python - 使用 appsink 时让 gstreamer 管道以读取/解码速度运行

Question

如果我创建一个简单的管道来将 mkv 文件读入 fakesink，它的运行速度将与机器读取和处理文件的速度一样快:

$ time gst-launch-1.0 filesrc location=bar.mkv ! matroskademux ! fakesink
Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
Pipeline is PREROLLED ...
Setting pipeline to PLAYING ...
New clock: GstSystemClock
Got EOS from element "pipeline0".
Execution ended after 0:00:00.000402211
Setting pipeline to PAUSED ...
Setting pipeline to READY ...
Setting pipeline to NULL ...
Freeing pipeline ...

real    0m0.020s
user    0m0.016s
sys     0m0.004s

相关文件长度为 5 秒，但运行时间不到一秒。现在，如果我想在程序中拥有相同的功能，并使用 appsink 而不是 fakesink，它会以播放速度运行。考虑这个简单的 Python 程序:

import sys
import gi
gi.require_version('Gst', '1.0')
from gi.repository import Gst
from gi.repository import GObject


def on_message(bus, msg, loop):
    if msg.type == Gst.MessageType.EOS:
        print("End of stream")
        loop.quit()

    return True


def new_sample(sink, data):
    sample = sink.emit('pull-sample')
    return Gst.FlowReturn.OK


def main():
    Gst.init(None)

    loop = GObject.MainLoop()

    pipeline = Gst.Pipeline()
    src = Gst.ElementFactory.make('filesrc')
    demux = Gst.ElementFactory.make('matroskademux')
    sink = Gst.ElementFactory.make(sys.argv[1])

    src.set_property('location', 'bar.mkv')

    if sys.argv[1] == 'appsink':
        sink.set_property('emit-signals', True)
        sink.connect('new-sample', new_sample, None)

    pipeline.add(src)
    pipeline.add(demux)
    pipeline.add(sink)

    src.link(demux)

    def pad_added(element, pad):
        stream = pad.query_caps(None).to_string()
        pad.link(sink.get_static_pad('sink'))
    demux.connect('pad-added', pad_added)

    bus = pipeline.get_bus()
    bus.add_signal_watch()
    bus.connect('message', on_message, loop)

    pipeline.set_state(Gst.State.PLAYING)
    loop.run()
    pipeline.set_state(Gst.State.NULL)


if __name__ == '__main__':
    main()

如果我用 fakesink 运行它，它仍然非常快:

$ time python ./foo.py fakesink
End of stream

real    0m0.095s
user    0m0.078s
sys     0m0.018s

但是如果我使用 appsink，只需要 5 秒多一点:

$ time python ./foo.py appsink
End of stream

real    0m5.068s
user    0m0.099s
sys     0m0.014s

这表明 gstreamer 正在同步帧，以便它们以文件中指定的帧速率运行。我们可以通过 appsink 获得 fakesink 行为吗？

Answer 1

此行为是通过接收器的 sync 属性控制的。默认情况下，fakesink 将此设置为 false。将此属性设置为 true 可以与 appsink 具有相同的行为。

$ time gst-launch-1.0 filesrc location=bar.mkv ! matroskademux ! fakesink sync=true