G

gamma ramp

A lookup table (LUT) of numeric values used to compensate for a particular monitor's color response. The table is divided into three parts (one each for red, green, and blue), with each part containing 256 lookup values.

A gamma ramp is used to map the range of possible intensities for each color to a limited number (256) of values. An IHV chooses these values so that the transition from one color intensity to the next is as smooth as possible, given that the eye perceives changes in light intensity nonlinearly.

DirectDraw® takes a pixel from the frame buffer, and examines it in terms of its individual red, green, and blue color components. Each component is represented by a value from 0 to 65535. DirectDraw takes the original value, and uses it as an index into a 256-element array (the ramp), where each element contains a value that replaces the original one. DirectDraw performs this "look-up and replace" process for each color component of each pixel within the frame buffer, thereby changing the final colors for all of the on-screen pixels. For more information, see the Platform SDK documentation.

generic printer description (GPD)

A text-based file used to describe a printer that is printing from Microsoft® Windows® 2000 and later using the Universal Printer Driver (Unidrv). A GPD contains information about the printer features that Unidrv presents in the user interface to allow the user to configure the printer.

A GPD also contains the printer commands and attributes that enable Unidrv to generate the correct printer-ready output data when the user prints from a Windows-based application. Creating a printer driver for Windows 2000 and later can be as simple as creating a GPD file if there is no custom functionality beyond what Unidrv provides.

glyph

The actual shape (bit pattern, outline, and so forth) of a character image. For example, an italic 'a' and a roman 'a' are two different glyphs that represent the same underlying character. Synonymous with "character image" or simply "image."

glyph translation table (GTT)

A table that maps characters with the bitmaps that are used to display those characters.

gradient fill

A technique used to fill a rectangular or triangular region with color that changes smoothly across the region. For rectangular regions, the gradient fill can be specified as top to bottom, or left to right.

See DrvGradientFill.

graphics capabilities bits (GCAPS)

A driver specifies its graphics capabilities by setting GCAPS_Xxx and GCAPS2_Xxx flags in the flGraphicsCaps and flGraphicsCaps2 members of the DEVINFO structure. GDI obtains the GCAPS when it calls the driver's DrvEnablePDEV routine.

graphics device interface (GDI)

Consists of Win32® GDI and kernel-mode GDI. Win32 GDI is a user-mode API used by Win32 applications that require graphics support. Kernel-mode GDI interfaces directly with kernel-mode graphics drivers, as well as the window manager. Kernel-mode GDI exports several services and functions that can be called by device drivers to perform a host of drawing and graphics-related operations. This eliminates the need for graphics drivers to implement much of the required graphics functionality.

The Win32 GDI library that is directly accessible to Win32 applications can be found in gdi32.dll. It fields calls to the functions listed in wingdi.h and passes the application-supplied information to kernel-mode GDI by way of executive system services in the NT-based operating system. The kernel-mode GDI library is found in win32k.sys. Kernel-mode GDI communicates with a graphics driver by calling the driver's implementations of the DDI functions listed in winddi.h.

graphics engine

Another name for GDI.

See also the kernel-mode GDI definition under GDI.

GUID

Globally unique identifier. A 16-byte quantity that is unique, having been generated from the unique identifier on a network card, the current date and time, and a sequence number. This combination of variables is used to allow any party to create identifiers that will be guaranteed not to match another GUID. For more information, see Using GUIDs in Drivers.