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Fix bubble level: sub-step spring-damper for stability at 500ms GUI rate

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The spring-damper (k=40, c=12) was unstable at dt=0.5s — position
overshot max_r on every frame, got clamped, then overshot again,
keeping the bubble stuck at the ring edge.

Fix: sub-step the physics at 20ms intervals (25 steps per 500ms
frame). The integration is now stable at any GUI update rate.
This commit is contained in:
GlassOnTin 2026-04-02 11:35:44 +01:00
commit fded16c19a
1 changed files with 10 additions and 5 deletions
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15
Gui.h
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@ -816,13 +816,18 @@ static void gui_update_data() {
float target_y = -tilt_dir_y * mapped_r;
// Spring-damper: overdamped for viscous fluid feel
// spring=40 damping=12 → settles in ~0.3s, no oscillation
// Sub-step at 20ms to keep integration stable at any frame rate
const float spring = 40.0f;
const float damping = 12.0f;
vel_x += (spring * (target_x - bub_x) - damping * vel_x) * dt;
vel_y += (spring * (target_y - bub_y) - damping * vel_y) * dt;
bub_x += vel_x * dt;
bub_y += vel_y * dt;
const float max_sub = 0.02f;
int steps = (int)(dt / max_sub) + 1;
float sdt = dt / steps;
for (int si = 0; si < steps; si++) {
vel_x += (spring * (target_x - bub_x) - damping * vel_x) * sdt;
vel_y += (spring * (target_y - bub_y) - damping * vel_y) * sdt;
bub_x += vel_x * sdt;
bub_y += vel_y * sdt;
}
// Clamp to ring boundary (bubble can't escape the fluid)
float dist = sqrtf(bub_x * bub_x + bub_y * bub_y);