diff --git a/src/bin/flat.rs b/src/bin/flat.rs
index 260b03b..4cad63f 100644
--- a/src/bin/flat.rs
+++ b/src/bin/flat.rs
@@ -629,28 +629,27 @@ trait FloatExt2 {
 }
 
 impl FloatExt2 for (f32, f32) {
-	fn lerp(&self, t: f32) -> f32 {
-		self.0.lerp(self.1, t)
-	}
+	fn lerp(&self, t: f32) -> f32 { f32::lerp(self.0, self.1, t) }
 }
 
 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 s = sx + sy - sx * sy;
 		assert!(sx.is_finite());
 		assert!(sy.is_finite());
 		assert!(sy > 0.0);
 		Decomp2 {
 			ortho: Mat2::IDENTITY,
-			diag: vec2(1.0, (sy, 1.0).lerp(sx)),
+			diag: vec2(1.0, s),
 		}
 	}
 
 	fn part_derivs_at(&self, pos: Vec2) -> Tens2 {
 		let sx = self.fx().value(pos.x);
 		let sy = self.fy().value(pos.y);
-		let s = (sy, 1.0).lerp(sx);
+		let s = sx + sy - sx * sy;
 		let dsx_dx = self.fx().derivative(pos.x);
 		let dsy_dy = self.fy().derivative(pos.y);
 		let ds2_dx = 2.0 * s * (1.0 - sy) * dsx_dx;