Window Coordinates

Every window has its own integer coordinate system with the origin (0,0) at the top left corner of the window. The x-coordinate increases to the right and the y-coordinate increases downwards. Applications specify coordinates relative to a particular window, and if that window is moved around on the screen, its coordinate system moves with it and hence the objects in that window remain in the same location with respect to the window. Finally, a window's position is taken to be the location of its top left corner within its parent window coordinate system.