FFmpeg v4l2 复制不稳定
FFmpeg v4l2 copying unstable
序曲
我在 Ubuntu 20.04 机器上使用 ffmpeg 4.2.2 克隆 USB 网络摄像头（v4l2 设备为 /dev/video0）的提要，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
本质上，这使用 /dev/video1 形式的 v4l2 环回设备（虚拟设备），并且消费应用程序只是将其视为常规 v4l2 设备。
之后效果很好，我运行进入需要改变输出像素格式（相关）。我的源设备提供 yuv420p 而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 -codec copy 时不支持此功能，因此我需要明确指定编解码器。这没问题，因为 v4l2 设备仅支持原始视频（至少据我所知）。
因此，我最终得到的是：
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。在启动此命令的大约 10 次中，它按预期工作：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，其他时候我会遇到非常陈旧的行为，除了第一帧（然后它冻结）之外，输出永远无法使用并且 ffmpeg 在短短几秒钟内提到超过 100k 重复帧以及报告超过 30'000 的 FPS。几秒钟后它就会崩溃或者只是挂起直到我 Ctrl^C 它:
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
28922285 frame duplication too large, skipping
28985573 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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29688271 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=29 speed=0.0238x
29759777 frame duplication too large, skipping
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30117317 frame duplication too large, skipping
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30255657 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=37 speed=0.0234x
30331325 frame duplication too large, skipping
30385245 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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31224713 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=51 speed=0.0227x
31297053 frame duplication too large, skipping
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31814019 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=59 speed=0.0223x
31876469 frame duplication too large, skipping
31917425 frame duplication too large, skipping
31995521 frame duplication too large, skipping
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32127183 frame duplication too large, skipping
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32267481 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=66 speed=0.022x
32358605 frame duplication too large, skipping
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32523429 frame duplication too large, skipping
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32753423 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=77 speed=0.0216x
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33255715 frame duplication too large, skipping:00:00.70 bitrate=N/A dup=707630 drop=92 speed=0.0213x
33292845 frame duplication too large, skipping
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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
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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
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35034011 frame duplication too large, skipping
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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，也不知道如何追踪并修复它。有没有人有过这种行为的经验，或者我的设置是否存在简单问题？
注意：我使用的是 DroidCam。 DroidCam 提供了一个 Linux 客户端，后者又提供了一个 v4l2 输出设备。除了一些小问题外，我没有注意到任何与普通网络摄像头不同的行为。不幸的是，我无法用普通的 USB UVC 网络摄像头重现这一点，因为网络摄像头已经提供了所需的输出像素格式。这种输出像素格式转换确实是必要的，因为 DroidCam 输出 yuv420p 但使用流的应用程序只能处理 yuyv422.
V4L2 设备是恒定帧率写入器，因此如果源帧率较低，ffmpeg 将复制帧以保持恒定帧率。在没有用户设置速率的情况下，ffmpeg 将其设置为输入的帧速率。对于没有明确表示帧速率的捕获设备，ffmpeg 通过寻找帧之间的最小间隔来推断速率。
当 ffmpeg 检测到 1000k tbr 时，就是您进行大量复制的时候。设置一个合理的数字 -r 15 或禁用帧复制，-vsync vfr.
序曲
我在 Ubuntu 20.04 机器上使用 ffmpeg 4.2.2 克隆 USB 网络摄像头（v4l2 设备为 /dev/video0）的提要，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
本质上，这使用 /dev/video1 形式的 v4l2 环回设备（虚拟设备），并且消费应用程序只是将其视为常规 v4l2 设备。
之后效果很好，我运行进入需要改变输出像素格式（相关yuv420p 而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 -codec copy 时不支持此功能，因此我需要明确指定编解码器。这没问题，因为 v4l2 设备仅支持原始视频（至少据我所知）。
因此，我最终得到的是：
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。在启动此命令的大约 10 次中，它按预期工作：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，其他时候我会遇到非常陈旧的行为，除了第一帧（然后它冻结）之外，输出永远无法使用并且 ffmpeg 在短短几秒钟内提到超过 100k 重复帧以及报告超过 30'000 的 FPS。几秒钟后它就会崩溃或者只是挂起直到我 Ctrl^C 它:
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
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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，也不知道如何追踪并修复它。有没有人有过这种行为的经验，或者我的设置是否存在简单问题？
注意：我使用的是 DroidCam。 DroidCam 提供了一个 Linux 客户端，后者又提供了一个 v4l2 输出设备。除了一些小问题外，我没有注意到任何与普通网络摄像头不同的行为。不幸的是，我无法用普通的 USB UVC 网络摄像头重现这一点，因为网络摄像头已经提供了所需的输出像素格式。这种输出像素格式转换确实是必要的，因为 DroidCam 输出 yuv420p 但使用流的应用程序只能处理 yuyv422.
V4L2 设备是恒定帧率写入器，因此如果源帧率较低，ffmpeg 将复制帧以保持恒定帧率。在没有用户设置速率的情况下，ffmpeg 将其设置为输入的帧速率。对于没有明确表示帧速率的捕获设备，ffmpeg 通过寻找帧之间的最小间隔来推断速率。
当 ffmpeg 检测到 1000k tbr 时，就是您进行大量复制的时候。设置一个合理的数字 -r 15 或禁用帧复制，-vsync vfr.