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262 lines
11 KiB
Text
262 lines
11 KiB
Text
What to do when a plugin is missing
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-----------------------------------
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The mechanism and API described in this document requires GStreamer core and
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gst-plugins-base versions >= 0.10.12. Further information on some aspects of
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this document can be found in the libgstbaseutils API reference.
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We only discuss playback pipelines for now.
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A three step process:
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1) GStreamer level
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- elements will use a "missing-plugin" element message to report missing
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plugins, with the following fields set:
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- type: (string) { "urisource", "urisink", "decoder", "encoder", "element" }
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(we do not distinguish between demuxer/decoders/parsers etc.)
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- detail: (string) or (caps) depending on the type { ANY }
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ex: "mms, "mmsh", "audio/x-mp3,rate=48000,..."
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- name: (string) { ANY }
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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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2) 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 the
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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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3) 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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4) 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"
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This identifier determines the format of the rest of the detail string.
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Automatic plugin installers should not process detail strings with
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unknown identifiers. This allows other plugin-based libraries to use
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the same mechanism for their automatic plugin installation needs, or
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for the format to be changed should it turn out to be insufficient.
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- plugin system version, e.g. "0.10"
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This is required so that when there is a GStreamer-0.12 or GStreamer-1.0
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at somem point in future, the different major versions can still co-exist
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and use the same plugin install mechanism in the same way.
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- application identifier, e.g. "totem"
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This may also be in the form of "pid/12345" if the program name can't
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be obtained for some reason.
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- human-readable localised description of the required component,
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e.g. "Vorbis audio decoder"
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- identifier string for the required component, e.g.
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- urisource-$(PROTOCOL_REQUIRED)
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e.g. urisource-http or urisource-mms
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- element-$(ELEMENT_REQUIRED),
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e.g. element-videoconvert
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- decoder-$(CAPS_REQUIRED)
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e.g. decoder-audio/x-vorbis or
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decoder-application/ogg or
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decoder-audio/mpeg, mpegversion=(int)4 or
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decoder-video/mpeg, systemstream=(boolean)true, mpegversion=(int)2
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- encoder-$(CAPS_REQUIRED)
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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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5) Using missing-plugin messages for error reporting:
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Missing-plugin messages are also useful for error reporting purposes, either
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in the case where the application does not support libgimme-codec, or the
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external installer is not available or not able to install the required
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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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6) Notes for packagers
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- An easy way to introspect plugin .so files is:
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$ gst-inspect --print-plugin-auto-install-info /path/to/libgstfoo.so
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The output will be something like:
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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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BUT could also be like this (from the faad element in this case):
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decoder-audio/mpeg, mpegversion=(int){ 2, 4 }
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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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decoder-audio/mpeg, mpegversion=(int)2
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decoder-audio/mpeg, mpegversion=(int)4
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- when introspecting, keep in mind that there are GStreamer plugins that
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in turn load external plugins. Examples of these are pitfdll, ladspa, or
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the GStreamer libvisual plugin. Those plugins will only announce elements
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for the currently installed external plugins at the time of introspection!
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With the exception of pitfdll, this is not really relevant to the playback
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case, but may become an issue in future when applications like buzztard,
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jokosher or pitivi start requestion elements by name, for example ladspa
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effect elements or 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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7) 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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