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BVH optimization: split on the longest bbox axis
I see about an 18% speedup on my box. Resolves #1007
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‎books/RayTracingTheNextWeek.html

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@@ -375,8 +375,10 @@
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<div class='together'>
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This gives the following result:
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<div id="image-bouncing-spheres">
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![<span class='num'>Image 1:</span> Bouncing spheres
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](../images/img-2.01-bouncing-spheres.png class='pixel')
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</div>
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</div>
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@@ -926,13 +928,14 @@
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public:
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...
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bvh_node(const std::vector<shared_ptr<hittable>>& src_objects, size_t start, size_t end) {
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auto objects = src_objects; // Create a modifiable array of the source scene objects
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int axis = random_int(0,2);
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auto comparator = (axis == 0) ? box_x_compare
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: (axis == 1) ? box_y_compare
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: box_z_compare;
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auto objects = src_objects; // A modifiable array of the source scene objects
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size_t object_span = end - start;
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if (object_span == 1) {
@@ -1038,6 +1041,111 @@
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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[Listing [random-spheres-bvh]: <kbd>[main.cc]</kbd> Random spheres, using BVH]
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The rendered image should be identical to the non-BVH version shown in
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[image 1](#image-bouncing-spheres). However, if you time the two versions, the BVH version should be
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faster. I see a speedup of almost _six and a half times_ the prior version.
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Another BVH Optimization
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-------------------------
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We can speed up the BVH optimization a bit more. Instead of choosing a random splitting axis, let's
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split the longest axis of the enclosing bounding box to get the most subdivision. The change is
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straight-forward, but we'll add a few things to the `aabb` class in the process.
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The first task is to construct an axis-aligned bounding box of the span of objects in the BVH
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constructor. Basically, we'll construct the `bvh_node`s bounding box from this span by initializing
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the bounding box to empty, and then augmenting it with each bounding box in the span of objects.
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We don't have a way yet to express an empty bounding box, so we'll imagine one for now and
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implementing it shortly.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
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class bvh_node : public hittable {
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public:
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...
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bvh_node(const std::vector<shared_ptr<hittable>>& src_objects, size_t start, size_t end) {
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
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// Build the bounding box of the span of source objects.
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bbox = aabb::empty;
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for (int object_index=start; object_index < end; ++object_index)
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bbox = aabb(bbox, src_objects[object_index]->bounding_box());
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
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...
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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[Listing [object-span-bbox]: <kbd>[bvh.h]</kbd> Building the bbox for the span of BVH objects]
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Now that we have the bounding box, set the splitting axis to the one with the longest side. Again,
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we'll imagine a function that does that for us: `aabb::longest_axis()`. Finally, since we're
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computing the bounding box of the object span up front, we can delete the original line that
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computed it as the union of the left and right sides.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
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class bvh_node : public hittable {
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public:
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...
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bvh_node(const std::vector<shared_ptr<hittable>>& src_objects, size_t start, size_t end) {
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// Build the bounding box of the span of source objects.
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bbox = aabb::empty;
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for (int object_index=start; object_index < end; ++object_index)
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bbox = aabb(bbox, src_objects[object_index]->bounding_box());
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
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int axis = bbox.longest_axis();
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
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auto comparator = (axis == 0) ? box_x_compare
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: (axis == 1) ? box_y_compare
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: box_z_compare;
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...
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ delete
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bbox = aabb(left->bounding_box(), right->bounding_box());
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
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}
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...
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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[Listing [object-span-bbox]: <kbd>[bvh.h]</kbd> Building the bbox for the span of BVH objects]
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Now to implement the empty `aabb` code and the new `aabb::longest_axis()` function:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
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class aabb {
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public:
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...
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
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int longest_axis() const {
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// Returns the index of the longest axis of the bounding box.
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if (x.size() > y.size())
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return x.size() > z.size() ? 0 : 2;
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else
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return y.size() > z.size() ? 1 : 2;
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}
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static const aabb empty, universe;
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};
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const aabb aabb::empty = aabb(interval::empty, interval::empty, interval::empty);
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const aabb aabb::universe = aabb(interval::universe, interval::universe, interval::universe);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
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...
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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[Listing [aabb-empty-and-axis]: <kbd>[aabb.h]</kbd>
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New aabb constants and longest_axis() function]
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As before, you should see identical results to [image 1](#image-bouncing-spheres), but rendering a
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little bit faster. On my system, this yields something like an additional 18% render speedup. Not
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bad for a little extra work.
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Texture Mapping

‎src/TheNextWeek/aabb.h

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Original file line numberDiff line numberDiff line change
@@ -72,8 +72,22 @@ class aabb {
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}
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return true;
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}
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int longest_axis() const {
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// Returns the index of the longest axis of the bounding box.
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if (x.size() > y.size())
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return x.size() > z.size() ? 0 : 2;
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else
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return y.size() > z.size() ? 1 : 2;
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}
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static const aabb empty, universe;
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};
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const aabb aabb::empty = aabb(interval::empty, interval::empty, interval::empty);
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const aabb aabb::universe = aabb(interval::universe, interval::universe, interval::universe);
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aabb operator+(const aabb& bbox, const vec3& offset) {
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return aabb(bbox.x + offset.x(), bbox.y + offset.y(), bbox.z + offset.z());
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}

‎src/TheNextWeek/bvh.h

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Original file line numberDiff line numberDiff line change
@@ -13,6 +13,7 @@
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#include "rtweekend.h"
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#include "aabb.h"
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#include "hittable.h"
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#include "hittable_list.h"
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@@ -24,13 +25,19 @@ class bvh_node : public hittable {
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bvh_node(const hittable_list& list) : bvh_node(list.objects, 0, list.objects.size()) {}
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bvh_node(const std::vector<shared_ptr<hittable>>& src_objects, size_t start, size_t end) {
27-
auto objects = src_objects; // Create a modifiable array of the source scene objects
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// Build the bounding box of the span of source objects.
29+
bbox = aabb::empty;
30+
for (int object_index=start; object_index < end; ++object_index)
31+
bbox = aabb(bbox, src_objects[object_index]->bounding_box());
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int axis = bbox.longest_axis();
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29-
int axis = random_int(0,2);
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auto comparator = (axis == 0) ? box_x_compare
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: (axis == 1) ? box_y_compare
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: box_z_compare;
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auto objects = src_objects; // A modifiable array of the source scene objects
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size_t object_span = end - start;
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3643
if (object_span == 1) {
@@ -50,8 +57,6 @@ class bvh_node : public hittable {
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left = make_shared<bvh_node>(objects, start, mid);
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right = make_shared<bvh_node>(objects, mid, end);
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}
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bbox = aabb(left->bounding_box(), right->bounding_box());
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}
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bool hit(const ray& r, interval ray_t, hit_record& rec) const override {

‎src/TheRestOfYourLife/aabb.h

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Original file line numberDiff line numberDiff line change
@@ -72,8 +72,22 @@ class aabb {
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}
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return true;
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}
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int longest_axis() const {
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// Returns the index of the longest axis of the bounding box.
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if (x.size() > y.size())
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return x.size() > z.size() ? 0 : 2;
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else
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return y.size() > z.size() ? 1 : 2;
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}
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static const aabb empty, universe;
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};
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const aabb aabb::empty = aabb(interval::empty, interval::empty, interval::empty);
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const aabb aabb::universe = aabb(interval::universe, interval::universe, interval::universe);
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aabb operator+(const aabb& bbox, const vec3& offset) {
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return aabb(bbox.x + offset.x(), bbox.y + offset.y(), bbox.z + offset.z());
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}

‎src/TheRestOfYourLife/bvh.h

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@@ -13,6 +13,7 @@
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#include "rtweekend.h"
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#include "aabb.h"
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#include "hittable.h"
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#include "hittable_list.h"
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@@ -24,13 +25,19 @@ class bvh_node : public hittable {
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bvh_node(const hittable_list& list) : bvh_node(list.objects, 0, list.objects.size()) {}
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bvh_node(const std::vector<shared_ptr<hittable>>& src_objects, size_t start, size_t end) {
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auto objects = src_objects; // Create a modifiable array of the source scene objects
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// Build the bounding box of the span of source objects.
29+
bbox = aabb::empty;
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for (int object_index=start; object_index < end; ++object_index)
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bbox = aabb(bbox, src_objects[object_index]->bounding_box());
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int axis = bbox.longest_axis();
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int axis = random_int(0,2);
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auto comparator = (axis == 0) ? box_x_compare
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: (axis == 1) ? box_y_compare
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: box_z_compare;
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auto objects = src_objects; // A modifiable array of the source scene objects
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size_t object_span = end - start;
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3643
if (object_span == 1) {
@@ -50,8 +57,6 @@ class bvh_node : public hittable {
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left = make_shared<bvh_node>(objects, start, mid);
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right = make_shared<bvh_node>(objects, mid, end);
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}
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bbox = aabb(left->bounding_box(), right->bounding_box());
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}
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bool hit(const ray& r, interval ray_t, hit_record& rec) const override {

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