- (void) calculate
{
    GLfloat clip[16];
    glGetFloatv(GL_PROJECTION_MATRIX, clip);

    top.n.x = clip[ 3] - clip[ 1];
    top.n.y = clip[ 7] - clip[ 5];
    top.n.z = clip[11] - clip[ 9];
    top.d = clip[15] - clip[13];

    bottom.n.x = clip[ 3] + clip[ 1];
    bottom.n.y = clip[ 7] + clip[ 5];
    bottom.n.z = clip[11] + clip[ 9];
    bottom.d = clip[15] + clip[13];

    left.n.x = clip[ 3] + clip[ 0];
    left.n.y = clip[ 7] + clip[ 4];
    left.n.z = clip[11] + clip[ 8];
    left.d = clip[15] + clip[12];

    right.n.x = clip[ 3] - clip[ 0];
    right.n.y = clip[ 7] - clip[ 4];
    right.n.z = clip[11] - clip[ 8];
    right.d = clip[15] - clip[12];

    front.n.x = clip[ 3] + clip[ 2];
    front.n.y = clip[ 7] + clip[ 6];
    front.n.z = clip[11] + clip[10];
    front.d = clip[15] + clip[14];

    back.n.x = clip[ 3] - clip[ 2];
    back.n.y = clip[ 7] - clip[ 6];
    back.n.z = clip[11] - clip[10];
    back.d = clip[15] - clip[14];


    top.n = normalize(top.n);
    bottom.n = normalize(bottom.n);
    left.n = normalize(left.n);
    right.n = normalize(right.n);
    front.n = normalize(front.n);
    back.n = normalize(back.n);
}

- (bool) pointInFrustum: (vec3) p withRadius: (float) r
{
    float topDistance = dot(top.n, p) + top.d;
    float bottomDistance = dot(bottom.n, p) + bottom.d;
    float leftDistance = dot(left.n, p) + left.d;
    float rightDistance = dot(right.n, p) + right.d;

    if(topDistance < 0) return false;
    if(bottomDistance < 0) return false;
    if(leftDistance < 0) return false;
    if(rightDistance < 0) return false;
    return true;
}