add "set-id" to Fixed constrained vertices. then in Remesher, if two Fixed vertices have same set ID, we (optionally) allow that edge to be collapsed to midpoint. This allows refining embedded loops to some degree (although still limited as we still don't smooth fixed verts)

Author: rms80

mesh/MeshConstraints.cs

@@ -47,13 +47,17 @@ public bool NoModifications {
     public struct VertexConstraint
     {
         public bool Fixed;
+        public int FixedSetID;      // in Remesher, we can allow two Fixed vertices with 
+                                    // same FixedSetID to be collapsed together
 
-        public VertexConstraint(bool isFixed)
+        public VertexConstraint(bool isFixed, int setID = InvalidSetID)
         {
             Fixed = isFixed;
+            FixedSetID = setID;
         }
 
-       static public readonly VertexConstraint Unconstrained = new VertexConstraint() { Fixed = false };
+        public const int InvalidSetID = -1;
+        static public readonly VertexConstraint Unconstrained = new VertexConstraint() { Fixed = false };
      }
 
 

mesh/Remesher.cs

@@ -14,7 +14,6 @@ public class Remesher {
 		public bool EnableSplits = true;
 		public bool EnableSmoothing = true;
 
-
 		public double MinEdgeLength = 0.001f;
 		public double MaxEdgeLength = 0.1f;
 
@@ -25,6 +24,14 @@ public enum SmoothTypes {
 		public SmoothTypes SmoothType = SmoothTypes.Uniform;
 
 
+        // other options
+
+        // if true, then when two Fixed vertices have the same non-invalid SetID,
+        // we treat them as not fixed and allow collapse
+        public bool AllowCollapseFixedVertsWithSameSetID = true;
+
+
+
 		public Remesher(DMesh3 m) {
 			mesh = m;
 		}
@@ -97,25 +104,30 @@ ProcessResult ProcessEdge(int edgeID)
 			Vector3d vB = mesh.GetVertex(b);
 			double edge_len_sqr = (vA-vB).LengthSquared;
 
-            bool aFixed = vertex_is_fixed(a);
-            bool bFixed = vertex_is_fixed(b);
-            bool bothFixed = (aFixed && bFixed);
+            // check if we should collapse, and also find which vertex we should collapse to,
+            // in cases where we have constraints/etc
+            int collapse_to = -1;
+            bool bCanCollapse = EnableCollapses
+                                && constraint.CanCollapse
+                                && edge_len_sqr < MinEdgeLength*MinEdgeLength
+                                && can_collapse(a, b, out collapse_to);
+
 
 			// optimization: if edge cd exists, we cannot collapse or flip. look that up here?
 			//  funcs will do it internally...
 			//  (or maybe we can collapse if cd exists? edge-collapse doesn't check for it explicitly...)
 
 			// if edge length is too short, we want to collapse it
 			bool bTriedCollapse = false;
-			if ( EnableCollapses && constraint.CanCollapse && bothFixed == false && edge_len_sqr < MinEdgeLength*MinEdgeLength ) {
+			if ( bCanCollapse ) {
 
                 int iKeep = b, iCollapse = a;
                 Vector3d vNewPos = (vA + vB) * 0.5;
 
                 // if either vtx is fixed, collapse to that position
-                if ( bFixed ) {
+                if ( collapse_to == b ) {
                     vNewPos = vB;
-                } else if ( aFixed ) {
+                } else if ( collapse_to == a ) {
                     iKeep = a; iCollapse = b;
                     vNewPos = vA;
                 }
@@ -214,13 +226,46 @@ void update_constraints_after_split(int edgeID, int va, int vb, DMesh3.EdgeSplit
                 //   cannot do outside loop because then pair of fixed verts connected
                 //   by unconstrained edge will produce a new fixed vert, which is bad
                 //   (eg on minimal triangulation of capped cylinder)
-                if (vertex_is_fixed(va) && vertex_is_fixed(vb))
+                VertexConstraint ca = constraints.GetVertexConstraint(va);
+                VertexConstraint cb = constraints.GetVertexConstraint(vb);
+                if (ca.Fixed && cb.Fixed) {
+                    int nSetID = (ca.FixedSetID > 0 && ca.FixedSetID == cb.FixedSetID) ?
+                        ca.FixedSetID : VertexConstraint.InvalidSetID;
                     constraints.SetOrUpdateVertexConstraint(splitInfo.vNew,
-                        new VertexConstraint(true));
+                        new VertexConstraint(true, nSetID));
+                }
             }
         }
 
 
+        bool can_collapse(int a, int b, out int collapse_to)
+        {
+            collapse_to = -1;
+            if (constraints == null)
+                return true;
+            VertexConstraint ca = constraints.GetVertexConstraint(a);
+            VertexConstraint cb = constraints.GetVertexConstraint(b);
+            if (ca.Fixed == false && cb.Fixed == false)
+                return true;
+            if ( ca.Fixed == true && cb.Fixed == false ) {
+                collapse_to = a;
+                return true;
+            }
+            if ( cb.Fixed == true && ca.Fixed == false) {
+                collapse_to = b;
+                return true;
+            }
+            // if both fixed, and options allow, treat this edge as unconstrained (eg collapse to midpoint)
+            // [RMS] tried picking a or b here, but something weird happens, where
+            //   eg cylinder cap will entirely erode away. Somehow edge lengths stay below threshold??
+            if ( AllowCollapseFixedVertsWithSameSetID 
+                    && ca.FixedSetID >= 0 
+                    && ca.FixedSetID == cb.FixedSetID) {
+                return true;
+            }
+
+            return false;            
+        }
 
 
         bool vertex_is_fixed(int vid)
@@ -258,7 +303,7 @@ void FullProjectionPass()
                 if (vertex_is_fixed(vID))
                     continue;
                 Vector3d curpos = mesh.GetVertex(vID);
-                Vector3d projected = target.Project(curpos);
+                Vector3d projected = target.Project(curpos, vID);
                 mesh.SetVertex(vID, projected);
             }
         }