diff --git a/src/bin/flat/main.rs b/src/bin/flat/main.rs
index 98db0bc..ab7ff31 100644
--- a/src/bin/flat/main.rs
+++ b/src/bin/flat/main.rs
@@ -573,18 +573,26 @@ mod fns {
 		pub external: f32,
 	}
 
+	/// Продолжает функцию f с [-lim, lim] линейно в предположении f(±lim) = ±val, f'(±lim) = 1.
+	fn extend_linear(t: f32, f: impl FnOnce(f32) -> f32, lim: f32, val: f32) -> f32 {
+		if t.abs() <= lim { f(t) } else { t + t.signum() * (val - lim) }
+	}
+
+	/// Продолжает функцию f с [-lim, lim] константой в предположении f(±lim) = val, f'(±lim) = 0.
+	fn extend_const(t: f32, f: impl FnOnce(f32) -> f32, lim: f32, val: f32) -> f32 {
+		if t.abs() <= lim { f(t) } else { val }
+	}
+
 	impl RectY {
 		fn a(&self) -> f32 { -(self.external - self.internal) / self.internal.powi(2) }
 		fn b(&self) -> f32 { 2.0 * self.external / self.internal - 1.0 }
 		fn root(&self, x: f32) -> f32 { (self.b().powi(2) + 4.0 * self.a() * x.abs()).sqrt() }
 
-		pub fn x(&self, u: f32) -> f32 { (self.a() * u.abs() + self.b()) * u }
-		pub fn u(&self, x: f32) -> f32 { 0.5 * x.signum() * (-self.b() + self.root(x)) / self.a() }
-		pub fn dx(&self, u: f32) -> f32 { 2.0 * self.a() * u.abs() + self.b() }
-		pub fn du(&self, x: f32) -> f32 { 1.0 / self.root(x) }
-
-		pub fn value(&self, x: f32) -> f32 { if x.abs() <= self.external { 1.0 / self.root(x) } else { 1.0 } }
-		pub fn derivative(&self, x: f32) -> f32 { if x.abs() <= self.external { -2.0 * x.signum() * self.a() * self.root(x).powi(-3) } else { 0.0 } }
+		pub fn x(&self, u: f32) -> f32 { extend_linear(u, |u| (self.a() * u.abs() + self.b()) * u, self.internal, self.external) }
+		pub fn u(&self, x: f32) -> f32 { extend_linear(x, |x| 0.5 * x.signum() * (-self.b() + self.root(x)) / self.a(), self.external, self.internal) }
+		pub fn dx(&self, u: f32) -> f32 { extend_const(u, |u| 2.0 * self.a() * u.abs() + self.b(), self.internal, 1.0) }
+		pub fn du(&self, x: f32) -> f32 { extend_const(x, |x| 1.0 / self.root(x), self.external, 1.0) }
+		pub fn d2u(&self, x: f32) -> f32 { extend_const(x, |x| -2.0 * x.signum() * self.a() * self.root(x).powi(-3), self.external, 0.0) }
 	}
 }
 
@@ -637,7 +645,7 @@ impl FloatExt2 for (f32, f32) {
 impl Metric for Rect {
 	fn sqrt_at(&self, pos: Vec2) -> Decomp2 {
 		let sx = self.fx().value(pos.x);
-		let sy = self.fy().value(pos.y);
+		let sy = self.fy().du(pos.y);
 		let s = sx + sy - sx * sy;
 		assert!(sx.is_finite());
 		assert!(sy.is_finite());
@@ -650,10 +658,10 @@ impl Metric for Rect {
 
 	fn part_derivs_at(&self, pos: Vec2) -> Tens2 {
 		let sx = self.fx().value(pos.x);
-		let sy = self.fy().value(pos.y);
+		let sy = self.fy().du(pos.y);
 		let s = sx + sy - sx * sy;
 		let dsx_dx = self.fx().derivative(pos.x);
-		let dsy_dy = self.fy().derivative(pos.y);
+		let dsy_dy = self.fy().d2u(pos.y);
 		let ds2_dx = 2.0 * s * (1.0 - sy) * dsx_dx;
 		let ds2_dy = 2.0 * s * (1.0 - sx) * dsy_dy;
 		[