backIn backInOut backOut bounceIn bounceInOut bounceOut byName circularIn circularInOut circularOut cubicIn cubicInOut cubicOut def elasticIn elasticInOut elasticOut exponentialIn exponentialInOut exponentialOut linear quadraticIn quadraticInOut quadraticOut quarticIn quarticInOut quarticOut quinticIn quinticInOut quinticOut sinusoidalIn sinusoidalInOut sinusoidalOut stepped

backIn detail::backInEasing backIn {} inlineconstexpr Easing function that creates anticipation by pulling back before moving forward. The backIn easing function begins by moving backwards slightly (overshooting in the negative direction), then reverses and accelerates toward the target value. This creates an anticipation effect similar to pulling back a slingshot or winding up before a throw. The backward motion at the start makes this easing particularly effective for animations that benefit from telegraphing or anticipation, such as: UI elements that "wind up" before sliding in Character movements that show preparation before action Camera movements that pull back before zooming forward Mathematically, this easing uses the formula: c * t * t * ((s + 1) * t - s) + b where s controls the overshoot amount (typically 1.70158). Note The animation will briefly have values less than the start value before proceeding to the target. Ensure your animation system can handle values outside the expected range. // Create a tween with back-in easing for anticipation effect auto tween = tweeny::from(0.0f) .to(100.0f) .via(easing::backIn) .during(60U) .build(); // The animation will dip below 0.0f briefly before accelerating to 100.0f See also backOut For overshoot at the end instead of the beginning backInOut For anticipation at the start and overshoot at the end Visualize at https://easings.net/#easeInBack