Can ffmpeg create a Prores .mov with an Alpha?

[Reuben Martin]

Yes, ffmpeg support quite a few pixel formats that include an alpha channel. You have to specify the pix_fmt, and obviously your source has to have an alpha channel.

[Chad Gilmour]

Thanks for the reply, I have been using the pix_fmt option. I’m curious though, have you personally made a prores 4444 with an alpha channel on a windows machine? So far the only people I’ve come across that have made prores 4444 files with an alpha have been on Mac’s.

This is the command I’ve been using, and I have verified that the source file has an alpha channel in AE, Quicktime, and VLC.

ffmpeg -y -i filename.mov -r 29.97 -codec prores_ks -pix_fmt yuva444p10le -alpha_bits 16 -profile:v 4444 -f mov filenameb.mov

When I run that command, ffmpeg creates a prores 4444 file with no visable alpha channel.

I’m using the latest static build from here: https://ffmpeg.zeranoe.com/builds/

[Chad Gilmour]

Made a little headway with this, I can make prores files with an alpha channel, but I’ve realized FFMPEG can’t decode the alpha channel from a DNxHD quicktime which means it can’t make a prores quicktime with an alpha if the input file is using the DNxHD codec, and since DNxHR also isn’t supported at all, I guess I’ll just have to use a different codec as an intermediary.

Hope this helps anyone else struggling with this.

[Reuben Martin]

There has been a ticket for this feature to be implemented for a while now. https://trac.ffmpeg.org/ticket/3707

[Chad Gilmour]

Yeah, it took me a long time before I realized it was actually the DNxHD codec that was the real issue.

Do you know if ffmpeg can combine the RGB or YUV info from one video file, the Alpha from another video file, and then encode 1 prores file with an alpha channel? I know it can kind of do that with audio, wasn’t sure if you could do that with video too.