Add random_distributions::CosineWeightedHemisphere

src/random_distributions/cosine_weighted_hemisphere.rs

@@ -0,0 +1,59 @@
+use std::f64::consts::PI;
+
+use crate::math::Vec3;
+
+use super::{RandomDistribution, UnitDisc};
+
+pub struct CosineWeightedHemisphere {
+    unit_disc: UnitDisc,
+}
+
+impl CosineWeightedHemisphere {
+    pub fn new() -> CosineWeightedHemisphere {
+        let unit_disc = UnitDisc::new();
+        CosineWeightedHemisphere { unit_disc }
+    }
+}
+
+impl RandomDistribution<Vec3> for CosineWeightedHemisphere {
+    fn value(&self) -> Vec3 {
+        let point_on_disc = self.unit_disc.value();
+        let z = 0.0f64
+            .max(
+                1.0 - point_on_disc.x() * point_on_disc.x() - point_on_disc.y() * point_on_disc.y(),
+            )
+            .sqrt();
+        Vec3::new(point_on_disc.x(), point_on_disc.y(), z)
+    }
+
+    fn pdf(&self, v: Vec3) -> f64 {
+        (v.x() * v.x() + v.y() * v.y()).sqrt() / PI
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[ignore]
+    fn print_values() {
+        let target = CosineWeightedHemisphere::new();
+        for _ in 0..1000 {
+            let value = target.value();
+            println!("{}, {}, {}", value.x(), value.y(), value.z());
+        }
+    }
+
+    #[test]
+    #[ignore]
+    fn integral_is_near_area() {
+        let target = CosineWeightedHemisphere::new();
+        let integral = (0..100000)
+            .map(|_| target.value())
+            .map(|value| 1.0 / target.pdf(value))
+            .sum::<f64>()
+            / 100000.0;
+        println!("Area: {}\nIntegral: {}", 2.0 * PI, integral);
+    }
+}

src/random_distributions/mod.rs

@@ -7,6 +7,9 @@ pub use unit_disc::UnitDisc;
 mod uniform_hemisphere;
 pub use uniform_hemisphere::UniformHemisphere;
 
+mod cosine_weighted_hemisphere;
+pub use cosine_weighted_hemisphere::CosineWeightedHemisphere;
+
 pub trait RandomDistribution<T> {
     fn value(&self) -> T;
     fn pdf(&self, value: T) -> f64;