如何使ffmpeg将带有alpha的vp9 webm重新编码为带alpha的vp9 webm

Question

在我的场景中，我试图找到使用ffmpeg编码透明视频的最佳参数，作为一个测试，我重新编码了一个预先制作的透明.webm (使用python制作的带有alpha通道的vp9)，如下所示：

$ /usr/bin/ffmpeg -i pre-made.webm pre-made-remade.webm
ffmpeg version 4.3.2-0york0~18.04 Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 7 (Ubuntu 7.5.0-3ubuntu1~18.04)
  configuration: --prefix=/usr --extra-version='0york0~18.04' --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-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-librabbitmq --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libsrt --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-libzimg --enable-pocketsphinx --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 51.100 / 56. 51.100
  libavcodec     58. 91.100 / 58. 91.100
  libavformat    58. 45.100 / 58. 45.100
  libavdevice    58. 10.100 / 58. 10.100
  libavfilter     7. 85.100 /  7. 85.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  7.100 /  5.  7.100
  libswresample   3.  7.100 /  3.  7.100
  libpostproc    55.  7.100 / 55.  7.100
Input #0, matroska,webm, from 'pre-made.webm':
  Metadata:
    ENCODER         : Lavf58.45.100
  Duration: 00:01:11.04, start: 0.000000, bitrate: 1107 kb/s
    Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv), 1514x1910, SAR 1:1 DAR 757:955, 25 fps, 25 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      alpha_mode      : 1
      ENCODER         : Lavc58.91.100 libvpx-vp9
      DURATION        : 00:01:11.040000000
Stream mapping:
  Stream #0:0 -> #0:0 (vp9 (native) -> vp9 (libvpx-vp9))
Press [q] to stop, [?] for help
[libvpx-vp9 @ 0x55927254b0c0] v1.7.0
[libvpx-vp9 @ 0x55927254b0c0] Neither bitrate nor constrained quality specified, using default CRF of 32
Output #0, webm, to 'pre-made-remade.webm':
  Metadata:
    encoder         : Lavf58.45.100
    Stream #0:0: Video: vp9 (libvpx-vp9), yuv420p, 1514x1910 [SAR 1:1 DAR 757:955], q=-1--1, 25 fps, 1k tbn, 25 tbc (default)
    Metadata:
      alpha_mode      : 1
      DURATION        : 00:01:11.040000000
      encoder         : Lavc58.91.100 libvpx-vp9
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
frame= 1776 fps=8.9 q=0.0 Lsize=    6159kB time=00:01:11.00 bitrate= 710.6kbits/s speed=0.354x
video:6146kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.214488%

根据ffmpeg日志，libvpx-vp9被正确选择，alpha_mode为1，这是个好兆头。然而，当实际观看视频时，视频不再是传送带。

当重新编码视频时，我怎么能强迫ffmpeg保留alpha通道呢？(这在尝试将音频合并到.webm时也适用)

额外的问题:对于高效的vp9编码，您建议使用哪些参数？我尝试过使用-threads 16 -speed 4 -tile-columns 6 -frame-parallel 1 -row-mt 1，但在用alpha通道编码时(当我的python脚本中的框架)编码时，它似乎无法充分利用我的所有CPU核心。

Answer 1

见线

Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv) ...

yuv420p表示输入中没有检测到alpha，否则将是yuva420p。要从VP9流导出alpha，需要使用libvpx解码器，因此

ffmpeg -c:v libvpx-vp9 -i pre-made.webm ...