The PixMap record for your window thereby contains the correct pixel specifications for the main screen. Color QuickDraw internally calculates the changes required for drawing to any other screens. By default, the main screen is the startup screen. However, by using the Monitors control panel, the user can specify a different startup screen. The Window Manager saves this region, called the gray region, as the global variable GrayRgn .
Users can drag windows from one screen to another and even across multiple screens.
Color QuickDraw calculates the global coordinates of the rectangle into which it must draw and issues the drawing command to each video device that the rectangle intersects. Your application generally never needs to create GDevice records. However, you may find it useful for your application to examine GDevice records to determine the capabilities of the user's screens. When zooming a window, for example, your application can use GDevice records to determine which screen contains the largest area of a window, and then determine the ideal window size for that screen. You may also wish to use the DeviceLoop procedure to optimize your application's drawing for screens with different capabilities.