Graphics Device Interface

The Graphics Device Interface (GDI) is a Microsoft Windows application programming interface and core operating system component responsible 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 Macintosh's QuickDraw and GNOME/GTK's GDK/Xlib.

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 center of all What You See Is What You Get applications for Microsoft Windows.

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

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 handle to the Device Context (HDC) is obtained before output is written and then released after elements have been written.

A DC, like most GDI objects, is opaque - its data cannot be accessed directly, but its handle can be passed to various GDI functions that will operate on it, either to draw an object, to retrieve information about it, or to change the object in some way.

Windows XP
With the introduction of Windows XP, GDI was deprecated in favor of its successor, the C++ based GDI+ subsystem. GDI+ adds anti-aliased 2D graphics, floating point coordinates, gradient shading, more complex path management, intrinsic support for modern graphics-file formats like JPEG and PNG, and support for composition of affine transformations in the 2D view pipeline. GDI+ uses ARGB values to represent color. Use of these features is apparent in Windows XP's user interface and several of its applications such as Microsoft Paint, Windows Picture and Fax Viewer, Photo Printing Wizard, My Pictures Slideshow screensaver, and their presence in the basic graphics layer greatly simplifies implementations of vector-graphics systems such as Flash or SVG.

GDI+ is included with all versions of Windows from Windows XP. The GDI+ dynamic library can also be shipped with an application and used under older versions of Windows from Windows 98 and Windows NT 4.0 onwards. Because of the additional text processing and resolution independence capabilities in GDI+, text rendering is performed by the CPU and it is nearly an order of magnitude slower than in GDI. Chris Jackson, an application compatibility expert working for Microsoft published some tests indicating that a piece of text rendering code he had written could render 99,000 glyphs per second in GDI, but the same code using GDI+ rendered 16,600 glyphs per second. The Microsoft .NET class library provides a managed interface for GDI+ via the  namespace.

GDI+ is similar (in purpose and structure) to Apple's Quartz 2D subsystem, and the open-source libart and Cairo libraries.

Windows Vista
In Windows Vista, all Windows applications including GDI and GDI+ applications run in the new compositing engine, Desktop Window Manager which is built atop the Windows Display Driver Model. The GDI render path is redirected through DWM, and GDI is no longer hardware-accelerated by the video card driver. However, due to the nature of desktop composition (internal management of moving bitmaps and transparency and anti-aliasing of GDI+ being handled at the DWM core), operations like window moves can be faster or more responsive because underlying content does not need to be re-rendered by the application.

Windows 7
Windows 7 includes GDI hardware acceleration for blitting operations. This improves GDI performance using new features in the Windows Display Driver Model v1.1. This allows the DWM engine to use local video memory for compositing, thereby reducing system memory footprint and increasing the performance of graphics operations. Most primitive GDI operations are still not hardware-accelerated, unlike Direct2D. As of November 2009, both ATI and Nvidia have released WDDM v1.1 compatible video drivers.

GDI+ will continue to rely on software rendering in Windows 7.

GDI printers
A GDI printer or 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 a page description language such as PostScript or XPS. A Winprinter uses GDI to prepare the output, which is then passed to the printer driver (usually supplied by the manufacturer) for further processing and only afterwards to the printer itself.

In general, the lowest-cost 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.

Compare how GDI printers rely on this 2-way constant communications between the PC and the printer to leach off the PC's processor, as opposed to standard printers, which send a completed file to a standard printer for it to create. Thus, a GDI printer cannot create a document without a PC attached. This is why a GDI printer won't print when attached to a standard stand-alone print server, though some manufacturers have created a solution to this problem by making print servers that are capable of processing data. Another solution would be to physically attach the GDI printer to a PC or file server, and share the printer via this connection (though the PC must be constantly left on for the GDI printer to be accessible).

Limitations
Each time a window is opened, it consumes GDI objects. As the complexity of the window increases, with additional features such as buttons and images, its GDI object usage also increases. When too many objects are in use, Windows is unable to draw any more GDI objects, leading to misbehaving software and frozen and unresponsive program operation. The total available GDI objects varies from one version of Windows to the next: Windows 95, 98, and Millennium had a limit of 1,200 total objects; Windows 2000 has a limit of 16,384 objects; and Windows XP, Vista, and Windows 7 have a configurable limit (via the registry) that defaults to 10,000 objects per process (but a theoretical maximum of 65,536 for the entire session).

Earlier versions of Windows such as Windows 3.1 and Windows 98 included a Resource Meter program to allow the user to monitor how much of the total system GDI resources were in use. Later versions such as Windows 2000 and Windows XP can report GDI object usage for each program in the Task Manager, but they cannot tell the user the total GDI capacity available.

Overflowing GDI capacity can affect Windows itself, preventing new windows from opening, menus from displaying, and alert boxes from appearing. The situation can be difficult to clear and can potentially require a forced hard-reset of the system, since it prevents core system programs from functioning.

For example, forcing a frozen process to end using the Task Manager normally makes an "Are you sure" alert window appear. With no free GDI, Windows beeps an error and the alert choice does not appear, so the GDI-overflowing processes cannot be terminated.

Sometimes a single application can consume the entire free desktop heap memory. There is a tool from Microsoft called Desktop Heap Monitor which can show which application consumes what percent of the desktop heap