Fix rendering bug due to zero normal faces

viewer/glow-extras/viewer/canvas.hh

@@ -819,15 +819,24 @@ public:
             centroid /= 3.0f * area;
         }
 
+        // try to find a non-zero normal
         tg::vec3 normal;
         {
-            auto e = face.any_halfedge();
-            auto const v0 = tg::pos3(e.vertex_from()[pos]);
-            auto const v1 = tg::pos3(e.vertex_to()[pos]);
+            auto const h_start = face.any_halfedge();
+            auto h_curr = h_start;
+            do
+            {
+                auto const v0 = tg::pos3(h_curr.vertex_from()[pos]);
+                auto const v1 = tg::pos3(h_curr.vertex_to()[pos]);
                 normal = cross(v0 - centroid, v1 - centroid);
                 auto l = length(normal);
                 normal = l == 0 ? tg::vec3::zero : normal / l;
+                h_curr = h_curr.next();
+
+            } while (h_curr != h_start && normal == tg::vec3::zero);
         }
+
+        // add triangle fan
         auto h0 = face.any_halfedge();
         auto he = h0.prev();
         auto h = h0.next();