1 files changed, 69 insertions, 16 deletions

diff --git a/OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs b/OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs
index 1feb153..712bd7d 100644
--- a/OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs
+++ b/OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs
@@ -705,22 +705,75 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api
         {
             //A and B should both be normalized
             m_host.AddScriptLPS(1);
-            double dotProduct = LSL_Vector.Dot(a, b);
-            LSL_Vector crossProduct = LSL_Vector.Cross(a, b);
-            double magProduct = LSL_Vector.Mag(a) * LSL_Vector.Mag(b);
-            double angle = Math.Acos(dotProduct / magProduct);
-            LSL_Vector axis = LSL_Vector.Norm(crossProduct);
-            double s = Math.Sin(angle / 2);
-
-            double x = axis.x * s;
-            double y = axis.y * s;
-            double z = axis.z * s;
-            double w = Math.Cos(angle / 2);
-
-            if (Double.IsNaN(x) || Double.IsNaN(y) || Double.IsNaN(z) || Double.IsNaN(w))
-                return new LSL_Rotation(0.0f, 0.0f, 0.0f, 1.0f);
-
-            return new LSL_Rotation((float)x, (float)y, (float)z, (float)w);
+            LSL_Rotation rotBetween;
+            // Check for zero vectors. If either is zero, return zero rotation. Otherwise,
+            // continue calculation.
+            if (a == new LSL_Vector(0.0f, 0.0f, 0.0f) || b == new LSL_Vector(0.0f, 0.0f, 0.0f))
+            {
+                rotBetween = new LSL_Rotation(0.0f, 0.0f, 0.0f, 1.0f);
+            }
+            else
+            {
+                a = LSL_Vector.Norm(a);
+                b = LSL_Vector.Norm(b);
+                double dotProduct = LSL_Vector.Dot(a, b);
+                // There are two degenerate cases possible. These are for vectors 180 or
+                // 0 degrees apart. These have to be detected and handled individually.
+                //
+                // Check for vectors 180 degrees apart.
+                // A dot product of -1 would mean the angle between vectors is 180 degrees.
+                if (dotProduct < -0.9999999f)
+                {
+                    // First assume X axis is orthogonal to the vectors.
+                    LSL_Vector orthoVector = new LSL_Vector(1.0f, 0.0f, 0.0f);
+                    orthoVector = orthoVector - a * (a.x / LSL_Vector.Dot(a, a));
+                    // Check for near zero vector. A very small non-zero number here will create
+                    // a rotation in an undesired direction.
+                    if (LSL_Vector.Mag(orthoVector) > 0.0001)
+                    {
+                        rotBetween = new LSL_Rotation(orthoVector.x, orthoVector.y, orthoVector.z, 0.0f);
+                    }
+                    // If the magnitude of the vector was near zero, then assume the X axis is not
+                    // orthogonal and use the Z axis instead.
+                    else
+                    {
+                        // Set 180 z rotation.
+                        rotBetween = new LSL_Rotation(0.0f, 0.0f, 1.0f, 0.0f);
+                    }
+                }
+                // Check for parallel vectors.
+                // A dot product of 1 would mean the angle between vectors is 0 degrees.
+                else if (dotProduct > 0.9999999f)
+                {
+                    // Set zero rotation.
+                    rotBetween = new LSL_Rotation(0.0f, 0.0f, 0.0f, 1.0f);
+                }
+                else
+                {
+                    // All special checks have been performed so get the axis of rotation.
+                    LSL_Vector crossProduct = LSL_Vector.Cross(a, b);
+                    // Quarternion s value is the length of the unit vector + dot product.
+                    double qs = 1.0 + dotProduct;
+                    rotBetween = new LSL_Rotation(crossProduct.x, crossProduct.y, crossProduct.z, qs);
+                    // Normalize the rotation.
+                    double mag = LSL_Rotation.Mag(rotBetween);
+                    // We shouldn't have to worry about a divide by zero here. The qs value will be
+                    // non-zero because we already know if we're here, then the dotProduct is not -1 so
+                    // qs will not be zero. Also, we've already handled the input vectors being zero so the
+                    // crossProduct vector should also not be zero.
+                    rotBetween.x = rotBetween.x / mag;
+                    rotBetween.y = rotBetween.y / mag;
+                    rotBetween.z = rotBetween.z / mag;
+                    rotBetween.s = rotBetween.s / mag;
+                    // Check for undefined values and set zero rotation if any found. This code might not actually be required
+                    // any longer since zero vectors are checked for at the top.
+                    if (Double.IsNaN(rotBetween.x) || Double.IsNaN(rotBetween.y) || Double.IsNaN(rotBetween.z) || Double.IsNaN(rotBetween.s))
+                    {
+                        rotBetween = new LSL_Rotation(0.0f, 0.0f, 0.0f, 1.0f);
+                    }
+                }
+            }
+            return rotBetween;
         }
 
         public void llWhisper(int channelID, string text)