diff --git a/crates/bevy_math/src/curve/easing.rs b/crates/bevy_math/src/curve/easing.rs index 0e5406fa732c7..81d5fb6df1d3a 100644 --- a/crates/bevy_math/src/curve/easing.rs +++ b/crates/bevy_math/src/curve/easing.rs @@ -176,9 +176,15 @@ pub enum EaseFunction { /// Behaves as `EaseFunction::CircularIn` for t < 0.5 and as `EaseFunction::CircularOut` for t >= 0.5 CircularInOut, - /// `f(t) = 2.0^(10.0 * (t - 1.0))` + /// `f(t) ≈ 2.0^(10.0 * (t - 1.0))` + /// + /// The precise definition adjusts it slightly so it hits both `(0, 0)` and `(1, 1)`: + /// `f(t) = 2.0^(10.0 * t - A) - B`, where A = log₂(2¹⁰-1) and B = 1/(2¹⁰-1). ExponentialIn, - /// `f(t) = 1.0 - 2.0^(-10.0 * t)` + /// `f(t) ≈ 1.0 - 2.0^(-10.0 * t)` + /// + /// As with `EaseFunction::ExponentialIn`, the precise definition adjusts it slightly + // so it hits both `(0, 0)` and `(1, 1)`. ExponentialOut, /// Behaves as `EaseFunction::ExponentialIn` for t < 0.5 and as `EaseFunction::ExponentialOut` for t >= 0.5 ExponentialInOut, @@ -324,20 +330,30 @@ mod easing_functions { } } + // These are copied from a high precision calculator; I'd rather show them + // with blatantly more digits than needed (since rust will round them to the + // nearest representable value anyway) rather than make it seem like the + // truncated value is somehow carefully chosen. + #[allow(clippy::excessive_precision)] + const LOG2_1023: f32 = 9.998590429745328646459226; + #[allow(clippy::excessive_precision)] + const FRAC_1_1023: f32 = 0.00097751710654936461388074291; #[inline] pub(crate) fn exponential_in(t: f32) -> f32 { - ops::powf(2.0, 10.0 * t - 10.0) + // Derived from a rescaled exponential formula `(2^(10*t) - 1) / (2^10 - 1)` + // See + ops::exp2(10.0 * t - LOG2_1023) - FRAC_1_1023 } #[inline] pub(crate) fn exponential_out(t: f32) -> f32 { - 1.0 - ops::powf(2.0, -10.0 * t) + (FRAC_1_1023 + 1.0) - ops::exp2(-10.0 * t - (LOG2_1023 - 10.0)) } #[inline] pub(crate) fn exponential_in_out(t: f32) -> f32 { if t < 0.5 { - ops::powf(2.0, 20.0 * t - 10.0) / 2.0 + ops::exp2(20.0 * t - (LOG2_1023 + 1.0)) - (FRAC_1_1023 / 2.0) } else { - (2.0 - ops::powf(2.0, -20.0 * t + 10.0)) / 2.0 + (FRAC_1_1023 / 2.0 + 1.0) - ops::exp2(-20.0 * t - (LOG2_1023 - 19.0)) } } @@ -459,3 +475,83 @@ impl EaseFunction { } } } + +#[cfg(test)] +mod tests { + use super::*; + const MONOTONIC_IN_OUT_INOUT: &[[EaseFunction; 3]] = { + use EaseFunction::*; + &[ + [QuadraticIn, QuadraticOut, QuadraticInOut], + [CubicIn, CubicOut, CubicInOut], + [QuarticIn, QuarticOut, QuarticInOut], + [QuinticIn, QuinticOut, QuinticInOut], + [SineIn, SineOut, SineInOut], + [CircularIn, CircularOut, CircularInOut], + [ExponentialIn, ExponentialOut, ExponentialInOut], + ] + }; + + // For easing function we don't care if eval(0) is super-tiny like 2.0e-28, + // so add the same amount of error on both ends of the unit interval. + const TOLERANCE: f32 = 1.0e-6; + const _: () = const { + assert!(1.0 - TOLERANCE != 1.0); + }; + + #[test] + fn ease_functions_zero_to_one() { + for ef in MONOTONIC_IN_OUT_INOUT.iter().flatten() { + let start = ef.eval(0.0); + assert!( + (0.0..=TOLERANCE).contains(&start), + "EaseFunction.{ef:?}(0) was {start:?}", + ); + + let finish = ef.eval(1.0); + assert!( + (1.0 - TOLERANCE..=1.0).contains(&finish), + "EaseFunction.{ef:?}(1) was {start:?}", + ); + } + } + + #[test] + fn ease_function_inout_deciles() { + // convexity gives these built-in tolerances + for [_, _, ef_inout] in MONOTONIC_IN_OUT_INOUT { + for x in [0.1, 0.2, 0.3, 0.4] { + let y = ef_inout.eval(x); + assert!(y < x, "EaseFunction.{ef_inout:?}({x:?}) was {y:?}"); + } + + for x in [0.6, 0.7, 0.8, 0.9] { + let y = ef_inout.eval(x); + assert!(y > x, "EaseFunction.{ef_inout:?}({x:?}) was {y:?}"); + } + } + } + + #[test] + fn ease_function_midpoints() { + for [ef_in, ef_out, ef_inout] in MONOTONIC_IN_OUT_INOUT { + let mid = ef_in.eval(0.5); + assert!( + mid < 0.5 - TOLERANCE, + "EaseFunction.{ef_in:?}(½) was {mid:?}", + ); + + let mid = ef_out.eval(0.5); + assert!( + mid > 0.5 + TOLERANCE, + "EaseFunction.{ef_out:?}(½) was {mid:?}", + ); + + let mid = ef_inout.eval(0.5); + assert!( + (0.5 - TOLERANCE..=0.5 + TOLERANCE).contains(&mid), + "EaseFunction.{ef_inout:?}(½) was {mid:?}", + ); + } + } +}