type`**: (string) { "urisource", "urisink", "decoder", "encoder",
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"element" } (we do not distinguish between demuxer/decoders/parsers etc.)
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* **`detail`**: (string) or (caps) depending on the type { ANY } ex: "mms,
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"mmsh", "audio/x-mp3,rate=48000,…"
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* **`name`**: (string) { ANY } ex: "MMS protocol handler",..
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### missing uri handler
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ex. mms://foo.bar/file.asf
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When no protocol handler is installed for mms://, the application will not be
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able to instantiate an element for that uri (`gst_element_make_from_uri()`
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returns NULL).
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Playbin will post a `missing-plugin` element message with the type set to
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"urisource", detail set to "mms". Optionally the friendly name can be filled
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in as well.
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### missing typefind function
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We don't recognize the type of the file, this should normally not happen
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because all the typefinders are in the basic GStreamer installation.
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There is not much useful information we can give about how to resolve this
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issue. It is possible to use the first N bytes of the data to determine the
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type (and needed plugin) on the server. We don't explore this option in this
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document yet, but the proposal is flexible enough to accommodate this in the
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future should the need arise.
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### missing demuxer
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Typically after running typefind on the data we determine the type of the
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file. If there is no plugin found for the type, a `missing-plugin` element
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message is posted by decodebin with the following fields: Type set to
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"decoder", detail set to the caps for witch no plugin was found. Optionally
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the friendly name can be filled in as well.
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### missing decoder
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The demuxer will dynamically create new pads with specific caps while it
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figures out the contents of the container format. Decodebin tries to find the
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decoders for these formats in the registry. If there is no decoder found, a
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`missing-plugin` element message is posted by decodebin with the following
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fields: Type set to "decoder", detail set to the caps for which no plugin
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was found. Optionally the friendly name can be filled in as well. There is
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no distinction made between the missing demuxer and decoder at the
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application level.
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### missing element
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Decodebin and playbin will create a set of helper elements when they set up
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their decoding pipeline. These elements are typically colorspace, sample rate,
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audio sinks,... Their presence on the system is required for the functionality
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of decodebin. It is typically a package dependency error if they are not
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present but in case of a corrupted system the following `missing-plugin`
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element message will be emitted: type set to "element", detail set to the
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element factory name and the friendly name optionally set to a description
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of the element's functionality in the decoding pipeline.
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Except for reporting the missing plugins, no further policy is enforced at the
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GStreamer level. It is up to the application to decide whether a missing
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plugin constitutes a problem or not.
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## Application level
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The application's job is to listen for the `missing-plugin` element messages
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and to decide on a policy to handle them. Following cases exist:
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### partially missing plugins
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The application will be able to complete a state change to PAUSED but there
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will be a `missing-plugin` element message on the `GstBus`.
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This means that it will be possible to play back part of the media file but not
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all of it.
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For example: suppose we have an .avi file with mp3 audio and divx video. If we
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have the mp3 audio decoder but not the divx video decoder, it will be possible
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to play only the audio part but not the video part. For an audio playback
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application, this is not a problem but a video player might want to decide on:
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- require the use to install the additionally required plugins.
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- inform the user that only the audio will be played back
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- ask the user if it should download the additional codec or only play
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the audio part.
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- …
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### completely unplayable stream
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The application will receive an ERROR message from GStreamer informing it that
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playback stopped (before it could reach PAUSED). This happens because none of
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the streams is connected to a decoder. The error code and domain should be one
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of the following in this case:
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- `GST_CORE_ERROR_MISSING_PLUGIN` (domain: `GST_CORE_ERROR`)
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- `GST_STREAM_ERROR_CODEC_NOT_FOUND` (domain: `GST_STREAM_ERROR`)
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The application can then see that there are a set of `missing-plugin` element
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messages on the `GstBus` and can decide to trigger the download procedure. It
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does that as described in the following section.
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`missing-plugin` element messages can be identified using the function
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`gst_is_missing_plugin_message()`.
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## Plugin download stage
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At this point the application has
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- collected one or more `missing-plugin` element messages
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- made a decision that additional plugins should be installed
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It will call a GStreamer utility function to convert each `missing-plugin`
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message into an identifier string describing the missing capability. This is
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done using the function `gst_missing_plugin_message_get_installer_detail()`.
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The application will then pass these strings to `gst_install_plugins_async()`
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or `gst_install_plugins_sync()` to initiate the download. See the API
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documentation there (`libgstbaseutils`, part of `gst-plugins-base`) for more
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details.
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When new plugins have been installed, the application will have to initiate
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a re-scan of the GStreamer plugin registry using `gst_update_registry()`.
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### Format of the (UTF-8) string ID passed to the external installer system
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The string is made up of several fields, separated by '|' characters.
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The fields are:
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- plugin system identifier, ie. "gstreamer" This identifier determines
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the format of the rest of the detail string. Automatic plugin
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installers should not process detail strings with unknown
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identifiers. This allows other plugin-based libraries to use the
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same mechanism for their automatic plugin installation needs, or for
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the format to be changed should it turn out to be insufficient.
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- plugin system version, e.g. "1.0" This is required so that when
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there is a GStreamer-2.0 or GStreamer-3.0 at some point in future,
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the different major versions can still co-exist and use the same
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plugin install mechanism in the same way.
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- application identifier, e.g. "totem" This may also be in the form of
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"pid/12345" if the program name can’t be obtained for some reason.
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- human-readable localised description of the required component, e.g.
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"Vorbis audio decoder"
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- identifier string for the required component, e.g.
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- urisource-(`PROTOCOL_REQUIRED`) e.g. `urisource-http` or `urisource-mms`
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- element-(`ELEMENT_REQUIRED`), e.g. `element-videoconvert`
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- decoder-(`CAPS_REQUIRED`) e.g. `decoder-audio/x-vorbis` or
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`decoder-application/ogg` or `decoder-audio/mpeg, mpegversion=(int)4` or
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`decoder-video/mpeg, systemstream=(boolean)true, mpegversion=(int)2`
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- encoder-(`CAPS_REQUIRED`) e.g. `encoder-audio/x-vorbis`
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- optional further fields not yet specified
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* An entire ID string might then look like this, for example:
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`gstreamer|0.10|totem|Vorbis audio decoder|decoder-audio/x-vorbis`
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* Plugin installers parsing this ID string should expect further fields also
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separated by '|' symbols and either ignore them, warn the user, or error
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out when encountering them.
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* The human-readable description string is provided by the libgstbaseutils
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library that can be found in gst-plugins-base versions >= 0.10.12 and can
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also be used by demuxers to find out the codec names for taglists from given
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caps in a unified and consistent way.
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* Applications can create these detail strings using the function
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`gst_missing_plugin_message_get_installer_detail()` on a given missing-plugin
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message.
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### Using missing-plugin messages for error reporting:
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Missing-plugin messages are also useful for error reporting purposes, either in
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the case where the application does not support libgimme-codec, or the external
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installer is not available or not able to install the required plugins.
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When creating error messages, applications may use the function
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`gst_missing_plugin_message_get_description()` to obtain a possibly translated
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description from each missing-plugin message (e.g. "Matroska demuxer" or
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"Theora video depayloader"). This can be used to report to the user exactly
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what it is that is missing.
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## Notes for packagers
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An easy way to introspect plugin .so files is:
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```
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$ gst-inspect --print-plugin-auto-install-info /path/to/libgstfoo.so
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```
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The output will be something like:
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```
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decoder-audio/x-vorbis
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element-vorbisdec
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element-vorbisenc
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element-vorbisparse
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element-vorbistag
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encoder-audio/x-vorbis
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```
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BUT could also be like this (from the faad element in this case):
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```
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decoder-audio/mpeg, mpegversion=(int){ 2, 4 }
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```
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NOTE that this does not exactly match the caps string that the installer
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will get from the application. The application will always ever ask for
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one of
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```
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decoder-audio/mpeg, mpegversion=(int)2
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decoder-audio/mpeg, mpegversion=(int)4
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```
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When introspecting, keep in mind that there are GStreamer plugins
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that in turn load external plugins. Examples of these are pitfdll,
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ladspa, or the GStreamer libvisual plugin. Those plugins will only
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announce elements for the currently installed external plugins at
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the time of introspection\! With the exception of pitfdll, this is
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not really relevant to the playback case, but may become an issue in
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future when applications like buzztard, jokosher or pitivi start
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requestion elements by name, for example ladspa effect elements or
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so.
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This case could be handled if those wrapper plugins would also provide a
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`gst-install-xxx-plugins-helper`, where xxx={ladspa|visual|...}. Thus if the
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distro specific `gst-install-plugins-helper` can't resolve a request for e.g.
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`element-bml-sonicverb` it can forward the request to
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`gst-install-bml-plugins-helper` (bml is the buzz machine loader).
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## Further references:
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<http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-libs/html/gstreamer-base-utils.html>
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