Add another wind model, that doesn't clamp the maximum force.

Thanks to saggitasaggita for the patch, see http://code.google.com/p/bullet/issues/detail?id=532

Demos/SoftDemo/SoftDemo.cpp

@@ -40,7 +40,7 @@ extern float eye[3];
 extern int glutScreenWidth;
 extern int glutScreenHeight;
 
-static bool sDemoMode = true;
+static bool sDemoMode = false;
 
 const int maxProxies = 32766;
 const int maxOverlap = 65535;
@@ -52,7 +52,7 @@ static btRigidBody* staticBody = 0;
 static float waveheight = 5.f;
 
 const float TRIANGLE_SIZE=8.f;
-unsigned	current_demo=23;
+unsigned	current_demo=12;
 #define DEMO_MODE_TIMEOUT 15.f //15 seconds for each demo
 
 
@@ -557,7 +557,58 @@ static void	Init_Aero(SoftDemo* pdemo)
 
 	}
 	pdemo->m_autocam=true;
+}
 
+static void	Init_Aero2(SoftDemo* pdemo)
+{
+	//TRACEDEMO
+	const btScalar	s=5;
+	const int		segments=10;
+	const int		count=5;
+	btVector3 pos(-s*segments, 0, 0);
+	btScalar gap = 0.5;
+
+	for(int i=0;i<count;++i)
+	{
+		btSoftBody*		psb=btSoftBodyHelpers::CreatePatch(	pdemo->m_softBodyWorldInfo,btVector3(-s,0,-s*3),
+			btVector3(+s,0,-s*3),
+			btVector3(-s,0,+s),
+			btVector3(+s,0,+s),
+			segments,segments*3,
+			1+2,true);
+		
+		psb->getCollisionShape()->setMargin(0.5);
+		btSoftBody::Material* pm=psb->appendMaterial();
+		pm->m_kLST		=	0.0004;
+		pm->m_flags		-=	btSoftBody::fMaterial::DebugDraw;
+		psb->generateBendingConstraints(2,pm);
+		
+		psb->m_cfg.kLF			=	0.05;
+		psb->m_cfg.kDG			=	0.01;
+
+		//psb->m_cfg.kLF			=	0.004;
+		//psb->m_cfg.kDG			=	0.0003;
+
+		psb->m_cfg.piterations = 2;
+		psb->m_cfg.aeromodel	=	btSoftBody::eAeroModel::V_TwoSidedLiftDrag;
+
+		
+		psb->setWindVelocity(btVector3(4, -12.0, -25.0));
+
+		btTransform		trs;
+		btQuaternion	rot;
+		pos += btVector3(s*2 + gap, 0, 0);
+		rot.setRotation(btVector3(1, 0, 0), btScalar(SIMD_PI/2));
+		trs.setIdentity();
+		trs.setOrigin(pos);
+		trs.setRotation(rot);
+		psb->transform(trs);
+		psb->setTotalMass(2.0);
+		
+		pdemo->getSoftDynamicsWorld()->addSoftBody(psb);
+	}
+
+	pdemo->m_autocam=true;
 }
 
 //
@@ -1273,6 +1324,7 @@ static void	Init_TetraCube(SoftDemo* pdemo)
 		Init_Collide3,
 		Init_Impact,
 		Init_Aero,
+		Init_Aero2,
 		Init_Friction,			
 		Init_Torus,
 		Init_TorusMatch,

src/BulletSoftBody/btSoftBody.cpp

@@ -2702,33 +2702,49 @@ void				btSoftBody::applyForces()
 				if(as_vaero)
 				{				
 					const btVector3	rel_v = n.m_v - medium.m_velocity;					
+					const btScalar rel_v_len = rel_v.length();
 					const btScalar	rel_v2 = rel_v.length2();
+
 					if(rel_v2>SIMD_EPSILON)
 					{
+						const btVector3 rel_v_nrm = rel_v.normalized();
 						btVector3	nrm = n.m_n;						
-						/* Setup normal		*/ 
+
-						switch(m_cfg.aeromodel)
+						if (m_cfg.aeromodel == btSoftBody::eAeroModel::V_TwoSidedLiftDrag)
 						{
-						case	btSoftBody::eAeroModel::V_Point:
-							nrm = NormalizeAny(rel_v);
-							break;
-						case	btSoftBody::eAeroModel::V_TwoSided:
 							nrm *= (btScalar)( (btDot(nrm,rel_v) < 0) ? -1 : +1);
-							break;							
+							btVector3 fDrag(0, 0, 0);
-						default:
+							btVector3 fLift(0, 0, 0);
-							{
+
-							}
+							btScalar n_dot_v = nrm.dot(rel_v_nrm);
+							btScalar tri_area = 0.5f * n.m_area;
+							
+							fDrag = 0.5f * kDG * medium.m_density * rel_v2 * tri_area * n_dot_v * (-rel_v_nrm);
+							
+							// Check angle of attack
+							// cos(10<31>) = 0.98480
+							if ( 0 < n_dot_v && n_dot_v < 0.98480f)
+								fLift = 0.5f * kLF * medium.m_density * rel_v_len * tri_area * btSqrt(1.0f-n_dot_v*n_dot_v) * (nrm.cross(rel_v_nrm).cross(rel_v_nrm));
+
+							n.m_f += fDrag;
+							n.m_f += fLift;
 						}
-						const btScalar dvn = btDot(rel_v,nrm);
+						else if (m_cfg.aeromodel == btSoftBody::eAeroModel::V_Point || m_cfg.aeromodel == btSoftBody::eAeroModel::V_OneSided || m_cfg.aeromodel == btSoftBody::eAeroModel::V_TwoSided)
-						/* Compute forces	*/ 
-						if(dvn>0)
 						{
-							btVector3		force(0,0,0);
+							if (btSoftBody::eAeroModel::V_TwoSided)
-							const btScalar	c0	=	n.m_area * dvn * rel_v2/2;
+								nrm *= (btScalar)( (btDot(nrm,rel_v) < 0) ? -1 : +1);
-							const btScalar	c1	=	c0 * medium.m_density;
+
-							force	+=	nrm*(-c1*kLF);
+							const btScalar dvn = btDot(rel_v,nrm);
-							force	+=	rel_v.normalized() * (-c1 * kDG);
+							/* Compute forces	*/ 
-							ApplyClampedForce(n, force, dt);
+							if(dvn>0)
+							{
+								btVector3		force(0,0,0);
+								const btScalar	c0	=	n.m_area * dvn * rel_v2/2;
+								const btScalar	c1	=	c0 * medium.m_density;
+								force	+=	nrm*(-c1*kLF);
+								force	+=	rel_v.normalized() * (-c1 * kDG);
+								ApplyClampedForce(n, force, dt);
+							}
 						}	
 					}
 				}
@@ -2754,31 +2770,63 @@ void				btSoftBody::applyForces()
 			const btVector3	v=(f.m_n[0]->m_v+f.m_n[1]->m_v+f.m_n[2]->m_v)/3;
 			const btVector3	x=(f.m_n[0]->m_x+f.m_n[1]->m_x+f.m_n[2]->m_x)/3;
 			EvaluateMedium(m_worldInfo,x,medium);
+			medium.m_velocity = m_windVelocity;
+			medium.m_density = m_worldInfo->air_density;
 			const btVector3	rel_v=v-medium.m_velocity;
+			const btScalar rel_v_len = rel_v.length();
 			const btScalar	rel_v2=rel_v.length2();
+
 			if(rel_v2>SIMD_EPSILON)
 			{
-				btVector3	nrm=f.m_normal;
+				const btVector3 rel_v_nrm = rel_v.normalized();
-				/* Setup normal		*/ 
+				btVector3	nrm = f.m_normal;
-				switch(m_cfg.aeromodel)
+
+				if (m_cfg.aeromodel == btSoftBody::eAeroModel::F_TwoSidedLiftDrag)
 				{
-				case	btSoftBody::eAeroModel::F_TwoSided:
+					nrm *= (btScalar)( (btDot(nrm,rel_v) < 0) ? -1 : +1);
-					nrm*=(btScalar)(btDot(nrm,rel_v)<0?-1:+1);break;
+
-					default:
+					btVector3 fDrag(0, 0, 0);
+					btVector3 fLift(0, 0, 0);
+
+					btScalar n_dot_v = nrm.dot(rel_v_nrm);
+					btScalar tri_area = 0.5f * f.m_ra;
+					
+					fDrag = 0.5f * kDG * medium.m_density * rel_v2 * tri_area * n_dot_v * (-rel_v_nrm);
+
+					// Check angle of attack
+					// cos(10<31>) = 0.98480
+					if ( 0 < n_dot_v && n_dot_v < 0.98480f)
+						fLift = 0.5f * kLF * medium.m_density * rel_v_len * tri_area * btSqrt(1.0f-n_dot_v*n_dot_v) * (nrm.cross(rel_v_nrm).cross(rel_v_nrm));
+
+					fDrag /= 3;
+					fLift /= 3;
+
+					for(int j=0;j<3;++j) 
 					{
+						if (f.m_n[j]->m_im>0)
+						{
+							f.m_n[j]->m_f += fDrag; 
+							f.m_n[j]->m_f += fLift;
+						}
 					}
 				}
-				const btScalar	dvn=btDot(rel_v,nrm);
+				else if (m_cfg.aeromodel == btSoftBody::eAeroModel::F_OneSided || m_cfg.aeromodel == btSoftBody::eAeroModel::F_TwoSided)
-				/* Compute forces	*/ 
-				if(dvn>0)
 				{
-					btVector3		force(0,0,0);
+					if (btSoftBody::eAeroModel::F_TwoSided)
-					const btScalar	c0	=	f.m_ra*dvn*rel_v2;
+						nrm *= (btScalar)( (btDot(nrm,rel_v) < 0) ? -1 : +1);
-					const btScalar	c1	=	c0*medium.m_density;
+
-					force	+=	nrm*(-c1*kLF);
+					const btScalar	dvn=btDot(rel_v,nrm);
-					force	+=	rel_v.normalized()*(-c1*kDG);
+					/* Compute forces	*/ 
-					force	/=	3;
+					if(dvn>0)
-					for(int j=0;j<3;++j) ApplyClampedForce(*f.m_n[j],force,dt);
+					{
+						btVector3		force(0,0,0);
+						const btScalar	c0	=	f.m_ra*dvn*rel_v2;
+						const btScalar	c1	=	c0*medium.m_density;
+						force	+=	nrm*(-c1*kLF);
+						force	+=	rel_v.normalized()*(-c1*kDG);
+						force	/=	3;
+						for(int j=0;j<3;++j) ApplyClampedForce(*f.m_n[j],force,dt);
+					}
 				}
 			}
 		}

src/BulletSoftBody/btSoftBody.h

@@ -80,11 +80,13 @@ public:
 
 	///eAeroModel 
 	struct eAeroModel { enum _ {
-		V_Point,	///Vertex normals are oriented toward velocity
+		V_Point,			///Vertex normals are oriented toward velocity
-		V_TwoSided,	///Vertex normals are fliped to match velocity	
+		V_TwoSided,			///Vertex normals are flipped to match velocity	
-		V_OneSided,	///Vertex normals are taken as it is	
+		V_TwoSidedLiftDrag, ///Vertex normals are flipped to match velocity and lift and drag forces are applied
-		F_TwoSided,	///Face normals are fliped to match velocity
+		V_OneSided,			///Vertex normals are taken as it is	
-		F_OneSided,	///Face normals are taken as it is
+		F_TwoSided,			///Face normals are flipped to match velocity
+		F_TwoSidedLiftDrag,	///Face normals are flipped to match velocity and lift and drag forces are applied 
+		F_OneSided,			///Face normals are taken as it is		
 		END
 	};};