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: https://drive.google.com/open?id=1IKThU4X_Ub1IqW-pnhds4Z_KwQEwUMtK
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```
