What is line plotting and point plotting system in display devices?

Answer 1

Point plotting is accomplished by converting a single coordinate position furnished

by an application program into appropriate operations for [he output device

in use. With a CRT monitor, for example, the electron beam is turned on to illuminate

the screen phosphor at the selected location. How the electron beam is

positioned depends on the display technology. A random-scan (vector) system

stores point-plotting instructions in the display list, and coordinate values in

these instructions are converted to deflection voltages that position the electron

beam at the screen locations to be plotted during each refresh cycle. For a blackand-

white raster system, on the other hand, a point is plotted by setting the bit

value corresponding to A specified screen position within the frame buffer to 1.

Then, as the electron beam sweeps across each horizontal scan line, it emits a

burst of electrons (plots a point) whenever a value of I is encounted in the sMian3-1

frame buffer. With an RGB system, the frame buffer is loaded with the color Pointsand hnes

codes for the intensities that are to be displayed at the s m n pixel positions.

Line drawing is accomplished by calculating intermediate positions along

the line path between two specified endpoint positions. An output device is then

directed to fill in these positions between the endpoints. For analog devices, such

as a vector pen plotter or a random-scan display, a straight line can be drawn

smoothly from one endpoint to the other. Linearly varying horizontal and vertical

deflection voltages are generated that are proportional to the required

changes in the x and y directions to produce the smooth line.

Digital devices display a straight line segment by plotting discrete points

between the two endpoints. Discrete coordinate positions along the line path are

calculated from the equation of the line. For a raster video display, the line color

(intensity) is then loaded into the frame buffer at the corresponding pixel coordinates.

Reading from the frame buffer, the video controller then "plots" the screen

pixels. Screen locations are xeferenced with integer values, so plotted positions

may only approximate actual Line positions between two specified endpoints. A

computed line position of (10.48,20.51), for example, would be converted to pixel

position (10,211. Tlus rounding of coordinate values to integers causes lines to be

displayed with a stairstep appearance ("the jaggies"), as represented in Fig 3-1.

The characteristic stairstep shape of raster lines is particularly noticeable on systems

with low resolution, and we can improve their appearance somewhat by

displaying them on high-resolution systems. More effective techniques for

smoothing raster lines are based on adjusting pixel intensities along the line

paths.

For the raster-graphics device-level algorithms discussed in this chapter, obp-

t positions are specified directly in integer device coordinates. For the time

being, we will assume that pixel positions are referenced according to scan-line

number and column number (pixel position across a scan line). This addressing

scheme is illustrated in Fig. 3-2. Scan lines are numbered consecutively from 0,

starting at the bottom of the screen; and pixel columns are numbered from 0, left

to right across each scan line. In Section 3-10, we consider alternative pixel addressing

schemes.

To load a specified color into the frame buffer at a position corresponding

to column x along scan line y, we will assume we have available a low-level procedure

of the form

Figure 3-1

Staintep effect (jaggies) produced

when a line is generated as a series

of pixel positions.

Line

Number

-

Plxd Column

Number

Figure 3-2

Pie1 positions referenced by scanline

number and column number.

We sometimes will also want to be able to retrieve the current framebuffer

intensity setting for a specified location. We accomplish this with the low-level

fundion

getpixel (x, y )