FFmpeg忽略输出像素格式 [英] FFmpeg ignoring output pixel format

问题描述

我正在Ubuntu 20.04计算机上使用 ffmpeg 4.2.2克隆USB网络摄像头的视频流，以便多个应用程序可以使用同一提要.为此，我只需克隆到 v4l2 回送设备:

I am using ffmpeg 4.2.2 on an Ubuntu 20.04 machine to clone the video stream of a USB webcam so that multiple applications can use the same feed. To achieve this, I simply clone to a v4l2 loop back device:

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

到目前为止，这相当有效.我能够成功访问/dev/video1 ，该提要与/dev/video0 相同.注意:要进行此工作，您需要确保已启用 v4l2loopback 设备内核模块:

So far, this works reasonably well. I am able to successfully access /dev/video1 which presents the same feed as /dev/video0. Note: To make this work you need to ensure that the v4l2loopback device kernel module is enabled:

modprobe v4l2loopback devices=1

接下来，我想转换虚拟设备的像素格式，因为正在访问虚拟设备的应用程序只能处理 yuv422p 或 RGB ，而我的源设备/dev/video0 提供了 yuv420p .我认为这将是一个简单的任务，只需在输出设备上向 ffmpeg 附加一个 -pix_fmt 参数，就可以轻松地完成该任务，如下所示:

Next I'd like to convert the pixel format of the dummy device as the application that is accessing the dummy device can only handle yuv422p or RGB whereas my source device /dev/video0 provides yuv420p. I thought that this would be a simple task that can easily be handled by presenting ffmpeg with an additional -pix_fmt argument on the output device like so:

ffmpeg -f v4l2 -i /dev/video0 -codec copy -f v4l2 -pix_fmt yuv422p /dev/video1

虽然 ffmpeg 开始克隆流而没有任何警告或错误，但仍在 yuv420p 中输出:

While ffmpeg starts cloning the stream without any warnings or errors, it is still outputting in yuv420p instead:

joel@joel-ubuntu:~$ ffmpeg -f v4l2 -i /dev/video0 -codec copy -f v4l2 -pix_fmt yuv422p /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 @ 0x55ca407b9700] Time per frame unknown
Input #0, video4linux2,v4l2, from '/dev/video0':
  Duration: N/A, start: 6726.737520, bitrate: N/A
    Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 640x480, 29.25 tbr, 1000k tbn, 1000k tbc
Output #0, video4linux2,v4l2, to '/dev/video1':
  Metadata:
    encoder         : Lavf58.29.100
    Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 640x480, q=2-31, 29.25 tbr, 1000k tbn, 1000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
frame=   76 fps= 34 q=-1.0 Lsize=N/A time=00:00:02.52 bitrate=N/A speed=1.14x

无论我通过什么 -pix_fmt ，我总是在输出中得到 yuv420p .

No matter what -pix_fmt I pass, I always end up with yuv420p on the output.

我使用适当的USB UVC网络摄像头和DroidCam进行了几次测试.输出像素格式永远不会按预期更改.这也不特定于请求 yuv422p 作为像素格式.其他格式也将被忽略.为什么会这样呢?我想念什么?

I did several tests with both proper USB UVC webcams as well as DroidCam. The output pixel format never changes as expected. This is also not specific to requesting yuv422p as a pixel format. Also other formats are being ignored. Why is this happening? What am I missing?

注意:我已经验证了 ffmpeg 能够支持 yuv422p 像素格式(在执行 ffmpeg -pix_fmts 时会列出该格式).

Note: I have verified that ffmpeg is capable of the yuv422p pixel format (it is being listed when executing ffmpeg -pix_fmts).

推荐答案

使用 -c:v复制时不能更改像素格式.更改为 -c:v rawvideo .

You can't change pixel formats when using -c:v copy. Change to -c:v rawvideo.

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