Depending on the color scheme used on your monitor, as you type your letter or play
your game, the text, cards, cars, or characters have contrast, colors, shapes, and shadows.
These elements of the display are generated in the lighting phase. In the transform phase,
the pixels and triangles are arranged to create the image desired by the application soft-
ware. Then any lighting effects are applied to the tips of the triangles.
As is discussed later, the CPU, the video card, or both may perform the transform and
lighting phases. In any case, the CPU sorts through the graphics instructions generated
by the software and either acts on them or sends it onto the video card for processing.
However, the final step, the setup phase, is always performed on the video card.Setup Phase
The setup phase of the video generation process maps out the image to specific pixels or
polygons on the screen. This very math-intensive process determines the vertical, horizontal,
and 3D placement of each bit of the data created by the transform and lighting phases to
describe the image. The graphic instructions are then mapped to specific locations on the
screen in what is called the hardware triangle setup, which prepares the data for display.Dividing Up the Work
If you are playing a video game on the PC and the scene shifts to the left, the software
running the game sends instructions to the video system that details the color and brightness
that each pixel in the display should be. These instructions are sent whether there is
movement or not. The display information is updated around 70 times per second to
eliminate screen flicker and to keep the animation on the screen from being choppy and
flowing smoothly. The information and instructions for the video display are embedded
in the data stream being sent to the CPU that may also include computation and data
retrieval requests.
The CPU separates the video display data from the graphics software’s data stream
and, depending on the age and technology of the video system, acts on the video instructions
or passes them onto the video card.
On older systems, the system CPU was used to perform the transform and lighting
phases on the graphic instructions generated by software. Of course, this meant that any
other tasks that needed the CPU, like moving data from a hard disk or performing a
computation, had to wait until the graphic instructions were processed and sent on to the
video card for the setup phase.
Newer video cards, such as graphics accelerators and 3D graphics cards, have the
processing power to perform the transform and lighting phases, along with the setup
phase. On a PC with a newer video card, the CPU is needed only to extract the transform
and lighting data from the graphic data stream and route it to the video card. This frees
the CPU to perform other tasks for the game, such as the physics or calculations related to
a game’s logic, or other applications running on the PC. The overall effect of the video
card processing the graphic data is that the entire PC performs more efficiently.Chapter 12: Video Cards^255