From ba40f3736c2db02e91d32a8d7c48a1b695c492cf Mon Sep 17 00:00:00 2001
From: Armin Friedl <dev@friedl.net>
Date: Mon, 18 Jul 2022 15:32:51 +0200
Subject: [PATCH] Adding a sphere

---
 rtiww/src/main.cpp | 39 ++++++++++++++++++++++++++++++---------
 1 file changed, 30 insertions(+), 9 deletions(-)

diff --git a/rtiww/src/main.cpp b/rtiww/src/main.cpp
index 2f9f2ce..a19ceaa 100644
--- a/rtiww/src/main.cpp
+++ b/rtiww/src/main.cpp
@@ -4,28 +4,49 @@
 
 #include <iostream>
 
-color ray_color(const ray& r) {
+double hit_sphere(const point3 &center, double radius, const ray &r) {
+    vec3 oc = r.origin() - center;
+
+    auto a = dot(r.direction(), r.direction());
+    auto b = 2.0 * dot(oc, r.direction());
+    auto c = dot(oc, oc) - radius * radius;
+    auto discriminant = b * b - 4 * a * c;
+    if (discriminant < 0) {
+        return -1.0;
+    } else {
+        return (-b - sqrt(discriminant)) / (2.0*a);
+    }
+}
+
+color ray_color(const ray &r) {
+    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 t = 0.5 * (unit_direction.y() + 1.0);
-    return (1.0-t)*color(1.0, 1.0, 1.0) + t*color(0.5, 0.7, 1.0);
+    t = 0.5*(unit_direction.y() + 1.0);
+    return (1.0 - t) * color(1.0, 1.0, 1.0) + t * color(0.5, 0.7, 1.0);
 }
 
 int main() {
 
     // Image
-    const auto aspect_ratio = 16.0/9.0;
+    const auto aspect_ratio = 16.0 / 9.0;
     const int image_width = 400;
     const int image_height = static_cast<int>(image_width / aspect_ratio);
 
     // Camera
     auto viewport_height = 2.0;
-    auto viewport_width = aspect_ratio*viewport_height;
+    auto viewport_width = aspect_ratio * viewport_height;
     auto focal_length = 1.0;
 
     auto origin = point3(0, 0, 0);
     auto horizontal = vec3(viewport_width, 0, 0);
     auto vertical = vec3(0, viewport_height, 0);
-    auto lower_left_corner = origin - horizontal/2 - vertical/2 - vec3(0, 0, focal_length);
+    auto lower_left_corner = origin - horizontal / 2 - vertical / 2 - vec3(0, 0, focal_length);
 
     // Render
     std::cout << "P3\n" << image_width << ' ' << image_height << "\n255\n";
@@ -33,10 +54,10 @@ int main() {
     for (int j = image_height - 1; j >= 0; --j) {
         std::cerr << "\rScanlines remaining: " << j << ' ' << std::flush;
         for (int i = 0; i < image_width; ++i) {
-            auto u = double(i)/(image_width-1);
-            auto v = double(j) / (image_height-1);
+            auto u = double(i) / (image_width - 1);
+            auto v = double(j) / (image_height - 1);
 
-            ray r(origin, lower_left_corner + u*horizontal + v*vertical - origin);
+            ray r(origin, lower_left_corner + u * horizontal + v * vertical - origin);
             color pixel_color = ray_color(r);
             write_color(std::cout, pixel_color);
         }