Improved force field effects on hair strands.

The previous calculation was modulated with the angle between the wind
direction and the segments, which leads to very oscillating behavior.

Now the formula includes an estimate for the geometric cross section
of a hair segment based on the incident angle and the hair thickness
(currently just the particle size). This gives a more stable behavior
and more realistic response to wind.

Conflicts:
	source/blender/blenkernel/intern/particle_system.c
	source/blender/physics/intern/BPH_mass_spring.cpp

1 files changed, 27 insertions, 7 deletions

diff --git a/source/blender/physics/intern/implicit_blender.c b/source/blender/physics/intern/implicit_blender.c
index cb115a2c10a..e6320507db0 100644
--- a/source/blender/physics/intern/implicit_blender.c
+++ b/source/blender/physics/intern/implicit_blender.c
@@ -1461,21 +1461,41 @@ void BPH_mass_spring_force_face_wind(Implicit_Data *data, int v1, int v2, int v3
 	}
 }
 
-void BPH_mass_spring_force_edge_wind(Implicit_Data *data, int v1, int v2, const float (*winvec)[3])
+static void edge_wind_vertex(const float dir[3], float length, float radius, const float wind[3], float f[3], float UNUSED(dfdx[3][3]), float UNUSED(dfdv[3][3]))
 {
-	const float effector_scale = 0.01;
-	float win[3], dir[3], nor[3], length;
+	const float density = 0.01f; /* XXX arbitrary value, corresponds to effect of air density */
+	float cos_alpha, sin_alpha, cross_section;
+	float windlen = len_v3(wind);
+	
+	if (windlen == 0.0f) {
+		zero_v3(f);
+		return;
+	}
+	
+	/* angle of wind direction to edge */
+	cos_alpha = dot_v3v3(wind, dir) / windlen;
+	sin_alpha = sqrt(1.0 - cos_alpha*cos_alpha);
+	cross_section = radius * (M_PI * radius * sin_alpha + length * cos_alpha);
+	
+	mul_v3_v3fl(f, wind, density * cross_section);
+}
+
+void BPH_mass_spring_force_edge_wind(Implicit_Data *data, int v1, int v2, float radius1, float radius2, const float (*winvec)[3])
+{
+	float win[3], dir[3], length;
+	float f[3], dfdx[3][3], dfdv[3][3];
 	
 	sub_v3_v3v3(dir, data->X[v1], data->X[v2]);
 	length = normalize_v3(dir);
 	
 	world_to_root_v3(data, v1, win, winvec[v1]);
-	madd_v3_v3v3fl(nor, win, dir, -dot_v3v3(win, dir));
-	madd_v3_v3fl(data->F[v1], nor, effector_scale * length);
+	edge_wind_vertex(dir, length, radius1, win, f, dfdx, dfdv);
+	add_v3_v3(data->F[v1], f);
 	
 	world_to_root_v3(data, v2, win, winvec[v2]);
-	madd_v3_v3v3fl(nor, win, dir, -dot_v3v3(win, dir));
-	madd_v3_v3fl(data->F[v2], nor, effector_scale * length);
+	/* use -length to invert edge direction */
+	edge_wind_vertex(dir, length, radius2, win, f, dfdx, dfdv);
+	add_v3_v3(data->F[v2], f);
 }
 
 BLI_INLINE void dfdx_spring(float to[3][3], const float dir[3], float length, float L, float k)