Update scene:render() in source to match book 3

Fixes #1048

Author: hollasch

src/TheRestOfYourLife/scene.h

@@ -24,15 +24,18 @@ class scene {
 
         std::cout << "P3\n" << image_width << ' ' << image_height << "\n255\n";
 
+        int sqrt_spp = int(sqrt(samples_per_pixel));
         for (int j = image_height-1; j >= 0; --j) {
             std::clog << "\rScanlines remaining: " << j << ' ' << std::flush;
             for (int i = 0; i < image_width; ++i) {
                 color pixel_color(0,0,0);
-                for (int sample = 0; sample < samples_per_pixel; ++sample) {
-                    auto u = (i + random_double()) / (image_width-1);
-                    auto v = (j + random_double()) / (image_height-1);
-                    ray r = cam.get_ray(u, v);
-                    pixel_color += ray_color(r, max_depth);
+                for (int s_j = 0; s_j < sqrt_spp; ++s_j) {
+                    for (int s_i = 0; s_i < sqrt_spp; ++s_i) {
+                        auto u = (i + (s_i + random_double()) / sqrt_spp) / (image_width-1);
+                        auto v = (j + (s_j + random_double()) / sqrt_spp) / (image_height-1);
+                        ray r = cam.get_ray(u, v);
+                        pixel_color += ray_color(r, max_depth);
+                    }
                 }
                 write_color(std::cout, pixel_color, samples_per_pixel);
             }