Feat: Rudimentary dielectric... but broken

Dielectric material matching Raytracing in a Weekend book chapter 10.2.
But it isn't right. The spheres turn into solid black holes.

As I'm making the commit, I've found the problem. I'm separating it out
so I can tag it and explore the code a bit more. See the step-by-step
changes, and such.

src/main.rs

@@ -34,8 +34,8 @@ fn main() {
     // world
 
     let mat_ground = Material::Lambertian{ albedo: Vec3::new(0.8, 0.8, 0.0) };
-    let mat_center = Material::Lambertian{ albedo: Vec3::new(0.7, 0.3, 0.3) };
-    let mat_left   = Material::Metal{ albedo: Vec3::new(0.8, 0.8, 0.8), fuzz: 0.3 };
+    let mat_center = Material::Dielectric { index_refraction: 1.5 };
+    let mat_left   = Material::Dielectric { index_refraction: 1.5 };
     let mat_right  = Material::Metal{ albedo: Vec3::new(0.8, 0.6, 0.2), fuzz: 1.0 };
 
     let mut world = HittableList::new();

src/material.rs

@@ -11,8 +11,9 @@ use rand::distributions::Uniform;
 
 #[derive(Copy, Clone, Debug)]
 pub enum Material{
-    Lambertian{ albedo: Vec3 },
-    Metal{ albedo:Vec3, fuzz: f32 },
+    Lambertian { albedo: Vec3 },
+    Metal { albedo:Vec3, fuzz: f32 },
+    Dielectric { index_refraction: f32 },
 }
 
 impl Material {
@@ -58,6 +59,19 @@ impl Material {
                 *attenuation = *albedo;
                 return Vec3::dot(scattered.dir, rec.normal) > 0.0;
             },
+            Material::Dielectric { index_refraction } => {
+                *attenuation = Vec3::ones();
+                let refraction_ratio = if rec.front_face { 1.0 / index_refraction } else { *index_refraction };
+                
+                let unit_direction = Vec3::as_unit(&ray_in.dir);
+                let refracted = Vec3::refract(unit_direction, rec.normal, refraction_ratio);
+
+                *scattered = Ray {
+                    orig: rec.p,
+                    dir: refracted
+                };
+                return true;
+            },
         }
     }
 }

src/vec3.rs

@@ -87,23 +87,30 @@ impl Vec3{
         let g = (self.y * scale).sqrt();
         let b = (self.z * scale).sqrt();
         
-        let ir = (Vec3::clamp(r, 0.0, 0.999) * 256.0) as i32;
-        let ig = (Vec3::clamp(g, 0.0, 0.999) * 256.0) as i32;
-        let ib = (Vec3::clamp(b, 0.0, 0.999) * 256.0) as i32;
+        let ir = (clamp(r, 0.0, 0.999) * 256.0) as i32;
+        let ig = (clamp(g, 0.0, 0.999) * 256.0) as i32;
+        let ib = (clamp(b, 0.0, 0.999) * 256.0) as i32;
         format!("{} {} {}", ir, ig, ib)
     }
     
     pub fn near_zero(&self) -> bool {
-        let epsilon: f32 = 1e-8;
+        let epsilon: f32 = 1e-4;
         return 
-            self.x < epsilon &&
-            self.y < epsilon &&
-            self.z < epsilon
+            self.x.abs() < epsilon &&
+            self.y.abs() < epsilon &&
+            self.z.abs() < epsilon
     }
     
     pub fn reflect(v: Vec3, n: Vec3) -> Vec3 {
         return v - n * Vec3::dot(v, n) * 2.0;
     }
+    
+    pub fn refract(uv: Vec3, n: Vec3, etai_over_etat: f32) -> Vec3 {
+        let cos_theta = min(Vec3::dot(-uv, n), 1.0);
+        let r_out_perp = (uv + n * cos_theta) * etai_over_etat;
+        let r_out_parallel = n * -(1.0 - r_out_perp.length_squared()).abs().sqrt();
+        r_out_perp + r_out_parallel
+    }
 
     pub fn dot(left: Vec3, right: Vec3) -> f32{
         left.x * right.x +
@@ -124,15 +131,6 @@ impl Vec3{
         *v / len
     }
 
-    fn clamp(input: f32, min: f32, max: f32) -> f32 {
-        if input < min {
-            return min;
-        } else if input > max {
-            return max;
-        } else {
-            return input;
-        }
-    }
 }
 impl Add for Vec3 {
 	type Output = Vec3;
@@ -281,6 +279,17 @@ impl Display for Vec3 {
 	}
 }
 
+pub fn clamp(input: f32, lower: f32, upper: f32) -> f32 {
+    min(max(input, lower), upper)
+}
+
+pub fn min(a: f32, b: f32) -> f32 {
+    if a < b { a } else { b }
+}
+
+pub fn max(a: f32, b: f32) -> f32 {
+    if a > b { a } else { b }
+}
 
 #[cfg(test)]
 mod test{