Add parameterized background color

This allows the scenes to specify their own background color, so they
match the results in the book.

Resolves #158

Author: hollasch

books/RayTracingTheNextWeek.html

@@ -1730,10 +1730,12 @@
 </div>
 
 <div class='together'>
-Next, let’s make the background black in our `ray_color` function, and pay attention to emitted:
+Next, we want a pure black background so the only light in the scene is coming from the emitters. To
+do this, we’ll add a background color parameter to our `ray_color` function, and pay attention to
+the new `emitted` value.
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
-    vec3 ray_color(const ray& r, hittable& world, int depth) {
+    vec3 ray_color(const ray& r, hittable& world, const vec3& background, int depth) {
         hit_record rec;
 
         // If we've exceeded the ray bounce limit, no more light is gathered.
@@ -1742,15 +1744,22 @@
 
         // If the ray hits nothing, return the background color.
         if (!world.hit(r, 0.001, infinity, rec))
-            return vec3(0,0,0);
+            return background;
 
         ray scattered;
         vec3 attenuation;
         vec3 emitted = rec.mat_ptr->emitted(rec.u, rec.v, rec.p);
         if (!rec.mat_ptr->scatter(r, rec, attenuation, scattered))
             return emitted;
 
-        return emitted + attenuation * ray_color(scattered, world, depth-1);
+        return emitted + attenuation * ray_color(scattered, world, background, depth-1);
+    }
+    ...
+
+    int main() {
+        ...
+                    color += ray_color(r, world, vec3(0,0,0), max_depth);
+        ...
     }
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     [Listing [ray-color-emitted]: <kbd>[main.cc]</kbd> ray_color function for emitting materials]

src/TheNextWeek/main.cc

@@ -23,7 +23,7 @@
 #include <iostream>
 
 
-vec3 ray_color(const ray& r, hittable& world, int depth) {
+vec3 ray_color(const ray& r, hittable& world, const vec3& background, int depth) {
     hit_record rec;
 
     // If we've exceeded the ray bounce limit, no more light is gathered.
@@ -32,7 +32,7 @@ vec3 ray_color(const ray& r, hittable& world, int depth) {
 
     // If the ray hits nothing, return the background color.
     if (!world.hit(r, 0.001, infinity, rec))
-        return vec3(0.5,0.5,0.5);
+        return background;
 
     ray scattered;
     vec3 attenuation;
@@ -41,7 +41,7 @@ vec3 ray_color(const ray& r, hittable& world, int depth) {
     if (!rec.mat_ptr->scatter(r, rec, attenuation, scattered))
         return emitted;
 
-    return emitted + attenuation * ray_color(scattered, world, depth-1);
+    return emitted + attenuation * ray_color(scattered, world, background, depth-1);
 }
 
 
@@ -360,34 +360,39 @@ int main() {
     auto vfov = 40.0;
     auto aperture = 0.0;
     auto dist_to_focus = 10.0;
+    vec3 background(0,0,0);
 
     switch (0) {
         case 1:
             world = random_scene();
             lookfrom = vec3(13,2,3);
             lookat = vec3(0,0,0);
             vfov = 20.0;
+            background = vec3(0.70, 0.80, 1.00);
             break;
 
         case 2:
             world = two_spheres();
             lookfrom = vec3(13,2,3);
             lookat = vec3(0,0,0);
             vfov = 20.0;
+            background = vec3(0.70, 0.80, 1.00);
             break;
 
         case 3:
             world = two_perlin_spheres();
             lookfrom = vec3(13,2,3);
             lookat = vec3(0,0,0);
             vfov = 20.0;
+            background = vec3(0.70, 0.80, 1.00);
             break;
 
         case 4:
             world = earth();
             lookfrom = vec3(0,0,12);
             lookat = vec3(0,0,0);
             vfov = 20.0;
+            background = vec3(0.70, 0.80, 1.00);
             break;
 
         case 5:
@@ -446,7 +451,7 @@ int main() {
                 auto u = (i + random_double()) / image_width;
                 auto v = (j + random_double()) / image_height;
                 ray r = cam.get_ray(u, v);
-                color += ray_color(r, world, max_depth);
+                color += ray_color(r, world, background, max_depth);
             }
             color.write_color(std::cout, samples_per_pixel);
         }