Problems with Bounding Box Frustum Culling

Question

I am having trouble getting correct frustum culling to work. I got a great suggestion from someone about AABB point testing and it works great but I have gotten to the point where a max and a min point testing do not always work because often they won't be in the frustum but a plane from one of the sides will be. So I need to move onto full bounding box detection. I already have the code which checks the bounding box against the planes of the frustum, just need to set up the bounding box.

Does anyone have a code example or an idea about how to do this. My two points I have with which to create my bounding box are a minimum x y and z and a maximum x y and z.

Thank you! :)

Are you asking how to get an AABB from the min and max points? Or how to get the min and max points from your object? — John Calsbeek
Incidentally, testing two points against a plane is equivalent to testing an AABB against the plane. You just have to pick the two points properly (i.e. instead of using the min and max points, you need to pick the nearest and farthest point from the plane). — John Calsbeek
@Josh Calsbeek Thanks for the reply. How does one do this? And yes, now with a new understanding of the AABB, how do I obtain it from my min/max. Thanks :) — Satchmo Brown
An AABB is defined by the min and max points. Unless the code you already have uses some different form to represent an AABB, there's nothing more you need to do than provide the min and max points. — John Calsbeek

user548569 user548569 · Accepted Answer · 2011-06-20T17:36:20

You need to have the planes of the frustrum looking inside (their normals must point to the center). Then the rendering discard can be done by checking if the object is completelly outside of one of any of the six planes of the frustrum. You can do that with a sphere or with a AABB, or any other container where you can calculate the distance (and the side) to the plane.

From my own plane code (so if any plane return -1 discard the rendering), I added some comments so it is easier to understand:

    int Side(const Sphere &e) const {
        float d=Distance(e.center);
        if(d-e.radius>Epsilon) return 1; // inside
        if(d+e.radius<-Epsilon) return -1; // outside
        return 0; // crossing the plane
    }

    int Side(const Vector3f &v) const { 
        float d=Distance(v);
        if(d>Epsilon) return 1;
        if(d<-Epsilon) return -1;
        return 0;
    }


    int Side(const Box3f &c) const {
        Vector3f a,b;
        if(Normal.x>=0.0)  { a.x=c.min.x; b.x=c.max.x; }
            else { b.x=c.min.x; a.x=c.max.x; }
        if(Normal.y>=0.0)  { a.y=c.min.y; b.y=c.max.y; }
            else { b.y=c.min.y; a.y=c.max.y; }
        if(Normal.z>=0.0)  { a.z=c.min.z; b.z=c.max.z; }
            else { b.z=c.min.z; a.z=c.max.z; }

        int l1 = Side(a), l2= Side(b);

        if(l1==l2) return l1; // both in the same side
        return 0; // we have the object crossing the plane
    }

Problems with Bounding Box Frustum Culling

1 Answers