Added two colored lines example

samples/basic/viewer/main.cc

@@ -182,15 +182,19 @@ void advanced_visualization(pm::vertex_attribute<tg::pos3> const& pos)
         // map positions to colors
         pm::vertex_attribute<tg::color3> vcolors = m.vertices().map([&](pm::vertex_handle v) { return tg::color3(pos[v] * 0.5f + 0.5f); });
 
+        // Normals are needed for two colored lines
+        pm::vertex_attribute<tg::vec3> vnormals = pm::vertex_normals_by_area(pos);
+
         gv::view(pos, tg::color3::red, "solid color for whole mesh");
         gv::view(pos, random_face_colors, "random face colors");
         gv::view(pos, random_vertex_colors, "random vertex colors");
         gv::view(pos, random_halfedge_colors, "random halfedge colors");
         gv::view(gv::points(pos), vcolors, "point cloud with xyz colors");
         gv::view(gv::points(pos), random_vertex_colors, gv::no_shading, "unlit points");
-        gv::view(gv::lines(pos), random_edge_colors, "lines with random edge colors");
+        gv::view(gv::lines(pos).normals(vnormals).force3D(), random_face_colors, "two colored lines with random face colors");
         gv::view(gv::lines(pos), random_vertex_colors, "lines with random vertex colors");
-        gv::view(gv::lines(pos), random_vertex_colors, gv::no_shading, "unlit lines");
+        gv::view(gv::lines(pos), random_edge_colors, "lines with random edge colors");
+        gv::view(gv::lines(pos), random_edge_colors, gv::no_shading, "unlit lines");
     }
 
     // data mapping