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202 lines
6.5 KiB
Text
202 lines
6.5 KiB
Text
A little explanation of the first autoplugger in GStreamer:
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Autoplugging is implemented in the following places:
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gstpipeline.c : construction of the pipeline
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gstautoplug.c : selection of the elementfactories needed for autoplugging
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1) pipeline setup
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-----------------
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before any autoplugging will take place, a new GstPipeline has to be created.
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The autoplugger needs to have a src element and one or more sink elements. the
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autoplugger will try to find the elements needed to connect the src element
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to the sinks.
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using:
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gst_pipeline_add_src (GstPipeline *pipeline, GstElement *element);
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a source element is added to the pipeline. only one src element can be added
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for now.
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using:
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gst_pipeline_add_sink (GstPipeline *pipeline, GstElement *element);
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a sink element can be added to the pipeline.
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2) starting autoplug
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--------------------
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when the pipeline has been set up as above, you will call
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gst_pipeline_autoplug (GstPipeline *pipeline);
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to start the autoplugger. this will be done in four phases
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ex. we are going to autoplug an mpeg1 system stream.
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2a) phase1: figure out the type (GstCaps) of the src element.
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-------------------------------------------------------------
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the gsttypefind element is connected to the "src" pad of the source
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element. gst_bin_iterate is called in a loop until gsttypefind
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signals "have_type". the gst_bin_iterate is stopped and the GstCaps
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is retrieved from the gsttypefind element.
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gsttypefind is disconnected from the src element and removed from the
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bin.
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the GstCaps of the source element is called src_caps later on.
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ex. all typefind functions are tried and the one in mpeg1types will
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return a GstCaps:
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video/mpeg,
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"systemstream", GST_PROPS_BOOLEAN (TRUE),
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"mpegversion", GST_PROPS_INT (1),
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NULL
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2b) phase2: create lists of factories.
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---------------------------------------
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for each sink:
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{
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sinkpad = take the first sinkpad of the sink (HACK)
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call
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list[i] = gst_autoplug_caps (src_caps, sinkpad->caps);
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I++;
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}
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gst_autoplug_caps will figure out (based on the padtemplates)
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which elementfactories are needed to connect src_caps to sinkpad->caps
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and will return them in a list.
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ex. we have two sinks with following caps:
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video/raw audio/raw
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"...." "...."
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gst_autoplug_caps will figure out that for the first sink the following
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elements are needed:
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mpeg1parse, mp1videoparse, mpeg_play
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for the second sink the following is needed:
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mpeg1parse, mp3parse, mpg123
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We now have two lists of elementfactories.
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2c) phase3: collect common elements from the lists.
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---------------------------------------------------
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the rationale is that from the lists we have created in phase2, there
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must be some element that is a splitter and that it has to come first (HACK)
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We try to find that element by comparing the lists until an element differs.
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we add the common elements to the bin and run gst_pipeline_pads_autoplug. this
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function will loop over the pads of the previous element and the one we
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just added, and tries to connect src to sink if possible.
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If a connection between the two elements could not be made, a signal "new_pad"
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is connected to the element so that pad connection can occur later on when
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the pad is actually created.
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ex. when we compare the two lists we see that we have common element: mpeg1parse.
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we add this element to the bin and try to connect it to the previous element in
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the bin, the disksrc.
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we see that the src pad of the disksrc and the sinkpad of the mpeg1parse element
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can be connected because they are compatible. We have a pipeline like:
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---------) (--------
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disksrc ! ! mpeg1parse
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src --- sink
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---------) (--------
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2d) phase4: add remaining elements
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----------------------------------
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now we loop over all the list and try to add the remaining elements
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(HACK) we always use a new thread for the elements when there is a common
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element found.
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if a new thread is needed (either bacuase the previous element is a common
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element or the object flag of the next element is set to GST_SUGGEST_THREAD)
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we add a queue to the bin and we add a new thread. We add the elements to
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the bin and connect them using gst_pipeline_pads_autoplug.
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If we add a queue, we have to copy the caps of the sink element of the queue
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to the src pad of the queue (else they won't connect)
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we finally arrive at the sink element and we're done.
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ex.
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we have just found our mpeg1parse common element, so we start a thread.
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We add a queue to the bin and a new thread, we add the elements
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mp1videoparse and mpeg_play to the thread. We arrive at the videosink, we
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see that the SUGGEST_THREAD flag is set, we add a queue and a thread and
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add the videosink in the thread.
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the same procedure happens for the audio part. We are now left with the
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following pipeline:
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We will also have set a signal "new_pad" on the mpeg1parse element bacause
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the element mp1videoparse could not be connected to the element just yet.
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(------------------------------------) (----------
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!thread ! ! thread
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! (-------------) (---------) ! ! (---------)
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! !mp1videoparse! !mpeg_play! ! ! !videosink!
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videoqueue--sink src -- sink src -- queue --- sink !
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---------) (-----------) ! (-------------) (---------) ! ! (---------)
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disksrc ! ! mpeg1parse! (------------------------------------) (-------------
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src --- sink !
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---------) (-----------)
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queue----- same for audio
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then we play, create_plan happens, data is flowing and the "new_pad" signal is called
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from mpeg1parse, gst_pipeline_pad_autoplug is called and the connection between
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mpeg1parse and the videoqueue is made. same for audio.
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voila. smame procedure for mp3/vorbis/avi/qt/mpeg2 etc...
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Problems:
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---------
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this is obviously a very naive solution. the creation of the elements actually happens
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beforehand. MPEG2, for one, fails bacause there are multiple possibilities to go
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from the mpeg demuxer to audio/raw (ac3, mp3)
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Also any intermedia elements like mixers (subtitles) are not possible because we
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assume that after the common elements, the streams to not converge anymore.
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