diff --git a/src/shading/surface_closure.rs b/src/shading/surface_closure.rs
index 5fc743b..3109acb 100644
--- a/src/shading/surface_closure.rs
+++ b/src/shading/surface_closure.rs
@@ -413,6 +413,9 @@ impl GTRClosure {
         (top / bottom).sqrt()
     }
 
+    /// Microfacet distribution function.
+    ///
+    /// nh: cosine of the angle between the surface normal and the microfacet normal.
     fn dist(&self, nh: f32, rough: f32) -> f32 {
         // Other useful numbers
         let roughness2 = rough * rough;
@@ -442,7 +445,7 @@ impl SurfaceClosure for GTRClosure {
               wavelength: f32)
               -> (Vector, SpectralSample, f32) {
         // Get normalized surface normal
-        let nn = if dot(nor.into_vector(), inc) <= 0.0 {
+        let nn = if dot(nor.into_vector(), inc) < 0.0 {
                 nor.normalized()
             } else {
                 -nor.normalized() // If back-facing, flip normal
@@ -472,10 +475,10 @@ impl SurfaceClosure for GTRClosure {
         let hh = (aa + bb).normalized(); // Half-way between aa and bb
 
         // Surface normal
-        let nn = if dot(nor.into_vector(), hh) <= 0.0 {
-                nor.normalized()
-            } else {
+        let nn = if dot(nor.into_vector(), hh) < 0.0 {
                 -nor.normalized() // If back-facing, flip normal
+            } else {
+                nor.normalized()
             }
             .into_vector();
 
@@ -561,10 +564,10 @@ impl SurfaceClosure for GTRClosure {
         let hh = (aa + bb).normalized(); // Half-way between aa and bb
 
         // Surface normal
-        let nn = if dot(nor.into_vector(), hh) <= 0.0 {
-                nor.normalized()
-            } else {
+        let nn = if dot(nor.into_vector(), hh) < 0.0 {
                 -nor.normalized() // If back-facing, flip normal
+            } else {
+                nor.normalized()
             }
             .into_vector();
 
@@ -588,7 +591,7 @@ impl SurfaceClosure for GTRClosure {
         assert!(cos_theta <= 1.0);
 
         // Surface normal
-        let nn = if dot(nor.into_vector(), inc) <= 0.0 {
+        let nn = if dot(nor.into_vector(), inc) < 0.0 {
                 nor.normalized()
             } else {
                 -nor.normalized() // If back-facing, flip normal
diff --git a/src/surface/triangle_mesh.rs b/src/surface/triangle_mesh.rs
index 88b29a0..d4fa217 100644
--- a/src/surface/triangle_mesh.rs
+++ b/src/surface/triangle_mesh.rs
@@ -7,7 +7,7 @@ use color::XYZ;
 use lerp::{lerp, lerp_slice, lerp_slice_with};
 use math::{Point, Matrix4x4, cross};
 use ray::{Ray, AccelRay};
-use shading::surface_closure::{SurfaceClosureUnion, LambertClosure};
+use shading::surface_closure::{SurfaceClosureUnion, GTRClosure};
 
 use super::{Surface, SurfaceIntersection};
 use super::triangle;
@@ -97,10 +97,17 @@ impl Surface for TriangleMesh {
                                 incoming: wr.dir,
                                 nor: cross(tri.0 - tri.1, tri.0 - tri.2).into_normal(),
                                 local_space: mat_space,
-                                // TODO
-                                closure: SurfaceClosureUnion::LambertClosure(
-                                    LambertClosure::new(XYZ::new(0.8, 0.8, 0.8))
-                                ),
+                                // TODO: get surface closure from surface shader.
+                                //closure: SurfaceClosureUnion::LambertClosure(
+                                //    LambertClosure::new(XYZ::new(0.8, 0.8, 0.8))
+                                //),
+                                closure:
+                                    SurfaceClosureUnion::GTRClosure(GTRClosure::new(XYZ::new(0.8,
+                                                                                             0.8,
+                                                                                             0.8),
+                                                                                    0.1,
+                                                                                    2.0,
+                                                                                    1.2)),
                             };
                             r.max_t = t;
                         }