gstreamer/docs/design/part-element-sink.txt

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Sink elements
-------------
Sink elements consume data. They normally have no source pads.
Typical sink elements include:
- audio/video renderers
- network sinks
- filesinks
Sinks are harder to construct than other element types as they are
treated specially by the GStreamer core.
state changes
~~~~~~~~~~~~~
A sink always returns ASYNC from the state change to PAUSED, this
includes a state change from READY->PAUSED and PLAYING->PAUSED. The
reason for this is that this way we can detect when the first buffer
or event arrives in the sink when the state change completes.
A sink should block on the first EOS event or buffer received in the
READY->PAUSED state before commiting the state to PAUSED.
FLUSHING events have to be handled out of sync with the buffer flow
and take no part in the preroll procedure.
Events other than EOS do not complete the preroll stage.
sink overview
~~~~~~~~~~~~~
- TODO: PREROLL_LOCK can be removed and we can safely use the STREAM_LOCK.
# commit the state. We return TRUE if we can continue
# streaming, FALSE in the case we go to a READY or NULL state.
# if we go to PLAYING, we don't need to block on preroll.
commit
{
LOCK
switch (pending)
case PLAYING:
need_preroll = FALSE
break
case PAUSED:
break
case READY:
case NULL:
return FALSE
case VOID:
return TRUE
# update state
state = pending
next = VOID
pending = VOID
UNLOCK
return TRUE
}
# sync an object. We have to wait for the element to reach
# the PLAYING state before we can wait on the clock.
# some items do not need synchronisation (most events) so the
# get_times method returns FALSE (not syncable)
# need_preroll indicates that we are not in the PLAYING state
# and therefore need to commit and potentially block on preroll
# if our clock_wait got interrupted we commit and block again.
# The reason for this is that the current item being rendered is
# not yet finished and we can use that item to finish preroll.
do_sync (obj)
{
# get timing information for this object
syncable = get_times (obj, &start, &stop)
if (!syncable)
return OK;
again:
while (need_preroll)
if (need_commit)
need_commit = FALSE
if (!commit)
return WRONG_STATE
if (need_preroll)
# release PREROLL_LOCK and wait. prerolled can be observed
# and will be TRUE
prerolled = TRUE
PREROLL_WAIT (releasing PREROLL_LOCK)
prerolled = FALSE
if (flushing)
return WRONG_STATE
if (valid (start || stop))
PREROLL_UNLOCK
end_time = stop
ret = wait_clock (obj,start)
PREROLL_LOCK
if (flushing)
return WRONG_STATE
# if the clock was unscheduled, we redo the
# preroll
if (ret == UNSCHEDULED)
goto again
}
# render a prerollable item (EOS or buffer). It is
# always called with the PREROLL_LOCK helt.
render_object (obj)
{
ret = do_sync (obj)
if (ret != OK)
return ret;
# preroll and syncing done, now we can render
render(obj)
}
| # sinks that sync on buffer contents do like this
| while (more_to_render)
| ret = render
| if (ret == interrupted)
| prerolled = TRUE
render (buffer) ----->| PREROLL_WAIT (releasing PREROLL_LOCK)
| prerolled = FALSE
| if (flushing)
| return WRONG_STATE
|
# queue a prerollable item (EOS or buffer). It is
# always called with the PREROLL_LOCK helt.
# This function will commit the state when receiving the
# first prerollable item.
# items are then added to the rendering queue or rendered
# right away if no preroll is needed.
queue (obj, prerollable)
{
if (need_preroll)
if (prerollable)
queuelen++
# first item in the queue while we need preroll
# will complete state change and call preroll
if (queuelen == 1)
preroll (obj)
if (need_commit)
need_commit = FALSE
if (!commit)
return WRONG_STATE
# then see if we need more preroll items before we
# can block
if (need_preroll)
if (queuelen <= maxqueue)
queue.add (obj)
return OK
# now clear the queue and render each item before
# rendering the current item.
while (queue.hasItem)
render_object (queue.remove())
render_object (obj)
queuelen = 0
}
# various event functions
event
EOS:
# events must complete preroll too
STREAM_LOCK
PREROLL_LOCK
if (flushing)
return FALSE
ret = queue (event, TRUE)
if (ret == WRONG_STATE)
return FALSE
PREROLL_UNLOCK
STREAM_UNLOCK
break
NEWSEGMENT:
# the newsegment must be used to clip incoming
# buffers. Then then go into the queue as non-prerollable
# items used for syncing the buffers
STREAM_LOCK
PREROLL_LOCK
if (flushing)
return FALSE
set_clip
ret = queue (event, FALSE)
if (ret == WRONG_STATE)
return FALSE
PREROLL_UNLOCK
STREAM_UNLOCK
break
FLUSH_START:
# set flushing and unblock all that is waiting
event ----> subclasses can interrupt render
PREROLL_LOCK
flushing = TRUE
unlock_clock
PREROLL_SIGNAL
PREROLL_UNLOCK
STREAM_LOCK
lost_state
STREAM_UNLOCK
break
FLUSH_END:
# unset flushing and clear all data and eos
STREAM_LOCK
event
PREROLL_LOCK
queue.clear
queuelen = 0
flushing = FALSE
eos = FALSE
PREROLL_UNLOCK
STREAM_UNLOCK
break
# the chain function checks the buffer falls within the
# configured segment and queues the buffer for preroll and
# rendering
chain
STREAM_LOCK
PREROLL_LOCK
if (flushing)
return WRONG_STATE
if (clip)
queue (buffer, TRUE)
PREROLL_UNLOCK
STREAM_UNLOCK
state
switch (transition)
READY_PAUSED:
# no datapassing is going on so we always return ASYNC
ret = ASYNC
need_commit = TRUE
eos = FALSE
flushing = FALSE
need_preroll = TRUE
prerolled = FALSE
break
PAUSED_PLAYING:
# we grab the preroll lock. This we can only do if the
# chain function is either doing some clock sync, we are
# waiting for preroll or the chain function is not being called.
PREROLL_LOCK
if (prerolled || eos)
ret = OK
need_commit = FALSE
need_preroll = FALSE
if (eos)
post_eos
else
PREROLL_SIGNAL
else
need_preroll = TRUE
need_commit = TRUE
ret = ASYNC
PREROLL_UNLOCK
break
PLAYING_PAUSED:
---> subclass can interrupt render
# we grab the preroll lock. This we can only do if the
# chain function is either doing some clock sync
# or the chain function is not being called.
PREROLL_LOCK
need_preroll = TRUE
unlock_clock
if (prerolled || eos)
ret = OK
else
ret = ASYNC
PREROLL_UNLOCK
break
PAUSED_READY:
---> subclass can interrupt render
# we grab the preroll lock. Set to flushing and unlock
# everything. This should exit the chain functions and stop
# streaming.
PREROLL_LOCK
flushing = TRUE
unlock_clock
queue.clear
queuelen = 0
PREROLL_SIGNAL
ret = OK
PREROLL_UNLOCK
break