Sorted out the animation vs. easing contradiction I believe. Now there's a separate easing_duration that's calculated from the total duration of all frames. Then a easing_position determines where in the total the animation is, which is fed to the asing functions as a ration of easing_position / easing_duration.

scene.cpp

@@ -131,7 +131,6 @@ namespace scene {
   inline void this_is_stupid_refactor(std::vector<Element>& elements) {
     for(auto& element : elements) {
       if(element.anim.playing) {
-        auto [ticks, alpha] = element.anim.commit();
         element.anim.update();
         element.anim.motion(*element.st.sprite, element.pos, element.scale);
         element.anim.apply(*element.st.sprite);