From ef96b413e73891b6f1c0f959cbcb3845db1a2c2e Mon Sep 17 00:00:00 2001
From: remibessard <remibessard@protonmail.com>
Date: Mon, 8 Sep 2025 08:20:14 +0200
Subject: [PATCH 1/4] UPDATE SpringForceField so that it handles configuration
 singularity in simulation by adding regularisation term (so that matrix can
 be inverted)

---
 .../component/solidmechanics/spring/SpringForceField.inl | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl b/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
index 6c926549d4e..bd8f85938be 100644
--- a/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
+++ b/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
@@ -542,11 +542,18 @@ auto SpringForceField<DataTypes>::computeSpringForce(
         // dF = k_s.U.U^T.dX + f/l.(I-U.U^T).dX = ((k_s-f/l).U.U^T + f/l.I).dX
         auto& m = springForce->dForce_dX;
         Real tgt = forceIntensity * inverseLength;
+        Real tol = 1e-8;
+        Real regParam = 1e-4;
+        bool isSingular = std::abs(elongation) < tol && std::abs(elongationVelocity) < tol;
+        if (isSingular)
+            msg_warning(this) << "!!! We detected a degenerated Spring configuration, we've to add a "
+                               "regularization term to make the matrix invertible !!!";
         for(sofa::Index j=0; j<N; ++j )
         {
             for(sofa::Index k=0; k<N; ++k )
             {
-                m(j,k) = ((Real)spring.ks-tgt) * u[j] * u[k];
+                m[j][k] = ((Real)spring.ks-tgt) * u[j] * u[k];
+                m[j][k] += (isSingular) ? regParam * spring.ks * (((j==k) ? 1.0: 0.0) - u[j] * u[k]) : 0.0;
             }
             m(j,j) += tgt;
         }

From 2edb4de4c97e5d0f769c22fe5afa540e57bbe411 Mon Sep 17 00:00:00 2001
From: hugtalbot <hugo.talbot@inria.fr>
Date: Fri, 23 Jan 2026 09:25:42 +0100
Subject: [PATCH 2/4] simplify ternary and use (,)

---
 .../component/solidmechanics/spring/SpringForceField.inl | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl b/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
index bd8f85938be..dadd52d661a 100644
--- a/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
+++ b/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
@@ -552,8 +552,13 @@ auto SpringForceField<DataTypes>::computeSpringForce(
         {
             for(sofa::Index k=0; k<N; ++k )
             {
-                m[j][k] = ((Real)spring.ks-tgt) * u[j] * u[k];
-                m[j][k] += (isSingular) ? regParam * spring.ks * (((j==k) ? 1.0: 0.0) - u[j] * u[k]) : 0.0;
+                m(j,k) = ((Real)spring.ks-tgt) * u[j] * u[k];
+                if(isSingular)
+                {
+                    const double diagonalTerm = (j == k) ? 1.0 : 0.0;
+                    m(j,k) +=  regParam * spring.ks * (diagonalTerm - u[j] * u[k]);
+                }
+                
             }
             m(j,j) += tgt;
         }

From 5a68d6ebb6e7169bef9547c52e2bbf487d498463 Mon Sep 17 00:00:00 2001
From: Hugo <hugo.talbot@sofa-framework.org>
Date: Fri, 23 Jan 2026 11:09:00 +0100
Subject: [PATCH 3/4] Apply suggestions from code review

---
 .../sofa/component/solidmechanics/spring/SpringForceField.inl  | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl b/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
index dadd52d661a..415062d9721 100644
--- a/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
+++ b/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
@@ -546,8 +546,7 @@ auto SpringForceField<DataTypes>::computeSpringForce(
         Real regParam = 1e-4;
         bool isSingular = std::abs(elongation) < tol && std::abs(elongationVelocity) < tol;
         if (isSingular)
-            msg_warning(this) << "!!! We detected a degenerated Spring configuration, we've to add a "
-                               "regularization term to make the matrix invertible !!!";
+            msg_warning(this) << "Degenerated spring configuration detected: a regularization term added making the matrix invertible";
         for(sofa::Index j=0; j<N; ++j )
         {
             for(sofa::Index k=0; k<N; ++k )

From 3189bdd8249239e71f63226fcea7466908abebc9 Mon Sep 17 00:00:00 2001
From: Hugo <hugo.talbot@sofa-framework.org>
Date: Tue, 27 Jan 2026 15:40:50 +0100
Subject: [PATCH 4/4] Apply suggestions from code review

Co-authored-by: Alex Bilger <alxbilger@users.noreply.github.com>
---
 .../sofa/component/solidmechanics/spring/SpringForceField.inl   | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl b/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
index 415062d9721..f2d9619bd04 100644
--- a/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
+++ b/Sofa/Component/SolidMechanics/Spring/src/sofa/component/solidmechanics/spring/SpringForceField.inl
@@ -554,7 +554,7 @@ auto SpringForceField<DataTypes>::computeSpringForce(
                 m(j,k) = ((Real)spring.ks-tgt) * u[j] * u[k];
                 if(isSingular)
                 {
-                    const double diagonalTerm = (j == k) ? 1.0 : 0.0;
+                    const Real diagonalTerm = (j == k) ? 1.0 : 0.0;
                     m(j,k) +=  regParam * spring.ks * (diagonalTerm - u[j] * u[k]);
                 }