Proper implementation of angular bending springs including jacobian

derivatives for stabilization.

The bending forces are based on a simplified torsion model where each
neighboring point of a vertex creates a force toward a local goal. This
can be extended later by defining the goals in a local curve frame, so
that natural hair shapes other than perfectly straight hair are
supported.

Calculating the jacobians for the bending forces analytically proved
quite difficult and doesn't work yet, so the fallback method for now
is a straightforward finite difference method. This works very well and
is not too costly. Even the original paper ("Artistic Simulation of
Curly Hair") suggests this approach.

1 files changed, 4 insertions, 1 deletions

diff --git a/source/blender/blenkernel/BKE_cloth.h b/source/blender/blenkernel/BKE_cloth.h
index b7be7c60af5..acda9e11265 100644
--- a/source/blender/blenkernel/BKE_cloth.h
+++ b/source/blender/blenkernel/BKE_cloth.h
@@ -134,7 +134,10 @@ typedef struct ClothSpring {
 	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) */
+	/* needed for implicit solver (fast lookup) */
+	int	matrix_ij_kl;
+	int	matrix_kl_mn;
+	int	matrix_ij_mn;
 	int	type;		/* types defined in BKE_cloth.h ("springType") */
 	int	flags; 		/* defined in BKE_cloth.h, e.g. deactivated due to tearing */
 	float dfdx[3][3];