序幕

我ffmpeg在Ubuntu 20.04计算机上使用4.2.2克隆USB网络摄像头的提要（v4l2设备为/dev/video0），以便多个应用程序可以访问提要。我使用了这个ffmpeg命令，效果很好：

ffmpeg -f v4l2 -i /dev/video0 -codec copy -f v4l2 /dev/video1

从本质上讲，这使用形式为的v4l2回送设备（虚拟设备），/dev/video1而消费应用程序只是将其视为常规v4l2设备。

在此之后，我遇到了更改输出像素格式的需求（相关的SO问题）。我的源设备提供了yuv420p，接收器应用程序只能处理yuyv422。为此，我使用了-pix_fmt参数。但是，使用时不支持此功能，-codec copy因此我需要明确指定编解码器。这是没有问题的，因为v4l2设备仅支持原始视频（至少据我所知）。因此，我最终得到以下结果：

ffmpeg -f v4l2 -i /dev/video0 -c:v rawvideo -pix_fmt yuyv422 -f v4l2 /dev/video1

问题

不幸的是，这似乎非常不稳定。在启动此命令的10次中，大约有1次按预期运行：ffmpeg开始克隆提要并转换颜色格式：

joel@joel-ubuntu:~$ ffmpeg -f v4l2 -i /dev/video0 -c:v rawvideo -pix_fmt yuyv422 -f v4l2 /dev/video1
ffmpeg version 4.2.2-1ubuntu1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.3.0-3ubuntu1)
  configuration: --prefix=/usr --extra-version=1ubuntu1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-nvenc --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
[video4linux2,v4l2 @ 0x55a4f38a5700] Time per frame unknown
Input #0, video4linux2,v4l2, from '/dev/video0':
  Duration: N/A, start: 11224.864514, bitrate: N/A
    Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 640x480, 14.33 tbr, 1000k tbn, 1000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> rawvideo (native))
Press [q] to stop, [?] for help
Output #0, video4linux2,v4l2, to '/dev/video1':
  Metadata:
    encoder         : Lavf58.29.100
    Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 640x480, q=2-31, 70451 kb/s, 14.33 fps, 14.33 tbn, 14.33 tbc
    Metadata:
      encoder         : Lavc58.54.100 rawvideo
frame=  214 fps= 15 q=-0.0 Lsize=N/A time=00:00:14.93 bitrate=N/A dup=1 drop=0 speed=1.05x    
video:128400kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Exiting normally, received signal 2.

但是，有时我会遇到非常过时的行为，即除了第一帧（然后冻结）之后，输出之后再也无法使用，并且ffmpeg在短短几秒钟内干草线就提到了超过10万个重复帧，并且报告了FPS超过30'000的情况。 。几秒钟后，它便崩溃了，或者只是挂起，直到我停下Ctrl^C来：

joel@joel-ubuntu:~$ ffmpeg -f v4l2 -i /dev/video0 -c:v rawvideo -pix_fmt yuyv422 -f v4l2 /dev/video1
ffmpeg version 4.2.2-1ubuntu1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.3.0-3ubuntu1)
  configuration: --prefix=/usr --extra-version=1ubuntu1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-nvenc --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
[video4linux2,v4l2 @ 0x561ef693c700] Time per frame unknown
Input #0, video4linux2,v4l2, from '/dev/video0':
  Duration: N/A, start: 11156.178415, bitrate: N/A
    Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 640x480, 1000k tbr, 1000k tbn, 1000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> rawvideo (native))
Press [q] to stop, [?] for help
[video4linux2,v4l2 @ 0x561ef6940400] Frame rate very high for a muxer not efficiently supporting it.
Please consider specifying a lower framerate, a different muxer or -vsync 2
Output #0, video4linux2,v4l2, to '/dev/video1':
  Metadata:
    encoder         : Lavf58.29.100
    Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 640x480, q=2-31, 4915200000 kb/s, 1000k fps, 1000k tbn, 1000k tbc
    Metadata:
      encoder         : Lavc58.54.100 rawvideo
More than 1000 frames duplicated
More than 10000 frames duplicated
More than 100000 frames duplicated=N/A time=00:00:00.15 bitrate=N/A dup=151050 drop=0 speed=0.0456x    
27653321 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=0 speed=0.0289x    
27733577 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=1 speed=0.0256x    
27787315 frame duplication too large, skipping
27859335 frame duplication too large, skipping
27941595 frame duplication too large, skipping
28006927 frame duplication too large, skipping
28069615 frame duplication too large, skipping
28140781 frame duplication too large, skipping
28217871 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=8 speed=0.0251x    
28282039 frame duplication too large, skipping
28347777 frame duplication too large, skipping
28449099 frame duplication too large, skipping
28491015 frame duplication too large, skipping
28565585 frame duplication too large, skipping
28630457 frame duplication too large, skipping
28710537 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=15 speed=0.0247x    
28770549 frame duplication too large, skipping
28843129 frame duplication too large, skipping
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29193197 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=22 speed=0.0243x    
29269443 frame duplication too large, skipping
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30740757 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=44 speed=0.023x    
30806259 frame duplication too large, skipping
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32358605 frame duplication too large, skipping
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33292845 frame duplication too large, skipping
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33720711 frame duplication too large, skipping
33749987 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=107 speed=0.021x    
33784387 frame duplication too large, skipping
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34263479 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=116 speed=0.0207x    
34403751 frame duplication too large, skipping
34470319 frame duplication too large, skipping
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34613587 frame duplication too large, skipping
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34755115 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=122 speed=0.0204x    
34823847 frame duplication too large, skipping
34893931 frame duplication too large, skipping
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35034011 frame duplication too large, skipping
35103715 frame duplication too large, skipping
35178691 frame duplication too large, skipping
frame=707641 fps=20090 q=-0.0 Lsize=N/A time=00:00:00.70 bitrate=N/A dup=707630 drop=129 speed=0.0201x    
video:424584600kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Exiting normally, received signal 2.

我不知道是什么导致了这种行为，也不知道如何对其进行跟踪和修复。是否有人对这种行为有经验，或者我的设置中有一个简单的问题？

注意：我正在使用DroidCam。DroidCam提供了Linux客户端，而Linux客户端又提供了v4l2输出设备。除了一些小问题以外，我没有发现任何与常规网络摄像头不同的行为。不幸的是，我无法使用普通的USB UVC网络摄像头来重现该视频，因为该网络摄像头已经提供了所需的输出像素格式。输出像素格式转换实际上仅是必需的，因为DroidCam输出yuv420p但消耗流的应用程序只能处理yuyv422。