[Libav-user] DLSR-esq 30-second-exposure filter (Light painting filter)

[Arthur Neufeld]

[Begin scene: in darkness]
Bright point lights moves through the frame, following complex paths.
[End scene]

My DSLR camera takes a 30-second exposure of the scene, accumulating the 
light over the entire 30 seconds into a single frame image.

My video camera captures the scene, as a 900-ish frame movie.

I'd like to convert my 30 second video, into a representation of the 
what the DSLR image sensor experiences.  As in:
     - Frame 1 is just frame 1.
     - Frame 2 becomes frame 1 + frame 2.
     - Frame 3 becomes frame 1 + frame 2 + frame 3
     - ...
     - Frame 900 becomes all frames added together.

I think FFmpeg's filter can do this ...
     "overlay input frame N onto output frame N-1."
.... but I can't get my head around the required syntax.