Implement spring animations

niri-config/src/lib.rs

@@ -527,20 +527,86 @@ impl Default for Animations {
     }
 }
 
-#[derive(knuffel::Decode, Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Animation {
-    #[knuffel(child)]
     pub off: bool,
-    #[knuffel(child, unwrap(argument))]
+    pub kind: AnimationKind,
-    pub duration_ms: Option<u32>,
-    #[knuffel(child, unwrap(argument))]
-    pub curve: Option<AnimationCurve>,
 }
 
 impl Animation {
     pub const fn unfilled() -> Self {
         Self {
             off: false,
+            kind: AnimationKind::Easing(EasingParams::unfilled()),
+        }
+    }
+
+    pub const fn default() -> Self {
+        Self {
+            off: false,
+            kind: AnimationKind::Easing(EasingParams::default()),
+        }
+    }
+
+    pub const fn default_workspace_switch() -> Self {
+        Self {
+            off: false,
+            kind: AnimationKind::Spring(SpringParams {
+                damping_ratio: 1.,
+                stiffness: 1000,
+                epsilon: 0.0001,
+            }),
+        }
+    }
+
+    pub const fn default_horizontal_view_movement() -> Self {
+        Self {
+            off: false,
+            kind: AnimationKind::Spring(SpringParams {
+                damping_ratio: 1.,
+                stiffness: 800,
+                epsilon: 0.0001,
+            }),
+        }
+    }
+
+    pub const fn default_config_notification_open_close() -> Self {
+        Self {
+            off: false,
+            kind: AnimationKind::Spring(SpringParams {
+                damping_ratio: 0.6,
+                stiffness: 1000,
+                epsilon: 0.001,
+            }),
+        }
+    }
+
+    pub const fn default_window_open() -> Self {
+        Self {
+            off: false,
+            kind: AnimationKind::Easing(EasingParams {
+                duration_ms: Some(150),
+                curve: Some(AnimationCurve::EaseOutExpo),
+            }),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum AnimationKind {
+    Easing(EasingParams),
+    Spring(SpringParams),
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct EasingParams {
+    pub duration_ms: Option<u32>,
+    pub curve: Option<AnimationCurve>,
+}
+
+impl EasingParams {
+    pub const fn unfilled() -> Self {
+        Self {
             duration_ms: None,
             curve: None,
         }
@@ -548,31 +614,10 @@ impl Animation {
 
     pub const fn default() -> Self {
         Self {
-            off: false,
             duration_ms: Some(250),
             curve: Some(AnimationCurve::EaseOutCubic),
         }
     }
-
-    pub const fn default_workspace_switch() -> Self {
-        Self::default()
-    }
-
-    pub const fn default_horizontal_view_movement() -> Self {
-        Self::default()
-    }
-
-    pub const fn default_config_notification_open_close() -> Self {
-        Self::default()
-    }
-
-    pub const fn default_window_open() -> Self {
-        Self {
-            duration_ms: Some(150),
-            curve: Some(AnimationCurve::EaseOutExpo),
-            ..Self::default()
-        }
-    }
 }
 
 #[derive(knuffel::DecodeScalar, Debug, Clone, Copy, PartialEq)]
@@ -581,6 +626,13 @@ pub enum AnimationCurve {
     EaseOutExpo,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct SpringParams {
+    pub damping_ratio: f64,
+    pub stiffness: u32,
+    pub epsilon: f64,
+}
+
 #[derive(knuffel::Decode, Debug, Default, Clone, PartialEq, Eq)]
 pub struct Environment(#[knuffel(children)] pub Vec<EnvironmentVariable>);
 
@@ -1012,6 +1064,229 @@ where
     }
 }
 
+fn parse_arg_node<S: knuffel::traits::ErrorSpan, T: knuffel::traits::DecodeScalar<S>>(
+    name: &str,
+    node: &knuffel::ast::SpannedNode<S>,
+    ctx: &mut knuffel::decode::Context<S>,
+) -> Result<T, DecodeError<S>> {
+    let mut iter_args = node.arguments.iter();
+    let val = iter_args.next().ok_or_else(|| {
+        DecodeError::missing(node, format!("additional argument `{name}` is required"))
+    })?;
+
+    let value = knuffel::traits::DecodeScalar::decode(val, ctx)?;
+
+    if let Some(val) = iter_args.next() {
+        ctx.emit_error(DecodeError::unexpected(
+            &val.literal,
+            "argument",
+            "unexpected argument",
+        ));
+    }
+    for name in node.properties.keys() {
+        ctx.emit_error(DecodeError::unexpected(
+            name,
+            "property",
+            format!("unexpected property `{}`", name.escape_default()),
+        ));
+    }
+    for child in node.children() {
+        ctx.emit_error(DecodeError::unexpected(
+            child,
+            "node",
+            format!("unexpected node `{}`", child.node_name.escape_default()),
+        ));
+    }
+
+    Ok(value)
+}
+
+impl<S> knuffel::Decode<S> for Animation
+where
+    S: knuffel::traits::ErrorSpan,
+{
+    fn decode_node(
+        node: &knuffel::ast::SpannedNode<S>,
+        ctx: &mut knuffel::decode::Context<S>,
+    ) -> Result<Self, DecodeError<S>> {
+        expect_only_children(node, ctx);
+
+        let mut off = false;
+        let mut easing_params = EasingParams::unfilled();
+        let mut spring_params = None;
+
+        for child in node.children() {
+            match &**child.node_name {
+                "off" => {
+                    knuffel::decode::check_flag_node(child, ctx);
+                    if off {
+                        ctx.emit_error(DecodeError::unexpected(
+                            &child.node_name,
+                            "node",
+                            "duplicate node `off`, single node expected",
+                        ));
+                    } else {
+                        off = true;
+                    }
+                }
+                "spring" => {
+                    if easing_params != EasingParams::unfilled() {
+                        ctx.emit_error(DecodeError::unexpected(
+                            child,
+                            "node",
+                            "cannot set both spring and easing parameters at once",
+                        ));
+                    }
+                    if spring_params.is_some() {
+                        ctx.emit_error(DecodeError::unexpected(
+                            &child.node_name,
+                            "node",
+                            "duplicate node `spring`, single node expected",
+                        ));
+                    }
+
+                    spring_params = Some(SpringParams::decode_node(child, ctx)?);
+                }
+                "duration-ms" => {
+                    if spring_params.is_some() {
+                        ctx.emit_error(DecodeError::unexpected(
+                            child,
+                            "node",
+                            "cannot set both spring and easing parameters at once",
+                        ));
+                    }
+                    if easing_params.duration_ms.is_some() {
+                        ctx.emit_error(DecodeError::unexpected(
+                            &child.node_name,
+                            "node",
+                            "duplicate node `duration-ms`, single node expected",
+                        ));
+                    }
+
+                    easing_params.duration_ms = Some(parse_arg_node("duration-ms", child, ctx)?);
+                }
+                "curve" => {
+                    if spring_params.is_some() {
+                        ctx.emit_error(DecodeError::unexpected(
+                            child,
+                            "node",
+                            "cannot set both spring and easing parameters at once",
+                        ));
+                    }
+                    if easing_params.curve.is_some() {
+                        ctx.emit_error(DecodeError::unexpected(
+                            &child.node_name,
+                            "node",
+                            "duplicate node `curve`, single node expected",
+                        ));
+                    }
+
+                    easing_params.curve = Some(parse_arg_node("curve", child, ctx)?);
+                }
+                name_str => {
+                    ctx.emit_error(DecodeError::unexpected(
+                        child,
+                        "node",
+                        format!("unexpected node `{}`", name_str.escape_default()),
+                    ));
+                }
+            }
+        }
+
+        let kind = if let Some(spring_params) = spring_params {
+            AnimationKind::Spring(spring_params)
+        } else {
+            AnimationKind::Easing(easing_params)
+        };
+
+        Ok(Self { off, kind })
+    }
+}
+
+impl<S> knuffel::Decode<S> for SpringParams
+where
+    S: knuffel::traits::ErrorSpan,
+{
+    fn decode_node(
+        node: &knuffel::ast::SpannedNode<S>,
+        ctx: &mut knuffel::decode::Context<S>,
+    ) -> Result<Self, DecodeError<S>> {
+        if let Some(type_name) = &node.type_name {
+            ctx.emit_error(DecodeError::unexpected(
+                type_name,
+                "type name",
+                "no type name expected for this node",
+            ));
+        }
+        if let Some(val) = node.arguments.first() {
+            ctx.emit_error(DecodeError::unexpected(
+                &val.literal,
+                "argument",
+                "unexpected argument",
+            ));
+        }
+        for child in node.children() {
+            ctx.emit_error(DecodeError::unexpected(
+                child,
+                "node",
+                format!("unexpected node `{}`", child.node_name.escape_default()),
+            ));
+        }
+
+        let mut damping_ratio = None;
+        let mut stiffness = None;
+        let mut epsilon = None;
+        for (name, val) in &node.properties {
+            match &***name {
+                "damping-ratio" => {
+                    damping_ratio = Some(knuffel::traits::DecodeScalar::decode(val, ctx)?);
+                }
+                "stiffness" => {
+                    stiffness = Some(knuffel::traits::DecodeScalar::decode(val, ctx)?);
+                }
+                "epsilon" => {
+                    epsilon = Some(knuffel::traits::DecodeScalar::decode(val, ctx)?);
+                }
+                name_str => {
+                    ctx.emit_error(DecodeError::unexpected(
+                        name,
+                        "property",
+                        format!("unexpected property `{}`", name_str.escape_default()),
+                    ));
+                }
+            }
+        }
+        let damping_ratio = damping_ratio
+            .ok_or_else(|| DecodeError::missing(node, "property `damping-ratio` is required"))?;
+        let stiffness = stiffness
+            .ok_or_else(|| DecodeError::missing(node, "property `stiffness` is required"))?;
+        let epsilon =
+            epsilon.ok_or_else(|| DecodeError::missing(node, "property `epsilon` is required"))?;
+
+        if !(0.1..=10.).contains(&damping_ratio) {
+            ctx.emit_error(DecodeError::conversion(
+                node,
+                "damping-ratio must be between 0.1 and 10.0",
+            ));
+        }
+        if stiffness < 1 {
+            ctx.emit_error(DecodeError::conversion(node, "stiffness must be >= 1"));
+        }
+        if !(0.00001..=0.1).contains(&epsilon) {
+            ctx.emit_error(DecodeError::conversion(
+                node,
+                "epsilon must be between 0.00001 and 0.1",
+            ));
+        }
+
+        Ok(SpringParams {
+            damping_ratio,
+            stiffness,
+            epsilon,
+        })
+    }
+}
+
 impl<S> knuffel::Decode<S> for Binds
 where
     S: knuffel::traits::ErrorSpan,
@@ -1345,12 +1620,16 @@ mod tests {
             animations {
                 slowdown 2.0
 
-                workspace-switch { off; }
+                workspace-switch {
+                    spring damping-ratio=1.0 stiffness=1000 epsilon=0.0001
+                }
 
                 horizontal-view-movement {
                     duration-ms 100
                     curve "ease-out-expo"
                 }
+
+                window-open { off; }
             }
 
             environment {
@@ -1507,12 +1786,22 @@ mod tests {
                 animations: Animations {
                     slowdown: 2.,
                     workspace_switch: Animation {
-                        off: true,
+                        off: false,
-                        ..Animation::unfilled()
+                        kind: AnimationKind::Spring(SpringParams {
+                            damping_ratio: 1.,
+                            stiffness: 1000,
+                            epsilon: 0.0001,
+                        }),
                     },
                     horizontal_view_movement: Animation {
-                        duration_ms: Some(100),
+                        off: false,
-                        curve: Some(AnimationCurve::EaseOutExpo),
+                        kind: AnimationKind::Easing(EasingParams {
+                            duration_ms: Some(100),
+                            curve: Some(AnimationCurve::EaseOutExpo),
+                        }),
+                    },
+                    window_open: Animation {
+                        off: true,
                         ..Animation::unfilled()
                     },
                     ..Default::default()

resources/default-config.kdl

@@ -254,18 +254,48 @@ animations {
     // slowdown 3.0
 
     // You can configure all individual animations.
-    // Available settings are the same for all of them:
+    // Available settings are the same for all of them.
     // - off disables the animation.
+    //
+    // Niri supports two animation types: easing and spring.
+    //
+    // Easing has the following settings:
     // - duration-ms sets the duration of the animation in milliseconds.
     // - curve sets the easing curve. Currently, available curves
     //   are "ease-out-cubic" and "ease-out-expo".
+    //
+    // Spring animations work better with touchpad gestures, because they
+    // take into account the velocity of your fingers as you release the swipe.
+    // The parameters are less obvious and generally should be tuned
+    // with trial and error. Notably, you cannot directly set the duration.
+    // You can use this app to help visualize how the spring parameters
+    // change the animation: https://flathub.org/apps/app.drey.Elastic
+    //
+    // A spring animation is configured like this:
+    // - spring damping-ratio=1.0 stiffness=1000 epsilon=0.0001
+    //
+    // The damping ratio goes from 0.1 to 10.0 and has the following properties:
+    // - below 1.0: underdamped spring, will oscillate in the end.
+    // - above 1.0: overdamped spring, won't oscillate.
+    // - 1.0: critically damped spring, comes to rest in minimum possible time
+    //        without oscillations.
+    //
+    // However, even with damping ratio = 1.0 the spring animation may oscillate
+    // if "launched" with enough velocity from a touchpad swipe.
+    //
+    // Lower stiffness will result in a slower animation more prone to oscillation.
+    //
+    // Set epsilon to a lower value if the animation "jumps" in the end.
+    //
+    // The spring mass is hardcoded to 1.0 and cannot be changed. Instead, change
+    // stiffness proportionally. E.g. increasing mass by 2x is the same as
+    // decreasing stiffness by 2x.
 
     // Animation when switching workspaces up and down,
     // including after the touchpad gesture.
     workspace-switch {
         // off
-        // duration-ms 250
+        // spring damping-ratio=1.0 stiffness=1000 epsilon=0.0001
-        // curve "ease-out-cubic"
     }
 
     // All horizontal camera view movement:
@@ -275,8 +305,7 @@ animations {
     // - And so on.
     horizontal-view-movement {
         // off
-        // duration-ms 250
+        // spring damping-ratio=1.0 stiffness=800 epsilon=0.0001
-        // curve "ease-out-cubic"
     }
 
     // Window opening animation. Note that this one has different defaults.
@@ -290,8 +319,7 @@ animations {
     // open/close animation.
     config-notification-open-close {
         // off
-        // duration-ms 250
+        // spring damping-ratio=0.6 stiffness=1000 epsilon=0.001
-        // curve "ease-out-cubic"
     }
 }
 

src/animation/mod.rs

@@ -6,6 +6,9 @@ use portable_atomic::{AtomicF64, Ordering};
 
 use crate::utils::get_monotonic_time;
 
+mod spring;
+pub use spring::{Spring, SpringParams};
+
 pub static ANIMATION_SLOWDOWN: AtomicF64 = AtomicF64::new(1.);
 
 #[derive(Debug)]
@@ -15,7 +18,19 @@ pub struct Animation {
     duration: Duration,
     start_time: Duration,
     current_time: Duration,
-    curve: Curve,
+    kind: Kind,
+}
+
+#[derive(Debug, Clone, Copy)]
+enum Kind {
+    Easing {
+        curve: Curve,
+    },
+    Spring(Spring),
+    Deceleration {
+        initial_velocity: f64,
+        deceleration_rate: f64,
+    },
 }
 
 #[derive(Debug, Clone, Copy)]
@@ -28,21 +43,63 @@ impl Animation {
     pub fn new(
         from: f64,
         to: f64,
+        initial_velocity: f64,
         config: niri_config::Animation,
         default: niri_config::Animation,
     ) -> Self {
+        if config.off {
+            return Self::ease(from, to, 0, Curve::EaseOutCubic);
+        }
+
+        // Resolve defaults.
+        let (kind, easing_defaults) = match (config.kind, default.kind) {
+            // Configured spring.
+            (configured @ niri_config::AnimationKind::Spring(_), _) => (configured, None),
+            // Configured nothing, defaults spring.
+            (
+                niri_config::AnimationKind::Easing(easing),
+                defaults @ niri_config::AnimationKind::Spring(_),
+            ) if easing == niri_config::EasingParams::unfilled() => (defaults, None),
+            // Configured easing or nothing, defaults easing.
+            (
+                configured @ niri_config::AnimationKind::Easing(_),
+                niri_config::AnimationKind::Easing(defaults),
+            ) => (configured, Some(defaults)),
+            // Configured easing, defaults spring.
+            (
+                configured @ niri_config::AnimationKind::Easing(_),
+                niri_config::AnimationKind::Spring(_),
+            ) => (configured, None),
+        };
+
+        match kind {
+            niri_config::AnimationKind::Spring(p) => {
+                let params = SpringParams::new(p.damping_ratio, f64::from(p.stiffness), p.epsilon);
+
+                let spring = Spring {
+                    from,
+                    to,
+                    initial_velocity,
+                    params,
+                };
+                Self::spring(spring)
+            }
+            niri_config::AnimationKind::Easing(p) => {
+                let defaults = easing_defaults.unwrap_or(niri_config::EasingParams::default());
+                let duration_ms = p.duration_ms.or(defaults.duration_ms).unwrap();
+                let curve = Curve::from(p.curve.or(defaults.curve).unwrap());
+                Self::ease(from, to, u64::from(duration_ms), curve)
+            }
+        }
+    }
+
+    pub fn ease(from: f64, to: f64, duration_ms: u64, curve: Curve) -> Self {
         // FIXME: ideally we shouldn't use current time here because animations started within the
         // same frame cycle should have the same start time to be synchronized.
         let now = get_monotonic_time();
 
-        let duration_ms = if config.off {
+        let duration = Duration::from_millis(duration_ms);
-            0
+        let kind = Kind::Easing { curve };
-        } else {
-            config.duration_ms.unwrap_or(default.duration_ms.unwrap())
-        };
-        let duration = Duration::from_millis(u64::from(duration_ms));
-
-        let curve = Curve::from(config.curve.unwrap_or(default.curve.unwrap()));
 
         Self {
             from,
@@ -50,7 +107,60 @@ impl Animation {
             duration,
             start_time: now,
             current_time: now,
-            curve,
+            kind,
+        }
+    }
+
+    pub fn spring(spring: Spring) -> Self {
+        // FIXME: ideally we shouldn't use current time here because animations started within the
+        // same frame cycle should have the same start time to be synchronized.
+        let now = get_monotonic_time();
+
+        let duration = spring.duration();
+        let kind = Kind::Spring(spring);
+
+        Self {
+            from: spring.from,
+            to: spring.to,
+            duration,
+            start_time: now,
+            current_time: now,
+            kind,
+        }
+    }
+
+    pub fn decelerate(
+        from: f64,
+        initial_velocity: f64,
+        deceleration_rate: f64,
+        threshold: f64,
+    ) -> Self {
+        // FIXME: ideally we shouldn't use current time here because animations started within the
+        // same frame cycle should have the same start time to be synchronized.
+        let now = get_monotonic_time();
+
+        let duration_s = if initial_velocity == 0. {
+            0.
+        } else {
+            let coeff = 1000. * deceleration_rate.ln();
+            (-coeff * threshold / initial_velocity.abs()).ln() / coeff
+        };
+        let duration = Duration::from_secs_f64(duration_s);
+
+        let to = from - initial_velocity / (1000. * deceleration_rate.ln());
+
+        let kind = Kind::Deceleration {
+            initial_velocity,
+            deceleration_rate,
+        };
+
+        Self {
+            from,
+            to,
+            duration,
+            start_time: now,
+            current_time: now,
+            kind,
         }
     }
 
@@ -118,10 +228,29 @@ impl Animation {
     }
 
     pub fn value(&self) -> f64 {
-        let passed = (self.current_time - self.start_time).as_secs_f64();
+        if self.is_done() {
-        let total = self.duration.as_secs_f64();
+            return self.to;
-        let x = (passed / total).clamp(0., 1.);
+        }
-        self.curve.y(x) * (self.to - self.from) + self.from
+
+        let passed = self.current_time - self.start_time;
+
+        match self.kind {
+            Kind::Easing { curve } => {
+                let passed = passed.as_secs_f64();
+                let total = self.duration.as_secs_f64();
+                let x = (passed / total).clamp(0., 1.);
+                curve.y(x) * (self.to - self.from) + self.from
+            }
+            Kind::Spring(spring) => spring.value_at(passed),
+            Kind::Deceleration {
+                initial_velocity,
+                deceleration_rate,
+            } => {
+                let passed = passed.as_secs_f64();
+                let coeff = 1000. * deceleration_rate.ln();
+                self.from + (deceleration_rate.powf(1000. * passed) - 1.) / coeff * initial_velocity
+            }
+        }
     }
 
     pub fn to(&self) -> f64 {

src/animation/spring.rs

@@ -0,0 +1,137 @@
+use std::time::Duration;
+
+#[derive(Debug, Clone, Copy)]
+pub struct SpringParams {
+    pub damping: f64,
+    pub mass: f64,
+    pub stiffness: f64,
+    pub epsilon: f64,
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct Spring {
+    pub from: f64,
+    pub to: f64,
+    pub initial_velocity: f64,
+    pub params: SpringParams,
+}
+
+impl SpringParams {
+    pub fn new(damping_ratio: f64, stiffness: f64, epsilon: f64) -> Self {
+        let damping_ratio = damping_ratio.max(0.);
+        let stiffness = stiffness.max(0.);
+        let epsilon = epsilon.max(0.);
+
+        let mass = 1.;
+        let critical_damping = 2. * (mass * stiffness).sqrt();
+        let damping = damping_ratio * critical_damping;
+
+        Self {
+            damping,
+            mass,
+            stiffness,
+            epsilon,
+        }
+    }
+}
+
+impl Spring {
+    pub fn value_at(&self, t: Duration) -> f64 {
+        self.oscillate(t.as_secs_f64())
+    }
+
+    // Based on libadwaita (LGPL-2.1-or-later):
+    // https://gitlab.gnome.org/GNOME/libadwaita/-/blob/1.4.4/src/adw-spring-animation.c,
+    // which itself is based on (MIT):
+    // https://github.com/robb/RBBAnimation/blob/master/RBBAnimation/RBBSpringAnimation.m
+    /// Computes and returns the duration until the spring is at rest.
+    pub fn duration(&self) -> Duration {
+        const DELTA: f64 = 0.001;
+
+        let beta = self.params.damping / (2. * self.params.mass);
+
+        if beta.abs() <= f64::EPSILON || beta < 0. {
+            return Duration::MAX;
+        }
+
+        let omega0 = (self.params.stiffness / self.params.mass).sqrt();
+
+        // As first ansatz for the overdamped solution,
+        // and general estimation for the oscillating ones
+        // we take the value of the envelope when it's < epsilon.
+        let mut x0 = -self.params.epsilon.ln() / beta;
+
+        // f64::EPSILON is too small for this specific comparison, so we use
+        // f32::EPSILON even though it's doubles.
+        if (beta - omega0).abs() <= f64::from(f32::EPSILON) || beta < omega0 {
+            return Duration::from_secs_f64(x0);
+        }
+
+        // Since the overdamped solution decays way slower than the envelope
+        // we need to use the value of the oscillation itself.
+        // Newton's root finding method is a good candidate in this particular case:
+        // https://en.wikipedia.org/wiki/Newton%27s_method
+        let mut y0 = self.oscillate(x0);
+        let m = (self.oscillate(x0 + DELTA) - y0) / DELTA;
+
+        let mut x1 = (self.to - y0 + m * x0) / m;
+        let mut y1 = self.oscillate(x1);
+
+        let mut i = 0;
+        while (self.to - y1).abs() > self.params.epsilon {
+            if i > 1000 {
+                return Duration::ZERO;
+            }
+
+            x0 = x1;
+            y0 = y1;
+
+            let m = (self.oscillate(x0 + DELTA) - y0) / DELTA;
+
+            x1 = (self.to - y0 + m * x0) / m;
+            y1 = self.oscillate(x1);
+            i += 1;
+        }
+
+        Duration::from_secs_f64(x1)
+    }
+
+    /// Returns the spring position at a given time in seconds.
+    fn oscillate(&self, t: f64) -> f64 {
+        let b = self.params.damping;
+        let m = self.params.mass;
+        let k = self.params.stiffness;
+        let v0 = self.initial_velocity;
+
+        let beta = b / (2. * m);
+        let omega0 = (k / m).sqrt();
+
+        let x0 = self.from - self.to;
+
+        let envelope = (-beta * t).exp();
+
+        // Solutions of the form C1*e^(lambda1*x) + C2*e^(lambda2*x)
+        // for the differential equation m*ẍ+b*ẋ+kx = 0
+
+        // f64::EPSILON is too small for this specific comparison, so we use
+        // f32::EPSILON even though it's doubles.
+        if (beta - omega0).abs() <= f64::from(f32::EPSILON) {
+            // Critically damped.
+            self.to + envelope * (x0 + (beta * x0 + v0) * t)
+        } else if beta < omega0 {
+            // Underdamped.
+            let omega1 = ((omega0 * omega0) - (beta * beta)).sqrt();
+
+            self.to
+                + envelope
+                    * (x0 * (omega1 * t).cos() + ((beta * x0 + v0) / omega1) * (omega1 * t).sin())
+        } else {
+            // Overdamped.
+            let omega2 = ((beta * beta) - (omega0 * omega0)).sqrt();
+
+            self.to
+                + envelope
+                    * (x0 * (omega2 * t).cosh() + ((beta * x0 + v0) / omega2) * (omega2 * t).sinh())
+        }
+    }
+}

src/layout/monitor.rs

@@ -94,6 +94,7 @@ impl<W: LayoutElement> Monitor<W> {
             return;
         }
 
+        // FIXME: also compute and use current velocity.
         let current_idx = self
             .workspace_switch
             .as_ref()
@@ -105,6 +106,7 @@ impl<W: LayoutElement> Monitor<W> {
         self.workspace_switch = Some(WorkspaceSwitch::Animation(Animation::new(
             current_idx,
             idx as f64,
+            0.,
             self.options.animations.workspace_switch,
             niri_config::Animation::default_workspace_switch(),
         )));
@@ -781,6 +783,7 @@ impl<W: LayoutElement> Monitor<W> {
             return true;
         }
 
+        let velocity = gesture.tracker.velocity() / WORKSPACE_GESTURE_MOVEMENT;
         let pos = gesture.tracker.projected_end_pos() / WORKSPACE_GESTURE_MOVEMENT;
 
         let min = gesture.center_idx.saturating_sub(1) as f64;
@@ -792,6 +795,7 @@ impl<W: LayoutElement> Monitor<W> {
         self.workspace_switch = Some(WorkspaceSwitch::Animation(Animation::new(
             gesture.current_idx,
             new_idx as f64,
+            velocity,
             self.options.animations.workspace_switch,
             niri_config::Animation::default_workspace_switch(),
         )));

src/layout/tile.rs

@@ -111,6 +111,7 @@ impl<W: LayoutElement> Tile<W> {
         self.open_animation = Some(Animation::new(
             0.,
             1.,
+            0.,
             self.options.animations.window_open,
             niri_config::Animation::default_window_open(),
         ));

src/layout/workspace.rs

@@ -436,9 +436,11 @@ impl<W: LayoutElement> Workspace<W> {
             return;
         }
 
+        // FIXME: also compute and use current velocity.
         self.view_offset_adj = Some(ViewOffsetAdjustment::Animation(Animation::new(
             self.view_offset as f64,
             new_view_offset as f64,
+            0.,
             self.options.animations.horizontal_view_movement,
             niri_config::Animation::default_horizontal_view_movement(),
         )));
@@ -1272,6 +1274,7 @@ impl<W: LayoutElement> Workspace<W> {
         // effort and bug potential.
 
         let norm_factor = self.working_area.size.w as f64 / VIEW_GESTURE_WORKING_AREA_MOVEMENT;
+        let velocity = gesture.tracker.velocity() * norm_factor;
         let pos = gesture.tracker.pos() * norm_factor;
         let current_view_offset = pos + gesture.delta_from_tracker;
 
@@ -1420,6 +1423,7 @@ impl<W: LayoutElement> Workspace<W> {
         self.view_offset_adj = Some(ViewOffsetAdjustment::Animation(Animation::new(
             current_view_offset + delta,
             target_view_offset as f64,
+            velocity,
             self.options.animations.horizontal_view_movement,
             niri_config::Animation::default_horizontal_view_movement(),
         )));

src/ui/config_error_notification.rs

@@ -62,6 +62,7 @@ impl ConfigErrorNotification {
         Animation::new(
             from,
             to,
+            0.,
             c.animations.config_notification_open_close,
             niri_config::Animation::default_config_notification_open_close(),
         )
@@ -118,7 +119,9 @@ impl ConfigErrorNotification {
             }
             State::Hiding(anim) => {
                 anim.set_current_time(target_presentation_time);
-                if anim.is_done() {
+                // HACK: prevent bounciness on hiding. This is better done with a clamp property on
+                // the spring animation.
+                if anim.is_done() || anim.value() <= 0. {
                     self.state = State::Hidden;
                 }
             }