From 7e450d664e1d8ad9d9ac691d974c6f1abefe107f Mon Sep 17 00:00:00 2001
From: Bill Thiede <git@xinu.tv>
Date: Sun, 18 Jul 2021 13:08:03 -0700
Subject: [PATCH] world & intersections:  handle shadows.

---
 rtchallenge/src/intersections.rs | 15 +++++++++++++++
 rtchallenge/src/world.rs         | 21 ++++++++++++++++++---
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/rtchallenge/src/intersections.rs b/rtchallenge/src/intersections.rs
index 313a260..49cd5bd 100644
--- a/rtchallenge/src/intersections.rs
+++ b/rtchallenge/src/intersections.rs
@@ -4,6 +4,7 @@ use crate::{
     rays::Ray,
     spheres::Sphere,
     tuples::{dot, Tuple},
+    EPSILON,
 };
 
 #[derive(Debug, Clone, PartialEq)]
@@ -138,6 +139,7 @@ pub struct PrecomputedData<'i> {
     pub t: f32,
     pub object: &'i Sphere,
     pub point: Tuple,
+    pub over_point: Tuple,
     pub eyev: Tuple,
     pub normalv: Tuple,
     pub inside: bool,
@@ -150,8 +152,10 @@ pub struct PrecomputedData<'i> {
 /// use rtchallenge::{
 ///     intersections::{prepare_computations, Intersection, Intersections},
 ///     rays::Ray,
+///     matrices::Matrix4x4,
 ///     spheres::{intersect, Sphere},
 ///     tuples::Tuple,
+///     EPSILON
 /// };
 ///
 /// // Precomputing the state of an intersection.
@@ -182,6 +186,15 @@ pub struct PrecomputedData<'i> {
 /// assert_eq!(comps.inside, true);
 //// // Normal would have been (0, 0, 1), but is inverted when inside.
 /// assert_eq!(comps.normalv, Tuple::vector(0., 0., -1.));
+///
+/// // The hit should offset the point.
+/// let r = Ray::new(Tuple::point(0., 0., -5.), Tuple::vector(0., 0., 1.));
+/// let mut shape = Sphere::default();
+/// shape .set_transform(Matrix4x4::translation(0.,0.,1.));
+/// let i = Intersection::new(5., &shape);
+/// let comps = prepare_computations(&i, &r);
+/// assert!(comps.over_point.z< -EPSILON/2.);
+/// assert!(comps.point.z>comps.over_point.z);
 /// ```
 pub fn prepare_computations<'i>(i: &'i Intersection, r: &Ray) -> PrecomputedData<'i> {
     let point = r.position(i.t);
@@ -192,10 +205,12 @@ pub fn prepare_computations<'i>(i: &'i Intersection, r: &Ray) -> PrecomputedData
     } else {
         (false, normalv)
     };
+    let over_point = point + normalv * EPSILON;
     PrecomputedData {
         t: i.t,
         object: i.object,
         point,
+        over_point,
         normalv,
         inside,
         eyev,
diff --git a/rtchallenge/src/world.rs b/rtchallenge/src/world.rs
index b69e3f4..29a73d8 100644
--- a/rtchallenge/src/world.rs
+++ b/rtchallenge/src/world.rs
@@ -91,7 +91,9 @@ impl World {
     /// use rtchallenge::{
     ///     intersections::{prepare_computations, Intersection},
     ///     lights::PointLight,
+    ///     matrices::Matrix4x4,
     ///     rays::Ray,
+    ///     spheres::Sphere,
     ///     tuples::{Color, Tuple},
     ///     world::World,
     ///     WHITE,
@@ -115,18 +117,31 @@ impl World {
     /// let comps = prepare_computations(&i, &r);
     /// let c = w.shade_hit(&comps);
     /// assert_eq!(c, Color::new(0.90498, 0.90498, 0.90498));
+    ///
+    /// // Shading with an intersection in shadow.
+    /// let mut w = World::default();
+    /// w.light = Some(PointLight::new(Tuple::point(0., 0., -10.), WHITE));
+    /// let s1 = Sphere::default();
+    /// let mut s2 = Sphere::default();
+    /// s2.set_transform(Matrix4x4::translation(0., 0., 10.));
+    /// w.objects = vec![s1, s2.clone()];
+    /// let r = Ray::new(Tuple::point(0., 0., 5.), Tuple::vector(0., 0., 1.));
+    /// let i = Intersection::new(4., &s2);
+    /// let comps = prepare_computations(&i, &r);
+    /// let c = w.shade_hit(&comps);
+    /// assert_eq!(c, Color::new(0.1, 0.1, 0.1));
     /// ```
     pub fn shade_hit(&self, comps: &PrecomputedData) -> Color {
         // TODO(wathiede): support multiple light sources by iterating over all
         // the light sources and summing the calls to lighting.
-        let in_shadow = false;
+        let shadowed = self.is_shadowed(comps.over_point);
         lighting(
             &comps.object.material,
             &self.light.as_ref().expect("World has no lights"),
-            comps.point,
+            comps.over_point,
             comps.eyev,
             comps.normalv,
-            in_shadow,
+            shadowed,
         )
     }
     /// Compute color for given ray fired at the world.