feat(constitution): EmbeddedCollisionMesh — barycentric coupling of passive surface mesh onto FEM tet mesh

include/uipc/constitution/embedded_collision_mesh.h

@@ -0,0 +1,30 @@
+#pragma once
+#include <uipc/geometry/simplicial_complex.h>
+#include <uipc/common/dllexport.h>
+
+namespace uipc::constitution
+{
+// Barycentric embedding of a dense surface mesh onto a coarse tet mesh.
+//
+// Usage (in IIPCSolver::build_scene):
+//   EmbeddedCollisionMesh ecm;
+//   ecm.apply_to(tet_sc, surface_sc);
+//
+// The surface_sc must be passive (no FEM constitution).
+// The tet_sc must already have a FEM constitution applied.
+//
+// At runtime the CUDA backend reads attributes written here:
+//   ecm_tet_index  (IndexT  per surface vertex) — containing tet index
+//   ecm_bary       (Vector4  per surface vertex) — barycentric coords
+//   ecm_driven     (IndexT  meta, value=1)       — marks the mesh as driven
+//
+// Reference: SOFA BarycentricMapping — apply() / applyJT()
+class UIPC_CONSTITUTION_API EmbeddedCollisionMesh
+{
+  public:
+    // Build barycentric embedding CPU-side (point-in-tet query).
+    // Returns false if any surface vertex falls outside all tets (clamped).
+    bool apply_to(geometry::SimplicialComplex& tet_sc,
+                  geometry::SimplicialComplex& surface_sc) const;
+};
+}

src/backends/cuda/engine/advance_ipc.cu

@@ -309,6 +309,9 @@ void SimEngine::advance()
             step_animation();
             m_time_integrator_manager->predict_dof();
 
+            // Forward pass: update passive surface positions from tet DOFs
+            event_before_collision_detection();
+
             // 3. Adaptive Parameter Calculation
             detect_dcd_candidates();
             compute_adaptive_kappa();
@@ -340,6 +343,8 @@ void SimEngine::advance()
                 m_state = SimEngineState::ComputeDyTopoEffect;
                 compute_dytopo_effect();
 
+                // Backward pass: scatter surface contact forces to tet DOFs
+                event_after_contact_assembly();
 
                 // 4) Solve Global Linear System => dx = A^-1 * b
                 m_state = SimEngineState::SolveGlobalLinearSystem;

src/backends/cuda/engine/sim_engine.cu

@@ -98,6 +98,18 @@ void SimEngine::event_write_scene()
         action();
 }
 
+void SimEngine::event_before_collision_detection()
+{
+    for(auto& action : m_on_before_collision_detection.view())
+        action();
+}
+
+void SimEngine::event_after_contact_assembly()
+{
+    for(auto& action : m_on_after_contact_assembly.view())
+        action();
+}
+
 void SimEngine::dump_global_surface()
 {
     BackendPathTool tool{workspace()};