python - python的gst.LinkError问题的gstreamer

Question

我正在用 Python 连接一个 gstreamer 应用程序。我得到一个带有以下代码的 LinkError：

import pygst
pygst.require('0.10')
import gst

import pygtk
pygtk.require('2.0')
import gtk

# this is very important, without this, callbacks from gstreamer thread
# will messed our program up
gtk.gdk.threads_init()

def main():
    pipeline = gst.Pipeline('pipleline')

    filesrc = gst.element_factory_make("filesrc", "filesrc")
    filesrc.set_property('location', 'C:/a.mp3')

    decode = gst.element_factory_make("decodebin", "decode")

    convert = gst.element_factory_make('audioconvert', 'convert')

    sink = gst.element_factory_make("autoaudiosink", "sink")

    pipeline.add(filesrc, decode, convert, sink)
    gst.element_link_many(filesrc, decode, convert, sink)

    pipeline.set_state(gst.STATE_PLAYING)

    gtk.main()

main()

和错误：

ImportError: could not import gio
Traceback (most recent call last):
  File "H:\workspace\ggg\src\test2.py", line 37, in <module>
    main()
  File "H:\workspace\ggg\src\test2.py", line 31, in main
    gst.element_link_many(filesrc, decode, convert, sink)
gst.LinkError: failed to link decode with convert

这很奇怪，使用相同的管道，但使用 parse_launch 构建，它可以工作。这是代码：

import pygst
pygst.require('0.10')
import gst

import pygtk
pygtk.require('2.0')
import gtk

# this is very important, without this, callbacks from gstreamer thread
# will messed our program up
gtk.gdk.threads_init()

def main():
    player = gst.parse_launch('filesrc location=C:/a.mp3 ! decodebin ! audioconvert ! autoaudiosink') 
    player.set_state(gst.STATE_PLAYING)
    gtk.main()

main()

问题来了，为什么手动失败，但解析成功？那有什么问题？我该如何解决？

谢谢。

Answer 1

your problem is here:

gst.element_link_many(filesrc, decode, convert, sink)

the reason is that not all elements have simple, static inputs and outputs. at this point in your program, your decodebin does not have any source pads (that is: no outputs).

a pad is like a nipple - it's an input / output to an element. pads can appear, disappear or just sit there. there are three classes of pads: static pads (the easiest and what you would expect), request pads (that appear only when you ask for them) and sometimes pads (that appear only when the element wants to make them appear). the outputs of decodebin are sometimes pads.

if you inspect the output of gst-inspect decodebin, you can see this for yourself:

Pad Templates:
  SINK template: 'sink'
    Availability: Always
    Capabilities:
      ANY

  SRC template: 'src%d'
    Availability: Sometimes
    Capabilities:
      ANY

at line 26 of your program, you can't link decode to anything, because it doesn't have any source pads to link with. source pads on a decodebin appear only as the input stream is decoded: this doesn't happen instantaneously. any number of source pads may appear (e.g one for an audio stream, two for a video stream with audio, none for an un-decodable stream).

you need to wait until the pads are created, and then link them. decodebin emits a signal, "new-decoded-pad" to tell you when this happens (this is also documented in gst-inspect decodebin). you must connect a callback function to this signal, and link your decode and audioconvert in the callback. here is your corrected code:

#!/usr/bin/python

import pygst
pygst.require('0.10')
import gst

import pygtk
pygtk.require('2.0')
import gtk

# this is very important, without this, callbacks from gstreamer thread
# will messed our program up
gtk.gdk.threads_init()

def on_new_decoded_pad(dbin, pad, islast):
    decode = pad.get_parent()
    pipeline = decode.get_parent()
    convert = pipeline.get_by_name('convert')
    decode.link(convert)
    pipeline.set_state(gst.STATE_PLAYING)
    print "linked!"

def main():
    pipeline = gst.Pipeline('pipleline')

    filesrc = gst.element_factory_make("filesrc", "filesrc")
    filesrc.set_property('location', 'C:/a.mp3')

    decode = gst.element_factory_make("decodebin", "decode")

    convert = gst.element_factory_make('audioconvert', 'convert')

    sink = gst.element_factory_make("autoaudiosink", "sink")

    pipeline.add(filesrc, decode, convert, sink)
    gst.element_link_many(filesrc, decode)
    gst.element_link_many(convert, sink)

    decode.connect("new-decoded-pad", on_new_decoded_pad)

    pipeline.set_state(gst.STATE_PAUSED)

    gtk.main()

main()

gst.parse_launch works because it takes care of all these niggly details for you. there is also the high level element playbin which automatically creates and links a decodebin internally.