Fix for Sintel hair bug.

The hair solver needs sane input to converge within reasonable time
steps. In particular the spring lengths must not be too difference
(factor 0.01..100 or so max, this is comparable to rigid body simulation
of vastly different masses, which is also unstable).

The basic hair system generate strands with equally spaced points, which
is good solver material. However, the hair edit operators, specifically
the cutting tool, can move points along the strands, creating tightly
packed hair points. This puts the solver under enormous stress and
causes the "explosions" observed already during the Sintel project.

The simple solution for now is to exclude very short hairs from the
simulation. Later the cutting tool should be modified such that it
keeps the segments roughly at the same length and throws away vertices
when the hair gets too short (same goes for the extension tool).

The hair system should have a general mechanism for making sure that
situations such as this don't occur. This will have to be a design
consideration for replacements in any future hair system.

1 files changed, 57 insertions, 18 deletions

diff --git a/source/blender/blenkernel/intern/particle_system.c b/source/blender/blenkernel/intern/particle_system.c
index f2e3c774cdc..6397c3739d9 100644
--- a/source/blender/blenkernel/intern/particle_system.c
+++ b/source/blender/blenkernel/intern/particle_system.c
@@ -3976,6 +3976,45 @@ static void psys_update_path_cache(ParticleSimulationData *sim, float cfra)
 		psys_free_path_cache(psys, NULL);
 }
 
+static bool psys_hair_use_simulation(ParticleData *pa, float max_length)
+{
+	/* Minimum segment length relative to average length.
+	 * Hairs with segments below this length will be excluded from the simulation,
+	 * because otherwise the solver will become unstable.
+	 * The hair system should always make sure the hair segments have reasonable length ratios,
+	 * but this can happen in old files when e.g. cutting hair.
+	 */
+	const float min_length = 0.1f * max_length;
+	
+	HairKey *key;
+	int k;
+	
+	if (pa->totkey < 2)
+		return false;
+	
+	for (k=1, key=pa->hair+1; k<pa->totkey; k++,key++) {
+		float length = len_v3v3(key->co, (key-1)->co);
+		if (length < min_length)
+			return false;
+	}
+	
+	return true;
+}
+
+static MDeformVert *hair_set_pinning(MDeformVert *dvert, float weight)
+{
+	if (dvert) {
+		if (!dvert->totweight) {
+			dvert->dw = MEM_callocN(sizeof(MDeformWeight), "deformWeight");
+			dvert->totweight = 1;
+		}
+		
+		dvert->dw->weight = weight;
+		dvert++;
+	}
+	return dvert;
+}
+
 static void do_hair_dynamics(ParticleSimulationData *sim)
 {
 	ParticleSystem *psys = sim->psys;
@@ -3990,6 +4029,7 @@ static void do_hair_dynamics(ParticleSimulationData *sim)
 	int k;
 	float hairmat[4][4];
 	float (*deformedVerts)[3];
+	float max_length;
 
 	if (!psys->clmd) {
 		psys->clmd = (ClothModifierData*)modifier_new(eModifierType_Cloth);
@@ -4000,6 +4040,16 @@ static void do_hair_dynamics(ParticleSimulationData *sim)
 		psys->clmd->coll_parms->flags |= CLOTH_COLLSETTINGS_FLAG_POINTS;
 	}
 
+	/* calculate maximum segment length */
+	max_length = 0.0f;
+	LOOP_PARTICLES {
+		for (k=1, key=pa->hair+1; k<pa->totkey; k++,key++) {
+			float length = len_v3v3(key->co, (key-1)->co);
+			if (max_length < length)
+				max_length = length;
+		}
+	}
+
 	/* create a dm from hair vertices */
 	LOOP_PARTICLES
 		totpoint += pa->totkey;
@@ -4034,6 +4084,7 @@ static void do_hair_dynamics(ParticleSimulationData *sim)
 	psys->particles->hair_index = 1;
 	LOOP_PARTICLES {
 		float root_mat[4][4];
+		bool use_hair = psys_hair_use_simulation(pa, max_length);
 
 		if (p)
 			pa->hair_index = (pa-1)->hair_index + (pa-1)->totkey + 1;
@@ -4062,15 +4113,7 @@ static void do_hair_dynamics(ParticleSimulationData *sim)
 				medge->v2 = pa->hair_index;
 				medge++;
 
-				if (dvert) {
-					if (!dvert->totweight) {
-						dvert->dw = MEM_callocN(sizeof(MDeformWeight), "deformWeight");
-						dvert->totweight = 1;
-					}
-
-					dvert->dw->weight = 1.0f;
-					dvert++;
-				}
+				dvert = hair_set_pinning(dvert, 1.0f);
 			}
 
 			/* store root transform in cloth data */
@@ -4088,15 +4131,11 @@ static void do_hair_dynamics(ParticleSimulationData *sim)
 				medge++;
 			}
 
-			if (dvert) {
-				if (!dvert->totweight) {
-					dvert->dw = MEM_callocN(sizeof(MDeformWeight), "deformWeight");
-					dvert->totweight = 1;
-				}
-				/* roots should be 1.0, the rest can be anything from 0.0 to 1.0 */
-				dvert->dw->weight = key->weight;
-				dvert++;
-			}
+			/* roots and disabled hairs should be 1.0, the rest can be anything from 0.0 to 1.0 */
+			if (use_hair)
+				dvert = hair_set_pinning(dvert, key->weight);
+			else
+				dvert = hair_set_pinning(dvert, 1.0f);
 		}
 	}