triangle.h

#ifndef TRIANGLE_H
#define TRIANGLE_H

#include "rtweekend.h"
#include "hittable.h"

class triangle : public hittable {
public:
    triangle(const point3& Q, const vec3& u, const vec3& v, shared_ptr<material> mat)
        : Q(Q), u(u), v(v), mat(mat)
    {
        auto n = cross(u, v);

        if (n.length_squared() < 1e-8) {
            normal = vec3(1, 0, 0);
            w = vec3(0, 0, 0);
            D = dot(normal, Q);
        }
        else {
            normal = unit_vector(n);
            D = dot(normal, Q);
            w = n / dot(n, n);
        }
        set_bounding_box();
    }

    virtual void set_bounding_box() {
        auto bbox_edge = aabb(Q, Q + u);
        auto bbox_point = aabb(Q + v, Q + v);
        bbox = aabb(bbox_edge, bbox_point);
    }

    aabb bounding_box() const override { return bbox; }

    bool hit(const ray& r, interval ray_t, hit_record& rec) const override {
        auto denom = dot(normal, r.direction());

        if (std::fabs(denom) < 1e-8)
            return false;

        auto t = (D - dot(normal, r.origin())) / denom;
        if (!ray_t.contains(t))
            return false;

        auto intersection = r.at(t);
        vec3 planar_hitpt_vector = intersection - Q;
        auto alpha = dot(w, cross(planar_hitpt_vector, v));
        auto beta = dot(w, cross(u, planar_hitpt_vector));

        if (!is_interior(alpha, beta, rec))
            return false;

        rec.t = t;
        rec.p = intersection;
        rec.mat = mat;
        rec.set_face_normal(r, normal);

        return true;
    }

    virtual bool is_interior(double a, double b, hit_record& rec) const {
        if ((a < 0) || (b < 0) || (a + b > 1))
            return false;

        rec.u = a;
        rec.v = b;
        return true;
    }

private:
    point3 Q;
    vec3 u, v;
    shared_ptr<material> mat;
    aabb bbox;
    vec3 normal;
    double D;
    vec3 w;
};

#endif