Efficient Q-Tree Algorithm: Understanding Parent-Child Relationships

[sayuri2009]

TL;DR

Cities

Hi,

I don't get what the conditions below exactly means.

Its which u they are talking about? Is the u the parent or children? Does it mean u = B? and the children are A,C,D?
if u is in the quadtree rooted by v:NW then u:x < v:x and u:y ≥ v:y;

Thanks,

 

[pbuk]

Its which u they are talking about?

$u$ stands for any node, not any particular one.

Is the u the parent or children?

Every node can be both a parent and a child.

Does it mean u = B? and the children are A,C,D?

There are many ways to build a quadtree, but if you start with B at the root then $B.parent = B.NE = B.SE = \mathrm{null}$, $B.NW = A$ and $B.SW$ can be either of $C$ or $D$ depending on how you build the tree.

Edit: quadtrees are less useful for representing scattered points as they are likely to be sparse and unbalanced, they are more useful for representing a square lattice.