Raster scan systems use a video controller to control the display device and refresh the screen. The video controller is given direct access to a frame buffer in system memory. It refreshes the screen by incrementing through pixel coordinates stored in the frame buffer and using the stored intensity values to set the intensity of each pixel on the CRT monitor. More advanced systems use separate display processors and multiple frame buffers to speed up processing and enable real-time animation.

1. DISPLAY SYSTEMS

2. RASTER SCAN SYSTEMS
• Interactive raster graphics employ several
processing units
• Apart from CPU, a special purpose
processor called video controller or display
controller  control operation of the
display device
• Simple raster graphics system

3. VIDEO CONTROLLER
• Fixed area of system memory reserved for
frame buffer
• Video controller given direct access to
frame buffer to refresh the screen
• Coordinate origin is defined at lower left
corner
• Scan lines labeled from ymax at the top of
the screen to 0 at the bottom

4. BASIC REFRESH
OPERATIONS
• Registers store the
Horizontal and
Raster Scan Vertical Deflection
Generator Voltages
coordinates of the
screen pixels
X
Register
Y
Register • X  0; Y  ymax
• Value stored in frame
Memory Pixel
Intensity buffer is used to set
Address Register
the intensity of the
CRT
Frame
Buffer

5. • X incremented by 1; process repeated for
each pixel along the scan line
• Procedure is repeated for each scan line
• After cycling through all pixels, refresh
process starts over
• To speed up processing, multiple pixel
intensities are retrieved
• Stored in separate register to control the
CRT for group of adjacent pixels

6. • In high quality systems, two frame buffers
are provided
• One used for refreshing and other is filled
with intensity values
• Thus provides a fast mechanism for real
time animation – different views
successively loaded into refresh buffers

7. RASTER SCAN DISPLAY
PROCESSOR
Display processor Frame Video Monitor
Memory Buffer Controller
CPU Display System
Processor Memory
System bus

8. RASTER SCAN DISPLAY
PROCESSOR
• Separate display processor free CPU from
graphics chores
• Separate display processor memory also
provided
• Major task  digitizing picture definition
into set of pixel intensities : scan
conversion

9. • Display processor perform additional
operations like generating various line
styles, displaying various color areas and
performing certain manipulations
• To reduce memory requirements
 Run length encoding : Store each
scan line as a set of integer
 Cell encoding : Encode as a set of
rectangular areas

10. RANDOM SCAN SYSTEMS
• Organization
• Application program is input and stored in
system memory along with the graphics
package
• Graphics commands in the application
pgm is translated by graphics package into
a display file which is accessed by display
processor
• Display processor  DPU or graphics
controller

12. SIMPLE RASTER SYSTEM
System Video
CPU Monitor
Memory Controller
System Bus
I/O Devices
BACK

13. COMMON ORGANIZATION
FOR RASTER SYSTEMS
System Frame Video
CPU Monitor
Memory Buffer Controller
System Bus
I/O Devices
BACK

14. RANDOM SCAN SYSTEMS
System Display
CPU Monitor
Memory Processor
System Bus
I/O Devices
BACK