diff --git a/common/mathlib.cc b/common/mathlib.cc
index 99ecee31..eda282ba 100644
--- a/common/mathlib.cc
+++ b/common/mathlib.cc
@@ -768,3 +768,26 @@ qvec3f ClosestPointOnLineSegment(const qvec3f &v, const qvec3f &w, const qvec3f&
     
     return ClosestPointOnLine(v, w, p);
 }
+
+/// Returns degrees of clockwise rotation from start to end, assuming `normal` is pointing towards the viewer
+float SignedDegreesBetweenUnitVectors(const vec3_t start, const vec3_t end, const vec3_t normal)
+{
+    const float cosangle = qmax(-1.0, qmin(1.0, DotProduct(start, end)));
+    const float unsigned_degrees = acos(cosangle) * (360.0 / (2.0 * Q_PI));
+    
+    if (unsigned_degrees < ANGLEEPSILON)
+        return 0;
+    
+    // get a normal for the rotation plane using the right-hand rule
+    vec3_t rotationNormal;
+    CrossProduct(start, end, rotationNormal);
+    VectorNormalize(rotationNormal);
+    
+    const float normalsCosAngle = DotProduct(rotationNormal, normal);
+    if (normalsCosAngle >= 0) {
+        // counterclockwise rotation
+        return -unsigned_degrees;
+    }
+    // clockwise rotation
+    return unsigned_degrees;
+}
diff --git a/include/common/mathlib.hh b/include/common/mathlib.hh
index 20c46204..852e2fb8 100644
--- a/include/common/mathlib.hh
+++ b/include/common/mathlib.hh
@@ -57,7 +57,7 @@ typedef struct {
 extern const vec3_t vec3_origin;
 
 #define EQUAL_EPSILON 0.001
-
+#define ANGLEEPSILON            0.000001
 #define ZERO_TRI_AREA_EPSILON 0.05f
 #define POINT_EQUAL_EPSILON 0.05f
 
@@ -313,6 +313,8 @@ float DistToLineSegment(const qvec3f &v, const qvec3f &w, const qvec3f &p);
 
 qvec3f ClosestPointOnLineSegment(const qvec3f &v, const qvec3f &w, const qvec3f &p);
 
+float SignedDegreesBetweenUnitVectors(const vec3_t start, const vec3_t end, const vec3_t normal);
+
 // Returns weights for f(0,0), f(1,0), f(0,1), f(1,1)
 // from: https://en.wikipedia.org/wiki/Bilinear_interpolation#Unit_Square
 static inline qvec4f bilinearWeights(const float x, const float y) {
diff --git a/light/test_light.cc b/light/test_light.cc
index 81445dce..551873ab 100644
--- a/light/test_light.cc
+++ b/light/test_light.cc
@@ -412,6 +412,15 @@ TEST(mathlib, ShrinkPoly2) {
     EXPECT_TRUE(polysEqual(shrunkPoly, actualShrunk));
 }
 
+TEST(mathlib, SignedDegreesBetweenUnitVectors) {
+    const vec3_t up = {0, 0, 1};
+    const vec3_t fwd = {0, 1, 0};
+    const vec3_t right = {1, 0, 0};
+    
+    EXPECT_FLOAT_EQ(-90, SignedDegreesBetweenUnitVectors(right, fwd, up));
+    EXPECT_FLOAT_EQ(90, SignedDegreesBetweenUnitVectors(fwd, right, up));
+}
+
 static const float MANGLE_EPSILON = 0.1f;
 
 TEST(light, vec_from_mangle) {