我正在尝试使用ffmpeg的filter_complex来将多个视频输入合并为一个输出。
我使用v4l2loopback创建虚拟视频设备：
sudo modprobe v4l2loopback video_nr=90
在此示例中，我首先仅尝试使用一个摄像头：
v4l2-ctl --device=/dev/video0 --info提供以下输出：
Driver Info (not using libv4l2):
Driver name : uvcvideo
Card type : HD USB Camera
Bus info : usb-0000:00:14.0-13.2
Driver version: 4.10.17
Capabilities : 0x84200001
Video Capture
Streaming
Extended Pix Format
Device Capabilities
Device Caps : 0x04200001
Video Capture
Streaming
Extended Pix Format
Priority: 2
Video input : 0 (Camera 1: ok)
Format Video Capture:
Width/Height : 640/480
Pixel Format : 'YUYV'
Field : None
Bytes per Line : 1280
Size Image : 614400
Colorspace : sRGB
Transfer Function : Default
YCbCr Encoding : Default
Quantization : Default
Flags :
Crop Capability Video Capture:
Bounds : Left 0, Top 0, Width 640, Height 480
Default : Left 0, Top 0, Width 640, Height 480
Pixel Aspect: 1/1
Selection: crop_default, Left 0, Top 0, Width 640, Height 480
Selection: crop_bounds, Left 0, Top 0, Width 640, Height 480
Streaming Parameters Video Capture:
Capabilities : timeperframe
Frames per second: 30.000 (30/1)
Read buffers : 0
brightness (int) : min=0 max=15 step=1 default=8 value=8
contrast (int) : min=0 max=15 step=1 default=8 value=8
saturation (int) : min=0 max=15 step=1 default=7 value=7
hue (int) : min=-10 max=10 step=1 default=0value=0
white_balance_temperature_auto (bool) : default=1 value=1
gamma (int) : min=1 max=10 step=1 default=7 value=7
gain (int) : min=0 max=0 step=0 default=0 value=0
power_line_frequency (menu) : min=0 max=2 default=2 value=2
white_balance_temperature (int) : min=2800 max=6500 step=1 default=2800 value=2800 flags=inactive
sharpness (int) : min=0 max=15 step=1 default=6 value=6
backlight_compensation (int) : min=0 max=1 step=1 default=0 value=0
exposure_auto (menu) : min=0 max=3 default=3 value=3
exposure_absolute (int) : min=4 max=5000 step=1 default=625 value=625 flags=inactive
focus_absolute (int) : min=0 max=21 step=1 default=16 value=16 flags=inactive
focus_auto (bool) : default=1 value=1
然后我使用此命令以特定的分辨率将/ dev / video0输出到左侧的/ dev / video90：
ffmpeg -loglevel verbose -y \
-f v4l2 -thread_queue_size 512 -i /dev/video0 \
-filter_complex "nullsrc=size=960x240 [base]; \
[0:v] format=pix_fmts=yuva420p, setpts=PTS-STARTPTS, scale=320x240 [left]; \
[base][left] overlay=shortest=1" \
-f v4l2 /dev/video90
这将导致以下错误：
[v4l2 @ 0x55fa67d02f60] Unknown V4L2 pixel format equivalent for yuva420p
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
Error initializing output stream 0:0 --
Conversion failed!
完整日志：
ffmpeg version 3.4.4-1~16.04.york0 Copyright (c) 2000-2018 the FFmpeg developers
built with gcc 5.4.0 (Ubuntu 5.4.0-6ubuntu1~16.04.10) 20160609
configuration: --prefix=/usr --extra-version='1~16.04.york0' --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --enable-gpl --disable-stripping --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-librsvg --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-omx --enable-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libopencv --enable-libx264 --enable-shared
WARNING: library configuration mismatch
avcodec configuration: --prefix=/usr --extra-version='1~16.04.york0'--toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --enable-gpl --disable-stripping --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-librsvg --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-omx --enable-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libopencv --enable-libx264 --enable-shared --enable-version3 --disable-doc --disable-programs --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libtesseract --enable-libvo_amrwbenc
libavutil 55. 78.100 / 55. 78.100
libavcodec 57.107.100 / 57.107.100
libavformat 57. 83.100 / 57. 83.100
libavdevice 57. 10.100 / 57. 10.100
libavfilter 6.107.100 / 6.107.100
libavresample 3. 7. 0 / 3. 7. 0
libswscale 4. 8.100 / 4. 8.100
libswresample 2. 9.100 / 2. 9.100
libpostproc 54. 7.100 / 54. 7.100
[video4linux2,v4l2 @ 0x55fa67cfca00] fd:5 capabilities:84200001
[video4linux2,v4l2 @ 0x55fa67cfca00] Querying the device for the current frame size
[video4linux2,v4l2 @ 0x55fa67cfca00] Setting frame size to 640x480
Input #0, video4linux2,v4l2, from '/dev/video0':
Duration: N/A, start: 273229.829138, bitrate: 147456 kb/s
Stream #0:0: Video: rawvideo, 1 reference frame (YUY2 / 0x32595559), yuyv422, 640x480, 147456 kb/s, 30 fps, 30 tbr, 1000k tbn, 1000k tbc
[Parsed_nullsrc_0 @ 0x55fa67d00680] size:960x240 rate:25/1 duration:-1.000000 sar:1/1
[Parsed_scale_3 @ 0x55fa67d02240] w:320 h:240 flags:'bilinear' interl:0
Stream mapping:
Stream #0:0 (rawvideo) -> format
overlay -> Stream #0:0 (rawvideo)
Press [q] to stop, [?] for help
[Parsed_nullsrc_0 @ 0x55fa67cfc760] size:960x240 rate:25/1 duration:-1.000000 sar:1/1
[Parsed_scale_3 @ 0x55fa67d055c0] w:320 h:240 flags:'bilinear' interl:0
[graph 0 input from stream 0:0 @ 0x55fa67d06a40] w:640 h:480 pixfmt:yuyv422 tb:1/1000000 fr:30/1 sar:0/1 sws_param:flags=2
[auto_scaler_0 @ 0x55fa67d05d80] w:iw h:ih flags:'bilinear' interl:0
[Parsed_format_1 @ 0x55fa67d04360] auto-inserting filter 'auto_scaler_0' between the filter 'graph 0 input from stream 0:0' and the filter 'Parsed_format_1'
[auto_scaler_0 @ 0x55fa67d05d80] w:640 h:480 fmt:yuyv422 sar:0/1 -> w:640 h:480 fmt:yuva420p sar:0/1 flags:0x2
[Parsed_format_1 @ 0x55fa67d04360] TB:0.000001 FRAME_RATE:30.000000 SAMPLE_RATE:nan
[Parsed_scale_3 @ 0x55fa67d055c0] w:640 h:480 fmt:yuva420p sar:0/1 -> w:320 h:240 fmt:yuva420p sar:0/1 flags:0x2
[Parsed_overlay_4 @ 0x55fa67d05e80] main w:960 h:240 fmt:yuva420p overlay w:320 h:240 fmt:yuva420p
[Parsed_overlay_4 @ 0x55fa67d05e80] [framesync @ 0x55fa67d05fa8] Selected 1/1000000 time base
[Parsed_overlay_4 @ 0x55fa67d05e80] [framesync @ 0x55fa67d05fa8] Sync level 2
[v4l2 @ 0x55fa67d02f60] Unknown V4L2 pixel format equivalent for yuva420p
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
Error initializing output stream 0:0 --
Conversion failed!
我尝试过但没有奏效的东西：
-f h264用于输出，结果以av_interleaved_write_frame(): Invalid argument错误结束答案 0 :(得分：0)
由于叠加层输入包含Alpha，因此叠加层过滤器会从nullsrc过滤器中请求一种alpha格式。现在，v4l2输出设备不接受带有 alpha 的YUV格式，并且与大多数编码器不同，它无法与过滤器自动协商，因此显示了错误。在叠加层之后强制使用受支持的格式，以免出现这种情况。
使用
ffmpeg -loglevel verbose -y \
-f v4l2 -thread_queue_size 512 -i /dev/video0 \
-filter_complex "nullsrc=size=960x240, format=yuv420p [base]; \
[0:v] format=pix_fmts=yuva420p, setpts=PTS-STARTPTS, scale=320x240 [left]; \
[base][left] overlay=shortest=1,format=yuv420p" \
-f v4l2 /dev/video90