I'm trying to convert a video with alpha channel using FFmpeg, but the transparency information gets lost in the output.

Here is the input video (prores, yuva444p) detail with ffprobe:

 Stream #0:1[0x2](und): Video: prores (4444) (ap4h / 0x68347061), yuva444p12le(bt709), 360x480, 73314 kb/s, 60.13 fps, 60 tbr, 600 tbn (default)
      Metadata:
        creation_time   : 2019-05-21T21:23:03.000000Z
        handler_name    : Core Media Video
        vendor_id       : appl
        encoder         : Apple ProRes 4444

And my command is below:

ffmpeg -i alpha_prores.mov -c:v libvpx-vp9 -pix_fmt yuva420p -auto-alt-ref 0 out.webm

There is the output info:

Output #0, webm, to 'out.webm':
  Metadata:
    major_brand     : qt
    minor_version   : 0
    compatible_brands: qt
    com.apple.quicktime.creationdate: 2019-05-14T13:47:17-0700
    com.apple.quicktime.location.ISO6709: +37.3367-122.0094/
    com.apple.quicktime.make: Apple
    com.apple.quicktime.model: iPhone X
    com.apple.quicktime.software: 12.1.2
    encoder         : Lavf61.7.100
  Stream #0:0(und): Video: vp9, yuva420p(tv, bt709, progressive), 360x480, q=2-31, 60 fps, 1k tbn (default)
      Metadata:
        creation_time   : 2019-05-21T21:23:03.000000Z
        handler_name    : Core Media Video
        vendor_id       : appl
        encoder         : Lavc61.19.101 libvpx-vp9
      Side data:
        cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A

We can see pix_fmt is still yuva420p, but after finished, I use ffprobe to check it, and the result changed!

  Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv, bt709, progressive), 360x480, SAR 1:1 DAR 3:4, 60 fps, 60 tbr, 1k tbn (default)
      Metadata:
        alpha_mode      : 1
        HANDLER_NAME    : Core Media Video
        VENDOR_ID       : appl
        ENCODER         : Lavc61.19.101 libvpx-vp9
        DURATION        : 00:00:05.950000000

The pix_fmt turn to yuv420p, which means lose alpha channel. Also we can see there is an extra info in metadata alpha_mode: 1.

By the way, the result webm video can display its transparent pixel correctly in browser.

When I try to overlay it on a mp4 video, transparent pixels turn to black pixels:

ffmpeg -i background.mp4 -i out.webm -filter_complex "[0][1]overlay=x=10:y=10" output2.mp4

And when I overlay the prores video on the same mp4, result is correct.

ffmpeg -i background.mp4 -i alpha_prores.mov -filter_complex "[0][1]overlay=x=10:y=10" output2.mp4

So I wonder the difference between yuva and alpha_mode:1.

And the reason why vp9 would lose alpha channel when overlay on a mp4 video.

It's better if you can provide other pixel format for my purpose(I want it can display correctly when overlay on mp4)!

Thanks!