refactor: separate single main.zig

10 files changed, 815 insertions, 709 deletions

diff --git a/src/main.zig b/src/main.zig
index eb537a8..e0e5702 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -3,422 +3,38 @@ const debug = std.debug;
 const math = std.math;
 const ArrayList = std.ArrayList;
 
-const Vec3 = struct {
-    x: f64,
-    y: f64,
-    z: f64,
-
-    pub fn norm(v: Vec3) f64 {
-        return @sqrt(v.normSquared());
-    }
-
-    pub fn normSquared(v: Vec3) f64 {
-        return v.x * v.x + v.y * v.y + v.z * v.z;
-    }
-
-    pub fn dot(u: Vec3, v: Vec3) f64 {
-        return u.x * v.x + u.y * v.y + u.z * v.z;
-    }
-
-    pub fn cross(u: Vec3, v: Vec3) Vec3 {
-        return Vec3{
-            .x = u.y * v.z - u.z * v.y,
-            .y = u.z * v.x - u.x * v.z,
-            .z = u.x * v.y - u.y * v.x,
-        };
-    }
-
-    pub fn normalized(v: Vec3) Vec3 {
-        const n = v.norm();
-        if (n == 0.0) {
-            return v;
-        } else {
-            return v.div(n);
-        }
-    }
-
-    pub fn add(u: Vec3, v: Vec3) Vec3 {
-        return Vec3{
-            .x = u.x + v.x,
-            .y = u.y + v.y,
-            .z = u.z + v.z,
-        };
-    }
-
-    pub fn sub(u: Vec3, v: Vec3) Vec3 {
-        return Vec3{
-            .x = u.x - v.x,
-            .y = u.y - v.y,
-            .z = u.z - v.z,
-        };
-    }
-
-    pub fn mul(v: Vec3, t: f64) Vec3 {
-        return Vec3{
-            .x = v.x * t,
-            .y = v.y * t,
-            .z = v.z * t,
-        };
-    }
-
-    pub fn mulV(u: Vec3, v: Vec3) Vec3 {
-        return Vec3{
-            .x = u.x * v.x,
-            .y = u.y * v.y,
-            .z = u.z * v.z,
-        };
-    }
-
-    pub fn div(v: Vec3, t: f64) Vec3 {
-        return Vec3{
-            .x = v.x / t,
-            .y = v.y / t,
-            .z = v.z / t,
-        };
-    }
-
-    pub fn random01(rand: std.rand.Random) Vec3 {
-        return Vec3{
-            .x = randomReal01(rand),
-            .y = randomReal01(rand),
-            .z = randomReal01(rand),
-        };
-    }
-
-    pub fn random(rand: std.rand.Random, min: f64, max: f64) Vec3 {
-        return Vec3{
-            .x = randomReal(rand, min, max),
-            .y = randomReal(rand, min, max),
-            .z = randomReal(rand, min, max),
-        };
-    }
-
-    // for debugging
-    pub fn pp(v: Vec3) void {
-        debug.print("{} {} {}\n", .{ v.x, v.y, v.z });
-    }
-
-    fn near_zero(v: Vec3) bool {
-        const epsilon = 1e-8;
-        return @fabs(v.x) < epsilon and @fabs(v.y) < epsilon and @fabs(v.z) < epsilon;
-    }
-};
-
-fn randomPointInUnitSphere(rand: std.rand.Random) Vec3 {
-    while (true) {
-        const p = Vec3.random(rand, -1.0, 1.0);
-        if (p.norm() >= 1) continue;
-        return p;
-    }
-}
-
-fn randomPointInUnitDisk(rand: std.rand.Random) Vec3 {
-    while (true) {
-        const p = Vec3{ .x = randomReal(rand, -1.0, 1.0), .y = randomReal(rand, -1.0, 1.0), .z = 0.0 };
-        if (p.norm() >= 1) continue;
-        return p;
-    }
-}
-
-fn randomUnitVector(rand: std.rand.Random) Vec3 {
-    return randomPointInUnitSphere(rand).normalized();
-}
-
-fn reflect(v: Vec3, n: Vec3) Vec3 {
-    return v.sub(n.mul(2 * v.dot(n)));
-}
-
-fn refract(uv: Vec3, n: Vec3, etai_over_etat: f64) Vec3 {
-    const cos_theta = @min(uv.mul(-1.0).dot(n), 1.0);
-    const r_out_perpendicular = uv.add(n.mul(cos_theta)).mul(etai_over_etat);
-    const r_out_parallel = n.mul(-@sqrt(@fabs(1.0 - r_out_perpendicular.normSquared())));
-    return r_out_perpendicular.add(r_out_parallel);
-}
-
-const Point3 = Vec3;
-const Color = Vec3;
-
-fn rgb(r: f64, g: f64, b: f64) Color {
-    return .{ .x = r, .y = g, .z = b };
-}
-
-const Ray = struct {
-    origin: Vec3,
-    dir: Vec3,
-    time: f64,
-
-    pub fn at(r: Ray, t: f64) Point3 {
-        return r.origin.add(r.dir.mul(t));
-    }
-};
-
-const MaterialTag = enum {
-    diffuse,
-    metal,
-    dielectric,
-};
-
-const Material = union(MaterialTag) {
-    diffuse: DiffuseMaterial,
-    metal: MetalMaterial,
-    dielectric: DielectricMaterial,
-
-    fn scatter(mat: Material, r_in: Ray, record: HitRecord, attenuation: *Color, scattered: *Ray, rand: std.rand.Random) bool {
-        return switch (mat) {
-            MaterialTag.diffuse => |diffuse_mat| diffuse_mat.scatter(r_in, record, attenuation, scattered, rand),
-            MaterialTag.metal => |metal_mat| metal_mat.scatter(r_in, record, attenuation, scattered, rand),
-            MaterialTag.dielectric => |dielectric_mat| dielectric_mat.scatter(r_in, record, attenuation, scattered, rand),
-        };
-    }
-};
-
-const DiffuseMaterial = struct {
-    albedo: Texture,
-
-    fn scatter(mat: DiffuseMaterial, r_in: Ray, record: HitRecord, attenuation: *Color, scattered: *Ray, rand: std.rand.Random) bool {
-        var scatter_direction = record.normal.add(randomUnitVector(rand));
-        if (scatter_direction.near_zero()) {
-            scatter_direction = record.normal;
-        }
-        scattered.* = .{ .origin = record.p, .dir = scatter_direction, .time = r_in.time };
-        attenuation.* = mat.albedo.value(record.u, record.v, record.p);
-        return true;
-    }
-};
-
-const MetalMaterial = struct {
-    albedo: Color,
-    fuzz: f64,
-
-    fn scatter(mat: MetalMaterial, r_in: Ray, record: HitRecord, attenuation: *Color, scattered: *Ray, rand: std.rand.Random) bool {
-        debug.assert(mat.fuzz <= 1.0);
-        const reflected = reflect(r_in.dir.normalized(), record.normal);
-        scattered.* = .{ .origin = record.p, .dir = reflected.add(randomPointInUnitSphere(rand).mul(mat.fuzz)), .time = r_in.time };
-        attenuation.* = mat.albedo;
-        return reflected.dot(record.normal) > 0.0;
-    }
-};
-
-const DielectricMaterial = struct {
-    // index of refraction.
-    ir: f64,
-
-    fn scatter(mat: DielectricMaterial, r_in: Ray, record: HitRecord, attenuation: *Color, scattered: *Ray, rand: std.rand.Random) bool {
-        const refraction_ratio = if (record.front_face) 1.0 / mat.ir else mat.ir;
-        const unit_dir = r_in.dir.normalized();
-
-        const cos_theta = @min(unit_dir.mul(-1.0).dot(record.normal), 1.0);
-        const sin_theta = @sqrt(1.0 - cos_theta * cos_theta);
-        // sin(theta') = refraction_ratio * sin(theta) <= 1
-        const can_refract = refraction_ratio * sin_theta <= 1.0;
-
-        const dir = if (can_refract and reflectance(cos_theta, refraction_ratio) < randomReal01(rand)) refract(unit_dir, record.normal, refraction_ratio) else reflect(unit_dir, record.normal);
-        scattered.* = .{ .origin = record.p, .dir = dir, .time = r_in.time };
-        attenuation.* = rgb(1.0, 1.0, 1.0);
-        return true;
-    }
-
-    fn reflectance(cos: f64, refraction_idx: f64) f64 {
-        const r0 = (1.0 - refraction_idx) / (1.0 + refraction_idx);
-        const r1 = r0 * r0;
-        return r1 + (1.0 - r1) * math.pow(f64, 1.0 - cos, 5.0);
-    }
-};
-
-const HitRecord = struct {
-    // The point where the ray and the hittable hits.
-    p: Point3,
-    // The normal of the hittable at p.
-    normal: Vec3,
-    // The material at p.
-    material: *const Material,
-    // p = ray.at(t)
-    t: f64,
-    // The coordinate of the surface where the ray intersects.
-    u: f64,
-    v: f64,
-    // True if the ray hits the hittable from the front face, i.e., outside of it.
-    front_face: bool,
-};
-
-const HittableTag = enum {
-    sphere,
-    movingSphere,
-    list,
-    bvhNode,
-};
-
-const Hittable = union(HittableTag) {
-    sphere: Sphere,
-    movingSphere: MovingSphere,
-    list: HittableList,
-    bvhNode: BvhNode,
-
-    fn hit(h: Hittable, r: Ray, tMin: f64, tMax: f64, record: *HitRecord) bool {
-        return switch (h) {
-            HittableTag.sphere => |sphere| sphere.hit(r, tMin, tMax, record),
-            HittableTag.movingSphere => |movingSphere| movingSphere.hit(r, tMin, tMax, record),
-            HittableTag.list => |list| list.hit(r, tMin, tMax, record),
-            HittableTag.bvhNode => |node| node.hit(r, tMin, tMax, record),
-        };
-    }
-
-    fn boudingBox(h: Hittable, time0: f64, time1: f64, outputBox: *Aabb) bool {
-        return switch (h.*) {
-            HittableTag.sphere => |sphere| sphere.boudingBox(time0, time1, outputBox),
-            HittableTag.movingSphere => |movingSphere| movingSphere.boudingBox(time0, time1, outputBox),
-            HittableTag.list => |list| list.boudingBox(time0, time1, outputBox),
-            HittableTag.bvhNode => |node| node.boudingBox(time0, time1, outputBox),
-        };
-    }
-
-    fn deinit(h: *const Hittable, allocator: anytype) void {
-        return switch (h.*) {
-            HittableTag.sphere => |sphere| sphere.deinit(allocator),
-            HittableTag.movingSphere => |movingSphere| movingSphere.deinit(allocator),
-            HittableTag.list => |list| list.deinit(allocator),
-            HittableTag.bvhNode => |node| node.deinit(allocator),
-        };
-    }
-};
-
-const Sphere = struct {
-    center: Point3,
-    radius: f64,
-    material: *const Material,
-
-    fn hit(sphere: Sphere, r: Ray, tMin: f64, tMax: f64, record: *HitRecord) bool {
-        const oc = r.origin.sub(sphere.center);
-        const a = r.dir.normSquared();
-        const half_b = Vec3.dot(oc, r.dir);
-        const c = oc.normSquared() - sphere.radius * sphere.radius;
-
-        const discriminant = half_b * half_b - a * c;
-        if (discriminant < 0.0) {
-            // r does not intersect the sphere.
-            return false;
-        }
-        const sqrtd = @sqrt(discriminant);
-
-        // Find the nearest root that lies in the acceptable range.
-        var root = (-half_b - sqrtd) / a;
-        if (root < tMin or tMax < root) {
-            root = (-half_b + sqrtd) / a;
-            if (root < tMin or tMax < root) {
-                // out of range
-                return false;
-            }
-        }
-
-        record.t = root;
-        record.p = r.at(root);
-        const outward_normal = (record.p.sub(sphere.center)).div(sphere.radius);
-        record.front_face = Vec3.dot(outward_normal, r.dir) < 0.0;
-        if (record.front_face) {
-            record.normal = outward_normal;
-        } else {
-            record.normal = outward_normal.mul(-1.0);
-        }
-        Sphere.getSphereUv(outward_normal, &record.u, &record.v);
-        record.material = sphere.material;
-
-        return true;
-    }
-
-    fn boudingBox(sphere: Sphere, time0: f64, time1: f64, outputBox: *Aabb) bool {
-        _ = time0;
-        _ = time1;
-        const o = sphere.center;
-        const r = sphere.radius;
-        outputBox.* = .{
-            .min = o.sub(.{ .x = r, .y = r, .z = r }),
-            .max = o.add(.{ .x = r, .y = r, .z = r }),
-        };
-        return true;
-    }
-
-    fn getSphereUv(p: Point3, u: *f64, v: *f64) void {
-        const phi = math.atan2(f64, -p.z, p.x) + pi;
-        const theta = math.acos(-p.y);
-        u.* = phi / (2.0 * pi);
-        v.* = theta / pi;
-    }
-
-    fn deinit(sphere: *const Sphere, allocator: anytype) void {
-        allocator.destroy(sphere.material);
-    }
-};
-
-const MovingSphere = struct {
-    center0: Point3,
-    center1: Point3,
-    time0: f64,
-    time1: f64,
-    radius: f64,
-    material: *const Material,
-
-    fn hit(sphere: MovingSphere, r: Ray, tMin: f64, tMax: f64, record: *HitRecord) bool {
-        const center_ = sphere.center(r.time);
-        const oc = r.origin.sub(center_);
-        const a = r.dir.normSquared();
-        const half_b = Vec3.dot(oc, r.dir);
-        const c = oc.normSquared() - sphere.radius * sphere.radius;
-
-        const discriminant = half_b * half_b - a * c;
-        if (discriminant < 0.0) {
-            // r does not intersect the sphere.
-            return false;
-        }
-        const sqrtd = @sqrt(discriminant);
-
-        // Find the nearest root that lies in the acceptable range.
-        var root = (-half_b - sqrtd) / a;
-        if (root < tMin or tMax < root) {
-            root = (-half_b + sqrtd) / a;
-            if (root < tMin or tMax < root) {
-                // out of range
-                return false;
-            }
-        }
-
-        record.t = root;
-        record.p = r.at(root);
-        const outward_normal = (record.p.sub(center_)).div(sphere.radius);
-        record.front_face = Vec3.dot(outward_normal, r.dir) < 0.0;
-        if (record.front_face) {
-            record.normal = outward_normal;
-        } else {
-            record.normal = outward_normal.mul(-1.0);
-        }
-        record.material = sphere.material;
-
-        return true;
-    }
-
-    fn boudingBox(sphere: MovingSphere, time0: f64, time1: f64, outputBox: *Aabb) bool {
-        const o0 = sphere.center(time0);
-        const o1 = sphere.center(time1);
-        const r = sphere.radius;
-        const box0 = .{
-            .min = o0.sub(.{ .x = r, .y = r, .z = r }),
-            .max = o0.add(.{ .x = r, .y = r, .z = r }),
-        };
-        const box1 = .{
-            .min = o1.sub(.{ .x = r, .y = r, .z = r }),
-            .max = o1.add(.{ .x = r, .y = r, .z = r }),
-        };
-        outputBox.* = surroundingBox(box0, box1);
-        return true;
-    }
-
-    fn center(sphere: MovingSphere, t: f64) Point3 {
-        return sphere.center0.add(sphere.center1.sub(sphere.center0).mul((t - sphere.time0) / (sphere.time1 - sphere.time0)));
-    }
-
-    fn deinit(sphere: *const MovingSphere, allocator: anytype) void {
-        allocator.destroy(sphere.material);
-    }
-};
+const rtw = @import("rtw.zig");
+
+const Ray = rtw.ray.Ray;
+const Vec3 = rtw.vec.Vec3;
+const Point3 = rtw.vec.Point3;
+const Color = rtw.vec.Color;
+const rgb = rtw.vec.rgb;
+const Random = rtw.rand.Random;
+const randomPointInUnitSphere = rtw.rand.randomPointInUnitSphere;
+const randomPointInUnitDisk = rtw.rand.randomPointInUnitDisk;
+const randomUnitVector = rtw.rand.randomUnitVector;
+const randomInt = rtw.rand.randomInt;
+const randomReal01 = rtw.rand.randomReal01;
+const randomReal = rtw.rand.randomReal;
+
+const Material = rtw.material.Material;
+const DiffuseMaterial = rtw.material.DiffuseMaterial;
+const MetalMaterial = rtw.material.MetalMaterial;
+const DielectricMaterial = rtw.material.DielectricMaterial;
+
+const Texture = rtw.texture.Texture;
+const SolidTexture = rtw.texture.SolidTexture;
+const CheckerTexture = rtw.texture.CheckerTexture;
+
+const Hittable = rtw.hittable.Hittable;
+const Sphere = rtw.hittable.Sphere;
+const MovingSphere = rtw.hittable.MovingSphere;
+const HittableList = rtw.hittable.HittableList;
+const BvhNode = rtw.hittable.BvhNode;
+
+const Aabb = rtw.aabb.Aabb;
+const HitRecord = rtw.hit_record.HitRecord;
 
 fn makeSphere(center: Point3, radius: f64, material: *const Material) Hittable {
     return .{
@@ -430,48 +46,6 @@ fn makeSphere(center: Point3, radius: f64, material: *const Material) Hittable {
     };
 }
 
-const HittableList = struct {
-    objects: ArrayList(Hittable),
-
-    fn hit(list: HittableList, r: Ray, tMin: f64, tMax: f64, record: *HitRecord) bool {
-        var hit_anything = false;
-        var closest_so_far = tMax;
-
-        for (list.objects.items) |object| {
-            var rec: HitRecord = undefined;
-            if (object.hit(r, tMin, closest_so_far, &rec)) {
-                hit_anything = true;
-                closest_so_far = rec.t;
-                record.* = rec;
-            }
-        }
-        return hit_anything;
-    }
-
-    fn boudingBox(list: HittableList, time0: f64, time1: f64, outputBox: *Aabb) bool {
-        if (list.objects.items.len == 0) {
-            return false;
-        }
-        var firstBox = true;
-        var tmpBox: Aabb = undefined;
-        for (list.objects.items) |object| {
-            if (!object.boudingBox(time0, time1, tmpBox)) {
-                return false;
-            }
-            outputBox = if (firstBox) tmpBox else surroundingBox(outputBox, tmpBox);
-            firstBox = false;
-        }
-        return true;
-    }
-
-    fn deinit(list: *const HittableList, allocator: anytype) void {
-        for (list.objects.items) |object| {
-            object.deinit(allocator);
-        }
-        list.objects.deinit();
-    }
-};
-
 const inf = math.inf(f64);
 const pi = math.pi;
 
@@ -479,233 +53,6 @@ fn deg2rad(degree: f64) f64 {
     return degree * pi / 180.0;
 }
 
-// [min, max)
-fn randomInt(comptime T: type, rand: std.rand.Random, min: T, max: T) T {
-    return rand.intRangeLessThan(T, min, max);
-}
-
-// [0, 1)
-fn randomReal01(rand: std.rand.Random) f64 {
-    return rand.float(f64);
-}
-
-// [min, max)
-fn randomReal(rand: std.rand.Random, min: f64, max: f64) f64 {
-    return min + randomReal01(rand) * (max - min);
-}
-
-const Aabb = struct {
-    min: Point3,
-    max: Point3,
-
-    fn hit(aabb: Aabb, r: Ray, tMin: f64, tMax: f64) bool {
-        var tMin_ = tMin;
-        var tMax_ = tMax;
-        {
-            const s0 = (aabb.min.x - r.origin.x) / r.dir.x;
-            const s1 = (aabb.max.x - r.origin.x) / r.dir.x;
-            const t0 = @min(s0, s1);
-            const t1 = @max(s0, s1);
-            tMin_ = @max(t0, tMin_);
-            tMax_ = @min(t1, tMax_);
-            if (tMax_ <= tMin_) {
-                return false;
-            }
-        }
-        {
-            const s0 = (aabb.min.y - r.origin.y) / r.dir.y;
-            const s1 = (aabb.max.y - r.origin.y) / r.dir.y;
-            const t0 = @min(s0, s1);
-            const t1 = @max(s0, s1);
-            tMin_ = @max(t0, tMin_);
-            tMax_ = @min(t1, tMax_);
-            if (tMax_ <= tMin_) {
-                return false;
-            }
-        }
-        {
-            const s0 = (aabb.min.z - r.origin.z) / r.dir.z;
-            const s1 = (aabb.max.z - r.origin.z) / r.dir.z;
-            const t0 = @min(s0, s1);
-            const t1 = @max(s0, s1);
-            tMin_ = @max(t0, tMin_);
-            tMax_ = @min(t1, tMax_);
-            if (tMax_ <= tMin_) {
-                return false;
-            }
-        }
-        return true;
-    }
-};
-
-fn surroundingBox(box0: Aabb, box1: Aabb) Aabb {
-    return .{
-        .min = .{
-            .x = @min(box0.min.x, box1.min.x),
-            .y = @min(box0.min.y, box1.min.y),
-            .z = @min(box0.min.z, box1.min.z),
-        },
-        .max = .{
-            .x = @max(box0.max.x, box1.max.x),
-            .y = @max(box0.max.y, box1.max.y),
-            .z = @max(box0.max.z, box1.max.z),
-        },
-    };
-}
-
-const BvhNode = struct {
-    left: *Hittable, // TODO
-    right: *Hittable, // TODO
-    box: Aabb,
-
-    fn init(
-        objects: ArrayList(*Hittable),
-        start: usize,
-        end: usize,
-        time0: f64,
-        time1: f64,
-    ) BvhNode {
-        var left: *Hittable = undefined;
-        var right: *Hittable = undefined;
-
-        var objects_ = objects;
-        const axis = randomInt(u8, 0, 3);
-
-        const objectSpan = end - start;
-        if (objectSpan == 1) {
-            left = objects.items[start];
-            right = objects.items[start];
-        } else if (objectSpan == 2) {
-            if (BvhNode.boxCompare(axis, objects[start], objects[start + 1])) {
-                left = objects[start];
-                right = objects[start + 1];
-            } else {
-                left = objects[start + 1];
-                right = objects[start];
-            }
-        } else {
-            std.sort.sort(*Hittable, objects_, axis, BvhNode.boxCompare);
-            const mid = start + objectSpan / 2;
-            left.* = .{ .bvhNode = BvhNode.init(objects, start, mid, time0, time1) };
-            right.* = .{ .bvhNode = BvhNode.init(objects, mid, end, time0, time1) };
-        }
-
-        var boxLeft: Aabb = undefined;
-        var boxRight: Aabb = undefined;
-
-        if (!left.boudingBox(time0, time1, boxLeft) || !right.boudingBox(time0, time1, boxRight)) {
-            // ERROR
-        }
-
-        return .{
-            .left = left,
-            .right = right,
-            .box = surroundingBox(boxLeft, boxRight),
-        };
-    }
-
-    fn boxCompare(axis: u8, a: *const Hittable, b: *const Hittable) bool {
-        if (axis == 0) {
-            return a.x < b.x;
-        } else if (axis == 1) {
-            return a.y < b.y;
-        } else {
-            return a.z < b.z;
-        }
-    }
-
-    fn hit(node: BvhNode, r: Ray, tMin: f64, tMax: f64, record: *HitRecord) bool {
-        if (!node.box.hit(r, tMin, tMax)) {
-            return false;
-        }
-
-        const hitLeft = node.left.hit(r, tMin, tMax, record);
-        const hitRight = node.right.hit(r, tMin, if (hitLeft) record.t else tMax, record);
-
-        return hitLeft or hitRight;
-    }
-
-    fn boudingBox(node: BvhNode, time0: f64, time1: f64, outputBox: *Aabb) bool {
-        _ = time0;
-        _ = time1;
-        outputBox.* = node.box;
-        return true;
-    }
-
-    fn deinit(node: *const BvhNode, allocator: anytype) void {
-        _ = node;
-        _ = allocator;
-    }
-};
-
-const TextureTag = enum {
-    solid,
-    checker,
-};
-
-const Texture = union(TextureTag) {
-    solid: SolidTexture,
-    checker: CheckerTexture,
-
-    fn makeSolid(color: Color) Texture {
-        return .{ .solid = .{ .color = color } };
-    }
-
-    fn makeChecker(allocator: std.mem.Allocator, odd: Color, even: Color) !Texture {
-        return .{ .checker = try CheckerTexture.init(
-            allocator,
-            Texture.makeSolid(odd),
-            Texture.makeSolid(even),
-        ) };
-    }
-
-    fn value(tx: Texture, u: f64, v: f64, p: Vec3) Color {
-        return switch (tx) {
-            TextureTag.solid => |solidTx| solidTx.value(u, v, p),
-            TextureTag.checker => |checkerTx| checkerTx.value(u, v, p),
-        };
-    }
-};
-
-const SolidTexture = struct {
-    color: Color,
-
-    fn value(tx: SolidTexture, u: f64, v: f64, p: Vec3) Color {
-        _ = u;
-        _ = v;
-        _ = p;
-        return tx.color;
-    }
-};
-
-const CheckerTexture = struct {
-    allocator: std.mem.Allocator,
-    odd: *const Texture,
-    even: *const Texture,
-
-    fn init(allocator: std.mem.Allocator, odd: Texture, even: Texture) !CheckerTexture {
-        var odd_ = try allocator.create(Texture);
-        var even_ = try allocator.create(Texture);
-        odd_.* = odd;
-        even_.* = even;
-        return .{
-            .allocator = allocator,
-            .odd = odd_,
-            .even = even_,
-        };
-    }
-
-    fn deinit(tx: CheckerTexture) void {
-        tx.allocator.destroy(tx.even);
-        tx.allocator.destroy(tx.odd);
-    }
-
-    fn value(tx: CheckerTexture, u: f64, v: f64, p: Vec3) Color {
-        const sines = @sin(10 * p.x) * @sin(10 * p.y) * @sin(10 * p.z);
-        return if (sines < 0) tx.odd.value(u, v, p) else tx.even.value(u, v, p);
-    }
-};
-
 const Camera = struct {
     origin: Point3,
     horizontal: Vec3,
@@ -757,19 +104,19 @@ const Camera = struct {
         };
     }
 
-    fn getRay(camera: Camera, rand: std.rand.Random, s: f64, t: f64) Ray {
-        const rd = randomPointInUnitDisk(rand).mul(camera.lens_radius);
+    fn getRay(camera: Camera, rng: Random, s: f64, t: f64) Ray {
+        const rd = randomPointInUnitDisk(rng).mul(camera.lens_radius);
         const offset = camera.u.mul(rd.x).add(camera.v.mul(rd.y));
         const dir = camera.lower_left_corner.add(camera.horizontal.mul(s)).add(camera.vertical.mul(t)).sub(camera.origin).sub(offset);
         return .{
             .origin = camera.origin.add(offset),
             .dir = dir,
-            .time = randomReal(rand, camera.time0, camera.time1),
+            .time = randomReal(rng, camera.time0, camera.time1),
         };
     }
 };
 
-fn rayColor(r: Ray, world: Hittable, rand: std.rand.Random, depth: u32) Color {
+fn rayColor(r: Ray, world: Hittable, rng: Random, depth: u32) Color {
     var rec: HitRecord = undefined;
     if (depth == 0) {
         // If we've exceeded the ray bounce limit, no more ligth is gathered.
@@ -778,8 +125,8 @@ fn rayColor(r: Ray, world: Hittable, rand: std.rand.Random, depth: u32) Color {
     if (world.hit(r, 0.001, inf, &rec)) {
         var scattered: Ray = undefined;
         var attenuation: Color = undefined;
-        if (rec.material.scatter(r, rec, &attenuation, &scattered, rand)) {
-            return attenuation.mulV(rayColor(scattered, world, rand, depth - 1));
+        if (rec.material.scatter(r, rec, &attenuation, &scattered, rng)) {
+            return attenuation.mulV(rayColor(scattered, world, rng, depth - 1));
         } else {
             return rgb(0.0, 0.0, 0.0);
         }
@@ -798,8 +145,8 @@ fn writeColor(out: anytype, c: Color, samples_per_pixel: u32) !void {
     });
 }
 
-fn generateTwoSpheres(rand: std.rand.Random, allocator: anytype) !Hittable {
-    _ = rand;
+fn generateTwoSpheres(rng: Random, allocator: anytype) !Hittable {
+    _ = rng;
     var hittable_objects = ArrayList(Hittable).init(allocator);
 
     const checker = try Texture.makeChecker(allocator, rgb(0.2, 0.3, 0.1), rgb(0.9, 0.9, 0.9));
@@ -815,7 +162,7 @@ fn generateTwoSpheres(rand: std.rand.Random, allocator: anytype) !Hittable {
     return .{ .list = .{ .objects = hittable_objects } };
 }
 
-fn generateRandomScene(rand: std.rand.Random, allocator: anytype) !Hittable {
+fn generateRandomScene(rng: Random, allocator: anytype) !Hittable {
     var hittable_objects = ArrayList(Hittable).init(allocator);
 
     var mat_ground = try allocator.create(Material);
@@ -839,11 +186,11 @@ fn generateRandomScene(rand: std.rand.Random, allocator: anytype) !Hittable {
     while (a < 3) : (a += 1) {
         var b: i32 = -3;
         while (b < 3) : (b += 1) {
-            const choose_mat = randomReal01(rand);
+            const choose_mat = randomReal01(rng);
             const center = Point3{
-                .x = @intToFloat(f64, a) + 0.9 * randomReal01(rand),
+                .x = @intToFloat(f64, a) + 0.9 * randomReal01(rng),
                 .y = 0.2,
-                .z = @intToFloat(f64, b) + 0.9 * randomReal01(rand),
+                .z = @intToFloat(f64, b) + 0.9 * randomReal01(rng),
             };
 
             if (center.sub(.{ .x = 4, .y = 0.2, .z = 0 }).norm() <= 0.9) {
@@ -853,9 +200,9 @@ fn generateRandomScene(rand: std.rand.Random, allocator: anytype) !Hittable {
             var mat_sphere = try allocator.create(Material);
             if (choose_mat < 0.8) {
                 // diffuse
-                const albedo = Color.random01(rand).mulV(Color.random01(rand));
+                const albedo = Color.random01(rng).mulV(Color.random01(rng));
                 mat_sphere.* = .{ .diffuse = .{ .albedo = .{ .solid = .{ .color = albedo } } } };
-                const center1 = center.add(.{ .x = 0, .y = randomReal(rand, 0, 0.5), .z = 0 });
+                const center1 = center.add(.{ .x = 0, .y = randomReal(rng, 0, 0.5), .z = 0 });
                 try hittable_objects.append(.{ .movingSphere = .{
                     .center0 = center,
                     .center1 = center1,
@@ -866,8 +213,8 @@ fn generateRandomScene(rand: std.rand.Random, allocator: anytype) !Hittable {
                 } });
             } else if (choose_mat < 0.95) {
                 // metal
-                const albedo = Color.random(rand, 0.5, 1);
-                const fuzz = randomReal(rand, 0, 0.5);
+                const albedo = Color.random(rng, 0.5, 1);
+                const fuzz = randomReal(rng, 0, 0.5);
                 mat_sphere.* = .{ .metal = .{ .albedo = albedo, .fuzz = fuzz } };
                 try hittable_objects.append(makeSphere(center, 0.2, mat_sphere));
             } else {
@@ -886,8 +233,8 @@ pub fn main() !void {
     const allocator = gpa.allocator();
     defer debug.assert(!gpa.deinit());
 
-    var rng = std.rand.DefaultPrng.init(42);
-    var rand = rng.random();
+    var rng_ = std.rand.DefaultPrng.init(42);
+    var rng = rng_.random();
 
     // Image
     const aspect_ratio = 3.0 / 2.0;
@@ -906,13 +253,13 @@ pub fn main() !void {
     var aperture: f64 = 0;
 
     if (scene == 1) {
-        world = try generateRandomScene(rand, allocator);
+        world = try generateRandomScene(rng, allocator);
         lookFrom = .{ .x = 13, .y = 2, .z = 3 };
         lookAt = .{ .x = 0, .y = 0, .z = 0 };
         vFov = 20.0;
         aperture = 0.1;
     } else if (scene == 2) {
-        world = try generateTwoSpheres(rand, allocator);
+        world = try generateTwoSpheres(rng, allocator);
         lookFrom = .{ .x = 13, .y = 2, .z = 3 };
         lookAt = .{ .x = 0, .y = 0, .z = 0 };
         vFov = 20.0;
@@ -947,10 +294,10 @@ pub fn main() !void {
             var s: u32 = 0;
             var pixelColor = rgb(0.0, 0.0, 0.0);
             while (s < samples_per_pixel) : (s += 1) {
-                const u = (@intToFloat(f64, i) + randomReal01(rand)) / (image_width - 1);
-                const v = (@intToFloat(f64, j) + randomReal01(rand)) / (image_height - 1);
-                const r = camera.getRay(rand, u, v);
-                pixelColor = pixelColor.add(rayColor(r, world, rand, max_depth));
+                const u = (@intToFloat(f64, i) + randomReal01(rng)) / (image_width - 1);
+                const v = (@intToFloat(f64, j) + randomReal01(rng)) / (image_height - 1);
+                const r = camera.getRay(rng, u, v);
+                pixelColor = pixelColor.add(rayColor(r, world, rng, max_depth));
             }
             try writeColor(stdout, pixelColor, samples_per_pixel);
         }
diff --git a/src/rtw.zig b/src/rtw.zig
new file mode 100644
index 0000000..7dd9c93
--- /dev/null
+++ b/src/rtw.zig
@@ -0,0 +1,8 @@
+pub const aabb = @import("rtw/aabb.zig");
+pub const hit_record = @import("rtw/hit_record.zig");
+pub const hittable = @import("rtw/hittable.zig");
+pub const material = @import("rtw/material.zig");
+pub const rand = @import("rtw/rand.zig");
+pub const ray = @import("rtw/ray.zig");
+pub const texture = @import("rtw/texture.zig");
+pub const vec = @import("rtw/vec.zig");
diff --git a/src/rtw/aabb.zig b/src/rtw/aabb.zig
new file mode 100644
index 0000000..2c283c0
--- /dev/null
+++ b/src/rtw/aabb.zig
@@ -0,0 +1,61 @@
+const Ray = @import("ray.zig").Ray;
+const Point3 = @import("vec.zig").Point3;
+
+pub const Aabb = struct {
+    min: Point3,
+    max: Point3,
+
+    pub fn hit(aabb: Aabb, r: Ray, tMin: f64, tMax: f64) bool {
+        var tMin_ = tMin;
+        var tMax_ = tMax;
+        {
+            const s0 = (aabb.min.x - r.origin.x) / r.dir.x;
+            const s1 = (aabb.max.x - r.origin.x) / r.dir.x;
+            const t0 = @min(s0, s1);
+            const t1 = @max(s0, s1);
+            tMin_ = @max(t0, tMin_);
+            tMax_ = @min(t1, tMax_);
+            if (tMax_ <= tMin_) {
+                return false;
+            }
+        }
+        {
+            const s0 = (aabb.min.y - r.origin.y) / r.dir.y;
+            const s1 = (aabb.max.y - r.origin.y) / r.dir.y;
+            const t0 = @min(s0, s1);
+            const t1 = @max(s0, s1);
+            tMin_ = @max(t0, tMin_);
+            tMax_ = @min(t1, tMax_);
+            if (tMax_ <= tMin_) {
+                return false;
+            }
+        }
+        {
+            const s0 = (aabb.min.z - r.origin.z) / r.dir.z;
+            const s1 = (aabb.max.z - r.origin.z) / r.dir.z;
+            const t0 = @min(s0, s1);
+            const t1 = @max(s0, s1);
+            tMin_ = @max(t0, tMin_);
+            tMax_ = @min(t1, tMax_);
+            if (tMax_ <= tMin_) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    pub fn surroundingBox(box0: Aabb, box1: Aabb) Aabb {
+        return .{
+            .min = .{
+                .x = @min(box0.min.x, box1.min.x),
+                .y = @min(box0.min.y, box1.min.y),
+                .z = @min(box0.min.z, box1.min.z),
+            },
+            .max = .{
+                .x = @max(box0.max.x, box1.max.x),
+                .y = @max(box0.max.y, box1.max.y),
+                .z = @max(box0.max.z, box1.max.z),
+            },
+        };
+    }
+};
diff --git a/src/rtw/hit_record.zig b/src/rtw/hit_record.zig
new file mode 100644
index 0000000..f71904e
--- /dev/null
+++ b/src/rtw/hit_record.zig
@@ -0,0 +1,21 @@
+const vec = @import("vec.zig");
+const material = @import("material.zig");
+const Vec3 = vec.Vec3;
+const Point3 = vec.Point3;
+const Material = material.Material;
+
+pub const HitRecord = struct {
+    // The point where the ray and the hittable hits.
+    p: Point3,
+    // The normal of the hittable at p.
+    normal: Vec3,
+    // The material at p.
+    material: *const Material,
+    // p = ray.at(t)
+    t: f64,
+    // The coordinate of the surface where the ray intersects.
+    u: f64,
+    v: f64,
+    // True if the ray hits the hittable from the front face, i.e., outside of it.
+    front_face: bool,
+};
diff --git a/src/rtw/hittable.zig b/src/rtw/hittable.zig
new file mode 100644
index 0000000..c84b3d0
--- /dev/null
+++ b/src/rtw/hittable.zig
@@ -0,0 +1,335 @@
+const std = @import("std");
+const debug = std.debug;
+const math = std.math;
+const pi = math.pi;
+const ArrayList = std.ArrayList;
+
+const Ray = @import("ray.zig").Ray;
+const Vec3 = @import("vec.zig").Vec3;
+const Point3 = @import("vec.zig").Point3;
+const Color = @import("vec.zig").Color;
+const rgb = @import("vec.zig").rgb;
+const randomPointInUnitSphere = @import("rand.zig").randomPointInUnitSphere;
+const randomPointInUnitDisk = @import("rand.zig").randomPointInUnitDisk;
+const randomUnitVector = @import("rand.zig").randomUnitVector;
+const randomInt = @import("rand.zig").randomInt;
+const randomReal01 = @import("rand.zig").randomReal01;
+const randomReal = @import("rand.zig").randomReal;
+
+const Material = @import("material.zig").Material;
+const DiffuseMaterial = @import("material.zig").DiffuseMaterial;
+const MetalMaterial = @import("material.zig").MetalMaterial;
+const DielectricMaterial = @import("material.zig").DielectricMaterial;
+
+const Texture = @import("texture.zig").Texture;
+const SolidTexture = @import("texture.zig").SolidTexture;
+const CheckerTexture = @import("texture.zig").CheckerTexture;
+
+const HitRecord = @import("hit_record.zig").HitRecord;
+const Aabb = @import("aabb.zig").Aabb;
+
+const HittableTag = enum {
+    sphere,
+    movingSphere,
+    list,
+    bvhNode,
+};
+
+pub const Hittable = union(HittableTag) {
+    sphere: Sphere,
+    movingSphere: MovingSphere,
+    list: HittableList,
+    bvhNode: BvhNode,
+
+    pub fn hit(h: Hittable, r: Ray, tMin: f64, tMax: f64, record: *HitRecord) bool {
+        return switch (h) {
+            HittableTag.sphere => |sphere| sphere.hit(r, tMin, tMax, record),
+            HittableTag.movingSphere => |movingSphere| movingSphere.hit(r, tMin, tMax, record),
+            HittableTag.list => |list| list.hit(r, tMin, tMax, record),
+            HittableTag.bvhNode => |node| node.hit(r, tMin, tMax, record),
+        };
+    }
+
+    fn boudingBox(h: Hittable, time0: f64, time1: f64, outputBox: *Aabb) bool {
+        return switch (h.*) {
+            HittableTag.sphere => |sphere| sphere.boudingBox(time0, time1, outputBox),
+            HittableTag.movingSphere => |movingSphere| movingSphere.boudingBox(time0, time1, outputBox),
+            HittableTag.list => |list| list.boudingBox(time0, time1, outputBox),
+            HittableTag.bvhNode => |node| node.boudingBox(time0, time1, outputBox),
+        };
+    }
+
+    pub fn deinit(h: *const Hittable, allocator: anytype) void {
+        return switch (h.*) {
+            HittableTag.sphere => |sphere| sphere.deinit(allocator),
+            HittableTag.movingSphere => |movingSphere| movingSphere.deinit(allocator),
+            HittableTag.list => |list| list.deinit(allocator),
+            HittableTag.bvhNode => |node| node.deinit(allocator),
+        };
+    }
+};
+
+const Sphere = struct {
+    center: Point3,
+    radius: f64,
+    material: *const Material,
+
+    fn hit(sphere: Sphere, r: Ray, tMin: f64, tMax: f64, record: *HitRecord) bool {
+        const oc = r.origin.sub(sphere.center);
+        const a = r.dir.normSquared();
+        const half_b = Vec3.dot(oc, r.dir);
+        const c = oc.normSquared() - sphere.radius * sphere.radius;
+
+        const discriminant = half_b * half_b - a * c;
+        if (discriminant < 0.0) {
+            // r does not intersect the sphere.
+            return false;
+        }
+        const sqrtd = @sqrt(discriminant);
+
+        // Find the nearest root that lies in the acceptable range.
+        var root = (-half_b - sqrtd) / a;
+        if (root < tMin or tMax < root) {
+            root = (-half_b + sqrtd) / a;
+            if (root < tMin or tMax < root) {
+                // out of range
+                return false;
+            }
+        }
+
+        record.t = root;
+        record.p = r.at(root);
+        const outward_normal = (record.p.sub(sphere.center)).div(sphere.radius);
+        record.front_face = Vec3.dot(outward_normal, r.dir) < 0.0;
+        if (record.front_face) {
+            record.normal = outward_normal;
+        } else {
+            record.normal = outward_normal.mul(-1.0);
+        }
+        Sphere.getSphereUv(outward_normal, &record.u, &record.v);
+        record.material = sphere.material;
+
+        return true;
+    }
+
+    fn boudingBox(sphere: Sphere, time0: f64, time1: f64, outputBox: *Aabb) bool {
+        _ = time0;
+        _ = time1;
+        const o = sphere.center;
+        const r = sphere.radius;
+        outputBox.* = .{
+            .min = o.sub(.{ .x = r, .y = r, .z = r }),
+            .max = o.add(.{ .x = r, .y = r, .z = r }),
+        };
+        return true;
+    }
+
+    fn getSphereUv(p: Point3, u: *f64, v: *f64) void {
+        const phi = math.atan2(f64, -p.z, p.x) + pi;
+        const theta = math.acos(-p.y);
+        u.* = phi / (2.0 * pi);
+        v.* = theta / pi;
+    }
+
+    fn deinit(sphere: *const Sphere, allocator: anytype) void {
+        allocator.destroy(sphere.material);
+    }
+};
+
+const MovingSphere = struct {
+    center0: Point3,
+    center1: Point3,
+    time0: f64,
+    time1: f64,
+    radius: f64,
+    material: *const Material,
+
+    fn hit(sphere: MovingSphere, r: Ray, tMin: f64, tMax: f64, record: *HitRecord) bool {
+        const center_ = sphere.center(r.time);
+        const oc = r.origin.sub(center_);
+        const a = r.dir.normSquared();
+        const half_b = Vec3.dot(oc, r.dir);
+        const c = oc.normSquared() - sphere.radius * sphere.radius;
+
+        const discriminant = half_b * half_b - a * c;
+        if (discriminant < 0.0) {
+            // r does not intersect the sphere.
+            return false;
+        }
+        const sqrtd = @sqrt(discriminant);
+
+        // Find the nearest root that lies in the acceptable range.
+        var root = (-half_b - sqrtd) / a;
+        if (root < tMin or tMax < root) {
+            root = (-half_b + sqrtd) / a;
+            if (root < tMin or tMax < root) {
+                // out of range
+                return false;
+            }
+        }
+
+        record.t = root;
+        record.p = r.at(root);
+        const outward_normal = (record.p.sub(center_)).div(sphere.radius);
+        record.front_face = Vec3.dot(outward_normal, r.dir) < 0.0;
+        if (record.front_face) {
+            record.normal = outward_normal;
+        } else {
+            record.normal = outward_normal.mul(-1.0);
+        }
+        record.material = sphere.material;
+
+        return true;
+    }
+
+    fn boudingBox(sphere: MovingSphere, time0: f64, time1: f64, outputBox: *Aabb) bool {
+        const o0 = sphere.center(time0);
+        const o1 = sphere.center(time1);
+        const r = sphere.radius;
+        const box0 = .{
+            .min = o0.sub(.{ .x = r, .y = r, .z = r }),
+            .max = o0.add(.{ .x = r, .y = r, .z = r }),
+        };
+        const box1 = .{
+            .min = o1.sub(.{ .x = r, .y = r, .z = r }),
+            .max = o1.add(.{ .x = r, .y = r, .z = r }),
+        };
+        outputBox.* = Aabb.surroundingBox(box0, box1);
+        return true;
+    }
+
+    fn center(sphere: MovingSphere, t: f64) Point3 {
+        return sphere.center0.add(sphere.center1.sub(sphere.center0).mul((t - sphere.time0) / (sphere.time1 - sphere.time0)));
+    }
+
+    fn deinit(sphere: *const MovingSphere, allocator: anytype) void {
+        allocator.destroy(sphere.material);
+    }
+};
+
+const HittableList = struct {
+    objects: ArrayList(Hittable),
+
+    fn hit(list: HittableList, r: Ray, tMin: f64, tMax: f64, record: *HitRecord) bool {
+        var hit_anything = false;
+        var closest_so_far = tMax;
+
+        for (list.objects.items) |object| {
+            var rec: HitRecord = undefined;
+            if (object.hit(r, tMin, closest_so_far, &rec)) {
+                hit_anything = true;
+                closest_so_far = rec.t;
+                record.* = rec;
+            }
+        }
+        return hit_anything;
+    }
+
+    fn boudingBox(list: HittableList, time0: f64, time1: f64, outputBox: *Aabb) bool {
+        if (list.objects.items.len == 0) {
+            return false;
+        }
+        var firstBox = true;
+        var tmpBox: Aabb = undefined;
+        for (list.objects.items) |object| {
+            if (!object.boudingBox(time0, time1, tmpBox)) {
+                return false;
+            }
+            outputBox = if (firstBox) tmpBox else Aabb.surroundingBox(outputBox, tmpBox);
+            firstBox = false;
+        }
+        return true;
+    }
+
+    fn deinit(list: *const HittableList, allocator: anytype) void {
+        for (list.objects.items) |object| {
+            object.deinit(allocator);
+        }
+        list.objects.deinit();
+    }
+};
+
+const BvhNode = struct {
+    left: *Hittable, // TODO
+    right: *Hittable, // TODO
+    box: Aabb,
+
+    fn init(
+        objects: ArrayList(*Hittable),
+        start: usize,
+        end: usize,
+        time0: f64,
+        time1: f64,
+    ) BvhNode {
+        var left: *Hittable = undefined;
+        var right: *Hittable = undefined;
+
+        var objects_ = objects;
+        const axis = randomInt(u8, 0, 3);
+
+        const objectSpan = end - start;
+        if (objectSpan == 1) {
+            left = objects.items[start];
+            right = objects.items[start];
+        } else if (objectSpan == 2) {
+            if (BvhNode.boxCompare(axis, objects[start], objects[start + 1])) {
+                left = objects[start];
+                right = objects[start + 1];
+            } else {
+                left = objects[start + 1];
+                right = objects[start];
+            }
+        } else {
+            std.sort.sort(*Hittable, objects_, axis, BvhNode.boxCompare);
+            const mid = start + objectSpan / 2;
+            left.* = .{ .bvhNode = BvhNode.init(objects, start, mid, time0, time1) };
+            right.* = .{ .bvhNode = BvhNode.init(objects, mid, end, time0, time1) };
+        }
+
+        var boxLeft: Aabb = undefined;
+        var boxRight: Aabb = undefined;
+
+        if (!left.boudingBox(time0, time1, boxLeft) || !right.boudingBox(time0, time1, boxRight)) {
+            // ERROR
+        }
+
+        return .{
+            .left = left,
+            .right = right,
+            .box = Aabb.surroundingBox(boxLeft, boxRight),
+        };
+    }
+
+    fn boxCompare(axis: u8, a: *const Hittable, b: *const Hittable) bool {
+        if (axis == 0) {
+            return a.x < b.x;
+        } else if (axis == 1) {
+            return a.y < b.y;
+        } else {
+            return a.z < b.z;
+        }
+    }
+
+    fn hit(node: BvhNode, r: Ray, tMin: f64, tMax: f64, record: *HitRecord) bool {
+        if (!node.box.hit(r, tMin, tMax)) {
+            return false;
+        }
+
+        const hitLeft = node.left.hit(r, tMin, tMax, record);
+        const hitRight = node.right.hit(r, tMin, if (hitLeft) record.t else tMax, record);
+
+        return hitLeft or hitRight;
+    }
+
+    fn boudingBox(node: BvhNode, time0: f64, time1: f64, outputBox: *Aabb) bool {
+        _ = time0;
+        _ = time1;
+        outputBox.* = node.box;
+        return true;
+    }
+
+    fn deinit(node: *const BvhNode, allocator: anytype) void {
+        _ = node;
+        _ = allocator;
+    }
+};
diff --git a/src/rtw/material.zig b/src/rtw/material.zig
new file mode 100644
index 0000000..87244ab
--- /dev/null
+++ b/src/rtw/material.zig
@@ -0,0 +1,100 @@
+const std = @import("std");
+const debug = std.debug;
+const math = std.math;
+
+const Ray = @import("ray.zig").Ray;
+const vec = @import("vec.zig");
+const Vec3 = vec.Vec3;
+const rgb = vec.rgb;
+const Color = vec.Color;
+const HitRecord = @import("hit_record.zig").HitRecord;
+const Texture = @import("texture.zig").Texture;
+const rand = @import("rand.zig");
+const Random = rand.Random;
+const randomPointInUnitSphere = rand.randomPointInUnitSphere;
+const randomUnitVector = rand.randomUnitVector;
+const randomReal01 = rand.randomReal01;
+
+const MaterialTag = enum {
+    diffuse,
+    metal,
+    dielectric,
+};
+
+pub const Material = union(MaterialTag) {
+    diffuse: DiffuseMaterial,
+    metal: MetalMaterial,
+    dielectric: DielectricMaterial,
+
+    pub fn scatter(mat: Material, r_in: Ray, record: HitRecord, attenuation: *Color, scattered: *Ray, rng: Random) bool {
+        return switch (mat) {
+            MaterialTag.diffuse => |diffuse_mat| diffuse_mat.scatter(r_in, record, attenuation, scattered, rng),
+            MaterialTag.metal => |metal_mat| metal_mat.scatter(r_in, record, attenuation, scattered, rng),
+            MaterialTag.dielectric => |dielectric_mat| dielectric_mat.scatter(r_in, record, attenuation, scattered, rng),
+        };
+    }
+};
+
+pub const DiffuseMaterial = struct {
+    albedo: Texture,
+
+    fn scatter(mat: DiffuseMaterial, r_in: Ray, record: HitRecord, attenuation: *Color, scattered: *Ray, rng: Random) bool {
+        var scatter_direction = record.normal.add(randomUnitVector(rng));
+        if (scatter_direction.near_zero()) {
+            scatter_direction = record.normal;
+        }
+        scattered.* = .{ .origin = record.p, .dir = scatter_direction, .time = r_in.time };
+        attenuation.* = mat.albedo.value(record.u, record.v, record.p);
+        return true;
+    }
+};
+
+pub const MetalMaterial = struct {
+    albedo: Color,
+    fuzz: f64,
+
+    fn scatter(mat: MetalMaterial, r_in: Ray, record: HitRecord, attenuation: *Color, scattered: *Ray, rng: Random) bool {
+        debug.assert(mat.fuzz <= 1.0);
+        const reflected = reflect(r_in.dir.normalized(), record.normal);
+        scattered.* = .{ .origin = record.p, .dir = reflected.add(randomPointInUnitSphere(rng).mul(mat.fuzz)), .time = r_in.time };
+        attenuation.* = mat.albedo;
+        return reflected.dot(record.normal) > 0.0;
+    }
+};
+
+pub const DielectricMaterial = struct {
+    // index of refraction.
+    ir: f64,
+
+    fn scatter(mat: DielectricMaterial, r_in: Ray, record: HitRecord, attenuation: *Color, scattered: *Ray, rng: Random) bool {
+        const refraction_ratio = if (record.front_face) 1.0 / mat.ir else mat.ir;
+        const unit_dir = r_in.dir.normalized();
+
+        const cos_theta = @min(unit_dir.mul(-1.0).dot(record.normal), 1.0);
+        const sin_theta = @sqrt(1.0 - cos_theta * cos_theta);
+        // sin(theta') = refraction_ratio * sin(theta) <= 1
+        const can_refract = refraction_ratio * sin_theta <= 1.0;
+
+        const dir = if (can_refract and reflectance(cos_theta, refraction_ratio) < randomReal01(rng)) refract(unit_dir, record.normal, refraction_ratio) else reflect(unit_dir, record.normal);
+        scattered.* = .{ .origin = record.p, .dir = dir, .time = r_in.time };
+        attenuation.* = rgb(1.0, 1.0, 1.0);
+        return true;
+    }
+
+    fn reflectance(cos: f64, refraction_idx: f64) f64 {
+        const r0 = (1.0 - refraction_idx) / (1.0 + refraction_idx);
+        const r1 = r0 * r0;
+        return r1 + (1.0 - r1) * math.pow(f64, 1.0 - cos, 5.0);
+    }
+};
+
+fn reflect(v: Vec3, n: Vec3) Vec3 {
+    return v.sub(n.mul(2 * v.dot(n)));
+}
+
+fn refract(uv: Vec3, n: Vec3, etai_over_etat: f64) Vec3 {
+    const cos_theta = @min(uv.mul(-1.0).dot(n), 1.0);
+    const r_out_perpendicular = uv.add(n.mul(cos_theta)).mul(etai_over_etat);
+    const r_out_parallel = n.mul(-@sqrt(@fabs(1.0 - r_out_perpendicular.normSquared())));
+    return r_out_perpendicular.add(r_out_parallel);
+}
diff --git a/src/rtw/rand.zig b/src/rtw/rand.zig
new file mode 100644
index 0000000..47a8d61
--- /dev/null
+++ b/src/rtw/rand.zig
@@ -0,0 +1,40 @@
+const std = @import("std");
+
+const Vec3 = @import("vec.zig").Vec3;
+
+pub const Random = std.rand.Random;
+
+// [min, max)
+pub fn randomInt(comptime T: type, rand: Random, min: T, max: T) T {
+    return rand.intRangeLessThan(T, min, max);
+}
+
+// [0, 1)
+pub fn randomReal01(rand: Random) f64 {
+    return rand.float(f64);
+}
+
+// [min, max)
+pub fn randomReal(rand: Random, min: f64, max: f64) f64 {
+    return min + randomReal01(rand) * (max - min);
+}
+
+pub fn randomPointInUnitSphere(rand: Random) Vec3 {
+    while (true) {
+        const p = Vec3.random(rand, -1.0, 1.0);
+        if (p.norm() >= 1) continue;
+        return p;
+    }
+}
+
+pub fn randomPointInUnitDisk(rand: Random) Vec3 {
+    while (true) {
+        const p = Vec3{ .x = randomReal(rand, -1.0, 1.0), .y = randomReal(rand, -1.0, 1.0), .z = 0.0 };
+        if (p.norm() >= 1) continue;
+        return p;
+    }
+}
+
+pub fn randomUnitVector(rand: Random) Vec3 {
+    return randomPointInUnitSphere(rand).normalized();
+}
diff --git a/src/rtw/ray.zig b/src/rtw/ray.zig
new file mode 100644
index 0000000..e46198f
--- /dev/null
+++ b/src/rtw/ray.zig
@@ -0,0 +1,13 @@
+const vec = @import("vec.zig");
+const Vec3 = vec.Vec3;
+const Point3 = vec.Point3;
+
+pub const Ray = struct {
+    origin: Vec3,
+    dir: Vec3,
+    time: f64,
+
+    pub fn at(r: Ray, t: f64) Point3 {
+        return r.origin.add(r.dir.mul(t));
+    }
+};
diff --git a/src/rtw/texture.zig b/src/rtw/texture.zig
new file mode 100644
index 0000000..47c3c4f
--- /dev/null
+++ b/src/rtw/texture.zig
@@ -0,0 +1,72 @@
+const std = @import("std");
+
+const Color = @import("vec.zig").Color;
+const Vec3 = @import("vec.zig").Vec3;
+
+const TextureTag = enum {
+    solid,
+    checker,
+};
+
+pub const Texture = union(TextureTag) {
+    solid: SolidTexture,
+    checker: CheckerTexture,
+
+    pub fn makeSolid(color: Color) Texture {
+        return .{ .solid = .{ .color = color } };
+    }
+
+    pub fn makeChecker(allocator: std.mem.Allocator, odd: Color, even: Color) !Texture {
+        return .{ .checker = try CheckerTexture.init(
+            allocator,
+            Texture.makeSolid(odd),
+            Texture.makeSolid(even),
+        ) };
+    }
+
+    pub fn value(tx: Texture, u: f64, v: f64, p: Vec3) Color {
+        return switch (tx) {
+            TextureTag.solid => |solidTx| solidTx.value(u, v, p),
+            TextureTag.checker => |checkerTx| checkerTx.value(u, v, p),
+        };
+    }
+};
+
+pub const SolidTexture = struct {
+    color: Color,
+
+    fn value(tx: SolidTexture, u: f64, v: f64, p: Vec3) Color {
+        _ = u;
+        _ = v;
+        _ = p;
+        return tx.color;
+    }
+};
+
+pub const CheckerTexture = struct {
+    allocator: std.mem.Allocator,
+    odd: *const Texture,
+    even: *const Texture,
+
+    fn init(allocator: std.mem.Allocator, odd: Texture, even: Texture) !CheckerTexture {
+        var odd_ = try allocator.create(Texture);
+        var even_ = try allocator.create(Texture);
+        odd_.* = odd;
+        even_.* = even;
+        return .{
+            .allocator = allocator,
+            .odd = odd_,
+            .even = even_,
+        };
+    }
+
+    fn deinit(tx: CheckerTexture) void {
+        tx.allocator.destroy(tx.even);
+        tx.allocator.destroy(tx.odd);
+    }
+
+    fn value(tx: CheckerTexture, u: f64, v: f64, p: Vec3) Color {
+        const sines = @sin(10 * p.x) * @sin(10 * p.y) * @sin(10 * p.z);
+        return if (sines < 0) tx.odd.value(u, v, p) else tx.even.value(u, v, p);
+    }
+};
diff --git a/src/rtw/vec.zig b/src/rtw/vec.zig
new file mode 100644
index 0000000..9b2c3f0
--- /dev/null
+++ b/src/rtw/vec.zig
@@ -0,0 +1,109 @@
+const std = @import("std");
+
+const rand = @import("rand.zig");
+const Random = rand.Random;
+const randomReal = rand.randomReal;
+const randomReal01 = rand.randomReal01;
+
+pub const Vec3 = struct {
+    x: f64,
+    y: f64,
+    z: f64,
+
+    pub fn norm(v: Vec3) f64 {
+        return @sqrt(v.normSquared());
+    }
+
+    pub fn normSquared(v: Vec3) f64 {
+        return v.x * v.x + v.y * v.y + v.z * v.z;
+    }
+
+    pub fn dot(u: Vec3, v: Vec3) f64 {
+        return u.x * v.x + u.y * v.y + u.z * v.z;
+    }
+
+    pub fn cross(u: Vec3, v: Vec3) Vec3 {
+        return Vec3{
+            .x = u.y * v.z - u.z * v.y,
+            .y = u.z * v.x - u.x * v.z,
+            .z = u.x * v.y - u.y * v.x,
+        };
+    }
+
+    pub fn normalized(v: Vec3) Vec3 {
+        const n = v.norm();
+        if (n == 0.0) {
+            return v;
+        } else {
+            return v.div(n);
+        }
+    }
+
+    pub fn add(u: Vec3, v: Vec3) Vec3 {
+        return Vec3{
+            .x = u.x + v.x,
+            .y = u.y + v.y,
+            .z = u.z + v.z,
+        };
+    }
+
+    pub fn sub(u: Vec3, v: Vec3) Vec3 {
+        return Vec3{
+            .x = u.x - v.x,
+            .y = u.y - v.y,
+            .z = u.z - v.z,
+        };
+    }
+
+    pub fn mul(v: Vec3, t: f64) Vec3 {
+        return Vec3{
+            .x = v.x * t,
+            .y = v.y * t,
+            .z = v.z * t,
+        };
+    }
+
+    pub fn mulV(u: Vec3, v: Vec3) Vec3 {
+        return Vec3{
+            .x = u.x * v.x,
+            .y = u.y * v.y,
+            .z = u.z * v.z,
+        };
+    }
+
+    pub fn div(v: Vec3, t: f64) Vec3 {
+        return Vec3{
+            .x = v.x / t,
+            .y = v.y / t,
+            .z = v.z / t,
+        };
+    }
+
+    pub fn random01(rng: Random) Vec3 {
+        return Vec3{
+            .x = randomReal01(rng),
+            .y = randomReal01(rng),
+            .z = randomReal01(rng),
+        };
+    }
+
+    pub fn random(rng: Random, min: f64, max: f64) Vec3 {
+        return Vec3{
+            .x = randomReal(rng, min, max),
+            .y = randomReal(rng, min, max),
+            .z = randomReal(rng, min, max),
+        };
+    }
+
+    pub fn near_zero(v: Vec3) bool {
+        const epsilon = 1e-8;
+        return @fabs(v.x) < epsilon and @fabs(v.y) < epsilon and @fabs(v.z) < epsilon;
+    }
+};
+
+pub const Point3 = Vec3;
+pub const Color = Vec3;
+
+pub fn rgb(r: f64, g: f64, b: f64) Color {
+    return .{ .x = r, .y = g, .z = b };
+}