如何使用gstreamer将两个视频(都有音频)混合到一个视频中？

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

# 初始化GStreamer
Gst.init(None)

# 创建管道
pipeline = Gst.Pipeline()

# 创建元素
filesrc1 = Gst.ElementFactory.make("filesrc", "filesrc1")
filesrc1.set_property("location", "video1.mp4")

filesrc2 = Gst.ElementFactory.make("filesrc", "filesrc2")
filesrc2.set_property("location", "video2.mp4")

decodebin1 = Gst.ElementFactory.make("decodebin", "decodebin1")
decodebin2 = Gst.ElementFactory.make("decodebin", "decodebin2")

videoscale1 = Gst.ElementFactory.make("videoscale", "videoscale1")
videoscale2 = Gst.ElementFactory.make("videoscale", "videoscale2")

videomixer = Gst.ElementFactory.make("videomixer", "videomixer")

audioconvert1 = Gst.ElementFactory.make("audioconvert", "audioconvert1")
audioconvert2 = Gst.ElementFactory.make("audioconvert", "audioconvert2")

adder = Gst.ElementFactory.make("adder", "adder")

filesink = Gst.ElementFactory.make("filesink", "filesink")
filesink.set_property("location", "output.mp4")

# 添加元素到管道
pipeline.add(filesrc1)
pipeline.add(filesrc2)
pipeline.add(decodebin1)
pipeline.add(decodebin2)
pipeline.add(videoscale1)
pipeline.add(videoscale2)
pipeline.add(videomixer)
pipeline.add(audioconvert1)
pipeline.add(audioconvert2)
pipeline.add(adder)
pipeline.add(filesink)

# 链接元素
filesrc1.link(decodebin1)
filesrc2.link(decodebin2)
decodebin1.connect("pad-added", on_pad_added, videoscale1)
decodebin2.connect("pad-added", on_pad_added, videoscale2)
videoscale1.link(videomixer)
videoscale2.link(videomixer)
videomixer.link(adder)
adder.link(filesink)

# 启动管道
pipeline.set_state(Gst.State.PLAYING)

# 等待管道完成
bus = pipeline.get_bus()
msg = bus.timed_pop_filtered(Gst.CLOCK_TIME_NONE, Gst.MessageType.ERROR | Gst.MessageType.EOS)

# 停止管道
pipeline.set_state(Gst.State.NULL)