mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-24 18:51:11 +00:00
110 lines
4.6 KiB
Markdown
110 lines
4.6 KiB
Markdown
# DRAFT Sparse Streams
|
|
|
|
## Introduction
|
|
|
|
In 0.8, there was some support for Sparse Streams through the use of
|
|
FILLER events. These were used to mark gaps between buffers so that
|
|
downstream elements could know not to expect any more data for that gap.
|
|
|
|
In 0.10, segment information conveyed through SEGMENT events can be used
|
|
for the same purpose.
|
|
|
|
In 1.0, there is a GAP event that works in a similar fashion as the
|
|
FILLER event in 0.8.
|
|
|
|
## Use cases
|
|
|
|
1) Sub-title streams Sub-title information from muxed formats such as
|
|
Matroska or MPEG consist of irregular buffers spaced far apart compared
|
|
to the other streams (audio and video). Since these usually only appear
|
|
when someone speaks or some other action in the video/audio needs
|
|
describing, they can be anywhere from 1-2 seconds to several minutes
|
|
apart. Downstream elements that want to mix sub-titles and video (and muxers)
|
|
have no way of knowing whether to process a video packet or wait a moment
|
|
for a corresponding sub-title to be delivered on another pad.
|
|
|
|
2) Still frame/menu support In DVDs (and other formats), there are
|
|
still-frame regions where the current video frame should be retained and
|
|
no audio played for a period. In DVD, these are described either as a
|
|
fixed duration, or infinite duration still frame.
|
|
|
|
3) Avoiding processing silence from audio generators Imagine a source
|
|
that from time to time produces empty buffers (silence or blank images).
|
|
If the pipeline has many elements next, it is better to optimise the
|
|
obsolete data processing in this case. Examples for such sources are
|
|
sound-generators (simsyn in gst-buzztard) or a source in a voip
|
|
application that uses noise-gating (to save bandwith).
|
|
|
|
## Details
|
|
|
|
### Sub-title streams
|
|
|
|
The main requirement here is to avoid stalling the
|
|
pipeline between sub-title packets, and is effectively updating the
|
|
minimum-timestamp for that
|
|
stream.
|
|
|
|
A demuxer can do this by sending an 'update' SEGMENT with a new start time
|
|
to the subtitle pad. For example, every time the SCR in MPEG data
|
|
advances more than 0.5 seconds, the MPEG demuxer can issue a SEGMENT with
|
|
(update=TRUE, start=SCR ). Downstream elements can then be aware not to
|
|
expect any data older than the new start time.
|
|
|
|
The same holds true for any element that knows the current position in the
|
|
stream - once the element knows that there is no more data to be presented
|
|
until time 'n' it can advance the start time of the current segment to 'n'.
|
|
|
|
This technique can also be used, for example, to represent a stream of
|
|
MIDI events spaced to a clock period. When there is no event present for
|
|
a clock time, a SEGMENT update can be sent in its place.
|
|
|
|
### Still frame/menu support
|
|
|
|
Still frames in DVD menus are not the same,
|
|
in that they do not introduce a gap in the timestamps of the data.
|
|
Instead, they represent a pause in the presentation of a stream.
|
|
Correctly performing the wait requires some synchronisation with
|
|
downstream elements.
|
|
|
|
In this scenario, an upstream element that wants to execute a still frame
|
|
performs the following steps:
|
|
|
|
- Send all data before the still frame wait
|
|
|
|
- Send a DRAIN event to ensure that all data has been played
|
|
downstream.
|
|
|
|
- wait on the clock for the required duration, possibly interrupting
|
|
if necessary due to an intervening activity (such as a user
|
|
navigation)
|
|
|
|
- FLUSH the pipeline using a normal flush sequence (FLUSH\_START,
|
|
chain-lock, FLUSH\_STOP)
|
|
|
|
- Send a SEGMENT to restart playback with the next timestamp in the
|
|
stream.
|
|
|
|
The upstream element performing the wait must only do so when in the PLAYING
|
|
state. During PAUSED, the clock will not be running, and may not even have
|
|
been distributed to the element yet.
|
|
|
|
DRAIN is a new event that will block on a src pad until all data downstream
|
|
has been played out.
|
|
|
|
Flushing after completing the still wait is to ensure that data after the wait
|
|
is played correctly. Without it, sinks will consider the first buffers
|
|
(x seconds, where x is the duration of the wait that occurred) to be
|
|
arriving late at the sink, and they will be discarded instead of played.
|
|
|
|
### For audio
|
|
|
|
It is the same case as the first one - there is a *gap* in the audio
|
|
data that needs to be presented, and this can be done by sending a
|
|
SEGMENT update that moves the start time of the segment to the next
|
|
timestamp when data will be sent.
|
|
|
|
For video, however it is slightly different. Video frames are typically
|
|
treated at the moment as continuing to be displayed after their indicated
|
|
duration if no new frame arrives. Here, it is desired to display a blank
|
|
frame instead, in which case at least one blank frame should be sent before
|
|
updating the start time of the segment.
|