frame-by-frame and reverse playback in python

[RiccardoCagnasso]

I'm trying to incorporate frame-by-frame playback a playbin based python
software, following this documentation
https://gstreamer.freedesktop.org/documentation/tutorials/basic/playback-speed.html?gi-language=c

I encountered some problems. First if I try to do

        self.pipeline.send_event(Gst.Event.new_seek(0.0, Gst.Format.TIME,
Gst.SeekFlags.FLUSH, Gst.SeekType.SET, position, Gst.SeekType.NONE, 0))
 
and then

    self.pipeline.send_event(Gst.Event.new_step(Gst.Format.BUFFERS, 1, 1,
True, False))

It doesn't work, the video just freezes. But it starts to work perfectly if
i change the seek to

        self.pipeline.send_event(Gst.Event.new_seek(0.00000001,
Gst.Format.TIME, Gst.SeekFlags.FLUSH, Gst.SeekType.SET, position,
Gst.SeekType.NONE, 0))

This seems to me some type conversion problem. But I don't have the
expertise on GI/Gobject/C to track it down.

More important, I can't seem to get reverse playback working.

            pos = self.player.pipeline.query_position(Gst.Format.TIME)[1]
           self.pipeline.send_event(Gst.Event.new_seek(-1.0,
Gst.Format.TIME, Gst.SeekFlags.FLUSH, Gst.SeekType.SET, 0,
Gst.SeekType.NONE, pos))

This should work, but the video just freezes and i get this error message:

0:00:20.992670044  6144 0x7f6f380165e0 WARN            videodecoder
gstvideodecoder.c:1140:gst_video_decoder_sink_event_default:<avdec_huffyuv0>
error: No valid frames decoded before end of stream
0:00:20.992697014  6144 0x7f6f380165e0 WARN            videodecoder
gstvideodecoder.c:1140:gst_video_decoder_sink_event_default:<avdec_huffyuv0>
error: no valid frames found









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[RiccardoCagnasso]

I kind of solved the second problem by sending the events to the videosink
instead that to the pipeline and changing the video from a motion jpeg to a
h264.

Problem is that in frame-by-frame reverse playback, it jumps from keyframe
to keyframe. Which... makes sense, but it's not what I need. I need to "go
back" actual frame by actual frame, that's why I was trying to use motion
jpeg. I also tried prores and raw video formats to no avail.

Is there any way to achieve this?



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[Mathieu Duponchelle]

Have you tried using the step API ([1], [2]) ?

[1]: https://gstreamer.freedesktop.org/documentation/gstreamer/gstevent.html?gi-language=c#gst_event_new_step
[2]: https://gstreamer.freedesktop.org/documentation/tutorials/basic/playback-speed.html?gi-language=c#a-trick-mode-player

On 6/25/19 12:17 AM, RiccardoCagnasso wrote:

> I kind of solved the second problem by sending the events to the videosink
> instead that to the pipeline and changing the video from a motion jpeg to a
> h264.
>
> Problem is that in frame-by-frame reverse playback, it jumps from keyframe
> to keyframe. Which... makes sense, but it's not what I need. I need to "go
> back" actual frame by actual frame, that's why I was trying to use motion
> jpeg. I also tried prores and raw video formats to no avail.
>
> Is there any way to achieve this?
>
>
>
> --
> Sent from: http://gstreamer-devel.966125.n4.nabble.com/
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[RiccardoCagnasso]

Yes, I'm using the step api for reverse playback also. Every step jumps from
one keyframe to the other.



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[Victor Jaquez]

On Tue, 25 Jun 2019 at 06:04, RiccardoCagnasso wrote:
> Yes, I'm using the step api for reverse playback also. Every step jumps from
> one keyframe to the other.

Are you using gstreamer-vaapi?

Because, as the decoder's context is bound to the number of surfaces in dpb, and
the reverse playback is based on holding a complete dpb it runs into a surface
exhaustion, thus in gstreamer-vaapi all non keyframe are discarded to avoid the
exhaustion.

We are discussing how to improve this increasing dynamically the context bounded
surfaces, but that might not be supported in other drivers.

vmjl
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[RiccardoCagnasso]

I changed computer and here it worked just fine. This workstation has a
quadro and no gstreamer-vaapi.
I guess that the reverse step playback doesn't work well with something in
vaapi.

The first behavior is still strange though.



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