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82 lines
3.6 KiB
Text
82 lines
3.6 KiB
Text
OUTDATED
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--------
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Since the plan generation only happens as a result of the state mechanism,
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I'll describe that first.
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It's supposed to be recursive, such that setting the state on a Bin
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recursively sets all the children. However, this needs to be rethought
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somewhat, in light of some recent ideas on the actual definition of some
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of the states.
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The mechanism is thus: When you call gst_element_set_state(element,state),
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it calls the change_state() class method. The basic Element-provided
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version just sets or unsets the state. A more complex element like the
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audiosink will switch on the state and do certain things like open or
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close the sound card on transition to/from various states. The success or
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failure of these actions can determine whether or not the state gets
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[un]set as requested.
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GtkObject signals enter in here, as whenever a state is successfully
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changed, the STATE_CHANGE signal is fired, which gives higher-level code
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the ability to do something based on the change.
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The Bin's change_state function walks through all its children and sets
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their state. This is where things get interesting, and where things are
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going to need to be changed.
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The issue is what the states are and mean. Currently the states are as
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follows (from gstelement.h):
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typedef enum {
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GST_STATE_COMPLETE = (1 << 0),
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GST_STATE_RUNNING = (1 << 1),
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GST_STATE_DISCOVERY = (1 << 2),
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GST_STATE_PREROLL = (1 << 3),
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GST_STATE_PLAYING = (1 << 4),
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GST_STATE_PAUSED = (1 << 5),
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GST_STATE_MAX = (1 << 15),
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} GstElementState;
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COMPLETE means all the necesary information is available to run, i.e. the
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filename for the disksrc, etc. RUNNING means that it's actually doing
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something, but that's fuzzy. PLAYING means there really is data flowing
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through the graph, where PAUSED temporary stops the flow. PLAYING &&
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PAUSED is the same idea as !PLAYING, but there are probably going to be
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many cases where there really is a distinction.
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DISCOVERY is intended for the autoconnect case, in those instances where
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the only way to determine the input or output type of some pad is for an
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element to actually process some data. The idea in that case is that the
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source element would be responsible for sending the data non-destructively
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(in the case of a network client, it would have to save it all up, unless
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it has seek capabilities over the network), and all downstream elements
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process it in such a way as to not hose their own state. Or rather, when
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they cease to do discovery, they completely wipe their state as if nothing
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ever happened.
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PREROLL is a local state, used for things like sending the first half of
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an MPEG GOP through the decoder in order to start playback at a frame
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somewhere in the middle of said GOP. Not sure how that will work,
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exactly.
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The issue is that these states aren't layered, and it most certainly isn't
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the case that a container isn't able to be of a certain state unless all
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of its children are. I guess I should explain the idea of reconfigurable
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pipelines:
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Build an MP3 player, give it the ability to use audio effects plugins.
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Since you don't want to have to start the stream over again (especially if
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it's a network stream) every time you change the effect. This means you
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need to be able to freeze the pipeline in place to change it, without
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taking too much time.
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This matters when you consider that certain state changes should render
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various state bits invalid. In the FROZEN state these won't happen,
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because the assumption is that they're temporary.
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If you haven't noticed by now, the state system isn't entirely
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self-consistent yet. It needs work, and it needs discussion.
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