`bvh_node` is broken with negative-radius spheres

[define-private-public]

I wanted to create some "Matryoshka hollow glass spheres", so I setup this scene:

    hittable_list world;

    const color ground_yellow(0.8, 0.8, 0);
    const color gold(0.8, 0.6, 0.2);
    const color blue(0.1, 0.2, 2);
    const color green(0.1, 0.9, 0.3);
    const double glass_refractive_index = 1.5;
    const auto ground_mat = make_shared<lambertian>(ground_yellow);
    const auto blue_mat   = make_shared<lambertian>(blue);
    const auto left_mat   = make_shared<dielectric>(glass_refractive_index);
    const auto center_mat = make_shared<lambertian>(green);
    const auto right_mat  = make_shared<metal>(gold, 1);
    const double tiny_y = 0.005;

    world.add(make_shared<sphere>(point3(-1,  tiny_y, -1),  0.5,  left_mat));
    world.add(make_shared<sphere>(point3(-1,  tiny_y, -1), -0.35, left_mat));
    world.add(make_shared<sphere>(point3(-1,  tiny_y, -1),  0.3,  left_mat));
    world.add(make_shared<sphere>(point3(-1,  tiny_y, -1), -0.2,  left_mat));
    world.add(make_shared<sphere>(point3(-1,  tiny_y, -1),  0.15, left_mat));
    world.add(make_shared<sphere>(point3(-1,  tiny_y, -1),  0.1,  blue_mat));
    world.add(make_shared<sphere>(point3( 0,  0.15 , -1),   .5,  center_mat));
    world.add(make_shared<sphere>(point3( 1,  tiny_y, -1),  0.5,  right_mat));
    world.add(make_shared<sphere>(point3( 0, -100.5, -1),   100,  ground_mat));   // The ground (Big boi)

    const bvh_node world_bvh(world, 0, 1); 

    point3 lookfrom(0, 0.25, 3.25);
    point3 lookat(0, 0, -1);
    auto vfov = 25.0;
    auto aperture = 0.0;
    color background = color(0.70, 0.80, 1.00);

    // Camera

    const vec3 vup(0,1,0);
    const auto dist_to_focus = (lookfrom - lookat).length();
    const int image_height = static_cast<int>(image_width / aspect_ratio);

    camera cam(lookfrom, lookat, vup, vfov, aspect_ratio, aperture, dist_to_focus, 0.0, 1.0);

When rendering on world, I get this, which is correct:

But when I render with world_bvh, this is the result:

I don't think that's right.

[hollasch]

Ref. #532

I believe that this is due to using negative radius spheres, and that you should be using appropriate indices of refraction for your air-inside-glass bubbles.

Can you try the following instead?

  hittable_list world;

  const color ground_yellow(0.8, 0.8, 0);
  const color gold(0.8, 0.6, 0.2);
  const color blue(0.1, 0.2, 2);
  const color green(0.1, 0.9, 0.3);
  const double glass_refractive_index = 1.5;
  const auto ground_mat = make_shared<lambertian>(ground_yellow);
  const auto blue_mat   = make_shared<lambertian>(blue);
  const auto left_mat   = make_shared<dielectric>(glass_refractive_index);
+ const auto left_air   = make_shared<dielectric>(1/glass_refractive_index);
  const auto center_mat = make_shared<lambertian>(green);
  const auto right_mat  = make_shared<metal>(gold, 1);
  const double tiny_y = 0.005;

  world.add(make_shared<sphere>(point3(-1,  tiny_y, -1),  0.5,  left_mat));
- world.add(make_shared<sphere>(point3(-1,  tiny_y, -1), -0.35, left_mat));
+ world.add(make_shared<sphere>(point3(-1,  tiny_y, -1),  0.35, left_air));
  world.add(make_shared<sphere>(point3(-1,  tiny_y, -1),  0.3,  left_mat));
- world.add(make_shared<sphere>(point3(-1,  tiny_y, -1), -0.2,  left_mat));
+ world.add(make_shared<sphere>(point3(-1,  tiny_y, -1),  0.2,  left_air));
  world.add(make_shared<sphere>(point3(-1,  tiny_y, -1),  0.15, left_mat));
  world.add(make_shared<sphere>(point3(-1,  tiny_y, -1),  0.1,  blue_mat));
  world.add(make_shared<sphere>(point3( 0,  0.15 , -1),   .5,  center_mat));
  world.add(make_shared<sphere>(point3( 1,  tiny_y, -1),  0.5,  right_mat));
  world.add(make_shared<sphere>(point3( 0, -100.5, -1),   100,  ground_mat));   // The ground (Big boi)

  const bvh_node world_bvh(world, 0, 1); 

  point3 lookfrom(0, 0.25, 3.25);
  point3 lookat(0, 0, -1);
  auto vfov = 25.0;
  auto aperture = 0.0;
  color background = color(0.70, 0.80, 1.00);

  // Camera

  const vec3 vup(0,1,0);
  const auto dist_to_focus = (lookfrom - lookat).length();
  const int image_height = static_cast<int>(image_width / aspect_ratio);

  camera cam(lookfrom, lookat, vup, vfov, aspect_ratio, aperture, dist_to_focus, 0.0, 1.0);

[hollasch]

Also, I'm assuming this is for Ray Tracing: The Next Week, right?

[define-private-public]

The "smaller negative" sphere is a trick that @petershirley mentioned as a way of producing a hollow glass sphere. I tried out you recommendation of doing the 1 / glass_refractive_index. It produced this:

No BVH:

With BVH:

It doesn't look right. The BVH node is from "Ray Tracing: The Next Week".

[define-private-public]

I figured out the fix. When making the aabb for the sphere to use be used for the bvh_node, std::abs() should be wrapped around the radius. I'll submit a patch.