diff --git a/common/mathlib.cc b/common/mathlib.cc
index e91c92ed..477b8f0f 100644
--- a/common/mathlib.cc
+++ b/common/mathlib.cc
@@ -835,17 +835,15 @@ qvec3f ClosestPointOnLineSegment(const qvec3f &v, const qvec3f &w, const qvec3f&
 }
 
 /// 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)
+float SignedDegreesBetweenUnitVectors(const qvec3f &start, const qvec3f &end, const qvec3f &normal)
 {
-    const float cosangle = qmax(-1.0, qmin(1.0, DotProduct(start, end)));
+    const float cosangle = qmax(-1.0, qmin(1.0, qv::dot(start, end)));
     const float unsigned_degrees = acos(cosangle) * (360.0 / (2.0 * Q_PI));
 
     // get a normal for the rotation plane using the right-hand rule
-    vec3_t rotationNormal;
-    CrossProduct(start, end, rotationNormal);
-    VectorNormalize(rotationNormal);
+    const qvec3f rotationNormal = qv::normalize(qv::cross(start, end));
     
-    const float normalsCosAngle = DotProduct(rotationNormal, normal);
+    const float normalsCosAngle = qv::dot(rotationNormal, normal);
     if (normalsCosAngle >= 0) {
         // counterclockwise rotation
         return -unsigned_degrees;
@@ -854,6 +852,24 @@ float SignedDegreesBetweenUnitVectors(const vec3_t start, const vec3_t end, cons
     return unsigned_degrees;
 }
 
+concavity_t FacePairConcavity(const qvec3f &face1Center,
+                      const qvec3f &face1Normal,
+                      const qvec3f &face2Center,
+                      const qvec3f &face2Normal)
+{
+    const qvec3f face1to2_dir = qv::normalize(face2Center - face1Center);
+    const qvec3f towards_viewer_dir = qv::cross(face1to2_dir, face1Normal);
+    
+    const float degrees = SignedDegreesBetweenUnitVectors(face1Normal, face2Normal, towards_viewer_dir);
+    if (fabs(degrees) < DEGREES_EPSILON) {
+        return concavity_t::Coplanar;
+    } else if (degrees < 0.0f) {
+        return concavity_t::Concave;
+    } else {
+        return concavity_t::Convex;
+    }
+}
+
 /**
  * do the line segments overlap at all?
  * - if not colinear, returns false.
diff --git a/include/common/mathlib.hh b/include/common/mathlib.hh
index b2cfdcb6..fb649614 100644
--- a/include/common/mathlib.hh
+++ b/include/common/mathlib.hh
@@ -60,6 +60,7 @@ extern const vec3_t vec3_origin;
 #define POINT_EQUAL_EPSILON 0.05f
 
 #define NORMAL_EPSILON          0.000001
+#define DEGREES_EPSILON		0.001
 
 qboolean VectorCompare(const vec3_t v1, const vec3_t v2, vec_t epsilon);
 
@@ -367,7 +368,18 @@ 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);
+float SignedDegreesBetweenUnitVectors(const qvec3f &start, const qvec3f &end, const qvec3f &normal);
+
+enum class concavity_t {
+    Coplanar,
+    Concave,
+    Convex
+};
+
+concavity_t FacePairConcavity(const qvec3f &face1Center,
+                      const qvec3f &face1Normal,
+                      const qvec3f &face2Center,
+                      const qvec3f &face2Normal);
 
 // 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
diff --git a/light/test_light.cc b/light/test_light.cc
index 5a2d4da6..8eeb2874 100644
--- a/light/test_light.cc
+++ b/light/test_light.cc
@@ -422,15 +422,64 @@ TEST(mathlib, ShrinkPoly2) {
 }
 
 TEST(mathlib, SignedDegreesBetweenUnitVectors) {
-    const vec3_t up = {0, 0, 1};
-    const vec3_t fwd = {0, 1, 0};
-    const vec3_t right = {1, 0, 0};
+    const qvec3f up {0, 0, 1};
+    const qvec3f fwd {0, 1, 0};
+    const qvec3f right {1, 0, 0};
     
     EXPECT_FLOAT_EQ(-90, SignedDegreesBetweenUnitVectors(right, fwd, up));
     EXPECT_FLOAT_EQ(90, SignedDegreesBetweenUnitVectors(fwd, right, up));
     EXPECT_FLOAT_EQ(0, SignedDegreesBetweenUnitVectors(right, right, up));
 }
 
+TEST(mathlib, ConcavityTest_concave) {
+    const qvec3f face1center {0, 0, 10};
+    const qvec3f face2center {10, 0, 200};
+    
+    const qvec3f face1normal {0, 0, 1};
+    const qvec3f face2normal {-1, 0, 0};
+    
+    EXPECT_EQ(concavity_t::Concave, FacePairConcavity(face1center, face1normal, face2center, face2normal));
+}
+
+TEST(mathlib, ConcavityTest_concave2) {
+    const qvec3f face1center {0, 0, 10};
+    const qvec3f face2center {-10, 0, 200};
+    
+    const qvec3f face1normal {0, 0, 1};
+    const qvec3f face2normal {1, 0, 0};
+    
+    EXPECT_EQ(concavity_t::Concave, FacePairConcavity(face1center, face1normal, face2center, face2normal));
+}
+
+TEST(mathlib, ConcavityTest_convex) {
+    const qvec3f face1center {0, 0, 10};
+    const qvec3f face2center {10, 0, 5};
+    
+    const qvec3f face1normal {0, 0, 1};
+    const qvec3f face2normal {1, 0, 0};
+    
+    EXPECT_EQ(concavity_t::Convex, FacePairConcavity(face1center, face1normal, face2center, face2normal));
+}
+
+TEST(mathlib, ConcavityTest_convex2) {
+    const qvec3f face1center {0, 0, 10};
+    const qvec3f face2center {-10, 0, 5};
+    
+    const qvec3f face1normal {0, 0, 1};
+    const qvec3f face2normal {-1, 0, 0};
+    
+    EXPECT_EQ(concavity_t::Convex, FacePairConcavity(face1center, face1normal, face2center, face2normal));
+}
+
+TEST(mathlib, ConcavityTest_coplanar) {
+    const qvec3f face1center {0, 0, 10};
+    const qvec3f face2center {100, 100, 10};
+    
+    const qvec3f face1normal {0, 0, 1};
+    const qvec3f face2normal {0, 0, 1};
+    
+    EXPECT_EQ(concavity_t::Coplanar, FacePairConcavity(face1center, face1normal, face2center, face2normal));
+}
 static const float MANGLE_EPSILON = 0.1f;
 
 TEST(light, vec_from_mangle) {