Objectives

Differentiate between raster scan display and random scan display.

Raster scan display

• The most common type of monitor employing a CRT is raster scan display.

• In Raster scan display the electron beam is swept across the screen one row at a time from top to bottom.

Disadvantage

Raster system produces jagged lines that are plotted as discrete points

Beam intensity is turned on and off to create a pattern of illuminated spots as the electron moves across each row .In a raster scan system entire screen is a matrix of pixels. Frame buffer or refresh buffer holds the set of all intensity values for all screen points. Stored intensity values are then retrieved from the refresh buffer and painted on the screen one row at a time.

Refreshing is carried out at the rate of 60 to 80 frames per sec.

A raster is a rectangular array of points or dots

• Horizontal Retrace

At the end of each scan line , the electron beam returns to the left side of the screen to begin displaying the next scan line. The return to the left of the screen , after refreshing each scan line is called horizontal retrace of electron beam.

• Vertical retrace

At the end of each frame the electron beam returns to the

top left corner of the screen to begin the next frame.

Random Scan Display

• The electron beam is directed only to the parts of the screen where a picture is to be drawn.

• Picture definition is stored as a set of line drawing commands in an area of memory referred to as refresh display file.

To display a specified picture ,the system cycles through a set of commands in the display file , drawing each component line . After processing all lines drawing commands the s/m cycle back to the first line command in the list.

AdvantageHas high resolution since picture definition is stored as

line drawing commands

o CRT has electron beam directed to the points of the screen where a picture is to be drawn.

o Refresh rate depends on the number of lines to be displayed.

o Picture definition is stored as a set of line drawing commands in an area of the memory called as refresh display file or frame buffer.

Adv:

Has high resolution since picture definition is stored as line drawing commands

• A special purpose processor called video controller or display controller is used to control the operation of the display device.

• Frame buffer can be anywhere in the s/m memory & video coordinate origin is upper left corner (in most pc’s) or lower left screen corner.

• Screen surface is represented as the first quadrant of a 2D system with positive x value increasing to right positive y value increasing from bottom to top.

•Scan lines are labelled from ymax at the top of the screen to 0 at the bottom and screen pixel positions from 0 to xmax

0 xmax

ymax

Raster Scan Display Processor• Raster scan system containing a separate

display processor is also called as graphics controller or display coprocessor.

• It frees the CDU from graphics function.

• In addition to the system memory a separate display processor memory is also provided.

• Display processor digitizes the picture definition into a set of pixel intensity values for storage in the frame buffer. This digitization process is called as scan conversion.

o Two registers are used to store the coordinates of screen pixels.

o Initially x register is set to zero and y register to ymax.

oThe value stored in the frame buffer for this pixel position is then retrieved and used to set the intensity of the CRT beam.

oThen x register is incremented by one and the process repeated for the next pixel on the top scan line. After the last pixel on the top scan line has been processed, the x register is reset to 0 and y register is decremented by one.

oAfter cycling through all pixels along the bottom scan line (y=0) video controller resets the register to the first pixel position on the top scan line and refresh process starts over.

Display Processor

Random

• Application program is the input and stored in the system memory along with a graphics package.

• The display file is accessed by the display processor to refresh the screen.

• Display processor is called as display processing unit or graphics controller

• Advantages and Disadvantages of raster scan system.

• Advantages – Low cost (memory has become cheap) – Refresh rate independent of image complexity – Can handle colour and filled areas

• Disadvantages – Uses more memory – Mathematical models of objects must be scan

converted by the host processor – aliasing – jagged lines that are plotted as discrete points

• Advantages and disadvantages of random scan system.

• Advantages are high resolution since picture definition is stored as line drawing commands, easy animation, and requires little memory (just display program),

• Disadvantages are limited colour capability and flicker occurs as complexity of image increases