ft: 11: dielectrics

src/objects/materials/dielectric.rs

@@ -1,7 +1,11 @@
+use core::f32::math::sqrt;
+
+use rand::RngExt;
+
 use crate::{
     objects::{hit::Hit, materials::traits::Material},
     ray::Ray,
-    vec3::Colour,
+    vec3::{Colour, Vec3},
 };
 
 pub struct Dielectric {
@@ -14,6 +18,13 @@ impl Dielectric {
             refraction_index: refraction_index,
         }
     }
+
+    fn _reflectance(cos: f32, refraction_index: f32) -> f32 {
+        // Shlick's approximation
+        let mut r0 = (1. - refraction_index) / (1. + refraction_index);
+        r0 *= r0;
+        return r0 + (1. - r0) * (1. - cos).powf(5.);
+    }
 }
 
 impl Material for Dielectric {
@@ -24,7 +35,21 @@ impl Material for Dielectric {
             self.refraction_index
         };
         let unit = ray.dir().get_unit();
-        let refr = unit.refract(hit.n(), ri);
-        Some((Ray::new(*hit.p(), refr), Colour::new(1., 1., 1.)))
+        let cos_theta = if -unit.dot(hit.n()) > 1. {
+            1.
+        } else {
+            -unit.dot(hit.n())
+        };
+        let sin_theta = sqrt(1. - cos_theta * cos_theta);
+        let cannot_refract = ri * sin_theta > 1.;
+
+        let dir: Vec3;
+        let mut rng = rand::rng();
+        if cannot_refract || Dielectric::_reflectance(cos_theta, ri) > rng.random::<f32>() {
+            dir = unit.reflect(hit.n());
+        } else {
+            dir = unit.refract(hit.n(), ri);
+        }
+        Some((Ray::new(*hit.p(), dir), Colour::new(1., 1., 1.)))
     }
 }