[fractallyte]

That's the problem with animation using bitmaps (or physical artwork): the in-betweens have to be manually drawn. Hence much animation is outsourced to studios - typically in Korea, and occasionally Japan - consisting of armies of animators and artists.

With vector graphics - where the lines and fills are mathematical objects - automatic 'tweening' becomes possible. Anime Studio (http://my.smithmicro.com/anime-studio-2D-animation-software....) is the zenith of this tech; there's also Synfig (http://www.synfig.org/cms/) and CACANi (https://cacani.sg/).

In Anime Studio it's possible to add all kinds of effects (including filter effects and motion blur) to animations, and to mix pure vector animation with cutout, or even frame-by-frame, animation.

[lotyrin]

There's a lot of work to take advantage of perceptual quirks of human vision that happens in tweens by humans that these algorithms don't account for (at least last I knew).

Sometimes a perfect interpolation, or even something based on a physical model doesn't feel right, isn't what is expected.

[bane]

I used to play around with just such a tool back in the 80s and 90s called Fantavision

https://en.wikipedia.org/wiki/Fantavision