Cloth: Componentize forces

This separates cloth stiffness and damping forces into tension,
compression, and shearing components, allowing more control over the
cloth behaviour.

This also adds a bending model selector (although the new bending model
itself is not implemented in this commit). This is because some of the
features implemented here only make sense within the new bending model,
while the old model is kept for compatibility.

This commit makes non-breaking changes, and thus maintains full
compatibility with existing simulations.

Reviewed By: brecht

Differential Revision: http://developer.blender.org/D3655

1 files changed, 27 insertions, 11 deletions

diff --git a/source/blender/physics/intern/implicit_blender.c b/source/blender/physics/intern/implicit_blender.c
index ddd71eb93e3..677e566ff39 100644
--- a/source/blender/physics/intern/implicit_blender.c
+++ b/source/blender/physics/intern/implicit_blender.c
@@ -1585,9 +1585,13 @@ BLI_INLINE void apply_spring(Implicit_Data *data, int i, int j, const float f[3]
 }
 
 bool BPH_mass_spring_force_spring_linear(Implicit_Data *data, int i, int j, float restlen,
-                                         float stiffness, float damping, bool no_compress, float clamp_force)
+                                         float stiffness_tension, float damping_tension,
+                                         float stiffness_compression, float damping_compression,
+                                         bool resist_compress, bool new_compress, float clamp_force)
 {
 	float extent[3], length, dir[3], vel[3];
+	float f[3], dfdx[3][3], dfdv[3][3];
+	float damping = 0;
 
 	// calculate elonglation
 	spring_length(data, i, j, extent, dir, &length, vel);
@@ -1595,29 +1599,41 @@ bool BPH_mass_spring_force_spring_linear(Implicit_Data *data, int i, int j, floa
 	/* This code computes not only the force, but also its derivative.
 	   Zero derivative effectively disables the spring for the implicit solver.
 	   Thus length > restlen makes cloth unconstrained at the start of simulation. */
-	if ((length >= restlen && length > 0) || no_compress) {
-		float stretch_force, f[3], dfdx[3][3], dfdv[3][3];
+	if ((length >= restlen && length > 0) || resist_compress) {
+		float stretch_force;
+
+		damping = damping_tension;
 
-		stretch_force = stiffness * (length - restlen);
+		stretch_force = stiffness_tension * (length - restlen);
 		if (clamp_force > 0.0f && stretch_force > clamp_force) {
 			stretch_force = clamp_force;
 		}
 		mul_v3_v3fl(f, dir, stretch_force);
 
-		// Ascher & Boxman, p.21: Damping only during elonglation
-		// something wrong with it...
-		madd_v3_v3fl(f, dir, damping * dot_v3v3(vel, dir));
+		dfdx_spring(dfdx, dir, length, restlen, stiffness_tension);
+	}
+	else if (new_compress) {
+		/* This is based on the Choi and Ko bending model, which works surprisingly well for compression. */
+		float kb = stiffness_compression;
+		float cb = kb; /* cb equal to kb seems to work, but a factor can be added if necessary */
 
-		dfdx_spring(dfdx, dir, length, restlen, stiffness);
-		dfdv_damp(dfdv, dir, damping);
+		damping = damping_compression;
 
-		apply_spring(data, i, j, f, dfdx, dfdv);
+		mul_v3_v3fl(f, dir, fbstar(length, restlen, kb, cb));
 
-		return true;
+		outerproduct(dfdx, dir, dir);
+		mul_m3_fl(dfdx, fbstar_jacobi(length, restlen, kb, cb));
 	}
 	else {
 		return false;
 	}
+
+	madd_v3_v3fl(f, dir, damping * dot_v3v3(vel, dir));
+	dfdv_damp(dfdv, dir, damping);
+
+	apply_spring(data, i, j, f, dfdx, dfdv);
+
+	return true;
 }
 
 /* See "Stable but Responsive Cloth" (Choi, Ko 2005) */