Output video to a file and to the screen (while also allow user to pause the recording)

[Радомир Хаџић]

Hello.

I want to save what camera "sees" in a file while also displaying it
on the screen. I have figured out that I can do that with this command

     gst-launch-1.0 -e v4l2src device=/dev/video0 ! videoconvert !
queue ! tee name=t t. ! queue ! matroskamux ! queue ! filesink
location=/tmp/test.mkv t. ! queue ! autovideosink sync=false

But since I'm working on a C program that uses GStreamer and GTK I
need to translate this command to C and I don't know how to properly
do it. I also want my program to allow users to pause the recording
but not the video output but I successfully figured that out.

In the end my pipeline looks like this


         /-> queue -> autovideosink
    v4l2src -> videoconvert -> queue -> tee

        \-> queue -> matroskamux -> queue -> filesink || fakesink

The reason for "filesink || fakesink" is because these two are swapped
when the recording state changes: when we're recording filesink is
present and when a user pauses the recording filesink is replaced with
fakesink (and when a user unpauses the recording fakesink is replaced
by filesink, etc.).

I wrote a small program (available as an attachment) that can
replicate all relevant problems that I have encountered in my project.
Use "Pause" / "Unpause" button to pause / unpause the recording and
"Stop" button to stop the recording and quit the application (normally
quitting the application can be achieved by also closing the control
window (the one with the buttons) but not by closing preview window (I
didn't bother changing the preview window to custom GTK window so this
is the default window provided by GStreamer)).

I hope it's OK that this example program uses a little bit of GTK,
it's only there for the control window and you don't have to know
anything about GTK to help me with my problem.

I used video player MPV to test the resulting MKV files. These are the
errors that I have found when running my program (all these errors
also exist in attached program if you find more please tell me):
    1. If I click on "Pause" then on "Unpause" the resulting file is
too corrupted to be recognized by MPV as an MKV file.
    2. If I do not click "Pause" button or click it only once before
stopping the program MPV displays errors like "[mkv] Expected element
0x1043a770 not found" when opening the file (usually three these). If
I use gst-launch-1.0 command mentioned at the beginning of this letter
opening the resulting MKV file does not produce such errors.
    3. When the resulting file produced as in error 2 and is played
using MPV, MPV seems unable to determine the duration of the file so
it constantly changes what it thinks the duration is. Again, if I use
command mentioned at the beginning of this letter this problem doesn't
occur.

I think that's it.
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