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docs/design/draft-latency.txt: Fix typo.

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Original commit message from CVS:
* docs/design/draft-latency.txt:
Fix typo.
* docs/design/part-buffering.txt:
Update design docs with more buffering ideas.
This commit is contained in:
Wim Taymans 2008-04-04 13:43:26 +00:00
parent 95c26e81bb
commit 878e4d9204
3 changed files with 200 additions and 26 deletions
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8
ChangeLog
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@ -1,3 +1,11 @@
2008-04-04 Wim Taymans <wim.taymans@collabora.co.uk>
* docs/design/draft-latency.txt:
Fix typo.
* docs/design/part-buffering.txt:
Update design docs with more buffering ideas.
2008-04-03 Tim-Philipp Müller <tim at centricular dot net>
* configure.ac:

2
docs/design/draft-latency.txt
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@ -293,7 +293,7 @@ A flush in a pipeline can happen in the following cases:
When a playing sink is flushed by a FLUSH_START event, an ASYNC_START message is
posted by the element. As part of the message, the fact that the element got
flushes is included. The element also goes to a pending PAUSED state and has to
flushed is included. The element also goes to a pending PAUSED state and has to
be set to the PLAYING state again later.
The ASYNC_START message is kept by the parent bin. When the element prerolls,

216
docs/design/part-buffering.txt
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@ -6,26 +6,109 @@ core that can be used by plugins and applications.
The purpose of buffering is to accumulate enough data in a pipeline so that
playback can occur smoothly and without interruptions. It is typically done
when reading from a (slow) and non-live network source.
when reading from a (slow) and non-live network source but can also be used for
live sources.
We want to be able to implement the following features:
- buffering up to a specifc amount of data, in memory, before starting playback
so that network fluctuations are minimized.
- download of the network file to a local disk with fast seeking in the
downloaded data. This is similar to the quicktime/youtube players.
- caching of semi-live streams to a local, on disk, ringbuffer with seeking in
the cached area. This is similar to tivo-like timeshifting.
- progress report about the buffering operations
- easy (backward compatible) application notification of buffering
- the possibility for the application to do more complex buffering
Some use cases:
+---------+ +--------+ +-------+
| httpsrc | | buffer | | demux |
| src - sink src - sink ....
+---------+ +--------+ +-------+
* Stream buffering:
+---------+ +---------+ +-------+
| httpsrc | | buffer | | demux |
| src - sink src - sink ....
+---------+ +---------+ +-------+
In this case we are reading from a slow network source into a buffer element
(such as queue).
(such as queue2).
The buffer element has a low and high watermark expressed in bytes. The
buffer uses the watermarks as follows:
- The buffer element will not produce data on the src pad until the high
watermark is hit. While accumulating data in the buffer, progress is
reported by posting BUFFERING messages.
- The source will stop to produce data on the src pad when the low watermark
is hit.
- The buffer element will post BUFFERING messages until the high watermark
is hit. This instructs the application to keep the pipeline PAUSED, which
will eventually block the srcpad from pushing while data is prerolled in
the sinks.
- When the high watermark is hit, a BUFFERING message with 100% will be
posted, which instructs the application to continue playback.
- When during playback, the low watermark is hit, the queue will start posting
BUFFERING messages again, making the application PAUSE the pipeline again
until the high watermark is hit again.
- during playback, the queue level will fluctuate between the high and the
low watermark as a way to compensate for network irregularities.
This buffering method is usable when the demuxer operates in push mode.
Seeking in the stream requires the seek to happen in the network source.
It is mostly desirable when the total duration of the file is not know, such
as in live streaming or when efficient seeking is not possible/required.
* Incremental download
+---------+ +---------+ +-------+
| httpsrc | | buffer | | demux |
| src - sink src - sink ....
+---------+ +----|----+ +-------+
V
file
In this case, we know the server is streaming a fixed length file to the
client. The application can choose to download the file on disk. The buffer
element will provide a push or pull based srcpad to the demuxer to navigate in
the downloaded file.
This mode is only suitable when the client can determine the length of the
file on the server.
In this case, buffering messages will be emited as usual when the requested
range is not within the downloaded area + buffersize. The buffering message
will also contain an indication that incremental download is being performed.
This flag can be used to let the application control the buffering in a more
intelligent way, using the BUFFERING query, for example.
The application can use the BUFFERING query to get the estimated download time
and match this time to the current/remaining playback time to control when
playback should start to have a non-interupted playback experience.
* Timeshifting
+---------+ +---------+ +-------+
| httpsrc | | buffer | | demux |
| src - sink src - sink ....
+---------+ +----|----+ +-------+
V
file-ringbuffer
In this mode, a fixed size ringbuffer is kept to download the server content.
This allows for seeking in the buffered data. Depending on the size of the
buffer one can seek further back in time.
This mode is suitable for all live streams.
As with the incremental download mode, buffering messages are emited along
with an indication that timeshifting download is in progress.
* Live buffering
In live pipelines we usually introduce some latency between the capture and
the playback elements. This latency can be introduced by a queue (such as a
jitterbuffer) or by other means (in the audiosink).
Buffering messages can be emited in those live pipelines as well and serve as
an indication to the user of the latency buffering. The application usually
does not react to these buffering messages with a state change.
Messages
@ -37,6 +120,46 @@ BUFFERING message is 100, buffering is done. Values less than 100 mean that
buffering is in progress.
The BUFFERING message should be intercepted and acted upon by the application.
The message contains at least one field that is sufficient for basic
functionality:
"buffer-percent", G_TYPE_INT, between 0 and 100
Several more clever ways of dealing with the buffering messages can be used when
in incremental or timeshifting download mode. For this purpose additional fields
are added to the buffering message:
"buffering-mode", GST_TYPE_BUFFERING_MODE,
enum { "none", "stream", "download", "timeshift", "live" }
- gives the buffering mode in use. See above for an explanation of the
different modes of buffering.
"avg-in-rate", G_TYPE_INT
- gives the average input buffering speed in bytes/second. -1 is unknown.
This is the average number of bytes per second that is received on the
buffering element input (sink) pads. If is a measurement of the network
speed in most cases.
"avg-out-rate", G_TYPE_INT
- gives the average consumption speed in bytes/second. -1 is unknown.
This is the average number of bytes per second that is consumed by the
downstream element of the buffering element.
"buffering-left", G_TYPE_INT
- gives the estimated time that bufferring will take in milliseconds.
-1 unknown.
This is measured based on the avg-in-rate and the filled level of the
queue. The application can use this hint to update the gui while buffering.
"estimated-time", G_TYPE_INT
- gives the estimated download time in milliseconds. -1 unknown.
When the size of the downloaded file is known, this value will contain
the latest estimate of the remaining download time. This value is usualy
only filled for the "download" buffering mode.
Application
@ -51,29 +174,72 @@ application should not set the pipeline to PLAYING before a BUFFERING message
with 100 percent value is received, which might only happen after the pipeline
prerolled.
An exception is made for live pipelines. The application may not change
the state of a live pipeline when a buffering message is received. Usually these
buffering messages contain the "buffering-mode" = "live".
The buffering message can also instruct the application to switch to a periodical
BUFFERING query instead to more precisely control the buffering process. The
application can, for example, choose to not act on the BUFFERING message with
100 percent fill level to resume playback but instead use the estimated download
time to resume playback to get uninterrupted playback.
Buffering Query
---------------
It is possible to query the amount of buffering performed in the pipeline, which
is defined as the amount of data made available at the source. This amount is
expressed in some GstFormat and is usually compared to the duration or position
of the media stream in the same GstFormat.
In addition to the BUFFERING messages posted by the buffering elements we want
to be able to query the same information from the application. We also want to
be able to present the user with information about the downloaded range in the
file so that the GUI can react on it.
The buffering query should return the following information:
In addition to all the fields present in the buffering message, the BUFFERING
query contains the following field, which indicate the available downloaded
range in a specific format:
- format
- position
- duration
"format", GST_TYPE_FORMAT
- the format of the "start" and "stop" values below
"start", G_TYPE_INT64, -1 unknown
- the start position of the available data
The format is of lesser importance, the ratio of position versus duration can be
used to calculate the percentage of available media. It should also be possible
for the application to calculate the expected time when the complete file will
be buffered.
"stop", G_TYPE_INT64, -1 unknown
- the stop position of the available data
For the "download" and "timeshift" buffering-modes, the start and stop positions
specify the ranges where efficient seeking in the downloaded media is possible.
Seeking outside of these ranges might be slow or not at all possible.
For the "stream" and "live" mode the start and stop values describe the oldest
and newest item (expressed in "format") in the buffer.
Defaults
--------
Incremental download
--------------------
Some defaults for common elements:
A GstBaseSrc with random access replies to the BUFFERING query with:
"buffer-percent" = 100
"buffering-mode" = "none"
"avg-in-rate" = -1
"avg-out-rate" = -1
"buffering-left" = 0
"estimated-time" = 0
"format" = GST_FORMAT_BYTES
"start" = 0
"stop" = the total filesize
A GstBaseSrc in push mode replies to the BUFFERING query with:
"buffer-percent" = 100
"buffering-mode" = "none"
"avg-in-rate" = -1
"avg-out-rate" = -1
"buffering-left" = 0
"estimated-time" = 0
"format" = a valid GST_TYPE_FORMAT
"start" = current position
"stop" = current position