Gstreamer到底是个啥？

GStreamer 是一个 基于pipeline的多媒体框架，基于GObject，以C语言写成。

应用GStreamer这个这个多媒体框架，你可以写出任意一种流媒体的应用来如：meidaplayer、音视频编辑器、VOIP、流媒体服务器、音视频编码等等。

关于视频快进/快退/循环播放的知识总结:

1.本地视频时长获取:

Gst.Pad.query_duration官方函数介绍:

def Gst.Pad.query_duration (self, format):
 #python wrapper for 'gst_pad_query_duration'

Queries a pad for the total stream duration.

Parameters:
pad ( Gst.Pad ) –a Gst.Pad to invoke the duration query on.
format ( Gst.Format ) –the Gst.Format requested

Returns a tuple made of:
( gboolean ) –TRUE (not introspectable) if the query could be performed.
duration ( gint64 ) –TRUE (not introspectable) if the query could be performed.

使用如下:

pipeline.query_duration(Gst.Format.TIME)[1]

其中pipeline为播放本地视频的管道,query_duration()函数返回一个元组,元组的形式为[Ture,duration:******]，******为以ns为单位的视频时长。

2.视频播放当前位置获取：

Gst.Pad.query_position官方函数介绍：

def Gst.Pad.query_position (self, format):
 #python wrapper for 'gst_pad_query_position'

Queries a pad for the stream position.

Parameters:
pad ( Gst.Pad ) –a Gst.Pad to invoke the position query on.
format ( Gst.Format ) –the Gst.Format requested

Returns a tuple made of:
( gboolean ) –TRUE (not introspectable) if the query could be performed.
cur ( gint64 ) –TRUE (not introspectable) if the query could be performed.

使用方法与时长获取函数query_duration()相同。

3.播放跳转函数：

Gst.Element.seek_simple官方函数介绍：

def Gst.Element.seek_simple (self, format, seek_flags, seek_pos):
 #python wrapper for 'gst_element_seek_simple'
 
Parameters:
element ( Gst.Element ) –a Gst.Element to seek on
format ( Gst.Format ) –a Gst.Format to execute the seek in, such as Gst.Format.TIME
seek_flags ( Gst.SeekFlags ) –seek options; playback applications will usually want to use GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_KEY_UNIT here
seek_pos ( gint64 ) –position to seek to (relative to the start); if you are doing a seek in Gst.Format.TIME this value is in nanoseconds - multiply with Gst.SECOND to convert seconds to nanoseconds or with Gst.MSECOND to convert milliseconds to nanoseconds.

Returns ( gboolean ) :
TRUE (not introspectable) if the seek operation succeeded. Flushing seeks will trigger a preroll, which will emit Gst.MessageType.ASYNC_DONE.

函数使用样例：

pipeline.seek_simple(Gst.Format.TIME, Gst.SeekFlags.FLUSH, time)

其中time的单位为nanoseconds。

有视频快进/快退/循环播放功能的小播放器.

import os, _thread, time
import gi
gi.require_version("Gst", "1.0")
gi.require_version('Gtk', '3.0')
from gi.repository import Gst, GObject, Gtk, Gdk
class GTK_Main:
 def __init__(self):
  window = Gtk.Window(Gtk.WindowType.TOPLEVEL)
  window.set_title("Vorbis-Player")
  window.set_default_size(500, -1)
  window.connect("destroy", Gtk.main_quit, "WM destroy")
  vbox = Gtk.VBox()
  window.add(vbox)
  self.entry = Gtk.Entry()
  vbox.pack_start(self.entry, False, False, 0)
  hbox = Gtk.HBox()
  vbox.add(hbox)
  buttonbox = Gtk.HButtonBox()
  hbox.pack_start(buttonbox, False, False, 0)
  rewind_button = Gtk.Button("Rewind")
  rewind_button.connect("clicked", self.rewind_callback)
  buttonbox.add(rewind_button)
  self.button = Gtk.Button("Start")
  self.button.connect("clicked", self.start_stop)
  buttonbox.add(self.button)
  forward_button = Gtk.Button("Forward")
  forward_button.connect("clicked", self.forward_callback)
  buttonbox.add(forward_button)
  self.time_label = Gtk.Label()
  self.time_label.set_text("00:00 / 00:00")
  hbox.add(self.time_label)
  window.show_all()
  self.player = Gst.Pipeline.new("player")
  source = Gst.ElementFactory.make("filesrc", "file-source")
  demuxer = Gst.ElementFactory.make("decodebin", "demuxer")
  videoconv = Gst.ElementFactory.make("videoconvert", "converter")
  videosink = Gst.ElementFactory.make("xvimagesink", "video-output")
  demuxer.connect("pad-added", self.demuxer_callback, videoconv)
  for ele in [source, demuxer, videoconv, videosink]:
   self.player.add(ele)
  source.link(demuxer)
  videoconv.link(videosink)
  bus = self.player.get_bus()
  bus.add_signal_watch()
  bus.connect("message", self.on_message)
 def start_stop(self, w):
  if self.button.get_label() == "Start":
   filepath = self.entry.get_text().strip()
   if os.path.isfile(filepath):
    filepath = os.path.realpath(filepath)
    self.button.set_label("Stop")
    self.player.get_by_name("file-source").set_property("location", filepath)
    self.player.set_state(Gst.State.PLAYING)
    self.play_thread_id = _thread.start_new_thread(self.play_thread, ())
  else:
   self.play_thread_id = None
   self.player.set_state(Gst.State.NULL)
   self.button.set_label("Start")
   self.time_label.set_text("00:00 / 00:00")
 def play_thread(self):
  play_thread_id = self.play_thread_id
  Gdk.threads_enter()
  self.time_label.set_text("00:00 / 00:00")
  Gdk.threads_leave()
  print(play_thread_id)
  print(self.play_thread_id)
  while play_thread_id == self.play_thread_id:
   time.sleep(0.2)
   dur_int = self.player.query_duration(Gst.Format.TIME)[1]
   if dur_int == -1:
    continue
   dur_str = self.convert_ns(dur_int)
   Gdk.threads_enter()
   self.time_label.set_text("00:00 / " + dur_str)
   Gdk.threads_leave()
   break
  time.sleep(0.2)
  while play_thread_id == self.play_thread_id:
   pos_int = self.player.query_position(Gst.Format.TIME)[1]
   pos_str = self.convert_ns(pos_int)
   if play_thread_id == self.play_thread_id:
    Gdk.threads_enter()
    self.time_label.set_text(pos_str + " / " + dur_str)
    Gdk.threads_leave()
   time.sleep(1)
 def on_message(self, bus, message):
  t = message.type
  if t == Gst.MessageType.EOS:
   self.player.seek_simple(Gst.Format.TIME, Gst.SeekFlags.FLUSH, 0000000000)
  elif t == Gst.MessageType.ERROR:
   err, debug = message.parse_error()
   print ("Error: %s" % err, debug)
   self.play_thread_id = None
   self.player.set_state(Gst.State.NULL)
   self.button.set_label("Start")
   self.time_label.set_text("00:00 / 00:00")
 def demuxer_callback(self, demuxer, pad, dst):
  caps = Gst.Pad.get_current_caps(pad)
  structure_name = caps.to_string()
  if structure_name.startswith("video"):
   videorate_pad = dst.get_static_pad("sink")
   pad.link(videorate_pad)
 def rewind_callback(self, w):
  rc, pos_int = self.player.query_position(Gst.Format.TIME)
  seek_ns = pos_int - 10 * 1000000000
  if seek_ns < 0:
   seek_ns = 0
  print ('Backward: %d ns -> %d ns' % (pos_int, seek_ns))
  self.player.seek_simple(Gst.Format.TIME, Gst.SeekFlags.FLUSH, seek_ns)
 def forward_callback(self, w):
  rc, pos_int = self.player.query_position(Gst.Format.TIME)
  seek_ns = pos_int + 10 * 1000000000
  print ('Forward: %d ns -> %d ns' % (pos_int, seek_ns))
  self.player.seek_simple(Gst.Format.TIME, Gst.SeekFlags.FLUSH, seek_ns)
 def convert_ns(self, t):
  s,ns = divmod(t, 1000000000)
  m,s = divmod(s, 60)
  if m < 60:
   return "%02i:%02i" %(m,s)
  else:
   h,m = divmod(m, 60)
   return "%i:%02i:%02i" %(h,m,s)
GObject.threads_init()
Gst.init(None)
GTK_Main()
Gtk.main()

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