Simple initial implementation of angular bending springs.

These are much better suited for creating stiff hair. The previous
bending springs are based on "push" type spring along the hypothenuse
of 3 hair vertices. This sort of spring requires a very large force
in the direction of the spring for any angular effect, and is still
unstable in the equilibrium.

The new bending spring model is based on "target" vectors defined in a
local hair frame, which generates a force perpendicular to the hair
segment. For further details see
"Artistic Simulation of Curly Hair" (Pixar technical memo #12-03a)
or
"A Mass Spring Model for Hair Simulation" (Selle, Lentine, Fedkiw 2008)

Currently the implementation uses a single root frame that is not yet
propagated along the hair, so the resulting rest shape is not very
natural. Also damping and derivatives are still missing.

1 files changed, 13 insertions, 1 deletions

diff --git a/source/blender/physics/intern/BPH_mass_spring.cpp b/source/blender/physics/intern/BPH_mass_spring.cpp
index db371997632..56de54e4192 100644
--- a/source/blender/physics/intern/BPH_mass_spring.cpp
+++ b/source/blender/physics/intern/BPH_mass_spring.cpp
@@ -373,7 +373,7 @@ BLI_INLINE void cloth_calc_spring_force(ClothModifierData *clmd, ClothSpring *s,
 		BPH_mass_spring_force_spring_goal(data, s->ij, s->matrix_index, goal_x, goal_v, k, parms->goalfrict * 0.01f, s->f, s->dfdx, s->dfdv);
 #endif
 	}
-	else {  /* calculate force of bending springs */
+	else if (s->type & CLOTH_SPRING_TYPE_BENDING) {  /* calculate force of bending springs */
 #ifdef CLOTH_FORCE_SPRING_BEND
 		float kb, cb, scaling;
 		
@@ -385,6 +385,18 @@ BLI_INLINE void cloth_calc_spring_force(ClothModifierData *clmd, ClothSpring *s,
 		BPH_mass_spring_force_spring_bending(data, s->ij, s->kl, s->matrix_index, s->restlen, kb, cb, s->f, s->dfdx, s->dfdv);
 #endif
 	}
+	else if (s->type & CLOTH_SPRING_TYPE_BENDING_ANG) {
+#ifdef CLOTH_FORCE_SPRING_BEND
+		float kb, cb, scaling;
+		
+		s->flags |= CLOTH_SPRING_FLAG_NEEDED;
+		
+		scaling = parms->bending + s->stiffness * fabsf(parms->max_bend - parms->bending);
+		cb = kb = scaling / (20.0f * (parms->avg_spring_len + FLT_EPSILON));
+		
+		BPH_mass_spring_force_spring_bending_angular(data, s->ij, s->kl, s->matrix_index, s->restlen, kb, cb, s->f, s->dfdx, s->dfdv);
+#endif
+	}
 }
 
 static void hair_get_boundbox(ClothModifierData *clmd, float gmin[3], float gmax[3])