Raster scan systems

Raster scan system The operation of the display device is

controlled by a special-purpose processor called video controller or display controller

Fig : Architecture of a simple raster graphics system

Raster scan systems

Frame buffer can be anywhere in the system memory & video controller access the frame buffer to refresh the screen

Other processors such as coprocessors & accelerators implement various graphics operations

Fig : Architecture of a raster system with a fixed portion of the system reserved for frame buffer

Video controller Screen positions in

frame buffer locations are referenced in Cartesian coordinates

Fig : Origin of coordinate system for identifying screen positions is usually specified in the lower left corner

Basic refresh operations 2 registers are used to store the coordinates

of the screen pixels (xreg,yreg) Initially xreg is set to zero & yreg is set ymax Value from frame buffer for this pixel position

is retrieved & used to set the intensity value of the CRT beam

xreg is incremented by 1 & process repeated for each pixel along the scan line

Then xreg is reset to zero & yreg is decremented by one

The process repeated till y=0, x=xmax

video controller Video controller resets the register to the first

pixel position on top scan line & the refresh process starts over

Basic video controller refresh operation Refreshing is 60 frames per second Cycle is too slow To speed up pixel processing, video controllers can

retrieve multiple pixel values from refresh buffer on each pass

Multiple pixel intensities (control beam intensity for a group of adjacent pixels) are then stored in pixel register

In high quality systems, 2 frame buffers are used 1 – refreshing 2- filling intensity values Fast mechanism for generating real-time animations

Fig : Architecture of Raster scan graphics system with a display processor

Raster scan display processor

Display processor or graphics controller or display coprocessor

Display processor – free the CPU from the graphics chores

In addition to the system memory, a separate display processor memory area can be provided

Display processor digitize a picture definition into a set of pixel intensity values for storage in frame buffer

This digitization process is called scan conversion

Character defined with rectangular grids of pixel positions (rectangular grid pattern – frame buffer)

Character defined as a curve outline (curve outlines, character shapes – frame buffer)

Display processor operation

Generating various line styles (dashed, dotted or solid)

display color area performing certain transformations manipulations on displayed objects To reduce memory

Frame buffer is organized as linked list Encoding intensity information

1. Run-length encoding

Store each scan line as a set of integer pairs First number of each pair indicates intensity

value Second number specifies the number of

adjacent pixels on the scan line with that intensity value

Advantage – save storage space if a picture is to be constructed with long runs of single color

2.Cell encoding

Encode the raster as set of rectangular areas Disadvantage

Intensity changes are difficult to make Storage requirements actually increase as length

of runs decrease

Random scan systems (Vector systems)

Fig : Architecture of random scan system

Random scan systems

Application program is input & stored in system memory along with a graphics package

Graphics commands in application program are translated to display file by graphics package & stored in system memory

Display processor access display file to refresh the screen

At each refresh cycle, display processor cycles through each command in display file

Random scan systems

Display processor in random scan system is also called as display processing unit or a graphics controller

Graphics patterns are drawn by directing e- beam along the component lines of the picture

Lines are defined by values for their coordinate endpoints & input coordinate values are converted to x& y deflection voltages

A scene is then drawn one line at a time by positioning the beam to fill in the line between specified endpoints