+#define M_PI 3.1415926535897932
+
+/* Adapted from BLI_math_vector.h */
+float angle_normalized_v2v2(vec2 v1, vec2 v2)
+{
+ v1 = normalize(v1 * aspect);
+ v2 = normalize(v2 * aspect);
+ /* this is the same as acos(dot_v3v3(v1, v2)), but more accurate */
+ bool q = (dot(v1, v2) >= 0.0);
+ vec2 v = (q) ? (v1 - v2) : (v1 + v2);
+ float a = 2.0 * asin(length(v) / 2.0);
+ return (q) ? a : M_PI - a;
+}
+