added a normal estimation algo

src/polymesh/algorithms/normal_estimation.hh

+#pragma once
+
+#include <polymesh/Mesh.hh>
+
+#include <polymesh/properties.hh>
+
+namespace polymesh
+{
+/// Compute per-vertex normals (stored on halfedges) by averaging per-face normals and respecting hard edges.
+/// is_hard_edge is a function (edge_handle) -> bool
+/// Normals are not smoothed over edges where is_hard_edge is true
+/// NOTE:
+///   - is_hard_edge is never called for boundaries
+///   - the normal belongs to halfedge.vertex_to()
+template <class Vec3, class IsHardEdgeF>
+[[nodiscard]] halfedge_attribute<Vec3> normal_estimation(face_attribute<Vec3> const& face_normals, IsHardEdgeF&& is_hard_edge)
+{
+    Mesh const& m = face_normals.mesh();
+
+    auto const hard_edges = m.edges().map([&](edge_handle e) { return e.is_boundary() ? true : is_hard_edge(e); });
+
+    return m.halfedges().map([&](halfedge_handle h) {
+        Vec3 n = face_normals[h.face()];
+
+        auto h0 = h;
+        auto h1 = h.next().opposite();
+
+        // iterate over h0
+        auto hh = h0;
+        while (hh != h1 && !hard_edges[hh])
+        {
+            hh = hh.opposite().prev();
+            n += face_normals[hh.face()];
+        }
+
+        // iterate over h1 if not reached around
+        if (hh != h1)
+        {
+            hh = h1;
+            while (hh != h0 && !hard_edges[hh])
+            {
+                n += face_normals[hh.face()];
+                hh = hh.next().opposite();
+            }
+        }
+
+        // normalize
+        auto l = std::sqrt(n.x * n.x + n.y * n.y + n.z * n.z);
+        return n / (l + 1e-30f);
+    });
+}
+
+/// same as normal_estimation(face_attribute<Vec3>, ...)
+/// but computes face normals first via pm::face_normals
+template <class Pos3, class IsHardEdgeF>
+[[nodiscard]] halfedge_attribute<typename field3<Pos3>::vec_t> normal_estimation(vertex_attribute<Pos3> const& pos, IsHardEdgeF&& is_hard_edge)
+{
+    return normal_estimation(pm::face_normals(pos), std::forward<IsHardEdgeF>(is_hard_edge));
+}
+}