2 files changed, 24 insertions, 2 deletions
diff --git a/material_dielectric.cpp b/material_dielectric.cpp
index 3aaf259..ed8e4eb 100644
--- a/material_dielectric.cpp
+++ b/material_dielectric.cpp
@@ -9,13 +9,24 @@ bool material_dielectric::scatter(ray3d &r, const object &hit_obj, double hit_t,
     assert(hit_obj.is_on(hit_p));
     auto n = hit_obj.normal_vector(hit_p);
     auto ri_ = ri_inv;
-    if (dot(r.direction(), n) > 0) {
+    auto cos1 = dot(r.direction(), n); // -cos(a1)
+    if (cos1 > 0) {
         // the ray is started from the object's inner,
         // use normal vector and ri on the surface's inner side
         n = -n;
         ri_ = 1.0 / ri_;
+    } else {
+        cos1 = -cos1;
+    }
+    vec3d r2;
+    // determine reflection or refraction using Schlick's Approximation.
+    if (reflectance(cos1, ri_) > ruvg.range01_scalar()) {
+        // reflect
+        r2 = n.reflect(r.direction());
+    } else {
+        // refract
+        r2 = n.refract<true>(r.direction(), ri_);
     }
-    const auto r2 = n.refract<true>(r.direction(), ri_); // refracted vector
     r.direction(r2.unit_vec());
     r.source(hit_p);
     return true;
diff --git a/material_dielectric.h b/material_dielectric.h
index 48382b4..46b17da 100644
--- a/material_dielectric.h
+++ b/material_dielectric.h
@@ -10,8 +10,19 @@
 
 class material_dielectric : public material {
     double ri_inv;
+
+    static double reflectance(double cosine, double ref_idx) {
+        assert(cosine > 0);
+        assert(ref_idx > 0);
+        // Use Schlick's approximation for reflectance.
+        auto r0 = (1 - ref_idx) / (1 + ref_idx);
+        r0 = r0 * r0;
+        return r0 + (1 - r0) * pow((1 - cosine), 5);
+    }
+
 public:
     explicit material_dielectric(double ri) : ri_inv{1.0 / ri} {}
+
     bool scatter(ray3d &r, const object &hit_obj, double hit_t, random_uv_gen_3d &ruvg) const override;
 };