VideoZ
I had a go at writing a simple 'media mixer' today. So far it's only video, but i'm already thinking about how to do the sound (hence some work on JOAL yesterday, I'm planning on using OpenAL-Soft to do the mixing, which gives me '3d sound' for free as well). Sound is a bit more difficult than video ...
As output it generates an encoded video file; using jjmpeg of course.
With a small amount of code i've got a slideshow generator, together with affine transforms, opacity, and video or still pictures. I'm just using Java2D for all the rendering: so the compositor is fairly slow, but it's workable.
But, the biggest part of any real application such as this is the user interface for setting up the animation parameters ...