Graphics card

Prior to the late 1970s, most computer applications (other than some scientific and experimental ones) did not use graphics. However, the early microcomputer systems such as the Apple II, Radio Shack TRS-80, and Commodore PET could all display graphics, either on a monitor or (with the aid of a video modulator) on an ordinary TV set. While primitive (low resolution; monochrome or just a handful of colors) this graphics capability allowed for a thriving mar-ket in games and educational software.

The earliest video displays for mainstream PCs pro-vided basic text display capabilities (such as the MDA, or monochrome display adapter, with 25 lines of text up to 80 characters per line) plus the ability to create graphics by setting the color of individual pixels. The typical low end graphics card of the early 1980s was the CGA (Color Graphics Adapter), which offered various modes such as 320 by 200 pixels with four colors. Computers marketed for professional use offered the EGA (Enhanced Graph-ics Adapter), which could show 640 by 350 pixels at 16 colors.

The ultimate video display standard during the time of IBM dominance was the VGA (Video Graphics Array), which offered a somewhat improved high resolution of 640 by 480 pixels at 16 colors, with an alternative of a lower 320 by 280 pixels but with 256 colors. Because of its use of a color palette containing index values, the 256 colors can actually be drawn from a range of 262,144 possible choices. VGA also marked a break from earlier standards because in order to accommodate such a range of colors it had to convert digital information to analog signals to drive the monitor, rather than using the digital circuitry found in earlier monitors.

Modern video cards can be loosely described as imple-menting SVGA (Super VGA), but there are no longer dis-crete standards. Typical display resolutions for desktop PCs today are 1024 by 768 or 1280 by 1024 pixels. (Laptops tra-ditionally have had a lower-resolution 800 by 600 display, but many are now comparable to desktop displays.) The range of colors is vast, with up to 16,777,216 possible colors stored as 32 bits per pixel.

Storing 32 bits (4 bytes) for each of the pixels on a 1024 by 768 screen requires more than 3 megabytes. However, this is just for static images. Games, simulations, and other applications use moving 3D graphics. Since a computer screen actually has only two dimensions, mathematical algorithms must be used to transform the representation of objects so they look as if they have three dimensions, appearing in proper perspective, with regard to what objects are behind other objects, and with realistic lighting and shading (see computer graphics).

Traditionally, all of the work of producing the actual screen data was undertaken by the PC’s main processor, executing instructions from the application program and display driver. By putting a separate processor on the video card (called a video accelerator), together with its own sup-ply of memory (now up to 256 MB), the main system was freed from this burden. A new high-bandwidth connection between the PC motherboard and the graphics card became available with the development of the AGP (Accelerated Graphics Port). (See bus.) Memory used on video cards is also optimized for video operations, such as by using types of memory such as Video RAM (VRAM) that do not need to be refreshed as frequently.

Increasingly, the algorithms for creating realistic images (such as lighting, shading, and texture mapping) are now supported by the software built into the video card. Of course, the applications program needs a way to tell the graphics routines what to draw and how to draw it. In systems running Microsoft Windows, a program function library called Direct3D (part of a suite called DirectX) has become the standard interface between applications and graphics hardware. Video card manufacturers in turn have optimized their cards to carry out the kinds of operations implemented in DirectX. (A nonproprietary standard called OpenGL has also achieved some acceptance, particularly on non-Windows systems.)

In evaluating video cards, the tradeoff is between the extent to which advanced graphic features are supported and the number of frames per second that can be calculated and sent to the display. If the processing becomes too com-plicated, the frame rate will slow down and the display will appear to be jerky instead of smooth.