From 63ac2182ae7fecfb552b3837aa631e4d84ddc210 Mon Sep 17 00:00:00 2001
From: erwincoumans <erwin.coumans@gmail.com>
Date: Sat, 22 Dec 2018 11:19:15 -0800
Subject: [PATCH] fix degerenate case in btHingeConstraint, see Issue 2031
 https://github.com/bulletphysics/bullet3/issues/2031

---
 src/BulletDynamics/ConstraintSolver/btHingeConstraint.cpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/BulletDynamics/ConstraintSolver/btHingeConstraint.cpp b/src/BulletDynamics/ConstraintSolver/btHingeConstraint.cpp
index 121290d0d..aa6f69000 100644
--- a/src/BulletDynamics/ConstraintSolver/btHingeConstraint.cpp
+++ b/src/BulletDynamics/ConstraintSolver/btHingeConstraint.cpp
@@ -764,6 +764,12 @@ void btHingeConstraint::getInfo2InternalUsingFrameOffset(btConstraintInfo2* info
 	btVector3 ax1A = trA.getBasis().getColumn(2);
 	btVector3 ax1B = trB.getBasis().getColumn(2);
 	btVector3 ax1 = ax1A * factA + ax1B * factB;
+	if (ax1.length2()<SIMD_EPSILON)
+	{
+		factA=0.f;
+		factB=1.f;
+		ax1 = ax1A * factA + ax1B * factB;
+	}
 	ax1.normalize();
 	// fill first 3 rows
 	// we want: velA + wA x relA == velB + wB x relB