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85 lines
3.9 KiB
Text
85 lines
3.9 KiB
Text
Stream selection
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1. Problem
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URIs (that being either a media stream or a media stream plus subtitle) can
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contain multiple streams of a type (audio, subtitle). A user has to be given
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the option of selecting one of those streams (or none altogether).
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2. Implementation ideas
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Stream selection, in GStreamer, has to be integrated at the player plugging
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level, which is (in the case of Totem) playbin. Playbin offers a feature to
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'mute' a stream (which means that no processing is done on that stream
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altogether, saving the decoding step). Currently, playbin will select the
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first occurrence of a stream type and mute all others. A queue is connected
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(for pre-roll) to the active stream. What is missing here is a way to change
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the active stream.
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Playbin interface - one possible interface could simply consist of a bunch of
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GObject properties: 'textstream' and 'audiostream', both integer. The number
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of available streams can be retrieved using the 'stream-info' GObject property.
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Similar to the 'nstreams' property, we could add utility GObject properties
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for getting the number of available audio/text streams ('naudiostreams' and
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'ntextstreams'). Names of these streams (like language name or so) can be
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added as an additional GObject property to streaminfo. Some container
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formats contain such names internally. Alternatively, we could allow those
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to be user-settable as well (for .sub files).
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On a set of either of these properties, playbasebin would mute the old
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selected stream (if any), unmute the newly selected stream (if any) and
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replug the preroll queue. The queue itself is disabled as well if no new
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stream was linked. Alternatively, a switch-like element is used, which
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requires no replugging. Pad disabling/enabling is then enough. This also
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makes relinking less painful. The switch-like element needs to proxy the
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active pads' caps. However, since those caps are (in practice) always the
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same across streams, caps setting will (inside the core) immediately
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return success.
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The switch-like element simply works like this:
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=
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static void
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loop_func (GstElement * element)
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{
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GList *inpads;
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GstPad *pad;
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GstData *data;
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for (inpads = ..; inpads != NULL; inpads = inpads->next) {
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pad = inpads->data;
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if (!GST_PAD_IS_USABLE (pad))
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continue;
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/* you'd also want some way to synchronize the inputs... */
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data = gst_pad_pull (pad);
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if (is_active_pad (pad))
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gst_pad_push (srcpad, data);
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else
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gst_data_unref (data);
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}
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}
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=
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It'd require an active-stream property itself, which (when set) takes
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care of doing renegotiation and so on. Using internal pad linkage is
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extremely useful here, and requires little code in the switch-like
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element itself. Note that there is a slight bit of duplication in the
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playbin interface and the switch-like element interface, but that's "just
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the way it is".
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The implementation of the switch-like element could initially be local to
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playbin, until it has been cleaned up and confirmed to be useful to a
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wider audience. This allows a lot of experimenting with interfaces because
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we won't be forced to maintain a stable interface.
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The current 'switch' element (gst-plugins/gst/switch/) already does a few
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of those operations, but stream synchronization, re-negotiation on stream
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changes, internal pad linkage and some other things are completely missing.
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If we're gonna use this element, it'll need a large overhaul. The choice of
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writing a new element or using an existing element as basis, and also the
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choice of whether or not to make this element local to playbin, should be
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based on technical merits and cost/effect analysis and not on personal
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pride.
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Notes:
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* seamless has the same switch-like element, but it's chain-based. Apart
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from scheduler considerations, this is a good idea, but limits its use
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(making either good docs and abuse-prevention [see multifilesrc] or
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private-to-playbin a prerequisite).
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* maybe text-* properties need to be renamed to subtitle-*.
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3. Written by
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Ronald S. Bultje <rbultje@ronald.bitfreak.net> - Jan. 2nd, 2005.
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