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tutorials: time-management: improve the example walkthrough

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Additionally: fix missing markup
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Reynaldo H. Verdejo Pinochet 2017-07-27 16:53:22 -07:00
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markdown/tutorials/basic/time-management.md
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@ -7,8 +7,7 @@ particular:
- How to query the pipeline for information like stream position or
duration.
- How to seek (jump) to a different position (time instant) inside the
- How to seek (jump) to a different position (time) inside the
stream.
## Introduction
@ -20,11 +19,11 @@ is allowed, then, once the movie has been running for ten seconds, we
skip to a different position using a seek.
In the previous tutorials, once we had the pipeline setup and running,
our main function just sat and waited to receive an ERROR or an EOS
through the bus. Here we modify this function to periodically wake up
our main function just sat and waited to receive an `ERROR` or an `EOS`
through the bus. Here, we modify this function to periodically wake up
and query the pipeline for the stream position, so we can print it on
screen. This is similar to what a media player would do, updating the
User Interface on a periodic basis.
the screen. This is similar to what a media player would do, updating the
user Interface on a periodic basis.
Finally, the stream duration is queried and updated whenever it changes.
@ -229,11 +228,11 @@ can pass it around to other functions. In particular, in this example we
move the message handling code to its own function
`handle_message` because it is growing a bit too big.
We would then build a pipeline composed of a single element, a
We then build a pipeline composed of a single element, a
`playbin`, which we already saw in [Basic tutorial 1: Hello
world!](tutorials/basic/hello-world.md). However,
`playbin` is in itself a pipeline, and in this case it is the only
element in the pipeline, so we use directly the `playbin` element. We
element in the pipeline, so we directly use the `playbin` element. We
will skip the details: the URI of the clip is given to `playbin` via
the URI property and the pipeline is set to the playing state.
@ -245,11 +244,13 @@ msg = gst_bus_timed_pop_filtered (bus, 100 * GST_MSECOND,
Previously we did not provide a timeout to
`gst_bus_timed_pop_filtered()`, meaning that it didn't return until a
message was received. Now we use a timeout of 100 milliseconds, so, if
no message is received, 10 times per second the function will return
with a NULL instead of a `GstMessage`. We are going to use this to
update our “UI”. Note that the timeout period is specified in
nanoseconds, so usage of the `GST_SECOND` or `GST_MSECOND` macros is
highly recommended.
no message is received during one tenth of a second, the function will return
`NULL`. We are going to use this logic to update our “UI”.
Note that the desired timeout must be specified as a `GstClockTime`, hence,
in nanoseconds. Numbers expressing different time units then, should be
multiplied by macros like `GST_SECOND` or `GST_MSECOND`. This also makes
your code more readable.
If we got a message, we process it in the `handle_message`` `function
(next subsection), otherwise:
@ -261,9 +262,9 @@ If we got a message, we process it in the `handle_message`` `function
if (data.playing) {
```
First off, if we are not in the `PLAYING` state, we do not want to do
anything here, since most queries would fail. Otherwise, it is time to
refresh the screen.
If the pipeline is in `PLAYING` state, it is time to refresh the screen.
We don't want to do anything if we are not in `PLAYING` state, because
most queries would fail.
We get here approximately 10 times per second, a good enough refresh
rate for our UI. We are going to print on screen the current media
@ -301,8 +302,7 @@ g_print ("Position %" GST_TIME_FORMAT " / %" GST_TIME_FORMAT "\r",
```
Note the usage of the `GST_TIME_FORMAT` and `GST_TIME_ARGS` macros to
provide user-friendly representation of GStreamer
times.
provide a user-friendly representation of GStreamer times.
``` c
/* If seeking is enabled, we have not done it yet, and the time is right, seek */
@ -322,9 +322,9 @@ thing!
Let's review the parameters:
`GST_FORMAT_TIME` indicates that we are specifying the destination in
time, as opposite to bytes (and other more obscure mechanisms).
time units. Other seek-formats use different units.
Then come the GstSeekFlags, let's review the most common:
Then come the `GstSeekFlags`, let's review the most common:
`GST_SEEK_FLAG_FLUSH`: This discards all data currently in the pipeline
before doing the seek. Might pause a bit while the pipeline is refilled
@ -333,14 +333,13 @@ and the new data starts to show up, but greatly increases the
“stale” data might be shown for a while until the new position appears
at the end of the pipeline.
`GST_SEEK_FLAG_KEY_UNIT`: Most encoded video streams cannot seek to
arbitrary positions, only to certain frames called Key Frames. When this
`GST_SEEK_FLAG_KEY_UNIT`: With most encoded video streams, seeking to
arbitrary positions is not possible but only to certain frames called Key Frames. When this
flag is used, the seek will actually move to the closest key frame and
start producing data straight away. If this flag is not used, the
pipeline will move internally to the closest key frame (it has no other
alternative) but data will not be shown until it reaches the requested
position. Not providing the flag is more accurate, but might take longer
to react.
position. This last alternative is more accurate, but might take longer.
`GST_SEEK_FLAG_ACCURATE`: Some media clips do not provide enough
indexing information, meaning that seeking to arbitrary positions is
@ -350,14 +349,14 @@ enough for your case (you see seeks not going to the exact time you
asked for), then provide this flag. Be warned that it might take longer
to calculate the seeking position (very long, on some files).
And finally we provide the position to seek to. Since we asked
for `GST_FORMAT_TIME` , this position is in nanoseconds, so we use
the `GST_SECOND` macro for simplicity.
Finally, we provide the position to seek to. Since we asked
for `GST_FORMAT_TIME`, the value must be in nanoseconds so we express
the time in seconds, for simplicity, and then multiply by `GST_SECOND`.
### Message Pump
The `handle_message` function processes all messages received through
the pipeline's bus. ERROR and EOS handling is the same as in previous
the pipeline's bus. `ERROR` and `EOS` handling is the same as in previous
tutorials, so we skip to the interesting part:
``` c
@ -384,12 +383,10 @@ case GST_MESSAGE_STATE_CHANGED: {
```
Seeks and time queries generally only get a valid reply when in the
PAUSED or PLAYING state, since all elements have had a chance to
receive information and configure themselves. Here we take note of
whether we are in the PLAYING state or not with the `playing`
variable.
Also, if we have just entered the PLAYING state, we do our first query.
`PAUSED` or `PLAYING` state, since all elements have had a chance to
receive information and configure themselves. Here, we use the `playing`
variable to keep track of whether the pipeline is in `PLAYING` state.
Also, if we have just entered the `PLAYING` state, we do our first query.
We ask the pipeline if seeking is allowed on this stream:
``` c