move deformation update to before explicit force calculation to prevent repetition of F calculation

src/BulletSoftBody/btDeformableBackwardEulerObjective.cpp

@@ -119,6 +119,11 @@ btScalar btDeformableBackwardEulerObjective::computeNorm(const TVStack& residual
 
 void btDeformableBackwardEulerObjective::applyExplicitForce(TVStack& force)
 {
+    for (int i = 0; i < m_softBodies.size(); ++i)
+    {
+        m_softBodies[i]->updateDeformation();
+    }
+    
     for (int i = 0; i < m_lf.size(); ++i)
     {
         m_lf[i]->addScaledExplicitForce(m_dt, force);

src/BulletSoftBody/btDeformableCorotatedForce.h

@@ -2,7 +2,7 @@
  Written by Xuchen Han <xuchenhan2015@u.northwestern.edu>
  
  Bullet Continuous Collision Detection and Physics Library
- Copyright (c) 2016 Google Inc. http://bulletphysics.org
+ 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,
@@ -63,10 +63,8 @@ public:
             for (int j = 0; j < psb->m_tetras.size(); ++j)
             {
                 btSoftBody::Tetra& tetra = psb->m_tetras[j];
-                updateDs(tetra);
-                btMatrix3x3 F = tetra.m_ds * tetra.m_Dm_inverse;
                 btMatrix3x3 P;
-                firstPiola(F,P);
+                firstPiola(tetra.m_F,P);
                 btVector3 force_on_node0 = P * (tetra.m_Dm_inverse.transpose()*grad_N_hat_1st_col);
                 btMatrix3x3 force_on_node123 = P * tetra.m_Dm_inverse.transpose();
                 
@@ -106,16 +104,6 @@ public:
             P += (F-R) * 2 * m_mu;
         }
     }
-    void updateDs(btSoftBody::Tetra& t)
-    {
-        btVector3 c1 = t.m_n[1]->m_q - t.m_n[0]->m_q;
-        btVector3 c2 = t.m_n[2]->m_q - t.m_n[0]->m_q;
-        btVector3 c3 = t.m_n[3]->m_q - t.m_n[0]->m_q;
-        btMatrix3x3 Ds(c1.getX(), c2.getX(), c3.getX(),
-                       c1.getY(), c2.getY(), c3.getY(),
-                       c1.getZ(), c2.getZ(), c3.getZ());
-        t.m_ds = Ds;
-    }
     
     virtual void addScaledForceDifferential(btScalar scale, const TVStack& dv, TVStack& df)
     {

src/BulletSoftBody/btSoftBody.cpp

@@ -2831,6 +2831,20 @@ void btSoftBody::initializeDmInverse()
     }
 }
 
+void btSoftBody::updateDeformation()
+{
+    for (int i = 0; i < m_tetras.size(); ++i)
+    {
+        btSoftBody::Tetra& t = m_tetras[i];
+        btVector3 c1 = t.m_n[1]->m_q - t.m_n[0]->m_q;
+        btVector3 c2 = t.m_n[2]->m_q - t.m_n[0]->m_q;
+        btVector3 c3 = t.m_n[3]->m_q - t.m_n[0]->m_q;
+        btMatrix3x3 Ds(c1.getX(), c2.getX(), c3.getX(),
+                       c1.getY(), c2.getY(), c3.getY(),
+                       c1.getZ(), c2.getZ(), c3.getZ());
+        t.m_F = Ds * t.m_Dm_inverse;
+    }
+}
 //
 void btSoftBody::Joint::Prepare(btScalar dt, int)
 {

src/BulletSoftBody/btSoftBody.h

@@ -293,7 +293,7 @@ public:
 		btScalar m_c1;       // (4*kVST)/(im0+im1+im2+im3)
 		btScalar m_c2;       // m_c1/sum(|g0..3|^2)
         btMatrix3x3 m_Dm_inverse; // rest Dm^-1
-        btMatrix3x3 m_ds;
+        btMatrix3x3 m_F;
         btScalar m_element_measure;
 	};
 	/* RContact		*/
@@ -1027,6 +1027,7 @@ public:
 	void dampClusters();
     void setSpringStiffness(btScalar k);
     void initializeDmInverse();
+    void updateDeformation();
 	void applyForces();
 	static void PSolve_Anchors(btSoftBody* psb, btScalar kst, btScalar ti);
 	static void PSolve_RContacts(btSoftBody* psb, btScalar kst, btScalar ti);