FFMPEG: create a transparent background with lavfi?

[Taapo]

I can create a transparent background using lavfi by setting colorkey - but that gives problems with edges around fonts, etc.

ffmpeg.exe -y -f lavfi -i color=s=1920x1080:color=0xff0000 
-an -vf "ass=test.ass,colorkey=0xff0000:0.5:0.5,format=yuva420p" 
-c:v png -t "00:01:40.544" -threads 0 "video.mov" -stats

How can I set the background to just transparent without resorting to colorkey?

[JVRaines]

Add an alpha component to your color argument:

0xff0000@0.0