//The two lines output by this script should report the same vector.

vector GetGeometricCenter()
{
    vector center = ZERO_VECTOR;
    integer p = llGetNumberOfPrims();
    //since we skip the root prim, we can ignore the single prim case.
    //If we weren't skipping the root, this would be more complicated.
    integer i = 1;

    while(i < p)
        center += llList2Vector(llGetLinkPrimitiveParams(++i, [PRIM_POS_LOCAL]), 0);
    return center / p;
}

default
{
    touch_start(integer touch_count)
    {
        llOwnerSay("LSL function returns " +
                    (string)llGetGeometricCenter());
        llOwnerSay("Function returns " +
                    (string)GetGeometricCenter());
    }
}

From 3e23243b02ad7287923864a5a29d7d6094c691df Mon Sep 17 00:00:00 2001
From: Magnuz Binder <magnuz@magnuz-se.com>
Date: Fri, 1 May 2015 07:00:45 +0200
Subject: [PATCH] Implement llGetGeometricCenter correctly.

---
 .../Shared/Api/Implementation/LSL_Api.cs            | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs b/OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs
index e20e4c4..096031b 100644
--- a/OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs
+++ b/OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs
@@ -10060,9 +10060,28 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api
             return prim;
         }
 
+        /// <summary>
+        /// Implementation of llGetGeometricCenter according to SL 2015-04-30.
+        /// http://wiki.secondlife.com/wiki/LlGetGeometricCenter
+        /// Returns the average position offset of all linked parts,
+        /// including the root prim and seated avatars,
+        /// relative to the root prim in local coordinates.
+        /// </summary>
         public LSL_Vector llGetGeometricCenter()
         {
-            return new LSL_Vector(m_host.GetGeometricCenter());
+            // Subtract whatever position the root prim has to make it zero
+            Vector3 offset = m_host.ParentGroup.RootPart.OffsetPosition * -1.0f;
+
+            // Add all prim/part position offsets
+            foreach (SceneObjectPart part in m_host.ParentGroup.Parts)
+                offset = offset + part.OffsetPosition;
+            // Add all avatar/scene presence position offsets
+            foreach (ScenePresence sp in m_host.ParentGroup.GetSittingAvatars())
+                offset = offset + sp.OffsetPosition;
+
+            // Calculate and return the average offset
+            offset = offset / (float)(m_host.ParentGroup.PrimCount + m_host.ParentGroup.GetSittingAvatarsCount());
+            return new LSL_Vector(offset);
         }
 
         public LSL_List GetEntityParams(ISceneEntity entity, LSL_List rules)
-- 
1.9.1