fix(animator): start the underdamped spring from rest - #285
Merged
Merged
Conversation
The closed-form underdamped step response is `1 - e^(-zeta*omega*t) * [cos(omega_d*t) + (zeta*omega/omega_d)*sin(omega_d*t)]`. The code feeds `zeta*omega*t/omega_d` into `sin()` instead of `omega_d*t` — the `t` was multiplied into the coefficient instead of the phase. I verified this numerically against a 400k-step ODE integration of `m*x'' + c*x' + k*x = k`: the correct formula matches the integrator to 5 decimals, the repo formula does not. With the shipped defaults (damping 15, stiffness 100, mass 1, zeta = 0.75), at the first 60fps frame after the start (t = 0.0167 s) the solver reports 0.10416 of the travel where the physics gives 0.01282 — 8x. Initial velocity is 6.21/s instead of 0, so the element visibly snaps on frame 1 rather than easing out of rest, and the overshoot peak is wrong too (1.0194 vs 1.0284). This affects every spring path in the engine: `EasingType::Spring` keyframes, the `bounce_in` preset (kf_anim_spring, zeta = 0.6, animator.rs:2092), `elastic_in` (kf_anim_spring_underdamped, zeta = 0.274, animator.rs:2106), and every author-supplied `spring` override applied through `apply_spring_to_motion`. `spring_settle_time`/`spring_rest_time` (what `rustmotion info` reports and what the `spring.duration` remap divides by) are derived from the same wrong curve. Refs #220
LeadcodeDev
added a commit
that referenced
this pull request
Sep 22, 2026
The closed-form underdamped step response is `1 - e^(-zeta*omega*t) * [cos(omega_d*t) + (zeta*omega/omega_d)*sin(omega_d*t)]`. The code feeds `zeta*omega*t/omega_d` into `sin()` instead of `omega_d*t` — the `t` was multiplied into the coefficient instead of the phase. I verified this numerically against a 400k-step ODE integration of `m*x'' + c*x' + k*x = k`: the correct formula matches the integrator to 5 decimals, the repo formula does not. With the shipped defaults (damping 15, stiffness 100, mass 1, zeta = 0.75), at the first 60fps frame after the start (t = 0.0167 s) the solver reports 0.10416 of the travel where the physics gives 0.01282 — 8x. Initial velocity is 6.21/s instead of 0, so the element visibly snaps on frame 1 rather than easing out of rest, and the overshoot peak is wrong too (1.0194 vs 1.0284). This affects every spring path in the engine: `EasingType::Spring` keyframes, the `bounce_in` preset (kf_anim_spring, zeta = 0.6, animator.rs:2092), `elastic_in` (kf_anim_spring_underdamped, zeta = 0.274, animator.rs:2106), and every author-supplied `spring` override applied through `apply_spring_to_motion`. `spring_settle_time`/`spring_rest_time` (what `rustmotion info` reports and what the `spring.duration` remap divides by) are derived from the same wrong curve. Refs #220
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
1 participant
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Audit finding carried by this chantier. Refs #220 (RM-09).