You need to have a format that supports an alpha channel.
You can export to an image sequence with a format such as .png or a video format that supports an alpha channel such as Quicktime Animation codec.
In the render output settings ensure you enable RGBA so an alpha channel is included in the output file
I already rendered all frames as individual *.png’s with RGBA.
The challenge arises when I want to make these png’s into a movie-format file within Blender. I find that none of the current options allow RGBA option! (is this correct?)
I can work around this - but this is a tedious way of working:
(i) Can import a movie file from blender and then in VP13 add a chroma keyer, ink-dropped background and threshold-removing background in video-file.
(ii) Can import a series of files (fx the *.pngs) into VP13 as individual frames and set still image duration so that it fits.
(iii) What I really would like is to just render a movie-file in Blender (with an alpha-channel) and import this into VP13.