Add random_distributions::SkyLightPdf

src/random_distributions/mod.rs

@@ -13,6 +13,9 @@ pub use cosine_weighted_hemisphere::CosineWeightedHemisphere;
 mod linear_weighted;
 pub use linear_weighted::LinearWeighted;
 
+mod sky_light_pdf;
+pub use sky_light_pdf::SkyLightPdf;
+
 pub trait RandomDistribution<T> {
     fn value(&self) -> T;
     fn pdf(&self, value: T) -> f64;

src/random_distributions/sky_light_pdf.rs

@@ -0,0 +1,65 @@
+use std::f64::consts::PI;
+
+use rand::distributions::Open01;
+use rand::{thread_rng, Rng};
+
+use crate::math::Vec3;
+
+use super::{LinearWeighted, RandomDistribution};
+
+pub struct SkyLightPdf {
+    z_distribution: LinearWeighted,
+}
+
+impl SkyLightPdf {
+    pub fn new() -> SkyLightPdf {
+        let z_distribution = LinearWeighted::new(1.0);
+        SkyLightPdf { z_distribution }
+    }
+}
+
+impl RandomDistribution<Vec3> for SkyLightPdf {
+    fn value(&self) -> Vec3 {
+        let mut rng = thread_rng();
+        let phi = rng.sample::<f64, _>(Open01) * 2.0 * PI;
+        let z = self.z_distribution.value();
+        let r = (1.0 - z * z).sqrt();
+        Vec3::new(r * phi.cos(), r * phi.sin(), z)
+    }
+
+    fn pdf(&self, value: Vec3) -> f64 {
+        let z = value.z();
+        if z < 0.0 {
+            0.0
+        } else {
+            z / PI
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[ignore]
+    fn print_values() {
+        let target = SkyLightPdf::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 = SkyLightPdf::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);
+    }
+}