Trace the index of x,y points across an indexed mask?

Hi guys,

I have an indexed mask (divided into 4 quadrants, see attached jpeg image and matrix 'Indexed mask') and I want to trace the index of individual x,y points of my matrix (see attached matrix 'Points x_y'), across this indexed mask.

In other words, I really want to measure how many times my x,y points change index (quadrant)? For example, say that my first x, y point is located in index 1 and my second x, y point is located in index 2. This is my first quadrant change (from index 1 to index 2). Then, my third x,y point goes back to index 1 again. This is my second quadrant change (from index 2 to 1). My fourth x,y point is located in index 3. This is my third quadrant change (from 1 to 3) and so on.

I would appreciate your help.

George