Reading frames one-by-one to python/numpy

[Hjalmar Turesson]

Hi,

The seek works when the format is TIME, e.g.:

pipeline.seek_simple(Gst.Format.TIME, Gst.SeekFlags.FLUSH, 0.3*Gst.SECOND)

but not when the format is BUFFERS, e.g.:

pipeline.seek_simple(Gst.Format.BUFFERS, Gst.SeekFlags.FLUSH, 1000)

Does this make any sense?

Thanks,
Hjalmar

On Mon, Dec 7, 2015 at 4:07 PM, Hjalmar Turesson <hturesson at gmail.com>
wrote:

> Perfect example. It does exactly what I wanted.
>
> But... The seek returns false (the other part works perfectly)
>
>     CLI='filesrc location=%s ! decodebin ! appsink name=sink' % filename
>     pipeline=Gst.parse_launch(CLI)
>     # get sink
>     appsink=pipeline.get_by_name("sink")
>     # set to PAUSED to make the first frame arrive in the sink
>     pipeline.set_state(Gst.State.PAUSED)
> *    pipeline.seek_simple(Gst.Format.BUFFERS, Gst.SeekFlags.FLUSH,
> frame_number)*
>     smp = appsink.emit('pull-preroll')
>     buf = smp.get_buffer()
>
>     data=buf.extract_dup(0, buf.get_size())[:307200] # Only 1/2 of
> expected RGB size
>     frame = np.fromstring(data,dtype='uint8').reshape((480,640,1))
>
>
> Thanks a lot,
>
> Hjalmar
>
> On Mon, Dec 7, 2015 at 1:47 PM, Tim Müller <tim at centricular.com> wrote:
>
>> On Mon, 2015-12-07 at 13:30 -0300, Hjalmar Turesson wrote:
>>
>> Hi Hjalmar,
>>
>> This (C) example shows how to retrieve frames from appsink, perhaps you
>> find it useful:
>>
>> http://cgit.freedesktop.org/gstreamer/gst-plugins-base/tree/tests/examp
>> les/snapshot/snapshot.c
>> <http://cgit.freedesktop.org/gstreamer/gst-plugins-base/tree/tests/examples/snapshot/snapshot.c>
>>
>> It uses the pull-preroll signal/API, that only works once, if you want
>> to retrieve one frame, and then another one, then just use the pull-
>> sample API N times.
>>
>> There are two ways to use appsink: one is to have it emit a signal
>> (new-sample) whenever a buffer arrives, and then you pull it in the
>> callback). The other is to simply set the pipeline to PLAYING and then
>> do pull-sample, which will block and wait for a simple if none is
>> available yet then.
>>
>> Note that from the "new-sample" signal callback you can also return
>> GST_FLOW_EOS if you don't want to be called again.
>>
>> Cheers
>>  -Tim
>>
>> > Hi,
>> >
>> > I would like read individual frames to python/numpy for analysis.
>> > That is, I want call a function that reads and returns 1 frame. I
>> > thought I could do this with appsink, but I have 2 problems. 1) I
>> > cannot return a frame from the callback function 2) It doesn't just
>> > read 1 frame.
>> >
>> > Simple example that does not do what I want:
>> >
>> >    import gi
>> >    gi.require_version('Gst', '1.0')
>> >    from gi.repository import GObject, Gst, Gtk
>> >    GObject.threads_init()
>> >    Gst.init(None)
>> >    ##########
>> >
>> >     CLI='filesrc location=testVideo.mp4 ! decodebin ! appsink
>> > name=sink'
>> >     pipeline=Gst.parse_launch(CLI)
>> >     appsink=pipeline.get_by_name("sink")
>> >     appsink.set_property("max-buffers",1)
>> >     appsink.set_property('emit-signals',True)
>> >     appsink.set_property('sync',False)
>> >
>> >     def on_new_buffer(appsink):
>> >         sample = appsink.emit('pull-sample')
>> >         #get the buffer
>> >         buf=sample.get_buffer()
>> >         data=buf.extract_dup(0,buf.get_size())
>> >         return False  # Here I would like to return the frame
>> >
>> >     appsink.connect('new-sample', on_new_buffer)
>> >     pipeline.set_state(Gst.State.PLAYING)
>> >     pipeline.set_state(Gst.State.NULL)
>> >
>> > Is it possible to do what I want to do using appsink?
>> >
>> > Another strategy I tried is to use fdsink and read the data from that
>> > location:
>> >
>> >     import os
>> >
>> >     readfd, writefd = os.pipe()
>> >     frame_size_bytes = 460800
>> >
>> >     CLI='filesrc location=testVideo.mp4 ! decodebin ! fdsink
>> > name=sink'
>> >     pipeline=Gst.parse_launch(CLI)
>> >     fdsink=pipeline.get_by_name("sink")
>> >     fdsink.set_property("fd", readfd)
>> >
>> >     pipeline.set_state(Gst.State.PLAYING)
>> >     data = os.read(readfd, frame_size_bytes)
>> >     pipeline.set_state(Gst.State.PAUSED)
>> >
>> > This does not work either. The code hang at the data = os.read...
>> > line, maybe trying to play the entire file before moving on.
>> >
>> > How can I read only one frame?
>> >
>> > Thanks,
>> > Hjalmar
>> > _______________________________________________
>> > gstreamer-devel mailing list
>> > gstreamer-devel at lists.freedesktop.org
>> > http://lists.freedesktop.org/mailman/listinfo/gstreamer-devel
>> --
>> Tim Müller, Centricular Ltd - http://www.centricular.com
>>
>>
>> _______________________________________________
>> gstreamer-devel mailing list
>> gstreamer-devel at lists.freedesktop.org
>> http://lists.freedesktop.org/mailman/listinfo/gstreamer-devel
>>
>
>
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