which point was got from bbox?

[lishi8168797]

hello,I can not view your blog at http://imagingsolution.blog107.fc2.com/blog-entry-137.html,but I want to know which point was got from bbox in this code.

VectorXd getCpFromBbox(PointCloud bBox){
PointXYZ p1 = bBox[0];
PointXYZ p2 = bBox[1];
PointXYZ p3 = bBox[2];
PointXYZ p4 = bBox[3];

double S1 = ((p4.x -p2.x)*(p1.y - p2.y) - (p4.y - p2.y)*(p1.x - p2.x))/2;
double S2 = ((p4.x -p2.x)*(p2.y - p3.y) - (p4.y - p2.y)*(p2.x - p3.x))/2;
double cx = p1.x + (p3.x-p1.x)*S1/(S1+S2);
double cy = p1.y + (p3.y-p1.y)*S1/(S1+S2);

VectorXd cp(2);
cp << cx, cy;
return cp;

}