I am the same as Blighty, never heard of/used REAPER but it is for digital audio production. Where does video fit in there?
Are you making a video demonstrating REAPER and if using a video editor, which one are you using?
Traditionally, video will not overlay transparency, it shows the 'background colour' as you discovered. The way was a grayscale mask (black/white) between the overlay and the base video to hide the bits not needed.
A lot easier these days where some editors do the work for you. example: http://www.openshotvideo.com/2012/09/tut...os-on.html
edit: amazing, there is an OSX version
Are you making a video demonstrating REAPER and if using a video editor, which one are you using?
Traditionally, video will not overlay transparency, it shows the 'background colour' as you discovered. The way was a grayscale mask (black/white) between the overlay and the base video to hide the bits not needed.
A lot easier these days where some editors do the work for you. example: http://www.openshotvideo.com/2012/09/tut...os-on.html
edit: amazing, there is an OSX version