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Video Pipeline
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.. include:: ../links.ref
.. include:: ../tags.ref
.. include:: ../abbrs.ref

============ ==========================
**Abstract** Video Pipeline
**Authors**  Walter Fan
**Status**   WIP
**Updated**  |date|
============ ==========================



.. contents::
   :local:

概述
================================

视频流水线是指视频从发送端摄像头捕捉视频图像，将视频流进行转换，编码，打包，发送，直到接收端进行解包，缓存，解码再到将视频图像渲染到画布上或者视频元素中

.. image:: ../_static/video_pipeline.webp

.. code-block::

   #show test video
   gst-launch-1.0 -vvv videotestsrc ! 'video/x-raw,width=1280,height=720,format=RGB,framerate=60/1' ! fpsdisplaysink

   #play audio of mp4
   gst-launch-1.0 filesrc location=`pwd`/obama_talk.mp4 \
   ! qtdemux name=demux  demux.audio_0 \
   ! decodebin ! audioconvert ! audioresample ! autoaudiosink

   #play video of mp4
   gst-launch-1.0 filesrc location=`pwd`/obama_talk.mp4 \
   ! qtdemux name=demux  demux.video_0 \
   ! decodebin ! videoconvert ! videoscale ! autovideosink



Video pipeline by gstreamer
-------------------------------


.. code-block::

   # linux send h264 rtp stream:
   gst-launch-1.0 -v ximagesrc \
   ! video/x-raw,framerate=20/1 \
   ! videoscale ! videoconvert \
   ! x264enc tune=zerolatency bitrate=500 speed-preset=superfast \
   ! rtph264pay \
   ! udpsink host=127.0.0.1 port=5000

   # Macos send h264 rtp stream:
   gst-launch-1.0 -v avfvideosrc capture-screen=true \
   ! video/x-raw,framerate=20/1 \
   ! videoscale ! videoconvert \
   ! x264enc tune=zerolatency bitrate=500 speed-preset=superfast \
   ! rtph264pay \
   ! udpsink host=127.0.0.1 port=5000

   # receive h264 rtp stream:
   gst-launch-1.0 -v udpsrc port=5000 caps = "application/x-rtp, media=(string)video, clock-rate=(int)90000, encoding-name=(string)H264, payload=(int)96" \
   ! rtph264depay \
   ! decodebin \
   ! videoconvert \
   ! autovideosink


大体步骤如下

1. capture 捕捉媒体流

getUserMedia
getDisplayMedia

2. convert 格式转换

RGB --> YUV

3. Encode 编码

H.264 Encode

4. Packetize 打包

H264 NAL pack into RTP packet

5. Transmit 传输

Transmit RTP packets from sender to receiver

6. Depacketize 解包

Depacketize RTP packets to H264 NAL

7. Jitter Buffer 缓存

Cache H264 NAL if there is no frame can be decoded

9. Decode 解码

Decode H264 NAL to VAL to image

10.  Render 渲染或回放

Render the image to Canvas, playback the images as video