From 86ad7845219e8db06fe47b62794180e1b40f90a5 Mon Sep 17 00:00:00 2001
From: Julian T <julian@jtle.dk>
Date: Sun, 1 Aug 2021 22:55:07 +0200
Subject: Implement dielectric material from RTIAW

---
 src/material/dielectric.rs | 59 ++++++++++++++++++++++++++++++++++++++++++++++
 src/material/mod.rs        |  4 ++--
 2 files changed, 61 insertions(+), 2 deletions(-)
 create mode 100644 src/material/dielectric.rs

(limited to 'src/material')

diff --git a/src/material/dielectric.rs b/src/material/dielectric.rs
new file mode 100644
index 0000000..41f47c2
--- /dev/null
+++ b/src/material/dielectric.rs
@@ -0,0 +1,59 @@
+use super::Material;
+use crate::core::{min, Vector3f, Spectrum, Ray};
+use crate::world::Intersection;
+use crate::sample::Sampler;
+use crate::Float;
+
+pub struct Dielectric {
+    ratio: Float,
+}
+
+fn reflect(v: Vector3f, n: Vector3f) -> Vector3f {
+    v - n * (2.0 * v.dot(&n))
+}
+
+// Implementation from RTIOW
+fn refract(v: Vector3f, n: Vector3f, r_ratio: Float, cos_theta: Float) -> Vector3f {
+    let r_perp = (v + n * cos_theta) * r_ratio;
+    let r_parallel = n * (-(1.0 - r_perp.len_squared()).abs().sqrt());
+
+    r_perp + r_parallel
+}
+
+// Schlick Approximation, explained in RTIOW
+fn fresnel(cos: Float, ratio: Float) -> Float {
+    let mut r0 = (1.0-ratio) / (1.0+ratio);
+    r0 = r0 * r0;
+
+    r0 + (1.0-r0)*(1.0-cos).powi(5)
+}
+
+impl Dielectric {
+    pub fn new(ratio: Float) -> Self {
+        Self { ratio }
+    }
+}
+
+impl Material for Dielectric {
+    // Implementation from RTIOW
+    fn scatter(&self, ray: &Ray, i: &Intersection, sampler: &mut dyn Sampler) -> Option<(Spectrum, Ray)> {
+        let ratio = if i.front {1.0/self.ratio} else {self.ratio};
+
+        let ray_dir = ray.direction.norm();
+        let cos_theta = min((-ray_dir).dot(&i.n), 1.0);
+        let sin_theta = (1.0 - cos_theta*cos_theta).sqrt();
+
+        // Test if it is possible for the ray the retract or if it must reflect.
+        let cannot_refract = (ratio * sin_theta) > 1.0;
+        let direction = if cannot_refract || (fresnel(cos_theta, ratio) > sampler.get_sample()) {
+            reflect(ray_dir, i.n)
+        } else {
+            refract(ray_dir, i.n, ratio, cos_theta)
+        };
+
+        Some((
+                Spectrum::WHITE,
+                Ray::new(i.p, direction),
+             ))
+    }
+}
diff --git a/src/material/mod.rs b/src/material/mod.rs
index 3e92bf6..d3c3154 100644
--- a/src/material/mod.rs
+++ b/src/material/mod.rs
@@ -3,14 +3,14 @@ use crate::world::Intersection;
 use crate::sample::Sampler;
 
 mod lambertian;
-mod reflectant;
 mod diffuse_light;
 mod sky_light;
+mod dielectric;
 
 pub use lambertian::Lambertian;
-pub use reflectant::Reflectant;
 pub use diffuse_light::DiffuseLight;
 pub use sky_light::SkyLight;
+pub use dielectric::Dielectric;
 
 pub trait Material: Sync + Send {
     fn scatter(&self, _: &Ray, _: &Intersection, _: &mut dyn Sampler) -> Option<(Spectrum, Ray)> {
-- 
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