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According to the specs and the options in the export panel webm should support transparency via the alpha channel.
The export window shows the checkerboard pattern, but the final file is completely black in those areas even though RGBA was selected.
I have not been able to get it to export correctly, either with VP8 or VP9 (similarly the H.264 option for mp4 does not work for this either).
Is this is a known issue and will there be a workaround?

Please, at first provide your informations.

Because it's not only a matter of codec specification, but also of operating system and decoder.

Which player do you use to verify the video transparency ? not all players do support it well.

deruji wrote: ↑12 Aug 2021, 14:56 Is this is a known issue and will there be a workaround?

Before any other attempt, I would suggest you to try with the PNG codec.

There seem to be a glitch in the GUI, RGBA is only available with VP9 (not VP8, though it should).

Xavier wrote: ↑19 Aug 2021, 11:23 Please, at first provide your informations.

Because it's not only a matter of codec specification, but also of operating system and decoder.

Which player do you use to verify the video transparency ? not all players do support it well.

I am using:

Operating System: Manjaro Linux
KDE Plasma Version: 5.22.4
KDE Frameworks Version: 5.85.0
Qt Version: 5.15.2
Kernel Version: 5.10.59-1-MANJARO (64-bit)
Graphics Platform: X11
Processors: 8 × Intel® Core™ i7-4770K CPU @ 3.50GHz
Memory: 31,2 GiB of RAM
Graphics Processor: NVIDIA GeForce GTX 960/PCIe/SSE2

TVPaint Pro STD 11.5.2 (64-bit)

I mainly use MPV for testing the output files, because for the files that do use transparency it shows a checkerboard pattern.
VLC I have as well, but it only shows a black background for all files, so it's no use to me for that.
The files I make are intended as digital assets for VTT such as roll20. Importing the files that do show the transparency work as they should. The files that do not work correctly there have been the same that do not show transparency when testing with MPV.
I already use mp4 with the PNG codec, which works well, though it has larger output files, it would be preferable to use webm as well.

deruji wrote: ↑19 Aug 2021, 14:22 I mainly use MPV for testing the output files, because for the files that do use transparency it shows a checkerboard pattern.

Great, that's good to know. This morning I've been looking for a player with such capability, and it's quite a struggle to find.

So indeed, using MPV I was able to see the transparency with AVI, PNG (MP4, MKV, MOV) and ProRes (MOV) but not VP9 (WebM) nor H.264.
Yet is TVPaint in cause or is it MPV ? when playing the WebM VP9 video, I noticed this warning from MPV : This message is actually produced by FFmpeg on which MPV relies ... as well as TVPaint.
Anyway, given that the support for H264 and VP9 are quite recent in TVPaint, it's possible that the transparency is not possible yet. So it'd be a bug.

About the PNG codec : this isn't great compression but a lot better than uncompressed.
If you prefer not to use PNG, I'd advise you to go either for AVI or MOV+ProRes.

In the meantime we'll try to fix the transparency, but it will take time.

Thank you for the information. I look forward to newer versions that improve this