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diff --git a/extern/bullet2/src/BulletSoftBody/btDeformableGravityForce.h b/extern/bullet2/src/BulletSoftBody/btDeformableGravityForce.h
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+++ b/extern/bullet2/src/BulletSoftBody/btDeformableGravityForce.h
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+/*
+ Written by Xuchen Han <xuchenhan2015@u.northwestern.edu>
+ 
+ Bullet Continuous Collision Detection and Physics Library
+ Copyright (c) 2019 Google Inc. http://bulletphysics.org
+ This software is provided 'as-is', without any express or implied warranty.
+ In no event will the authors be held liable for any damages arising from the use of this software.
+ Permission is granted to anyone to use this software for any purpose,
+ including commercial applications, and to alter it and redistribute it freely,
+ subject to the following restrictions:
+ 1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
+ 2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
+ 3. This notice may not be removed or altered from any source distribution.
+ */
+
+#ifndef BT_DEFORMABLE_GRAVITY_FORCE_H
+#define BT_DEFORMABLE_GRAVITY_FORCE_H
+
+#include "btDeformableLagrangianForce.h"
+
+class btDeformableGravityForce : public btDeformableLagrangianForce
+{
+public:
+	typedef btAlignedObjectArray<btVector3> TVStack;
+	btVector3 m_gravity;
+
+	btDeformableGravityForce(const btVector3& g) : m_gravity(g)
+	{
+	}
+
+	virtual void addScaledForces(btScalar scale, TVStack& force)
+	{
+		addScaledGravityForce(scale, force);
+	}
+
+	virtual void addScaledExplicitForce(btScalar scale, TVStack& force)
+	{
+		addScaledGravityForce(scale, force);
+	}
+
+	virtual void addScaledDampingForce(btScalar scale, TVStack& force)
+	{
+	}
+
+	virtual void addScaledElasticForceDifferential(btScalar scale, const TVStack& dx, TVStack& df)
+	{
+	}
+
+	virtual void addScaledDampingForceDifferential(btScalar scale, const TVStack& dv, TVStack& df)
+	{
+	}
+
+	virtual void buildDampingForceDifferentialDiagonal(btScalar scale, TVStack& diagA) {}
+
+	virtual void addScaledGravityForce(btScalar scale, TVStack& force)
+	{
+		int numNodes = getNumNodes();
+		btAssert(numNodes <= force.size());
+		for (int i = 0; i < m_softBodies.size(); ++i)
+		{
+			btSoftBody* psb = m_softBodies[i];
+			if (!psb->isActive())
+			{
+				continue;
+			}
+			for (int j = 0; j < psb->m_nodes.size(); ++j)
+			{
+				btSoftBody::Node& n = psb->m_nodes[j];
+				size_t id = n.index;
+				btScalar mass = (n.m_im == 0) ? 0 : 1. / n.m_im;
+				btVector3 scaled_force = scale * m_gravity * mass * m_softBodies[i]->m_gravityFactor;
+				force[id] += scaled_force;
+			}
+		}
+	}
+
+	virtual btDeformableLagrangianForceType getForceType()
+	{
+		return BT_GRAVITY_FORCE;
+	}
+
+	// the gravitational potential energy
+	virtual double totalEnergy(btScalar dt)
+	{
+		double e = 0;
+		for (int i = 0; i < m_softBodies.size(); ++i)
+		{
+			btSoftBody* psb = m_softBodies[i];
+			if (!psb->isActive())
+			{
+				continue;
+			}
+			for (int j = 0; j < psb->m_nodes.size(); ++j)
+			{
+				const btSoftBody::Node& node = psb->m_nodes[j];
+				if (node.m_im > 0)
+				{
+					e -= m_gravity.dot(node.m_q) / node.m_im;
+				}
+			}
+		}
+		return e;
+	}
+};
+#endif /* BT_DEFORMABLE_GRAVITY_FORCE_H */