From c5bb3d6660320c6b13256311f42ae8e6fd0ed422 Mon Sep 17 00:00:00 2001
From: Alex Bilger <alexbilger0@gmail.com>
Date: Tue, 3 Feb 2026 15:25:56 +0100
Subject: [PATCH] [Mass] Fix kinetic energy in MeshMatrixMass when lumped

---
 .../sofa/component/mass/MeshMatrixMass.inl    | 20 +++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/Sofa/Component/Mass/src/sofa/component/mass/MeshMatrixMass.inl b/Sofa/Component/Mass/src/sofa/component/mass/MeshMatrixMass.inl
index aa3fa6f348a..32317b5a4f7 100644
--- a/Sofa/Component/Mass/src/sofa/component/mass/MeshMatrixMass.inl
+++ b/Sofa/Component/Mass/src/sofa/component/mass/MeshMatrixMass.inl
@@ -2100,23 +2100,23 @@ SReal MeshMatrixMass<DataTypes, GeometricalTypes>::getKineticEnergy( const core:
 
     helper::ReadAccessor< DataVecDeriv > v = vv;
 
-    const unsigned int nbEdges=l_topology->getNbEdges();
-    unsigned int v0,v1;
+    const sofa::Size nbEdges = l_topology->getNbEdges();
 
     SReal e = 0;
+    const auto lumpingCoef = isLumped() ? m_massLumpingCoeff : static_cast<Real>(1.0);
 
-    for (unsigned int i=0; i<v.size(); i++)
+    for (unsigned int i = 0; i < v.size(); i++)
     {
-        e += dot(v[i],v[i]) * vertexMass[i]; // v[i]*v[i]*masses[i] would be more efficient but less generic
+        e += dot(v[i],v[i]) * vertexMass[i] * lumpingCoef; // v[i]*v[i]*masses[i] would be more efficient but less generic
     }
 
-    for (unsigned int i = 0; i < nbEdges; ++i)
+    if (!isLumped())
     {
-        v0 = l_topology->getEdge(i)[0];
-        v1 = l_topology->getEdge(i)[1];
-
-        e += 2 * dot(v[v0], v[v1])*edgeMass[i];
-
+        for (unsigned int i = 0; i < nbEdges; ++i)
+        {
+            const auto& [v0, v1] = l_topology->getEdge(i).array();
+            e += 2 * dot(v[v0], v[v1]) * edgeMass[i];
+        }
     }
 
     return e/2;