GStreamer: Pipeline to write data to file based on condition - c

Question

Is there a way to write data in a GStreamer pipeline to a file based on an (external) condition?
I have an application/code, which streams/displays video to the screen and continuously writes it to a file (it works fine).
I would like to have the GStreamer pipeline to only write to a file if an external condition is true (at runtime - I don't know the condition in advance).
What I have done so far:
I carefully searched the official GStreamer documentation, where I found some information on appsink, but I don't really see a way how to apply it based on an (external) conditional.
I also used 'dynamic pipelines' as a search term, which seems describe the modification of GStreamer pipelines based on conditions.
I also searched the GStreamer mailing list and found this post, which uses the gst_element_set_locked_state() function.
I added a
if (condition) {
gst_element_set_locked_state(videosink, 'TRUE');
} else {
gst_element_set_locked_state(videosink, 'FALSE');
}
to my code by then the pipeline would not work at all (displaying a black image).
Another way is described on https://coaxion.net/blog/2014/01/gstreamer-dynamic-pipelines/ in Example 2 with the corresponding code being available on GitHub (https://github.com/sdroege/gst-snippets/blob/217ae015aaddfe3f7aa66ffc936ce93401fca04e/dynamic-tee-vsink.c).
It seems to use a callback and the gst_element_set_state (sink->sink, GST_STATE_NULL) function call to write to a file based on an (external) condition.
Applying this function in analogy to the function above causes the pipeline to display find, but also results in continuous (and not conditional) output to a file:
if (condition) {
gst_element_set_state(videosink, GST_STATE_PLAYING);
} else {
gst_element_set_state(videosink, GST_STATE_NULL);
}
Also gst_pad_add_probe () could be a possibility to dynamically change output to a file, but despite having loocked in the GStreamer documentation, I don't know how use this function correctly.

Answer 1

For your requirement you need tee and valve elements.
Tee will seperate the pipeline for both displaying to window and writing to a file. Valve is the condition you are looking. Its drop attribute drops the frame where the valve is.
Your pipeline will be like:
gst-launch-1.0 ksvideosrc ! videoconvert ! tee name=t ! queue ! valve drop=false ! autovideosink t. ! queue ! valve drop=false ! openh264enc ! h264parse ! mp4mux ! filesink location="test.mp4" -v --eos-on-shutdown
When your condition occurres, set your specific valve's drop attribute as true for not continuing to write file.
In C/C++:
if(condition)
g_object_set(videoValve,"drop",true,nullptr);
else
g_object_set(videoValve,"drop",false,nullptr);
WARNING:
Valve elements must be false until data will pass inside everything in the pipeline. Which means, you can set valve's drop attribute as true when the pipeline is on PLAYING State. You can adjust your code accordingly such as trigger the mechanism on BusCallback, you can reach pipeline states inside that.
Note: ksvideosrc (Windows) if you use Unix try v4lsrc.
If you build your application like this, it will work, I use similar scenario.