About Graphics Devices
When you turn on the machine, the system gathers all of the gDevice records
into a DeviceList that shows all of the video cards attached to the Mac. The
graphics environment is defined by GrafPorts, which contain information about windows, and graphics device records, which contain information about
graphics devices such as screens that are attached to the system. Unless you
design startup procedures that specify otherwise, the gDevice record that
points to the video card in the lowest- numbered slot is the first one
en countered on the DeviceList, is designated as the active device and, hence,
the default startup screen. The startup screen is also known at the main
screen in multi-display setups and is the one that carries the menu bar.
An entire new set of functions and procedures have been created to deal with
multi-screen devices. With them, the user can set the desired depth, display
mode, presence or absence of a menu bar, and relative position of each video
display. All of these capabilities are available to the user through the Control Panel. A new resource type ('scrn') was created to store all of this information in the system file.
When you start a Quickdraw operation, InitGraf looks for 'scrn', which has the job of organizing the screens in a multiple display setup. If 'scrn' is
missing, InitGraf assumes the presence of only a single screen.
InitWindows puts together an area that combines all active screens and saves
it as the GrayRgn global variable. GrayRgn, in turn, gives the parameters of
the desktop.
Use the construction, FillRgn( GrayRgn,myPattern ), for programs that
paint the desktop; and write your programs so they pin moveable objects to
the GrayRgn instead of the the screen boundary. The latter point will let the
Window Manager stretch or drag windows from screen to screen while still letting Quickdraw draw to the window's port as if it were only looking at a single monitor. It accomplishes this by setting the window's base address field
equal to the base address of the main screen. Quickdraw then compares the screen. When they're equal, Quickdraw knows it may have to draw on more than one screen.
When there is more than one screen, Quickdraw calculates the entire drawing area, intersects the destination rectangle with the device's rectangle
and issues the drawing command to that portion of the total area when there's
an overlap. Color Quickdraw also provides the correct pixel value for the
foreground and background colors and re-expands patterns and other
structures appropriately for each device.
If you are developing a graphics-intensive application, you may need to create
Devices Manager's high-level routines probably provide all the features
you need. With these routines you can create an offscreen graphics world:
a graphics environment which you control specifications such as pixel depth
and whether the pixel values are indexed or direct. Using these routines
minimizes the possibility of compatibility problems, because these routines
and data structures, collectively called the offscreen graphics world interface,
will be maintained by Apple as the Macintosh graphics systems evolve. With
system software version 7.0, the offscreen graphics routines are available on
black-and-white machines (lacking Color QuickDraw), but they create an