world: implement World::is_shadowed.

rtchallenge/src/world.rs

@@ -177,4 +177,48 @@ impl World {
             None => BLACK,
         }
     }
+
+    /// Determine if point in world is in a shadow.
+    ///
+    /// # Examples
+    /// ```
+    /// use rtchallenge::{
+    ///     tuples::{ Tuple},
+    ///     world::World,
+    /// };
+    ///
+    /// let w = World::test_world();
+    ///
+    /// // There is no shadow when nothing is collinear with point and light.
+    /// let p = Tuple::point(0.,10.,0.);
+    /// assert_eq!(w.is_shadowed(p), false);
+    ///
+    /// // There shadow when an object is between the point and the light.
+    /// let p = Tuple::point(10.,-10.,10.);
+    /// assert_eq!(w.is_shadowed(p), true);
+    ///
+    /// // There is no shadow when an object is behind the light.
+    /// let p = Tuple::point(-20.,20.,-20.);
+    /// assert_eq!(w.is_shadowed(p), false);
+    ///
+    /// // There is no shadow when an object is behind the point.
+    /// let p = Tuple::point(-2.,2.,-2.);
+    /// assert_eq!(w.is_shadowed(p), false);
+    pub fn is_shadowed(&self, point: Tuple) -> bool {
+        // TODO(wathiede): how to make this multi light aware?
+        let light = self
+            .light
+            .as_ref()
+            .expect("cannot compute is_shadowed in world with no light");
+        let v = light.position - point;
+        let distance = v.magnitude();
+        let direction = v.normalize();
+
+        let r = Ray::new(point, direction);
+        let intersections = self.intersect(&r);
+        if let Some(h) = intersections.hit() {
+            return h.t < distance;
+        }
+        false
+    }
 }