What can I use to compress a webm file without losing transparency?

[supercain]

Hi,

I have a transparent background webm video and I want to make it smaller but I cant seem to find an application that allows me to compress it without losing its transparent background. What can I use for this task?

Thank you.

[poisondeathray]

Not much you can do in terms of terms of making it smaller in filesize and retaining alpha channel -

If it's lossless webm, then you can use lossy webm profile but it will be chroma subsampled (sharp color edges will become blurred)

If it's lossy profile webm already, you can use a lower quality webm but it will obviously be lower in quality

There is a hevc variant that supports alpha channel, but it's only supported by macs

Or depending on your scenario or how you are planning to use it, you can use 2 streams with separate alpha

[supercain]

I plan it to use it on the web so I would prefer a smaller file if possible.

[rrats]

I think it's possible with premiere pro

[poisondeathray]

I plan it to use it on the web so I would prefer a smaller file if possible.

Web - as in embedding ?

There aren't really any other decent options for web that preserve transparency and that is compatible with most browsers except webm , such as using vp9 compression

You could use ffmpeg, libvpx-vp9

pixel format would be yuva420p ("a" to retain the alpha channel)

If you have a specific target filesize (bitrate) in mind, then use 2-pass mode. Otherwise use a higher -crf value with -b:v 0 until the quality is too low for you
https://trac.ffmpeg.org/wiki/Encode/VP9

AV1 in the future should support alpha channel, but currently does not officially yet (although AVIF does, the AV1 based animated image format - but it's temporal compression is not as good as vp9 - because AVIF is really a still image format)

[supercain]

I have avidemux and virtualdub, so if I use Vp9 codec I can reduce the size with those and keep the transparency?

[poisondeathray]

I have avidemux and virtualdub, so if I use Vp9 codec I can reduce the size with those and keep the transparency?

Both do not preserve the alpha channel on export. I do not see any options to keep the alpha channel for vdub or avidemux using vp9 in the settings

[supercain]

Any online solution that can do it? I tried many but none of them worked.

[ProWo]

As poisondeathray mentioned above you could try

ffmpeg -i your.webm -c:v libvpx-vp9 -crf 30 -b:v 0 -pix_fmt yuva420p -c:a copy output.webm

[supercain]

Thank you but that didnt preserve the transparency. It adds a white background.

[poisondeathray]

Are you using a recent ffmpeg version ?

How are you viewing it ? Post a cut stream copy of your source

[supercain]

Yes, I just downloaded the latest version. This is the file I tried to compress by using the command ProWo provided but it only added a white background. You can test the transparency here: https://rotato.app/tools/transparent-video

[Selur]

ffmpeg -i input.webm -c:v libvpx-vp9 -crf 30  -vf "colorkey=0x000000:0.1:0.1,format=yuva420p" -pix_fmt yuva420p -c:a copy output.webm

works fine here,...

[supercain]

Can I see the output please?

[poisondeathray]

You have to specify the decoder as libvpx-vp9 (as in input option, before the -i) . FFmpeg's default decoder doesn't read the alpha channel

And it doesn't hurt to tag the alpha mode metadata, it can be more compatible with some applications, and there is no down side

ffmpeg -c:v libvpx-vp9 -i aa73a2.webm -c:v libvpx-vp9 -crf 30 -b:v 0 -pix_fmt yuva420p -metadata:s:v:0 alpha_mode="1" -c:a copy output.webm

[supercain]

Thanks, this time the alpha transparency was preserved. I guess the alpha_mode="1" line did the trick. However I tried the same on a similar sample but it wont get compressed. In fact, it increases in size. I applied the same exact command so I dont get it. I understand libvpx-vp9 is the codec, -crf 30 is the frame rate? I dont know whats the -b:v 0 line and as for yuva420p I guess its the video dimensions. This is my sample:

[poisondeathray]

However I tried the same on a similar sample but it wont get compressed. In fact, it increases in size. I applied the same exact command so I dont get it. I understand libvpx-vp9 is the codec, -crf 30 is the frame rate? I dont know whats the -b:v 0 line and as for yuva420p I guess its the video dimensions. This is my sample:

-crf is essentially the "quality" . It means "constant rate factor" . Higher values yield smaller filesizes, lower quality . Try 35 or 40. 35 gives slightly smaller filesize than a2.webm. You can look into advanced vp9 encoding settings as well

-b:v 0 is required for ffmpeg libvpx-vp9, to select crf mode. You can use other rate control methods outlined in the link earlier.

yuva420p is the pixel format . YUV 4:2:0 , "a" means alpha channel.

[supercain]

I finally managed to compress it the right way. Thank you so much guys.