diff --git a/perf.data b/perf.data
deleted file mode 100644
index 2796874..0000000
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diff --git a/perf.data.old b/perf.data.old
deleted file mode 100644
index 3090232..0000000
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diff --git a/src/colour.h b/src/colour.h
index 77ecfe7..6e07b9f 100644
--- a/src/colour.h
+++ b/src/colour.h
@@ -75,9 +75,14 @@ class Colour {
 };
 
 constexpr Colour Colour::kBlack{0.0, 0.0, 0.0};
+constexpr Colour Colour::kWhite{1.0, 1.0, 1.0};
 
-constexpr Colour operator*(f64 t, Colour col) { return {t * col.r(), t * col.g(), t * col.b()}; }
-constexpr Colour operator/(Colour col, f64 t) { return (1.0 / t) * col; }
+constexpr Colour operator*(f64 t, Colour col) {
+    return {t * col.r(), t * col.g(), t * col.b()};
+}
+constexpr Colour operator/(Colour col, f64 t) {
+    return (1.0 / t) * col;
+}
 
 constexpr Colour operator+(Colour c1, Colour c2) {
     Colour out{c1};
diff --git a/src/main.cc b/src/main.cc
index 3fac189..c96e20f 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -1,6 +1,5 @@
 #include <iostream>
 #include <memory>
-#include <utility>
 
 #include "camera.h"
 #include "colour.h"
@@ -26,21 +25,26 @@ int main(int /* argc */, char* /* argv */[]) {
     world.Add(std::move(ground));
     world.Add(std::move(sphere)); */
 
-    auto material_ground = std::make_shared<Lambertian>(Colour(0.8, 0.8, 0.0));
-    auto material_center = std::make_shared<Lambertian>(Colour(0.7, 0.3, 0.3));
-    auto material_left = std::make_shared<Metal>(Colour(0.8, 0.8, 0.8), 0.05);
-    auto material_right = std::make_shared<Metal>(Colour(0.8, 0.5, 0.4), 0.5);
+    auto mat_ground = std::make_shared<Lambertian>(Colour(0.8, 0.8, 0.0));
+    auto mat_lamb = std::make_shared<Lambertian>(Colour(0.1, 0.2, 0.5));
+    auto mat_metal = std::make_shared<Metal>(Colour(0.8, 0.6, 0.2), 0.0);
+    auto mat_dielec = std::make_shared<Dielectric>(1.5, 0.0);
+    auto mat_dielec2 = std::make_shared<Dielectric>(0.66, 0.0);
 
-    world.Add(std::make_unique<Sphere>(Point3(0.0, -100.5, -1.0), 100.0, material_ground));
-    world.Add(std::make_unique<Sphere>(Point3(0.0, 0.0, -1.0), 0.5, material_center));
-    world.Add(std::make_unique<Sphere>(Point3(-1.0, 0.0, -1.0), 0.5, material_left));
-    world.Add(std::make_unique<Sphere>(Point3(1.0, 0.0, -1.0), 0.5, material_right));
+    world.Add(std::make_unique<Sphere>(Point3(0.0, -100.5, -1.0), 100.0, mat_ground));
+
+    world.Add(std::make_unique<Sphere>(Point3(0.0, 0.0, -1.0), 0.5, mat_lamb));
+
+    world.Add(std::make_unique<Sphere>(Point3(-1.0, 0.0, -1.0), 0.5, mat_dielec));
+    world.Add(std::make_unique<Sphere>(Point3(-1.0, 0.0, -1.0), 0.4, mat_dielec2));
+
+    world.Add(std::make_unique<Sphere>(Point3(1.0, 0.0, -1.0), 0.5, mat_metal));
 
     // camera
 
     constexpr f64 aspect_ratio = 16.0 / 9.0;
 
-    constexpr u32 image_width = 600;
+    constexpr u32 image_width = 800;
     constexpr auto image_height = static_cast<u32>(image_width / aspect_ratio);
 
     // constexpr u32 image_width = 1920;
diff --git a/src/material.h b/src/material.h
index 4fd22f3..c8fe960 100644
--- a/src/material.h
+++ b/src/material.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <cassert>
+#include <cmath>
 #include <optional>
 #include <tuple>
 
@@ -9,6 +10,7 @@
 #include "ray.h"
 #include "raytracer.h"
 #include "renderobject.h"
+#include "vec3.h"
 
 class Material {
    public:
@@ -22,13 +24,10 @@ class Material {
 
     virtual ~Material() = default;
 
-    RandomGen& rand() const { return rand_; }
-
     virtual std::optional<std::tuple<Colour, Ray>> Scatter(const Ray& in,
                                                            const HitRecord& hit_record) const = 0;
 
    private:
-    mutable RandomGen rand_{};
 };
 
 class Lambertian : public Material {
@@ -37,7 +36,7 @@ class Lambertian : public Material {
 
     std::optional<std::tuple<Colour, Ray>> Scatter(const Ray& /* in */,
                                                    const HitRecord& hit_record) const override {
-        auto scatter_dir = hit_record.normal + rand().GenOnUnitSphere();
+        auto scatter_dir = hit_record.normal + RandomGen::GenInstance().GenOnUnitSphere();
 
         if (scatter_dir.almost_zero()) {
             scatter_dir = hit_record.normal;
@@ -60,7 +59,7 @@ class Metal : public Material {
                                                    const HitRecord& hit_record) const override {
         auto reflect_dir = in.direction().reflect(hit_record.normal).normed();
 
-        reflect_dir += fuzz_ * rand().GenOnUnitSphere();
+        reflect_dir += fuzz_ * RandomGen::GenInstance().GenOnUnitSphere();
 
         Ray scattered{hit_record.p, reflect_dir};
 
@@ -71,3 +70,44 @@ class Metal : public Material {
     Colour albedo_;
     f64 fuzz_;
 };
+
+class Dielectric : public Material {
+   public:
+    constexpr explicit Dielectric(f64 eta, f64 fuzz) : eta_{eta}, fuzz_{fuzz} {}
+
+    std::optional<std::tuple<Colour, Ray>> Scatter(const Ray& in,
+                                                   const HitRecord& hit_record) const override {
+        auto& rand = RandomGen::GenInstance();
+
+        f64 cos_theta = dot(-in.direction(), hit_record.normal);
+        f64 sin_theta = std::sqrt(1.0 - cos_theta * cos_theta);
+
+        f64 eta = hit_record.front_face ? 1 / eta_ : eta_;
+
+        Vec3 out_dir;
+
+        constexpr auto reflectance = [](f64 cos, f64 eta) {
+            auto r0 = (1 - eta) / (1 + eta);
+            r0 = r0 * r0;
+            return r0 * (1 - r0) * std::pow(1 - cos, 5);
+        };
+
+        if (eta * sin_theta > 1.0 || reflectance(cos_theta, eta) > rand.GenUniform()) {
+            // can't refract, must reflect or Schlick approximation
+            out_dir = in.direction().reflect(hit_record.normal);
+        } else {
+            // reflect
+            out_dir = in.direction().refract(hit_record.normal, eta);
+        }
+
+        out_dir += fuzz_ * rand.GenOnUnitSphere();
+
+        Ray out{hit_record.p, out_dir};
+
+        return std::make_tuple(Colour::kWhite, out);
+    }
+
+   private:
+    f64 eta_;  // index of refraction inside over outside
+    f64 fuzz_;
+};
diff --git a/src/rand.h b/src/rand.h
index 13b68da..4b1cdd9 100644
--- a/src/rand.h
+++ b/src/rand.h
@@ -7,6 +7,20 @@
 
 class RandomGen {
    public:
+    constexpr RandomGen(const RandomGen&) = delete;
+    constexpr RandomGen& operator=(const RandomGen&) = delete;
+
+    constexpr RandomGen(RandomGen&&) = delete;
+    constexpr RandomGen& operator=(RandomGen&&) = delete;
+
+    constexpr ~RandomGen() = default;
+
+    static RandomGen& GenInstance() {
+        static thread_local RandomGen rand{};
+
+        return rand;
+    }
+
     constexpr u64 GenU64() { return philox_.Next(); }
     // constexpr u32 GenU32() { return static_cast<u32>(GenU64()); }
 
@@ -43,5 +57,7 @@ class RandomGen {
     }
 
    private:
+    RandomGen() = default;
+
     Philox philox_;
 };
diff --git a/src/ray.h b/src/ray.h
index 67f7eb7..6f260d5 100644
--- a/src/ray.h
+++ b/src/ray.h
@@ -7,7 +7,8 @@ class Ray {
    public:
     constexpr Ray() = default;
 
-    constexpr Ray(const Point3& origin, const Vec3& direction) : orig_{origin}, dir_{direction} {}
+    constexpr Ray(const Point3& origin, const Vec3& direction)
+        : orig_{origin}, dir_{direction.normed()} {}
 
     Point3 origin() const { return orig_; }
     Vec3 direction() const { return dir_; }
diff --git a/src/renderer.h b/src/renderer.h
index 0843811..2fad7b7 100644
--- a/src/renderer.h
+++ b/src/renderer.h
@@ -1,7 +1,11 @@
 #pragma once
 
+#include <sys/sysinfo.h>
+
 #include <array>
+#include <atomic>
 #include <cassert>
+#include <chrono>
 #include <iostream>
 #include <thread>
 
@@ -30,12 +34,12 @@ class Renderer {
     Image Render(const RenderObject& world) {
         Image img{camera_.image_width(), camera_.image_height()};
 
-        constexpr u32 kNumThreads = 4;
+        constexpr u32 kNumThreads = 16;
 
-        auto render_thread = [this, &world, &img](u32 thread) {
+        std::atomic_uint32_t lines_done = 0;
+
+        auto render_thread = [this, &world, &img, &lines_done](u32 thread) {
             for (u32 j = thread; j < img.height(); j += kNumThreads) {
-                std::clog << "\rWriting line " << j << " of " << img.height() << std::flush;
-
                 for (u32 i = 0; i < img.width(); i++) {
                     Colour colour_sum{0.0, 0.0, 0.0};
 
@@ -47,6 +51,8 @@ class Renderer {
 
                     img[i, j] = colour_sum / samples_per_pixel_;
                 }
+
+                lines_done.fetch_add(1);
             }
         };
 
@@ -58,21 +64,39 @@ class Renderer {
             render_threads[i] = std::thread{render_thread, i};
         }
 
+        std::atomic_bool done = false;
+
+        auto writer_thread = std::thread([&done, &lines_done, total_lines = img.height()] {
+            using std::literals::chrono_literals::operator""ms;
+
+            while (!done.load()) {
+                std::cerr << "\rWriting line " << lines_done.load() << " of " << total_lines
+                          << std::flush;
+                std::this_thread::sleep_for(std::chrono::milliseconds(10));
+            }
+
+            std::clog << "\rDone                                                " << newline;
+        });
+
         for (auto& t : render_threads) {
             t.join();
         }
 
-        std::clog << "\rDone                                                " << newline;
+        done.store(true);
+
+        writer_thread.join();
 
         return img;
     }
 
    private:
-    constexpr Ray SampleRay(u32 i, u32 j) {
+    Ray SampleRay(u32 i, u32 j) {
         auto pixel_centre = camera_.PixelToWorld(i, j);
 
-        Vec3 random_shift = rand_.GenUniform(-0.5, 0.5) * camera_.d_u_pixel() +
-                            rand_.GenUniform(-0.5, 0.5) * camera_.d_v_pixel();
+        auto& rand = RandomGen::GenInstance();
+
+        Vec3 random_shift = rand.GenUniform(-0.5, 0.5) * camera_.d_u_pixel() +
+                            rand.GenUniform(-0.5, 0.5) * camera_.d_v_pixel();
 
         pixel_centre += random_shift;
 
@@ -86,7 +110,7 @@ class Renderer {
 
         // background
         if (!hit_record.has_value()) {
-            auto unit_dir = ray.direction().normed();
+            auto unit_dir = ray.direction();
 
             f64 a = 0.5 * (unit_dir.y() + 1.0);
 
@@ -95,7 +119,7 @@ class Renderer {
 
         // hit
 
-        if (!hit_record->front_face || bounces == 0) {
+        if (bounces == 0) {
             return Colour::kBlack;
         }
 
@@ -114,8 +138,6 @@ class Renderer {
 
     Camera camera_;
 
-    RandomGen rand_{};
-
     u32 samples_per_pixel_;
     u32 max_bounces_;
 };
diff --git a/src/vec3.h b/src/vec3.h
index 35b304a..dc64ed1 100644
--- a/src/vec3.h
+++ b/src/vec3.h
@@ -73,6 +73,8 @@ class Vec3 {
 
     constexpr Vec3 reflect(Vec3 normal) const;
 
+    constexpr Vec3 refract(Vec3 normal, f64 eta) const;
+
    private:
     std::array<f64, 3> xyz_;
 };
@@ -93,7 +95,9 @@ constexpr Vec3 operator+(const Vec3& u, const Vec3& v) {
     return out;
 }
 
-constexpr Vec3 operator-(const Vec3& u, const Vec3& v) { return u + (-v); }
+constexpr Vec3 operator-(const Vec3& u, const Vec3& v) {
+    return u + (-v);
+}
 
 constexpr Vec3 operator*(const Vec3& u, const Vec3& v) {
     return {u.x() * v.x(), u.y() * v.y(), u.z() * v.z()};
@@ -105,7 +109,9 @@ constexpr Vec3 operator*(f64 t, const Vec3& v) {
     return out;
 }
 
-constexpr Vec3 operator/(const Vec3& v, f64 t) { return (1 / t) * v; }
+constexpr Vec3 operator/(const Vec3& v, f64 t) {
+    return (1 / t) * v;
+}
 
 constexpr f64 dot(const Vec3& u, const Vec3& v) {
     Vec3 tmp = u * v;
@@ -117,10 +123,27 @@ constexpr Vec3 cross(const Vec3& u, const Vec3& v) {
             u.x() * v.y() - u.y() - v.x()};
 }
 
-constexpr f64 Vec3::squared() const { return dot(*this, *this); }
+constexpr f64 Vec3::squared() const {
+    return dot(*this, *this);
+}
 
-constexpr f64 Vec3::norm() const { return std::sqrt(squared()); }
+constexpr f64 Vec3::norm() const {
+    return std::sqrt(squared());
+}
 
-constexpr Vec3 Vec3::normed() const { return *this / norm(); }
+constexpr Vec3 Vec3::normed() const {
+    return *this / norm();
+}
 
-constexpr Vec3 Vec3::reflect(Vec3 normal) const { return *this - 2 * dot(*this, normal) * normal; }
+constexpr Vec3 Vec3::reflect(Vec3 normal) const {
+    return *this - 2 * dot(*this, normal) * normal;
+}
+
+constexpr Vec3 Vec3::refract(Vec3 normal, f64 eta) const {
+    f64 cos_theta = dot(-*this, normal);
+
+    Vec3 out_orth = eta * (*this + cos_theta * normal);
+    Vec3 out_par = -std::sqrt(std::fabs(1.0 - out_orth.squared())) * normal;
+
+    return out_orth + out_par;
+}
diff --git a/subprojects/rand b/subprojects/rand
index 3b8796e..8cba484 160000
--- a/subprojects/rand
+++ b/subprojects/rand
@@ -1 +1 @@
-Subproject commit 3b8796ee2775f3f1fe735a19c0f041ea144ae7b4
+Subproject commit 8cba4843cff103ae458b814d3ae0a4244a041710