diff --git a/ray-tracing-core/src/material.rs b/ray-tracing-core/src/material.rs
index e836016..79eae71 100644
--- a/ray-tracing-core/src/material.rs
+++ b/ray-tracing-core/src/material.rs
@@ -1,16 +1,26 @@
 use crate::prelude::*;
 use std::fmt::Debug;
 
+/// Trait to model bxdf.
+///
+/// f(w_in, w_out) * cos(theta) / pdf
+///
 /// All calculations for the material are done a tangent space of the intersection.
+///
+/// The sample function has to sample according to the pdf function
 pub trait Material<R: Rng>: Send + Sync + Debug {
+    /// evaluate f(w_in, w_out)
     fn eval(&self, w_in: Dir3, w_out: Dir3, rng: &mut R) -> Color;
 
+    /// sample w_out and return:
+    /// f(w_in, w_out) * cos(theta) / pdf
     fn sample(&self, w_in: Dir3, rng: &mut R) -> SampleResult {
         let w_out = Dir3::sample_cosine_hemisphere(rng);
 
         SampleResult::new(w_out, self.eval(w_in, w_out, rng) * FloatConsts::PI, false)
     }
 
+    /// returns the pdf for (w_in, w_out)
     fn pdf(&self, w_in: Dir3, w_out: Dir3) -> Float {
         let _ = w_out;
         FloatConsts::FRAC_1_PI * Float::max(w_in.y(), 0.0)
@@ -25,6 +35,7 @@ pub struct SampleResult {
 }
 
 impl SampleResult {
+    /// In case the sample is according to a delta distribution the delta flat has to be set.
     pub fn new(w_out: Dir3, color: Color, delta: bool) -> Self {
         Self {
             w_out,