1 files changed, 65 insertions, 0 deletions
diff --git a/src/hittable/constant_medium.rs b/src/hittable/constant_medium.rs
new file mode 100644
index 0000000..b047c23
--- /dev/null
+++ b/src/hittable/constant_medium.rs
@@ -0,0 +1,65 @@
+use std::sync::Arc;
+
+use crate::{hittable::{HitRecord, Hittable, AABB}, material::{Isotropic, Material}, ray::Ray, texture::Texture, vec3::Vec3};
+
+pub struct ConstantMedium {
+    boundary: Arc<dyn Hittable>,
+    phase_function: Arc<dyn Material>,
+    neg_inv_density: f64,
+}
+
+impl ConstantMedium {
+    pub fn new(boundary: Arc<dyn Hittable>, density: f64, texture: Arc<dyn Texture>) -> Self {
+        Self {
+            boundary,
+            phase_function: Arc::new(Isotropic::from_texture(texture)),
+            neg_inv_density: -1.0/density,
+        }
+    }
+}
+
+impl Hittable for ConstantMedium {
+    // TODO: this only support convex shapes.
+    fn hit(&self, ray: &Ray, t_min: f64, t_max: f64) -> Option<HitRecord> {
+        let mut record_1 = self.boundary.hit(ray, -f64::INFINITY, f64::INFINITY)?;
+        let mut record_2 = self.boundary.hit(ray, record_1.t + 0.0001, f64::INFINITY)?;
+
+        if record_1.t < t_min {
+            record_1.t = t_min;
+        }
+        if record_2.t > t_max {
+            record_2.t = t_max;
+        }
+
+        if record_1.t >= record_2.t {
+            return None;
+        }
+
+        if record_1.t < 0.0 {
+            record_1.t = 0.0;
+        }
+
+        let ray_length = ray.direction.length();
+        let distance_inside_boundary = (record_2.t - record_1.t) * ray_length;
+        let hit_distance = self.neg_inv_density * rand::random::<f64>().ln();
+
+        if hit_distance > distance_inside_boundary {
+            return None;
+        }
+
+        let t = record_1.t + hit_distance / ray_length;
+        Some(HitRecord {
+            p: ray.at(t),
+            t,
+            material: Some(self.phase_function.clone()),
+            normal: Vec3 { x: 1.0, y: 0.0, z: 0.0 }, // arbitrary
+            front_face: true, // arbitrary
+            u: 0.0, // arbitrary
+            v: 0.0, // arbitrary
+        })
+    }
+
+    fn bounding_box(&self, time_start: f64, time_end: f64) -> Option<AABB> {
+        self.boundary.bounding_box(time_start, time_end)
+    }
+}