From 8adf1bcadbb3ecd63836f502c8240e94bdbc1b86 Mon Sep 17 00:00:00 2001
From: Bill Thiede <git@xinu.tv>
Date: Sat, 13 Aug 2022 17:17:35 -0700
Subject: [PATCH] zigrtiow: some refraction with dielectric.

---
 zigrtiow/src/main.zig     |  6 ++----
 zigrtiow/src/material.zig | 31 ++++++++++++++++++++-----------
 2 files changed, 22 insertions(+), 15 deletions(-)

diff --git a/zigrtiow/src/main.zig b/zigrtiow/src/main.zig
index 3d19d96..6a294e5 100644
--- a/zigrtiow/src/main.zig
+++ b/zigrtiow/src/main.zig
@@ -90,11 +90,9 @@ pub fn main() anyerror!void {
     var tmp_world = HittableList.init();
 
     const material_ground = Material{ .labertian = Labertian{ .albedo = Color.init(0.8, 0.8, 0.0) } };
-    const material_center = Material{ .dielectric = Dielectric{ .ir = 1.5 } };
-    //const material_center = Material{ .labertian = Labertian{ .albedo = Color.init(0.7, 0.3, 0.3) } };
+    const material_center = Material{ .labertian = Labertian{ .albedo = Color.init(0.1, 0.2, 0.5) } };
     const material_left = Material{ .dielectric = Dielectric{ .ir = 1.5 } };
-    //const material_left = Material{ .metal = Metal{ .albedo = Color.init(0.8, 0.8, 0.8), .fuzz = 0.3 } };
-    const material_right = Material{ .metal = Metal{ .albedo = Color.init(0.8, 0.6, 0.2), .fuzz = 1.0 } };
+    const material_right = Material{ .metal = Metal{ .albedo = Color.init(0.8, 0.6, 0.2), .fuzz = 0.0 } };
 
     try tmp_world.add(Hittable{ .sphere = Sphere.init(Point3.init(0, -100.5, -1), 100, material_ground) });
     try tmp_world.add(Hittable{ .sphere = Sphere.init(Point3.init(0, 0, -1), 0.5, material_center) });
diff --git a/zigrtiow/src/material.zig b/zigrtiow/src/material.zig
index 98798a2..fa2360d 100644
--- a/zigrtiow/src/material.zig
+++ b/zigrtiow/src/material.zig
@@ -23,6 +23,17 @@ pub const Material = union(enum) {
 
 pub const ScatterRec = struct { attenuation: Color, scattered: Ray };
 
+fn reflect(v: Vec3, n: Vec3) Vec3 {
+    return v.sub(n.scale(v.dot(n) * 2));
+}
+fn refract(uv: Vec3, n: Vec3, etai_over_etat: f32) Vec3 {
+    const cos_theta = @minimum(-uv.dot(n), 1.0);
+
+    const r_out_perp = uv.add(n.scale(cos_theta)).scale(etai_over_etat);
+    const r_out_parallel = n.scale(-@sqrt(@fabs(1.0 - r_out_perp.length_squared())));
+    return r_out_perp.add(r_out_parallel);
+}
+
 pub const Labertian = struct {
     albedo: Vec3,
     pub fn scatter(labertian: Labertian, r_in: Ray, rec: HitRecord) ?ScatterRec {
@@ -55,9 +66,6 @@ pub const Metal = struct {
             return null;
         }
     }
-    fn reflect(v: Vec3, n: Vec3) Vec3 {
-        return v.sub(n.scale(v.dot(n) * 2));
-    }
 };
 
 pub const Dielectric = struct {
@@ -68,18 +76,19 @@ pub const Dielectric = struct {
         const refraction_ratio = if (rec.front_face) 1.0 / dielectric.ir else dielectric.ir;
 
         const unit_direction = r_in.direction().unit();
-        const refracted = refract(unit_direction, rec.normal, refraction_ratio);
+        const cos_theta = @minimum(-unit_direction.dot(rec.normal), 1.0);
+        const sin_theta = @sqrt(1 - cos_theta * cos_theta);
 
-        const scattered = Ray.init(rec.p, refracted);
+        const cannot_refract = refraction_ratio * sin_theta > 1;
+        const direction = if (cannot_refract)
+            reflect(unit_direction, rec.normal)
+        else
+            refract(unit_direction, rec.normal, refraction_ratio);
+
+        const scattered = Ray.init(rec.p, direction);
         return ScatterRec{
             .attenuation = attenuation,
             .scattered = scattered,
         };
     }
-    fn refract(uv: Vec3, n: Vec3, etai_over_etat: f32) Vec3 {
-        const cos_theta = @minimum(-uv.dot(n), 1.0);
-        const r_out_perp = uv.add(n.scale(cos_theta)).scale(etai_over_etat);
-        const r_out_parallel = n.scale(-@sqrt(@fabs(1.0 - r_out_perp.length_squared())));
-        return r_out_perp.add(r_out_parallel);
-    }
 };