GAMES101 作业6——光线追踪2（Ray-Bounding Volume求交与BVH查找）

参考资料：
https://blog.csdn.net/qq_41765657/article/details/121865049

作业描述

在之前的编程练习中，我们实现了基础的光线追踪算法，具体而言是光线传输、光线与三角形求交。我们采用了这样的方法寻找光线与场景的交点：遍历场景中的所有物体，判断光线是否与它相交。在场景中的物体数量不大时，该做法可以取得良好的结果，但当物体数量增多、模型变得更加复杂，该做法将会变得非常低效。因此，我们需要加速结构来加速求交过程。在本次练习中，我们重点关注物体划分算法 Bounding Volume Hierarchy (BVH)。本练习要求你实现 Ray-Bounding Volume 求交与 BVH 查找。

本次代码的流程为：

1. 从 main 函数开始，定义了场景的参数。
2. 生成物体对象（object），包含顶点，材质以及对应的 bounding box 和加速结构 BVH
3. 向场景中添加物体，并构建场景的 BVH
4. 调用渲染器对物体进行渲染，其中需要通过 BVH 来加速判断光线与场景中物体的求交

首先，你需要从上一次编程练习中引用以下函数：

Render() in Renderer.cpp: 将你的光线生成过程粘贴到此处，并且按照新框架更新相应调用的格式。

// The main render function. This where we iterate over all pixels in the image,
// generate primary rays and cast these rays into the scene. The content of the
// framebuffer is saved to a file.
void Renderer::Render(const Scene &scene) {
    std::vector<Vector3f> framebuffer(scene.width * scene.height);

    float scale = tan(deg2rad(scene.fov * 0.5));
    float imageAspectRatio = scene.width / (float) scene.height;
    Vector3f eye_pos(-1, 5, 10);
    int m = 0;
    for (uint32_t j = 0; j < scene.height; ++j) {
        for (uint32_t i = 0; i < scene.width; ++i) {
            // generate primary ray direction
            float x = (2 * (i + 0.5) / (float) scene.width - 1) * imageAspectRatio * scale;
            float y = (1 - 2 * (j + 0.5) / (float) (scene.height - 1)) * scale;

            // Find the x and y positions of the current pixel to get the
            // direction
            //  vector that passes through it.
            // Also, don't forget to multiply both of them with the variable
            // *scale*, and x (horizontal) variable with the *imageAspectRatio*
            // Don't forget to normalize this direction!
            // Ray 方向
            Vector3f dir = Vector3f(x, y, -1);
            dir = normalize(dir);

            // Ray
            Ray ray(eye_pos, dir);

            framebuffer[m++] = scene.castRay(ray, 0);

        }
        UpdateProgress(j / (float) scene.height);
    }
    UpdateProgress(1.f);

    // save framebuffer to file
    FILE *fp = fopen("binary.ppm", "wb");
    (void) fprintf(fp, "P6\n%d %d\n255\n", scene.width, scene.height);
    for (auto i = 0; i < scene.height * scene.width; ++i) {
        static unsigned char color[3];
        color[0] = (unsigned char) (255 * clamp(0, 1, framebuffer[i].x));
        color[1] = (unsigned char) (255 * clamp(0, 1, framebuffer[i].y));
        color[2] = (unsigned char) (255 * clamp(0, 1, framebuffer[i].z));
        fwrite(color, 1, 3, fp);
    }
    fclose(fp);
}

Triangle::getIntersection() in Triangle.hpp: 将你的光线-三角形相交函数粘贴到此处，并且按照新框架更新相应相交信息的格式。

inline Intersection Triangle::getIntersection(Ray ray)
{
    Intersection inter;

    //如果结果大于0，两向量夹角小于90度
    //这里normal是从三角形指向外的，若同方向则不可能穿过三角形
    if (dotProduct(ray.direction, normal) > 0)
        return inter;
    double u, v, t_tmp = 0;
    Vector3f pvec = crossProduct(ray.direction, e2); //S1
    double det = dotProduct(e1, pvec); //S1*E1
    if (fabs(det) < EPSILON) //这里分母太小会导致t特别大，相当于距离很远什么都看不到
        return inter;

    double det_inv = 1. / det; // 1/(S1*E1)
    Vector3f tvec = ray.origin - v0; //S
    u = dotProduct(tvec, pvec) * det_inv; // b1 = S1*S/S1*E1
    if (u < 0 || u > 1)
        return inter;
    Vector3f qvec = crossProduct(tvec, e1); //S2
    v = dotProduct(ray.direction, qvec) * det_inv; //b2
    if (v < 0 || u + v > 1)
        return inter;
    t_tmp = dotProduct(e2, qvec) * det_inv; //t = S2*E2/S1*E1

    //find ray triangle intersection
    if (t_tmp < 0) {
        return inter;
    }

    inter.distance = t_tmp;
    inter.happened = true;
    inter.m = m;
    inter.obj = this;
    inter.normal = normal;
    inter.coords = ray(t_tmp);
    return inter;
}

在本次编程练习中，你需要实现以下函数：

IntersectP(const Ray& ray, const Vector3f& invDir, const std::array<int, 3>& dirIsNeg) in the Bounds3.hpp: 这个函数的作用是判断包围盒 BoundingBox 与光线是否相交，你需要按照课程介绍的算法实现求交过程。

inline bool Bounds3::IntersectP(const Ray& ray, const Vector3f& invDir,
                                const std::array<int, 3>& dirIsNeg) const
{
    // invDir: ray direction(x,y,z), invDir=(1.0/x,1.0/y,1.0/z), use this because Multiply is faster that Division
    // dirIsNeg: ray direction(x,y,z), dirIsNeg=[int(x>0),int(y>0),int(z>0)], use this to simplify your logic
    // test if ray bound intersects
    float min_x = (pMin.x - ray.origin.x) * invDir[0];
    float max_x = (pMax.x - ray.origin.x) * invDir[0];

    float min_y = (pMin.y - ray.origin.y) * invDir[1];
    float max_y = (pMax.y - ray.origin.y) * invDir[1];

    float min_z = (pMin.z - ray.origin.z) * invDir[2];
    float max_z = (pMax.z - ray.origin.z) * invDir[2];

    if(dirIsNeg[0]) {
        std::swap(min_x, max_x);
    }

    if (dirIsNeg[1]) {
        std::swap(min_y, max_y);
    }

    if (dirIsNeg[2]) {
        std::swap(min_z, max_z);
    }

    float enter = std::max(min_x, std::max(min_y, min_z));
    float exit = std::max(max_x, std::max(max_y, max_z));

    if (enter < exit && exit >= 0) {
        return true;
    } else {
        return false;
    }

}

getIntersection(BVHBuildNode* node, const Ray ray)in BVH.cpp: 建立 BVH 之后，我们可以用它加速求交过程。该过程递归进行，你将在其中调用你实现的 Bounds3::IntersectP。

Intersection BVHAccel::getIntersection(BVHBuildNode *node, const Ray &ray) const {
    //Traverse the BVH to find intersection
    Vector3f invDir(1.0 / ray.direction.x, 1.0 / ray.direction.y, 1.0 / ray.direction.z);
    std::array<int, 3> dirIsNeg;

    dirIsNeg[0] = ray.direction.x > 0 ? 0 : 1;
    dirIsNeg[1] = ray.direction.y > 0 ? 0 : 1;
    dirIsNeg[2] = ray.direction.z > 0 ? 0 : 1;

    //如果光线没有与碰撞盒相交，直接返回一个空值
    if (!node->bounds.IntersectP(ray, invDir, dirIsNeg)) {
        return {};
    }

    //如果碰撞盒不再继续细分,测试碰撞盒内的所有物体是否与光纤相交，返回最早相交的
    if (node->left == nullptr && node->right == nullptr) {
        return node->object->getIntersection(ray);
    }

    //测试细分的碰撞盒
    Intersection leaf1 = BVHAccel::getIntersection(node->left, ray);
    Intersection leaf2 = BVHAccel::getIntersection(node->right, ray);

    return leaf1.distance < leaf2.distance ? leaf1 : leaf2;
}