I'm looking for something (prereably FOSS software) which can take an animation with N images as input, and which will output an animation with M frames, where M is in the range 2N to 5N or so.

I believe the general technique is called "temporal upsampling" or possibly "inbetweening" (or "'tweening" for short). Note that it does need to make some effort to do motion tracking of things in the scene ("optical flow"); just fading ("dissolve") between keyframes isn't going to cut it. Googling "temporal upsampling" turns up any number of papers on the subject, but I've yet to discover any code/software I can just use to try the technique out.

Any suggestions ?