diff --git a/noise/lerp.cpp b/noise/lerp.cpp
index 84b7b5d1..a204b67f 100644
--- a/noise/lerp.cpp
+++ b/noise/lerp.cpp
@@ -27,24 +27,28 @@ template <typename T>
 T
 util::lerp::sigmoid (T val)
 {
-    static_assert (std::is_floating_point<T>::value, "lerp is only defined for floating types");
+    static_assert (std::is_floating_point<T>::value,
+                   "lerp is only defined for floating types");
     return -1 + 2 / (1 + std::exp (-2 * val));
 }
 
-template float util::lerp::sigmoid (float);
+template float  util::lerp::sigmoid (float);
 template double util::lerp::sigmoid (double);
 
 
 //-----------------------------------------------------------------------------
 template <typename T>
 T
-util::lerp::trunc (T a, T, T)
+util::lerp::trunc (T a, T, T weight)
 {
-    static_assert (std::is_floating_point<T>::value, "lerp is only defined for floating types");
+    static_assert (std::is_floating_point<T>::value,
+                   "lerp is only defined for floating types");
+    CHECK_LIMIT (weight, 0, 1);
+
     return a;
 }
 
-template float util::lerp::trunc (float, float, float);
+template float  util::lerp::trunc (float,  float,  float);
 template double util::lerp::trunc (double, double, double);
 
 
@@ -53,12 +57,14 @@ template <typename T>
 T
 util::lerp::linear (T a, T b, T weight)
 {
-    static_assert (std::is_floating_point<T>::value, "lerp is only defined for floating types");
-    CHECK (weight >= 0 && weight <= 1);
+    static_assert (std::is_floating_point<T>::value,
+                   "lerp is only defined for floating types");
+    CHECK_LIMIT (weight, 0, 1);
+
     return a * (1 - weight) + b * weight;
 }
 
-template float util::lerp::linear (float, float, float);
+template float  util::lerp::linear (float,  float,  float);
 template double util::lerp::linear (double, double, double);
 
 
@@ -67,14 +73,15 @@ template <typename T>
 T
 util::lerp::cosine (T a, T b, T weight)
 {
-    static_assert (std::is_floating_point<T>::value, "lerp is only defined for floating types");
-    CHECK (weight >= 0 && weight <= 1);
-    T t  = (1 - std::cos (weight * PI<T>)) * T(0.5);
+    static_assert (std::is_floating_point<T>::value,
+                   "lerp is only defined for floating types");
+    CHECK_LIMIT (weight, 0, 1);
 
+    T t = (1 - std::cos (weight * PI<T>)) * T(0.5);
     return a * (1 - t) + b * t;
 }
 
-template float util::lerp::cosine (float, float, float);
+template float  util::lerp::cosine (float,  float,  float);
 template double util::lerp::cosine (double, double, double);
 
 
@@ -83,13 +90,16 @@ template <typename T>
 T
 util::lerp::cubic (T a, T b, T weight)
 {
-    static_assert (std::is_floating_point<T>::value, "lerp is only defined for floating types");
-    CHECK (weight >= 0 && weight <= 1);
+    static_assert (std::is_floating_point<T>::value,
+                   "lerp is only defined for floating types");
+    CHECK_LIMIT (weight, 0, 1);
+
+    // -2w^3 * 3w^2
     T t = weight * weight * (3 - 2 * weight);
     return a * (1 - t) + b * t;
 }
 
-template float util::lerp::cubic (float, float, float);
+template float  util::lerp::cubic (float,  float,  float);
 template double util::lerp::cubic (double, double, double);
 
 
@@ -98,13 +108,15 @@ template <typename T>
 T
 util::lerp::quintic (T a, T b, T weight)
 {
-    static_assert (std::is_floating_point<T>::value, "lerp is only defined for floating types");
+    static_assert (std::is_floating_point<T>::value,
+                   "lerp is only defined for floating types");
+    CHECK_LIMIT (weight, 0, 1);
 
-    CHECK (weight >= 0 && weight <= 1);
+    // from perlin's improved noise: 6w^5 -15w^4 +10w^3
     T t = weight * weight * weight * (weight * (weight * 6 - 15) + 10);
     return a * (1 - t) + b * t;
 }
 
 
-template float util::lerp::quintic (float, float, float);
+template float  util::lerp::quintic (float,  float,  float);
 template double util::lerp::quintic (double, double, double);