From 4a478b08a09c6b69683651ae2928b350df09fbc2 Mon Sep 17 00:00:00 2001
From: Fabien Freling <public@ffreling.com>
Date: Thu, 27 May 2021 22:03:28 +0200
Subject: [PATCH] add total internal reflection

---
 in_one_weekend/src/main.zig     |  4 ++--
 in_one_weekend/src/material.zig | 11 +++++++++--
 in_one_weekend/src/vec3.zig     | 14 +++++++-------
 3 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/in_one_weekend/src/main.zig b/in_one_weekend/src/main.zig
index ecf6152..c9378b1 100644
--- a/in_one_weekend/src/main.zig
+++ b/in_one_weekend/src/main.zig
@@ -97,8 +97,8 @@ pub fn main() anyerror!void {
         }
     };
     const materialCenter = Material{
-        .dielectric = material.Dielectric{
-            .refraction_index = 1.5,
+        .lambertian = material.Lambertian{
+            .color = Color{ .x = 0.1, .y = 0.2, .z = 0.5 },
         }
     };
     const materialLeft = Material{
diff --git a/in_one_weekend/src/material.zig b/in_one_weekend/src/material.zig
index 0396294..33df95e 100644
--- a/in_one_weekend/src/material.zig
+++ b/in_one_weekend/src/material.zig
@@ -1,5 +1,6 @@
 const std = @import("std");
 const Random = std.rand.Random;
+const math = std.math;
 
 const vec3 = @import("vec3.zig");
 const Color = @import("color.zig").Color;
@@ -52,9 +53,15 @@ pub const Dielectric = struct {
         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);
+
+        const cos_theta = math.min(unit_direction.mul_s(-1).dot(hit.normal), 1.0);
+        const sin_theta = math.sqrt(1.0 - cos_theta * cos_theta);
+
+        const cant_refract = refraction_ratio * sin_theta > 1.0;
+        const direction = if (cant_refract) unit_direction.reflect(hit.normal)
+            else unit_direction.refract(hit.normal, refraction_ratio);
         return ScatteredRay{
-            .ray = Ray{ .origin = hit.p, .direction = refracted},
+            .ray = Ray{ .origin = hit.p, .direction = direction},
             .color = attenuation,
         };
     }
diff --git a/in_one_weekend/src/vec3.zig b/in_one_weekend/src/vec3.zig
index 0ada129..8b45c40 100644
--- a/in_one_weekend/src/vec3.zig
+++ b/in_one_weekend/src/vec3.zig
@@ -82,6 +82,13 @@ pub const Vec3 = packed struct {
     pub fn reflect(self: Vec3, n: Vec3) Vec3 {
         return self.sub(n.mul_s(self.dot(n) * 2));
     }
+
+    pub fn refract(self: Vec3, n: Vec3, etai_over_etat: f32) Vec3 {
+        const cos_theta = math.min(self.mul_s(-1).dot(n), 1.0);
+        const r_out_perp =  self.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 fn random(rng: *Random, min: f32, max: f32) Vec3 {
@@ -116,13 +123,6 @@ 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;