Introduction - Computer Graphics and Geometric Modeling - page 7

Figure 1.7. The raster scan CRT.

particular, one does not have to worry about refresh rates or flicker and they are not
as bulky.
The hardware assumption made in this topic, one that should apply to two-dimen-
sional displays in the foreseeable future, is that the reader is working on a raster scan
device. This assumption has an important consequence. Raster scan devices use a
refresh buffer to specify which dots on the screen are to be lit and how. To get the
picture we want, we only have to set the values in that buffer correctly. Therefore, our
abstract representation problem specializes to representing subsets of Euclidean
space as (discrete) subsets of a rectangle in Z 2 . Less formally, we shall talk about rep-
resenting objects in a “raster.” A raster refers to a two-dimensional rectangular array
of pixels, where a pixel is an abbreviation for “picture element,” which could, in theory,
be any value. In practice, a pixel is represented in computer memory by one or more
bits that specify a color. A monochrome picture is where each pixel is represented by
only one bit. A row in a raster is called a scan line . If the raster has m columns and
n rows, then we say that the resolution of the picture is m ¥ n.
The hardware graphics standards for computers have evolved over time. The stan-
dards for the IBM personal computer (PC) are listed in chronological order below:

Type Resolution Number of colors

CGA 640 ¥ 200 2 (black plus one other)

Hercules 720 ¥ 348 2 (black and white)

EGA 640 ¥ 350 16

VGA 640 ¥ 480 16

super VGA ≥ 800 ¥ 600 ≥ 256

For more details about these standards see [Wilt87] or [Ferr94].
The refresh buffer of a raster scan device is usually called a frame buffer . In
general, the term “frame buffer” refers to an array of memory (separate from main
memory) thought of as a two-dimensional array of pixels (a raster). Frame buffers
serve two functions:

(1) as a place where the image is stored as it is computed
(2) as a refresh buffer from which the image is displayed on a raster device