normals!

src/main.rs

@@ -82,8 +82,12 @@ impl Ray {
         self.origin + t * self.direction
     }
     fn color(&self) -> DVec3 {
-        if hit_sphere(&DVec3::new(0., 0., -1.), 0.5, self) {
-            return DVec3::new(1., 0., 0.);
+        let t =
+            hit_sphere(&DVec3::new(0., 0., -1.), 0.5, self);
+        if t > 0.0 {
+            let N = (self.at(t) - DVec3::new(0., 0., -1.))
+                .normalize();
+            return 0.5 * (N + 1.0);
         };
 
         let unit_direction: DVec3 =
@@ -98,11 +102,16 @@ fn hit_sphere(
     center: &DVec3,
     radius: f64,
     ray: &Ray,
-) -> bool {
+) -> f64 {
     let oc: DVec3 = ray.origin - *center;
     let a = ray.direction.dot(ray.direction);
     let b = 2.0 * oc.dot(ray.direction);
     let c = oc.dot(oc) - radius * radius;
     let discriminant = b * b - 4. * a * c;
-    discriminant >= 0.
+
+    if discriminant < 0. {
+        -1.0
+    } else {
+        (-b - discriminant.sqrt()) / (2.0 * a)
+    }
 }