Lossless VP9 Webm Encoding of PNG Sequence with Alpha in FFmpeg

I'm trying to encode a sequence of 32-bit 8bpc PNG images (RGB with an alpha channel) using the lossless VP9 codec with ffmpeg; specifically:

ffmpeg -framerate 60 -i out%04d.png -c:v libvpx-vp9 -lossless 1 -pix_fmt yuva420p output.webm

This seems to encode the video losslessly; however, when I run:

ffmpeg -i output.webm out%04d.png

the resulting PNG images that it spits out are missing the alpha channel (they are 24 bit, 8bpc)! What's going wrong here?

These are the files that I'm testing this on if you want to try it for yourself:

If it helps, I'm using this build of ffmpeg on Windows 10:

 built with gcc 9.1.1 (GCC) 20190621 configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt libavutil 56. 30.100 / 56. 30.100 libavcodec 58. 53.101 / 58. 53.101 libavformat 58. 28.101 / 58. 28.101 libavdevice 58. 7.100 / 58. 7.100 libavfilter 7. 55.100 / 7. 55.100 libswscale 5. 4.101 / 5. 4.101 libswresample 3. 4.100 / 3. 4.100 libpostproc 55. 4.100 / 55. 4.100

1 Answer

The native VP9 decoder does not support alpha, so the VPX decoder has to be forced.

ffmpeg -c:v libvpx-vp9 -i output.webm out%04d.png