gistfile1.pde

// which part of the screen is my mouse in?
 
int grid;
 
void setup ()
{
    size( 500, 500 );
    grid = 3;
}
 
void draw ()
{
    background(205);
    int rx = mouseX / (width/grid);  // should precalc width/grid, height/grid
    int ry = mouseY / (height/grid);
    rect( rx*(width/grid), ry*(height/grid), (width/grid), (height/grid) );
}