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.

2 files changed, 65 insertions, 31 deletions

diff --git a/source/blender/blenkernel/BKE_cloth.h b/source/blender/blenkernel/BKE_cloth.h
index 571c0ec89a1..838388f428c 100644
--- a/source/blender/blenkernel/BKE_cloth.h
+++ b/source/blender/blenkernel/BKE_cloth.h
@@ -124,6 +124,7 @@ ClothVertex;
 typedef struct ClothSpring {
 	int	ij;		/* Pij from the paper, one end of the spring.	*/
 	int	kl;		/* Pkl from the paper, one end of the spring.	*/
+	int mn;
 	float	restlen;	/* The original length of the spring.	*/
 	int	matrix_index; 	/* needed for implicit solver (fast lookup) */
 	int	type;		/* types defined in BKE_cloth.h ("springType") */
@@ -175,7 +176,8 @@ typedef enum {
 	CLOTH_SPRING_TYPE_SHEAR       = (1 << 2),
 	CLOTH_SPRING_TYPE_BENDING     = (1 << 3),
 	CLOTH_SPRING_TYPE_GOAL        = (1 << 4),
-	CLOTH_SPRING_TYPE_SEWING      = (1 << 5)
+	CLOTH_SPRING_TYPE_SEWING      = (1 << 5),
+	CLOTH_SPRING_TYPE_BENDING_ANG = (1 << 6),
 } CLOTH_SPRING_TYPES;
 
 /* SPRING FLAGS */
diff --git a/source/blender/blenkernel/intern/cloth.c b/source/blender/blenkernel/intern/cloth.c
index 4132fc86608..1b3b668f524 100644
--- a/source/blender/blenkernel/intern/cloth.c
+++ b/source/blender/blenkernel/intern/cloth.c
@@ -1287,39 +1287,71 @@ static int cloth_build_springs ( ClothModifierData *clmd, DerivedMesh *dm )
 		}
 	}
 	else if (struct_springs > 2) {
-		/* bending springs for hair strands */
-		/* The current algorightm only goes through the edges in order of the mesh edges list	*/
-		/* and makes springs between the outer vert of edges sharing a vertice. This works just */
-		/* fine for hair, but not for user generated string meshes. This could/should be later	*/
-		/* extended to work with non-ordered edges so that it can be used for general "rope		*/
-		/* dynamics" without the need for the vertices or edges to be ordered through the length*/
-		/* of the strands. -jahka */
-		search = cloth->springs;
-		search2 = search->next;
-		while (search && search2) {
-			tspring = search->link;
-			tspring2 = search2->link;
-
-			if (tspring->ij == tspring2->kl) {
-				spring = (ClothSpring *)MEM_callocN ( sizeof ( ClothSpring ), "cloth spring" );
+		if (G.debug_value != 1112) {
+			search = cloth->springs;
+			search2 = search->next;
+			while (search && search2) {
+				tspring = search->link;
+				tspring2 = search2->link;
 				
-				if (!spring) {
-					cloth_free_errorsprings(cloth, edgelist);
-					return 0;
+				if (tspring->ij == tspring2->kl) {
+					spring = (ClothSpring *)MEM_callocN ( sizeof ( ClothSpring ), "cloth spring" );
+					
+					if (!spring) {
+						cloth_free_errorsprings(cloth, edgelist);
+						return 0;
+					}
+					
+					spring->ij = tspring->kl;
+					spring->kl = tspring->ij;
+					spring->mn = tspring2->ij;
+					spring->restlen = len_v3v3(cloth->verts[spring->kl].xrest, cloth->verts[spring->ij].xrest);
+					spring->type = CLOTH_SPRING_TYPE_BENDING_ANG;
+					spring->stiffness = (cloth->verts[spring->kl].bend_stiff + cloth->verts[spring->ij].bend_stiff) / 2.0f;
+					bend_springs++;
+					
+					BLI_linklist_prepend ( &cloth->springs, spring );
 				}
-
-				spring->ij = tspring2->ij;
-				spring->kl = tspring->kl;
-				spring->restlen = len_v3v3(cloth->verts[spring->kl].xrest, cloth->verts[spring->ij].xrest);
-				spring->type = CLOTH_SPRING_TYPE_BENDING;
-				spring->stiffness = (cloth->verts[spring->kl].bend_stiff + cloth->verts[spring->ij].bend_stiff) / 2.0f;
-				bend_springs++;
-
-				BLI_linklist_prepend ( &cloth->springs, spring );
+				
+				search = search->next;
+				search2 = search2->next;
+			}
+		}
+		else {
+			/* bending springs for hair strands */
+			/* The current algorightm only goes through the edges in order of the mesh edges list	*/
+			/* and makes springs between the outer vert of edges sharing a vertice. This works just */
+			/* fine for hair, but not for user generated string meshes. This could/should be later	*/
+			/* extended to work with non-ordered edges so that it can be used for general "rope		*/
+			/* dynamics" without the need for the vertices or edges to be ordered through the length*/
+			/* of the strands. -jahka */
+			search = cloth->springs;
+			search2 = search->next;
+			while (search && search2) {
+				tspring = search->link;
+				tspring2 = search2->link;
+				
+				if (tspring->ij == tspring2->kl) {
+					spring = (ClothSpring *)MEM_callocN ( sizeof ( ClothSpring ), "cloth spring" );
+					
+					if (!spring) {
+						cloth_free_errorsprings(cloth, edgelist);
+						return 0;
+					}
+					
+					spring->ij = tspring2->ij;
+					spring->kl = tspring->kl;
+					spring->restlen = len_v3v3(cloth->verts[spring->kl].xrest, cloth->verts[spring->ij].xrest);
+					spring->type = CLOTH_SPRING_TYPE_BENDING;
+					spring->stiffness = (cloth->verts[spring->kl].bend_stiff + cloth->verts[spring->ij].bend_stiff) / 2.0f;
+					bend_springs++;
+					
+					BLI_linklist_prepend ( &cloth->springs, spring );
+				}
+				
+				search = search->next;
+				search2 = search2->next;
 			}
-			
-			search = search->next;
-			search2 = search2->next;
 		}
 	}