Tweeny is an inbetweening library designed for the creation of complex
animations for games and other beautiful interactive software. It leverages
features of modern C++ to empower developers with an intuitive API for declaring
tweenings of any type of value, as long as they support arithmetic operations.

The goal of Tweeny is to provide means to create fluid interpolations when
animating position, scale, rotation, frames or other values of screen objects,
by setting their values as the tween starting point and then, after each tween
step, plugging back the result.

WWW: https://mobius3.github.io/tweeny/