How to change this code from recursion to iteration - c++

Question

I've trying to make the following code implement by iteration instead of recursive(the "hit" function of the BVH_node class),but I've no idea how to do this.I did found some topic about this,but they are not very helpful.Could anybody help me?thanks so much.(by the way this is the third time I ask this question ,people says that I'm not making my question clear,please give me some advice on asking question)
class Ray {
public:
vec3 origin;
vec3 direction;
float t = 10000.0f;
};
class AABB{
public:
bool hit(Ray ray){
//test if ray hit itself
//but don't write the t into the ray
}
vec3 min;
vec3 max;
};
class Geometry {
public:
bool hit(Ray& ray) {
float t;
//test if the ray hit this geometry
//ray.origin + t * ray.direction = ......
//then solve the t
if (t < ray.t && t>0.0f) {
ray.t = t;
return true;
}
return false;
}
private:
//...some private data
};
class BVH_node {
public:
void construction(/*......some data for construction*/) {
//is_node_or_geo = ......
//if(is_node_or_geo == 0)
//L_node = ......; R_node = ......;
//if(is_node_or_geo == 1)
//L_geo = ......; R_geo = ......;
}
bool hit(Ray& ray) {
if(aabb.hit(ray)){
if (is_node_or_geo == 0) {
bool hit_left = L_node->hit(ray);
bool hit_right = R_node->hit(ray);
return hit_left || hit_right;
}
if (is_node_or_geo == 1) {
bool hit_left = L_geo->hit(ray);
bool hit_right = R_geo->hit(ray);
return hit_left || hit_right;
}
}
return false;
}
private:
int is_node_or_geo;//node = 0,geo = 1
AABB aabb;
BVH_node* L_node;
BVH_node* R_node;
Geometry* L_geo;
Geometry* R_geo;
};
int main(){
BVH_node* root = new BVH_node;
root->construction(/*.......*/);
Ray ray;//ray = .......;
bool hit = root->hit(ray);
delete root;
}
most tutorial I found on how to change recursion to iteration(like this http://blog.moertel.com/posts/2013-05-11-recursive-to-iterative.html) don't consider some situation where member variable is involved.

Answer 1

Follow up on the approach mentioned in a comment of Alan Birtles:
Let us focus on the hit function. In order to make our life easier, it's better to first rewrite it as follows:
bool hit(Ray& ray) {
if(!aabb.hit(ray)) return false;
if (is_node_or_geo == 0) {
if (L_node->hit(ray)) return true;
if (R_node->hit(ray)) return true;
return false;
} else if (is_node_or_geo == 1) {
if (L_geo->hit(ray)) return true;
if (R_geo->hit(ray)) return true;
return false;
}
}
Now we can add a loop and a stack (implemented with a std::vector, since I'm not familiar with std::stack). We also make the method static, as it does not relate to a single object:
static bool hit(Ray& ray, BVH_node * root) {
std::vector<BVH_node *> nodes {root};
while (!nodes.empty()){
BVH_node & top = *nodes.back();
nodes.pop_back();
if(!top.aabb.hit(ray)) continue;
if (top.is_node_or_geo == 0) {
nodes.push_back(top.L_node);
nodes.push_back(top.R_node);
continue;
} else if (top.is_node_or_geo == 1) {
if (top.L_geo->hit(ray)) return true;
if (top.R_geo->hit(ray)) return true;
continue;
}
}
return false;
}
This code wasn't tested so it might require minor tweacking.
Note that depending on your implementation, you might want to add a bunch of consts - after every mention of BVH_node