more optimal method of calculating the normal for the solidify modifier.

When adding 2 unit length vectors, the length can be used to calculate the angle.

source/blender/blenlib/BLI_math_vector.h

@@ -182,6 +182,9 @@ void mid_v3_v3v3(float r[3], const float a[3], const float b[3]);
 void mid_v2_v2v2(float r[2], const float a[2], const float b[2]);
 void mid_v3_v3v3v3(float v[3], const float v1[3], const float v2[3], const float v3[3]);
 
+void mid_v3_v3v3_angle_weighted(float r[3], const float a[3], const float b[3]);
+void mid_v3_angle_weighted(float r[3]);
+
 void flip_v4_v4v4(float v[4], const float v1[4], const float v2[4]);
 void flip_v3_v3v3(float v[3], const float v1[3], const float v2[3]);
 void flip_v2_v2v2(float v[2], const float v1[2], const float v2[2]);

source/blender/blenlib/intern/math_vector.c

@@ -129,6 +129,57 @@ void mid_v3_v3v3v3(float v[3], const float v1[3], const float v2[3], const float
 	v[2] = (v1[2] + v2[2] + v3[2]) / 3.0f;
 }
 
+/**
+ * Specialized function for calculating normals.
+ * fastpath for:
+ *
+* \code{.c}
+ * add_v3_v3v3(r, a, b);
+ * normalize_v3(r)
+ * mul_v3_fl(r, angle_normalized_v3v3(a, b) / M_PI_2);
+ * \endcode
+ *
+ * We can use the length of (a + b) to calculate the angle.
+ */
+void mid_v3_v3v3_angle_weighted(float r[3], const float a[3], const float b[3])
+{
+	/* trick, we want the middle of 2 normals as well as the angle between them
+	 * avoid multiple calculations by */
+	float angle;
+
+	/* double check they are normalized */
+	BLI_ASSERT_UNIT_V3(a);
+	BLI_ASSERT_UNIT_V3(b);
+
+	add_v3_v3v3(r, a, b);
+	angle = ((float)(1.0 / (M_PI / 2.0)) *
+	         /* normally we would only multiply by 2,
+	          * but instead of an angle make this 0-1 factor */
+	         2.0f) *
+	        acosf(normalize_v3(r) / 2.0f);
+	mul_v3_fl(r, angle);
+}
+/**
+ * Same as mid_v3_v3v3_angle_weighted
+ * but \a r is assumed to be accumulated normals, divided by their total.
+ */
+void mid_v3_angle_weighted(float r[3])
+{
+	/* trick, we want the middle of 2 normals as well as the angle between them
+	 * avoid multiple calculations by */
+	float angle;
+
+	/* double check they are normalized */
+	BLI_assert(len_squared_v3(r) <= 1.0f + FLT_EPSILON);
+
+	angle = ((float)(1.0 / (M_PI / 2.0)) *
+	         /* normally we would only multiply by 2,
+	          * but instead of an angle make this 0-1 factor */
+	         2.0f) *
+	        acosf(normalize_v3(r));
+	mul_v3_fl(r, angle);
+}
+
 /**
  * Equivalent to:
  * interp_v3_v3v3(v, v1, v2, -1.0f);

source/blender/bmesh/intern/bmesh_polygon.c

@@ -534,11 +534,6 @@ void BM_face_normal_update(BMFace *f)
 			normal_tri_v3(f->no, co1, co2, co3);
 			break;
 		}
-		case 0:
-		{
-			zero_v3(f->no);
-			break;
-		}
 		default:
 		{
 			bm_face_calc_poly_normal(f);

source/blender/modifiers/intern/MOD_solidify.c

@@ -142,15 +142,19 @@ static void dm_calc_normal(DerivedMesh *dm, float (*temp_nors)[3])
 			if (edge_ref->f2 != -1) {
 				/* We have 2 faces using this edge, calculate the edges normal
 				 * using the angle between the 2 faces as a weighting */
+#if 0
 				add_v3_v3v3(edge_normal, face_nors[edge_ref->f1], face_nors[edge_ref->f2]);
 				normalize_v3(edge_normal);
+
 				mul_v3_fl(edge_normal, angle_normalized_v3v3(face_nors[edge_ref->f1], face_nors[edge_ref->f2]));
+#else
+				mid_v3_v3v3_angle_weighted(edge_normal, face_nors[edge_ref->f1], face_nors[edge_ref->f2]);
+#endif
 			}
 			else {
 				/* only one face attached to that edge */
-				/* an edge without another attached- the weight on this is
-				 * undefined, M_PI/2 is 90d in radians and that seems good enough */
-				mul_v3_v3fl(edge_normal, face_nors[edge_ref->f1], M_PI / 2);
+				/* an edge without another attached- the weight on this is undefined */
+				copy_v3_v3(edge_normal, face_nors[edge_ref->f1]);
 			}
 			add_v3_v3(temp_nors[ed_v1], edge_normal);
 			add_v3_v3(temp_nors[ed_v2], edge_normal);