Trying to understand Quadtree concept and apply it to storing coloring info of an image

Question

I've read so many articles, but none seem to answer this question. Or maybe I'm just not understanding. I'm attempting to build a quadtree so that it can represent an image. The leaf nodes are to hold pixels, and non-leaf nodes will hold the average value pixel of its children.

My question is:

How does it work that the leaf nodes only hold pixels? Why don't the other nodes hold pixels? And how do we know how many times to subdivide our original root node to represent that given image? Do we just subdivide it n times, where n is the height and width (for a square)?

Edit: So how do I keep track of leaf nodes, so I know when to add pixels at that location? Right now I have a helper function that divides the regions for me, keeping track of width and height.

Answer 1

Quadtrees work best for square images whose size is a power of 2 (for example, most textures). You shouldn't think of each node as representing a "pixel". Instead, think of it as representing a "square block of pixels of size 2^k". In the case of final leaves, k is 0, so each leaf node represents a square block of pixels of size 1, that is, a single pixel. Internal nodes in the tree represent increasingly large sections of image.