Listing 12 Rendering surface normals on a sphere

Author: hollasch

src/InOneWeekend/main.cc

@@ -4,18 +4,26 @@
 
 #include <iostream>
 
-bool hit_sphere(const point3& center, double radius, const ray& r) {
+double hit_sphere(const point3& center, double radius, const ray& r) {
     vec3 oc = center - r.origin();
     auto a = dot(r.direction(), r.direction());
     auto b = -2.0 * dot(r.direction(), oc);
     auto c = dot(oc, oc) - radius*radius;
     auto discriminant = b*b - 4*a*c;
-    return (discriminant >= 0);
+
+    if (discriminant < 0) {
+        return -1.0;
+    } else {
+        return (-b - sqrt(discriminant) ) / (2.0*a);
+    }
 }
 
 color ray_color(const ray& r) {
-    if (hit_sphere(point3(0,0,-1), 0.5, r))
-        return color(1, 0, 0);
+    auto t = hit_sphere(point3(0,0,-1), 0.5, r);
+    if (t > 0.0) {
+        vec3 N = unit_vector(r.at(t) - vec3(0,0,-1));
+        return 0.5*color(N.x()+1, N.y()+1, N.z()+1);
+    }
 
     vec3 unit_direction = unit_vector(r.direction());
     auto a = 0.5*(unit_direction.y() + 1.0);