Using scenechange to obtain scene timestamps programatically

[Francisco Ayala Le Brun]

I have resolved the issue. I was using the wrong approach to intercept
the events. Using a pad probe instead, I can retrieve the event correctly.
Updated code: https://gist.github.com/fayalalebrun/6bb57cb6f3d730ec7d359cff69d5adcf

Of course, the question remains of whether an upstream element could
emit the GstForceKeyUnit event, thus causing a false positive in terms
of a scene change.

Francisco

Francisco Ayala Le Brun <francisco at videowindow.eu> writes:

> To whom it may concern,
>
> I am trying to use gstreamer in order to take screenshots of a video
> each time the scene changes. As a first step, I am attempting to get a
> list of timestamps corresponding to each scene change.
>
> In the documentation of the scenechange element, it is stated that a
> signal is emitted every time the scene changes. However, the name of the
> signal is not mentioned, and looking at the code, specifically lines
> 245-257 of scenechange.c, I can see no signal being emitted.
> Source file is here:
> https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/a10b35c011b8291bae7b5da959f5a9d6b9a7cc0f/subprojects/gst-plugins-bad/gst/videofilters/gstscenechange.c
>
> Thus I am now trying to catch the GstKeyForceUnit event in order to find
> out when the scene changes. I do this by using the playbin element,
> setting its video filter as the scenechange element, and then setting
> the video-sink as a bin with a fakesink. The bin has a GhostPad which
> prints events passing through it. However, while I can see scenechange's
> debug messages, no GstKeyForceUnit events seem to pass through the GhostPad.
> Here is the code: https://gist.github.com/fayalalebrun/6bb57cb6f3d730ec7d359cff69d5adcf
>
> The log I see is as follows:
> Reconfigure(Reconfigure(Event { ptr: 0x56036f43e030, type: "reconfigure", seqnum: Seqnum(4), structure: None }))
> Latency(Latency(Event { ptr: 0x7fa5680a06c0, type: "latency", seqnum: Seqnum(345), structure: Some(GstEventLatency, latency=(guint64)0;) }))
> 0:00:00.806090337 26542 0x7fa580023920 INFO             scenechange gstscenechange.c:246:gst_scene_change_transform_frame_ip:<scenechange0> 0 407.956 22.8788 0.0560816 1
> 0:00:01.080411411 26542 0x7fa580023920 INFO             scenechange gstscenechange.c:246:gst_scene_change_transform_frame_ip:<scenechange0> 0 115.526 31.0005 0.268341 1
> 0:00:01.274231698 26542 0x7fa580023920 INFO             scenechange gstscenechange.c:246:gst_scene_change_transform_frame_ip:<scenechange0> 0 755.079 22.5827 0.0299078 1
> 0:00:01.349182752 26542 0x7fa580023920 INFO             scenechange gstscenechange.c:246:gst_scene_change_transform_frame_ip:<scenechange0> 0 27.9868 30.45 1.08801 1
> 0:00:01.689698365 26542 0x7fa580023920 INFO             scenechange gstscenechange.c:246:gst_scene_change_transform_frame_ip:<scenechange0> 0 801.204 37.4627 0.046758 1
> 0:00:02.063346606 26542 0x7fa580023920 INFO             scenechange gstscenechange.c:246:gst_scene_change_transform_frame_ip:<scenechange0> 0 14.1078 38.7118 2.74401 1
> 0:00:02.797156784 26542 0x7fa580023920 INFO             scenechange gstscenechange.c:246:gst_scene_change_transform_frame_ip:<scenechange0> 0 6.56696 29.0186 4.41888 1
> 0:00:03.083369457 26542 0x7fa580023920 INFO             scenechange gstscenechange.c:246:gst_scene_change_transform_frame_ip:<scenechange0> 0 14.522 37.9089 2.61045 1
> 0:00:03.347006242 26542 0x7fa580023920 INFO             scenechange gstscenechange.c:246:gst_scene_change_transform_frame_ip:<scenechange0> 0 10.5185 35.7488 3.39866 1
>   C-c C-c
>
> As you can see, a couple of events are printed at the beginning, but not
> the GstKeyForceUnit event I am interested in.
>
> Thank you in advance,
> Francisco Ayala