A.1. Coordinate Systems

The screen coordinate system is the basis for all other coordinate systems used for positioning, sizing, drawing, and event handling. You can think of the entire screen as occupying the upper-right quadrant of a two-dimensional coordinate grid as shown in Figure 1.1. The other three quadrants, which are invisible to users, take negative values along their x-axis, their y-axis, or both axes. The screen's quadrant has its origin in the lower-left corner; the positive x-axis extends horizontally to the right and the positive y-axis extends vertically upward. A unit along either axis is expressed as a pixel.

Note that although Figure 1.1 represents the screen coordinate system using the example of a single display device, the screen coordinate system is really a logical rectangular area that is determined by the union of all screen rectangles of all physical framebuffers attached to the computer. The origin lies at the lower-left corner of that unioned rectangle.