Reading frames one-by-one to python/numpy

[Hjalmar Turesson]

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
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