Lecture+ +raster+&+random+scan+systems
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  • 1. Raster scan systems
  • 2. 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
  • 3. 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
  • 4. Fig : Architecture of a raster system with a fixed portion of the system reserved for frame buffer
  • 5. 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
  • 6. 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
  • 7. video controller
    • Video controller resets the register to the first pixel position on top scan line & the refresh process starts over
  • 8. 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
  • 9. Fig : Architecture of Raster scan graphics system with a display processor
  • 10. 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
  • 11.
    • 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)
  • 12. 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
  • 13. 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
  • 14. 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
  • 15. Random scan systems (Vector systems)
    • Fig : Architecture of random scan system
  • 16. 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
  • 17. 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