Convert WEBM to HEVC with alpha

Question

I'm trying to convert a simple WEBM video with transparency to HEVC with alpha. I couldn't find any resources about how to convert without losing the alpha channel.

I don't know if it's possible with FFMPEG:

encoding requests cannot be made here as FFmpeg does not contain a native hevc encoder.

cf. https://trac.ffmpeg.org/ticket/7965

Answer 1

VideoToolbox supports alpha / transparency

FFmpeg does not have its own HEVC/H.265 encoder. It supports 7 HEVC/H.265 external encoders (most are hardware based) including libx265 which does not currently support alpha.

As of now only the VideoToolbox HEVC Encoder (-c:v hevc_videotoolbox) supports alpha. This is a hardware accelerated encoder for Apple hardware. This has been updated after FFmpeg 4.3 was released, so you'll have to use a build from the current git branch, or wait for 4.4.

Example command adapted from the relevant commit:

ffmpeg -i input%03d.png -c:v hevc_videotoolbox -allow_sw 1 -alpha_quality 0.75 -vtag hvc1 output.mov

The "HEVC Video with Alpha" profile will only be used when the -alpha_quality value is not 0 (default is 0, range 0-1.). See ffmpeg -h encoder=hevc_videotoolbox for more info.

This answer may become outdated due to continuous developments, so make sure to research the current situation.

Checking if an encoder supports alpha / transparency

$ ffmpeg -h encoder=libx265
Supported pixel formats: yuv420p yuvj420p yuv422p yuvj422p yuv444p yuvj444p gbrp yuv420p10le yuv422p10le yuv444p10le gbrp10le yuv420p12le yuv422p12le yuv444p12le gbrp12le gray gray10le gray12le

If it supported alpha it would have pixel format with an a in the name, such as yuva420p or rgba. As you can see libx265 does not yet support alpha.

Related bug reports to watch

• #7965 - Support for HEVC decoding with Alpha
• #9088 - HEVC bitstream with alpha layer to mp4

Answer 2

This is my example how to convert transparent VP9 webm video to mov. Instead of libvpx better to use a new libvpx-vp9 (the more advanced version of libvpx).

Step 1. Split webm video to frames

ffmpeg -vcodec libvpx-vp9 -i input.webm -pix_fmt rgba image_%04d.png

Step 2. Combine all frames to a new mov video.

ffmpeg -i image_%04d.png -c:v hevc_videotoolbox -allow_sw 1 -alpha_quality 0.75 -vtag hvc1 output.mov

Now you can use it on web

<video autoplay muted loop playsinline>
    <source src="output.mov" type="video/quicktime">
    <source src="input.webm" type="video/webm">
</video>

Answer 3

I also racked my brains over this problem but finally got it to work. Here are the steps I did (on a Mac):

1. Convert the transparent webm to a set of transparent pngs (with 25fps) with this command:

   ffmpeg -vcodec libvpx -i transparent.webm -pix_fmt rgba -r 25 out/image_%d.png

2. Create a transparent mov video from the pngs:

   cd out
   ffmpeg -i image_%d.png -c:v prores_ks -profile:v 5  -bits_per_mb 8000 -pix_fmt yuva444p10le output.mov
   

(taken from: how can I convert a stack of PNGs with transparency to a video playable with Keynote )

3. This mov is transparent but has a huge filesize (10gb in my case). Open the video with quicktime, click File > Export as. Select the desired resolution and then select HEVC and keep “Preserve Transparency” checked.