I am trying to find optimal parameters for encoding a transparent video using ffmpeg in my scenario, and as a test, I have been re-encoding a pre-made transparent .webm (vp9 with alpha channel, made using python vidgear) like this:
$ /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%
According to the ffmpeg logs, the libvpx-vp9 is correctly being chosen, and alpha_mode is 1, which bodes well. However, when actually viewing the video, the video is no longer trasparent.
How could I force ffmpeg to keep the alpha channel when re-encoding the video? (This also applies when trying to merge audio into the .webm)
Bonus question: what arguments do you recommend for efficient vp9 encoding? I have tried using -threads 16 -speed 4 -tile-columns 6 -frame-parallel 1 -row-mt 1 but it seems to fail at fully utilizing all my CPU cores when encoding with the alpha channel (when piping frames from my python script).
See the line
Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv) ...
The yuv420p indicates that ffmpeg hasn't detected alpha in the input, else it would be yuva420p. To export alpha from VP9 streams, you need to use libvpx decoders, so
ffmpeg -c:v libvpx-vp9 -i pre-made.webm ...