Graphics Device Interface

Headline text

The Graphics Device Interface (GDI, sometimes called Graphical Device Interface) is one of the three core components or "subsystems", together with the kernel and the user (window manager), of Microsoft Windows.

GDI is a Microsoft Windows interface for representing graphical objects and transmitting them to output devices such as monitors and printers.

GDI is responsible for tasks such as drawing lines and curves, rendering fonts and handling palettes. It is not directly responsible for drawing windows, menus, etc.; that task is reserved for the user subsystem, which resides in user32.dll and is built atop GDI. GDI is similar to Apple's classic QuickDraw.

Perhaps the most significant capability of GDI over more direct methods of accessing the hardware is its scaling capabilities, and abstraction of target devices. Using GDI, it is very easy to draw on multiple devices, such as a screen and a printer, and expect proper reproduction in each case. This capability is at the centre of all WYSIWYG applications for Microsoft Windows.

Simple games which do not require fast graphics rendering, such as Freecell or Minesweeper, use GDI. However, GDI cannot animate properly (no notion of synchronizing with the framebuffer) and lacks rasterization for 3D. Modern games use DirectX or OpenGL, which give programmers access to more hardware capabilities.

GDI printers

A Winprinter (similar to a Winmodem) is a print processor that uses software to do all the print processing instead of requiring the printer hardware to do it. It works by rendering an image to a bitmap on the host computer and then sending the bitmap to the printer.

This allows low-cost printers to be built by printer manufacturers, because all the page composition is done in software. Usually, such printers do not natively support PostScript and use a Unidrv-based printer driver. A Winprinter uses GDI to prepare the output. Hence, the printer is often also called a GDI printer.

In general, usually, the lowest-cost current-model laser printers are GDI devices. Most manufacturers also produce more flexible models that add PCL compatibility, or PostScript, or both. In most cases it is only the very lowest-cost models in any given manufacturer's range that are GDI-only.

Technical details

A Device Context (DC) is used to define the attributes of text and images that are output to the screen or printer. The actual context is maintained by GDI. A DC, which is a handle to the structure, is obtained before output is written and released after the elements have been written.

A DC, like most GDI objects, is opaque, meaning that you can't access its data directly, but you can pass it to various GDI functions that will operate on it, either to draw something, to get information about it, or to change the object in some way........rama krishna