feat: cubic-bezier curve for animation (#2059)

* feat: bezier curve for animation

* fixes

---------

Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

docs/wiki/Configuration:-Animations.md

@@ -84,14 +84,24 @@ animations {
 }
 ```
 
-Currently, niri only supports four curves:
+Currently, niri only supports five curves.
+You can get a feel for them on pages like [easings.net](https://easings.net/).
 
 - `ease-out-quad` <sup>Since: 0.1.5</sup>
 - `ease-out-cubic`
 - `ease-out-expo`
 - `linear` <sup>Since: 0.1.6</sup>
-
-You can get a feel for them on pages like [easings.net](https://easings.net/).
+- `cubic-bezier` <sup>Since: next release</sup>
+    A custom [cubic Bézier curve](https://www.w3.org/TR/css-easing-1/#cubic-bezier-easing-functions). You need to set 4 numbers defining the control points of the curve, for example:
+    ```kdl
+    animations {
+        window-open {
+            // Same as CSS cubic-bezier(0.05, 0.7, 0.1, 1)
+            curve "cubic-bezier" 0.05 0.7 0.1 1
+        }
+    }
+    ```
+    You can tweak the cubic-bezier parameters on pages like [easings.co](https://easings.co?curve=0.05,0.7,0.1,1).
 
 #### Spring
 

niri-config/src/animations.rs

@@ -69,12 +69,13 @@ pub struct EasingParams {
     pub curve: Curve,
 }
 
-#[derive(knuffel::DecodeScalar, Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub enum Curve {
     Linear,
     EaseOutQuad,
     EaseOutCubic,
     EaseOutExpo,
+    CubicBezier(f64, f64, f64, f64),
 }
 
 #[derive(Debug, Clone, Copy, PartialEq)]
@@ -540,7 +541,94 @@ impl Animation {
                         ));
                     }
 
-                    easing_params.curve = Some(parse_arg_node("curve", child, ctx)?);
+                    let mut iter_args = child.arguments.iter();
+                    let val = iter_args.next().ok_or_else(|| {
+                        DecodeError::missing(child, "additional argument `curve` is required")
+                    })?;
+                    let animation_curve_string: String =
+                        knuffel::traits::DecodeScalar::decode(val, ctx)?;
+
+                    let animation_curve = match animation_curve_string.as_str() {
+                        "linear" => Some(Curve::Linear),
+                        "ease-out-quad" => Some(Curve::EaseOutQuad),
+                        "ease-out-cubic" => Some(Curve::EaseOutCubic),
+                        "ease-out-expo" => Some(Curve::EaseOutExpo),
+                        "cubic-bezier" => {
+                            let val = iter_args.next().ok_or_else(|| {
+                                DecodeError::missing(
+                                    child,
+                                    "missing x1 coordinate for cubic Bézier curve control point",
+                                )
+                            })?;
+                            // the X axis represents time frame so it cannot be negative
+                            // or larger than 1
+                            let x1: FloatOrInt<0, 1> =
+                                knuffel::traits::DecodeScalar::decode(val, ctx)?;
+                            let val = iter_args.next().ok_or_else(|| {
+                                DecodeError::missing(
+                                    child,
+                                    "missing y1 coordinate for cubic Bézier curve control point",
+                                )
+                            })?;
+                            let y1: FloatOrInt<{ i32::MIN }, { i32::MAX }> =
+                                knuffel::traits::DecodeScalar::decode(val, ctx)?;
+                            let val = iter_args.next().ok_or_else(|| {
+                                DecodeError::missing(
+                                    child,
+                                    "missing x2 coordinate for cubic Bézier curve control point",
+                                )
+                            })?;
+                            let x2: FloatOrInt<0, 1> =
+                                knuffel::traits::DecodeScalar::decode(val, ctx)?;
+                            let val = iter_args.next().ok_or_else(|| {
+                                DecodeError::missing(
+                                    child,
+                                    "missing y2 coordinate for cubic Bézier curve control point",
+                                )
+                            })?;
+                            let y2: FloatOrInt<{ i32::MIN }, { i32::MAX }> =
+                                knuffel::traits::DecodeScalar::decode(val, ctx)?;
+
+                            Some(Curve::CubicBezier(x1.0, y1.0, x2.0, y2.0))
+                        }
+                        unexpected_curve => {
+                            ctx.emit_error(DecodeError::unexpected(
+                                &val.literal,
+                                "argument",
+                                format!(
+                                    "unexpected animation curve `{unexpected_curve}`. \
+                                    Niri only supports five animation curves: \
+                                    `ease-out-quad`, `ease-out-cubic`, `ease-out-expo`, `linear` and `cubic-bezier`."
+                                ),
+                            ));
+
+                            None
+                        }
+                    };
+
+                    if let Some(val) = iter_args.next() {
+                        ctx.emit_error(DecodeError::unexpected(
+                            &val.literal,
+                            "argument",
+                            "unexpected argument",
+                        ));
+                    }
+                    for name in child.properties.keys() {
+                        ctx.emit_error(DecodeError::unexpected(
+                            name,
+                            "property",
+                            format!("unexpected property `{}`", name.escape_default()),
+                        ));
+                    }
+                    for child in child.children() {
+                        ctx.emit_error(DecodeError::unexpected(
+                            child,
+                            "node",
+                            format!("unexpected node `{}`", child.node_name.escape_default()),
+                        ));
+                    }
+
+                    easing_params.curve = animation_curve;
                 }
                 name_str => {
                     if !process_children(child, ctx)? {

niri-config/src/lib.rs

@@ -441,6 +441,10 @@ mod tests {
                 }
 
                 window-open { off; }
+
+                window-close {
+                    curve "cubic-bezier" 0.05 0.7 0.1 1  
+                }
             }
 
             gestures {
@@ -1038,7 +1042,12 @@ mod tests {
                         kind: Easing(
                             EasingParams {
                                 duration_ms: 150,
-                                curve: EaseOutQuad,
+                                curve: CubicBezier(
+                                    0.05,
+                                    0.7,
+                                    0.1,
+                                    1.0,
+                                ),
                             },
                         ),
                     },

src/animation/bezier.rs

@@ -0,0 +1,60 @@
+use keyframe::EasingFunction;
+
+#[derive(Debug, Clone, Copy)]
+pub struct CubicBezier {
+    x1: f64,
+    y1: f64,
+    x2: f64,
+    y2: f64,
+}
+
+impl CubicBezier {
+    pub fn new(x1: f64, y1: f64, x2: f64, y2: f64) -> Self {
+        Self { x1, y1, x2, y2 }
+    }
+
+    // Based on libadwaita (LGPL-2.1-or-later):
+    // https://gitlab.gnome.org/GNOME/libadwaita/-/blob/1.7.6/src/adw-easing.c?ref_type=tags#L469-531
+
+    fn x_for_t(&self, t: f64) -> f64 {
+        let omt = 1. - t;
+        3. * omt * omt * t * self.x1 + 3. * omt * t * t * self.x2 + t * t * t
+    }
+
+    fn y_for_t(&self, t: f64) -> f64 {
+        let omt = 1. - t;
+        3. * omt * omt * t * self.y1 + 3. * omt * t * t * self.y2 + t * t * t
+    }
+
+    fn t_for_x(&self, x: f64) -> f64 {
+        let mut min_t = 0.;
+        let mut max_t = 1.;
+
+        for _ in 0..=30 {
+            let guess_t = (min_t + max_t) / 2.;
+            let guess_x = self.x_for_t(guess_t);
+
+            if x < guess_x {
+                max_t = guess_t;
+            } else {
+                min_t = guess_t;
+            }
+        }
+
+        (min_t + max_t) / 2.
+    }
+}
+
+impl EasingFunction for CubicBezier {
+    fn y(&self, x: f64) -> f64 {
+        if x <= f64::EPSILON {
+            return 0.;
+        }
+
+        if 1. - f64::EPSILON <= x {
+            return 1.;
+        }
+
+        self.y_for_t(self.t_for_x(x))
+    }
+}

src/animation/mod.rs

@@ -3,6 +3,9 @@ use std::time::Duration;
 use keyframe::functions::{EaseOutCubic, EaseOutQuad};
 use keyframe::EasingFunction;
 
+mod bezier;
+use bezier::CubicBezier;
+
 mod spring;
 pub use spring::{Spring, SpringParams};
 
@@ -43,6 +46,7 @@ pub enum Curve {
     EaseOutQuad,
     EaseOutCubic,
     EaseOutExpo,
+    CubicBezier(CubicBezier),
 }
 
 impl Animation {
@@ -342,6 +346,7 @@ impl Curve {
             Curve::EaseOutQuad => EaseOutQuad.y(x),
             Curve::EaseOutCubic => EaseOutCubic.y(x),
             Curve::EaseOutExpo => 1. - 2f64.powf(-10. * x),
+            Curve::CubicBezier(b) => b.y(x),
         }
     }
 }
@@ -353,6 +358,9 @@ impl From<niri_config::animations::Curve> for Curve {
             niri_config::animations::Curve::EaseOutQuad => Curve::EaseOutQuad,
             niri_config::animations::Curve::EaseOutCubic => Curve::EaseOutCubic,
             niri_config::animations::Curve::EaseOutExpo => Curve::EaseOutExpo,
+            niri_config::animations::Curve::CubicBezier(x1, y1, x2, y2) => {
+                Curve::CubicBezier(CubicBezier::new(x1, y1, x2, y2))
+            }
         }
     }
 }