games101-hw/07/Assignment7/BVH.cpp

#include <algorithm>
#include <cassert>
#include "BVH.hpp"

BVHAccel::BVHAccel(std::vector<Object*> p, int maxPrimsInNode,
                   SplitMethod splitMethod)
    : maxPrimsInNode(std::min(255, maxPrimsInNode)), splitMethod(splitMethod),
      primitives(std::move(p))
{
    time_t start, stop;
    time(&start);
    if (primitives.empty())
        return;

    root = recursiveBuild(primitives);

    time(&stop);
    double diff = difftime(stop, start);
    int hrs = (int)diff / 3600;
    int mins = ((int)diff / 60) - (hrs * 60);
    int secs = (int)diff - (hrs * 3600) - (mins * 60);

    printf(
        "\rBVH Generation complete: \nTime Taken: %i hrs, %i mins, %i secs\n\n",
        hrs, mins, secs);
}

BVHBuildNode* BVHAccel::recursiveBuild(std::vector<Object*> objects)
{
    BVHBuildNode* node = new BVHBuildNode();

    // Compute bounds of all primitives in BVH node
    Bounds3 bounds;
    for (int i = 0; i < objects.size(); ++i)
        bounds = Union(bounds, objects[i]->getBounds());
    if (objects.size() == 1) {
        // Create leaf _BVHBuildNode_
        node->bounds = objects[0]->getBounds();
        node->object = objects[0];
        node->left = nullptr;
        node->right = nullptr;
        node->area = objects[0]->getArea();
        return node;
    }
    else if (objects.size() == 2) {
        node->left = recursiveBuild(std::vector{objects[0]});
        node->right = recursiveBuild(std::vector{objects[1]});

        node->bounds = Union(node->left->bounds, node->right->bounds);
        node->area = node->left->area + node->right->area;
        return node;
    }
    else {
        Bounds3 centroidBounds;
        for (int i = 0; i < objects.size(); ++i)
            centroidBounds =
                Union(centroidBounds, objects[i]->getBounds().Centroid());
        int dim = centroidBounds.maxExtent();
        switch (dim) {
        case 0:
            std::sort(objects.begin(), objects.end(), [](auto f1, auto f2) {
                return f1->getBounds().Centroid().x <
                       f2->getBounds().Centroid().x;
            });
            break;
        case 1:
            std::sort(objects.begin(), objects.end(), [](auto f1, auto f2) {
                return f1->getBounds().Centroid().y <
                       f2->getBounds().Centroid().y;
            });
            break;
        case 2:
            std::sort(objects.begin(), objects.end(), [](auto f1, auto f2) {
                return f1->getBounds().Centroid().z <
                       f2->getBounds().Centroid().z;
            });
            break;
        }

        auto beginning = objects.begin();
        auto middling = objects.begin() + (objects.size() / 2);
        auto ending = objects.end();

        auto leftshapes = std::vector<Object*>(beginning, middling);
        auto rightshapes = std::vector<Object*>(middling, ending);

        assert(objects.size() == (leftshapes.size() + rightshapes.size()));

        node->left = recursiveBuild(leftshapes);
        node->right = recursiveBuild(rightshapes);

        node->bounds = Union(node->left->bounds, node->right->bounds);
        node->area = node->left->area + node->right->area;
    }

    return node;
}

Intersection BVHAccel::Intersect(const Ray& ray) const
{
    Intersection isect;
    if (!root)
        return isect;
    isect = BVHAccel::getIntersection(root, ray);
    return isect;
}

Intersection BVHAccel::getIntersection(BVHBuildNode* node, const Ray& ray) const
{
    // TODO Traverse the BVH to find intersection

    Intersection intr;
    if(!node)
        return intr;

    Vector3f invDir(1.0/ray.direction.x,1.0/ray.direction.y,1.0/ray.direction.z);
    const std::array<int,3> & dirIsNeg ={ray.direction.x<0,ray.direction.y<0,ray.direction.z<0};
    if(!node->bounds.IntersectP(ray,invDir,dirIsNeg))
        return intr;
    if(node->object != nullptr)
    {
        return node->object->getIntersection(ray);

    }
    Intersection hit1 = getIntersection(node->left,ray);
    Intersection hit2 = getIntersection(node->right,ray);
    if(hit1.happened||hit2.happened)
    {
        return hit1.distance<=hit2.distance ? hit1:hit2;
    }
    return intr;
    
}


void BVHAccel::getSample(BVHBuildNode* node, float p, Intersection &pos, float &pdf){
    if(node->left == nullptr || node->right == nullptr){
        node->object->Sample(pos, pdf);
        pdf *= node->area;
        return;
    }
    if(p < node->left->area) getSample(node->left, p, pos, pdf);
    else getSample(node->right, p - node->left->area, pos, pdf);
}

void BVHAccel::Sample(Intersection &pos, float &pdf){
    float p = std::sqrt(get_random_float()) * root->area;
    getSample(root, p, pos, pdf);
    pdf /= root->area;
}