From 92dabdc07d484e668df1b2138408bfbcfe0e0393 Mon Sep 17 00:00:00 2001
From: emMichael Alexander Ewert/em <emmewert99@yahoo.com/em>
Date: Fri, 12 Dec 2014 15:39:16 -0800
Subject: [PATCH] Fix a couple of bugs in 2dConvex Hull algorithm. * Need to
 use atan2 so 3d angles are calculated properly after projection. * Need to
 handle case where the first tripple of points is non-convex, previously this
 would cause the algorithm to fail with only 1 point.

---
 src/LinearMath/btGrahamScan2dConvexHull.h | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/src/LinearMath/btGrahamScan2dConvexHull.h b/src/LinearMath/btGrahamScan2dConvexHull.h
index e658c5cf0..13a79aa58 100644
--- a/src/LinearMath/btGrahamScan2dConvexHull.h
+++ b/src/LinearMath/btGrahamScan2dConvexHull.h
@@ -85,9 +85,17 @@ inline void GrahamScanConvexHull2D(btAlignedObjectArray<GrahamVector3>& original
 	originalPoints[0].m_angle = -1e30f;
 	for (int i=1;i<originalPoints.size();i++)
 	{
-		btVector3 xvec = axis0;
-		btVector3 ar = originalPoints[i]-originalPoints[0];
-		originalPoints[i].m_angle = btCross(xvec, ar).dot(normalAxis) / ar.length();
+	    btVector3 ar = originalPoints[i]-originalPoints[0];
+	    btScalar ar1 = axis1.dot(ar);
+	    btScalar ar0 = axis0.dot(ar);
+	    if( ar1*ar1+ar0*ar0 < FLT_EPSILON ) 
+	    {
+	      originalPoints[i].m_angle = 0.0f;
+	    }
+	    else
+	    {
+	      originalPoints[i].m_angle = btAtan2Fast(ar1, ar0);
+	    }
 	}
 
 	//step 2: sort all points, based on 'angle' with this anchor
@@ -111,6 +119,11 @@ inline void GrahamScanConvexHull2D(btAlignedObjectArray<GrahamVector3>& original
 			else 
 				hull.push_back(originalPoints[i]);
 		}
+
+	    if( hull.size() == 1 )
+	    {
+	      hull.push_back( originalPoints[i] );
+	    }
 	}
 }