add Dielectric material

in_one_weekend/src/main.zig

@@ -97,14 +97,13 @@ pub fn main() anyerror!void {
         }
     };
     const materialCenter = Material{
-        .lambertian = material.Lambertian{
-            .color = Color{ .x = 0.7, .y = 0.3, .z = 0.3 },
+        .dielectric = material.Dielectric{
+            .refraction_index = 1.5,
         }
     };
     const materialLeft = Material{
-        .metal = material.Metal{
-            .color = Color{ .x = 0.8, .y = 0.8, .z = 0.8 },
-            .fuzz = 0.3,
+        .dielectric = material.Dielectric{
+            .refraction_index = 1.5,
         }
     };
     const materialRight = Material{

in_one_weekend/src/material.zig

@@ -9,6 +9,7 @@ const HitRecord = @import("hittable.zig").HitRecord;
 pub const MaterialType = enum {
     lambertian,
     metal,
+    dielectric,
 };
 
 pub const Lambertian = struct {
@@ -44,6 +45,21 @@ pub const Metal = struct {
     }
 };
 
+pub const Dielectric = struct {
+    refraction_index: f32,
+
+    pub fn scatter(self: Dielectric, ray: Ray, hit: HitRecord, rng: *Random) ?ScatteredRay {
+        const attenuation = Color{ .x = 1.0, .y = 1.0, .z = 1.0 };
+        const refraction_ratio = if (hit.front_face) 1.0 / self.refraction_index else self.refraction_index;
+        const unit_direction = ray.direction.unit();
+        const refracted = vec3.refract(unit_direction, hit.normal, refraction_ratio);
+        return ScatteredRay{
+            .ray = Ray{ .origin = hit.p, .direction = refracted},
+            .color = attenuation,
+        };
+    }
+};
+
 pub const ScatteredRay = struct {
     ray: Ray,
     color: Color,
@@ -52,11 +68,13 @@ pub const ScatteredRay = struct {
 pub const Material = union(MaterialType) {
     lambertian: Lambertian,
     metal: Metal,
+    dielectric: Dielectric,
 };
 
 pub fn scatter(ray: Ray, hit: HitRecord, rng: *Random) ?ScatteredRay {
     return switch (hit.material) {
         .lambertian => |m| m.scatter(ray, hit, rng),
         .metal => |m| m.scatter(ray, hit, rng),
+        .dielectric => |m| m.scatter(ray, hit, rng),
     };
 }

in_one_weekend/src/vec3.zig

@@ -116,6 +116,13 @@ pub fn random_in_hemisphere(rng: *Random, normal: Vec3) Vec3 {
     }
 }
 
+pub fn refract(uv: Vec3, n: Vec3, etai_over_etat: f32) Vec3 {
+    const cos_theta = math.min(uv.mul_s(-1).dot(n), 1.0);
+    const r_out_perp =  uv.add(n.mul_s(cos_theta)).mul_s(etai_over_etat);
+    const r_out_parallel = n.mul_s(-math.sqrt(math.absFloat(1.0 - r_out_perp.length_squared())));
+    return r_out_perp.add(r_out_parallel);
+}
+
 pub const Point3 = Vec3;
 
 const assert = @import("std").debug.assert;