lecture+ +raster+&+random+scan+systems
DESCRIPTION
includes a quick reference of raster and random scanTRANSCRIPT
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