Add initial MIS implementation

ray-tracing-core/src/material.rs

@@ -10,6 +10,11 @@ pub trait Material<R: Rng>: Send + Sync + Debug {
 
         SampleResult::new(w_out, self.eval(w_in, w_out, rng) * FloatConsts::PI)
     }
+
+    fn pdf(&self, w_in: Dir3, w_out: Dir3) -> Float {
+        let _ = w_out;
+        FloatConsts::FRAC_1_PI * Float::max(w_in.y(), 0.0)
+    }
 }
 
 #[derive(Debug)]

ray-tracing-core/src/scene.rs

@@ -18,6 +18,7 @@ pub struct Intersection<'sc, R: Rng> {
     normal: Dir3,
     material: Option<&'sc dyn Material<R>>,
     light: Option<&'sc dyn Light<R>>,
+    light_pdf: Float,
 }
 
 impl<'sc, R: Rng> Intersection<'sc, R> {
@@ -26,12 +27,14 @@ impl<'sc, R: Rng> Intersection<'sc, R> {
         normal: Dir3,
         material: Option<&'sc dyn Material<R>>,
         light: Option<&'sc dyn Light<R>>,
+        light_pdf: Float,
     ) -> Self {
         Self {
             t,
             normal,
             material,
             light,
+            light_pdf,
         }
     }
 
@@ -51,6 +54,10 @@ impl<'sc, R: Rng> Intersection<'sc, R> {
         self.light
     }
 
+    pub fn light_pdf(&self) -> Float {
+        self.light_pdf
+    }
+
     pub fn tangent_frame(&self) -> Frame {
         Frame::from_normal(self.normal)
     }