To draw an image on a CRT, one must first specify
the image mathematically. Most CRT use Cartesian coordinates to
specify a point on the CRT screen. Given an
origin (a point) and
and
axes (vectors),
any point
can be express in the following form
where
and
are scalars. The tuple
are the Cartesian coordinate of
with respect to
,
, and
.
By convention, the origin is usually placed in a corner of the screen.
For most hardware devices, the origin is at the upper left corner
of the screen. The axis points to the right and the
axis points
down. Unfortunately, most graphics and math software does not obey
this convention. In software, the origin is usually place at the lower left
corner with the
axis pointing to the right and
axis pointing upward.
There are two main types of CRT displays used in computer graphics. The first type, random-scan displays, are used primarily to draw sequences of line segments. The controller for the CRT maintains a display list that consists of a sequence of line segments specified by their endpoints in screen coordinates. By continuously varying the voltage applied to the horizontal and vertical deflection plates, the electron beam traces out each line segment. Due to the simplicity of this process, the electron beam can traced out a large (up to 100K) number of line segments at up to 60 times per second. By continuous redrawing the line segments, a solid line segment appears visible to the viewer. Random-scan displays were popular in the late 60's and early 70's. Popular video games such as ``Asteriods'' used random-scan display. Engineering workstations also used random-scan displays to display 2D blueprints.
A second type of CRT display became popular in the laste 70's and early 80's, the raster-scan display. Raster-scan displays represent the screen as a logical collection of blocks known as pixels. Typically, pixels correspond to integer coordinates in the screen coordinate system. A standard raster-scan display might use a 1024 by 1024 grid of pixels. The CRT maintains a 2D array of intensities associated with these pixels. This array is the frame buffer.
To draw an actual image, the electron beam for the CRT focuses on each pixel for a time proportionate to the contents of that pixel's entry in the frame buffer. On black and white displays, this entry is either a zero (black) or one (white). On a gray scale display, this entry is typically an eight bit number that corresponds varies from black to white.
Raster-scan CRTs can also produce arbitrary colors in a given pixel. Usually, these colors are represented as combinations of red, green and blue light of various intensities. These three lights are generated by splitting each pixel in three different areas. Each area is covered by a type of phosphor that flouresces either red, green, or blue. The three areas are close enough to be visually indistinguishable. The three color blend smoothly to form a single color.
The idea of representing a color are a linear
combination of red, green and blue is known
as the RGB color model. This model is not
the only possible color model. For example, most
artists done paint using combinations of only red, green and
blue paint. Later in the course we will discuss other possible color models.
However, most color CRTs use the RGB model in their hardware.
Typically, bits are allocated
for each pixel;
bits for red intensity,
bits for green
intensity and
bits for blue intensity.