Create File with Alpha

I tried to find a solution but somehow i dont catch the options and settings for ffmpeg. Here is what I want to do:
I have a html template that works fine. i want to render it to a video file with a transparent background. As I understand this I need an Alpha Channel for it. The HTML “has” a transparent background (plays white bg in browser and black in ccg).
I tried to use
add 1 file testfile.mp4
but that comes out without alpha. or lets say it has a black background.
in another discussion vimlesh uses qtrle as a flag/option for ffmpeg, but that puts out an empty file. I think that has to do with a missing quicktime encoder? because of security issues i dont want to install quicktime. if move is the only way to output a video with alpha, well i dont know, i have to rethink my project.
thanks for some help and corrections


ADD 1 FILE "" -vcodec libx264 SEPARATE_KEY

That gives you a file “” with the fill and a file “” with the key. If you play “” the key file will be played automatically.

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Thank you, @didikunz
I tried that, but i get only one file,, no “a” file.
but this mov file at least has a content which is great.
when i insert the file in my Video Program it seems that the move file has still a black background instead of a transparent one.
i was looking for a tool to make sure that the video file is with a proper background, i tried mediainfo, but i dont know what it should say. that i try because i somehow want to check on every level where the problem might be…
thanks for help

Sure you will never get a transparent background in a h264 file, as that is not supported with that codec. What version of Caspar do you use? I today tried on a 2.0.7 and got both files with that command.

It depend where you want to use the file. If it is in a Adobe product then I would record it as NDI in NDI Tools Studio Monitor. Super simple.

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Thanks to you both:
@didikunz Server Version is 2.3.2 5e6e83f5 Dev
do you have an idea of a codec working with transparency like png against jpeg?
size does not matter that much

@markusnygard :
in the end it will be lightworks
ndi might be interresting as well. i will give it a try, but is ndi recording transparency?
i will try.

i read here about the version thing, that acmp changed a bit from version to version.
i now have a word file with my notes and if there is a way for me as a newby to help with the documention, i try to help

I tried with version 2.3.2 Beta 1 (I have no other around) and it seems, that that is not working. On my system I need to kill Caspar server to get it to stop recording, but I do not get any transparency anyway.

You can use ProRes or the like to get a file format with transparency.

That stop recording thing works, i tried remove 1 file filename
how would prores command look like? i dont know about all those fancy parameters.

I guess it’s Prores instead of libx264. I use the official client to send these commands, They also end up in the log file, so that you can see them there and copy paste to whatever you use.