diff --git a/voxel.cpp b/voxel.cpp
index 12fc891..b5d69b6 100644
--- a/voxel.cpp
+++ b/voxel.cpp
@@ -3,7 +3,7 @@
 #include "voxel_mesh_builder.h"
 
 Voxel::Voxel() : Reference(), 
-    _id(0), 
+    _id(-1), 
     _material_id(0), 
     _is_transparent(false), 
     _library(NULL), 
@@ -12,7 +12,11 @@ Voxel::Voxel() : Reference(),
 
 Ref<Voxel> Voxel::set_id(int id) {
     ERR_FAIL_COND_V(id < 0 || id >= 256, Ref<Voxel>(this));
+    // Cannot modify ID after creation
+    ERR_FAIL_COND_V(_id != -1, Ref<Voxel>(this));
+
     _id = id;
+
     return Ref<Voxel>(this); 
 }
 
@@ -173,6 +177,9 @@ void Voxel::_bind_methods() {
     ObjectTypeDB::bind_method(_MD("set_transparent:Voxel", "color"), &Voxel::set_transparent, DEFVAL(true));
     ObjectTypeDB::bind_method(_MD("is_transparent"), &Voxel::is_transparent);
 
+    ObjectTypeDB::bind_method(_MD("set_material_id", "id"), &Voxel::set_material_id);
+    ObjectTypeDB::bind_method(_MD("get_material_id"), &Voxel::get_material_id);
+
     ObjectTypeDB::bind_method(_MD("set_cube_geometry:Voxel", "height"), &Voxel::set_cube_geometry, DEFVAL(1.f));
     ObjectTypeDB::bind_method(_MD("set_cube_uv_all_sides:Voxel", "atlas_pos"), &Voxel::set_cube_uv_all_sides);
     ObjectTypeDB::bind_method(_MD("set_cube_uv_tbs_sides:Voxel", "top_atlas_pos", "side_atlas_pos", "bottom_atlas_pos"), &Voxel::set_cube_uv_tbs_sides);
diff --git a/voxel_library.cpp b/voxel_library.cpp
index 801ca28..19960dd 100644
--- a/voxel_library.cpp
+++ b/voxel_library.cpp
@@ -1,7 +1,7 @@
 #include "voxel_library.h"
 
 VoxelLibrary::VoxelLibrary() : Reference(), _atlas_size(1) {
-
+    create_voxel(0, "air")->set_transparent(true);
 }
 
 VoxelLibrary::~VoxelLibrary() {
diff --git a/voxel_mesh_builder.cpp b/voxel_mesh_builder.cpp
index 6de1760..aa32464 100644
--- a/voxel_mesh_builder.cpp
+++ b/voxel_mesh_builder.cpp
@@ -149,6 +149,22 @@ void VoxelMeshBuilder::set_occlusion_enabled(bool enable) {
 
 inline Color Color_greyscale(float c) { return Color(c, c, c); }
 
+inline bool is_face_visible(const VoxelLibrary & lib, const Voxel & vt, int other_voxel_id) {
+    if (other_voxel_id == 0) // air
+        return true;
+    if (lib.has_voxel(other_voxel_id)) {
+        const Voxel & other_vt = lib.get_voxel_const(other_voxel_id);
+        return other_vt.is_transparent() && vt.get_id() != other_voxel_id;
+    }
+    return true;
+}
+
+inline bool is_transparent(const VoxelLibrary & lib, int voxel_id) {
+    if (lib.has_voxel(voxel_id))
+        return lib.get_voxel_const(voxel_id).is_transparent();
+    return true;
+}
+
 Ref<Mesh> VoxelMeshBuilder::build(Ref<VoxelBuffer> buffer_ref) {
     ERR_FAIL_COND_V(buffer_ref.is_null(), Ref<Mesh>());
     ERR_FAIL_COND_V(_library.is_null(), Ref<Mesh>());
@@ -194,7 +210,7 @@ Ref<Mesh> VoxelMeshBuilder::build(Ref<VoxelBuffer> buffer_ref) {
 
                             int neighbor_voxel_id = buffer.get_voxel_local(nx, ny, nz, 0);
                             // TODO Better face visibility test
-                            if (neighbor_voxel_id == 0) {
+                            if (is_face_visible(library, voxel, neighbor_voxel_id)) {
 
                                 // The face is visible
 
@@ -210,7 +226,7 @@ Ref<Mesh> VoxelMeshBuilder::build(Ref<VoxelBuffer> buffer_ref) {
                                         unsigned ex = x + edge_normal.x;
                                         unsigned ey = y + edge_normal.y;
                                         unsigned ez = z + edge_normal.z;
-                                        if (buffer.get_voxel_local(ex, ey, ez) != 0) {
+                                        if (!is_transparent(library, buffer.get_voxel_local(ex, ey, ez))) {
                                             shaded_corner[g_edge_corners[edge][0]] += 1;
                                             shaded_corner[g_edge_corners[edge][1]] += 1;
                                         }
@@ -225,7 +241,7 @@ Ref<Mesh> VoxelMeshBuilder::build(Ref<VoxelBuffer> buffer_ref) {
                                             unsigned int cx = x + corner_normal.x;
                                             unsigned int cy = y + corner_normal.y;
                                             unsigned int cz = z + corner_normal.z;
-                                            if (buffer.get_voxel_local(cx, cy, cz) != 0) {
+                                            if (!is_transparent(library, buffer.get_voxel_local(cx, cy, cz))) {
                                                 shaded_corner[corner] += 1;
                                             }
                                         }
diff --git a/voxel_mesh_builder.h b/voxel_mesh_builder.h
index 2fd4c93..0838c21 100644
--- a/voxel_mesh_builder.h
+++ b/voxel_mesh_builder.h
@@ -36,6 +36,7 @@ public:
     Ref<Mesh> build(Ref<VoxelBuffer> buffer_ref);
     
 protected:
+
     static void _bind_methods();
 
 };