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Lecture+ +raster+&+random+scan+systems

includes a quick reference of raster and random scan

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Lecture+ +raster+&+random+scan+systems

  1. 1. Raster scan systems
  2. 2. Raster scan system <ul><li>The operation of the display device is controlled by a special-purpose processor called video controller or display controller </li></ul><ul><li>Fig : Architecture of a simple raster graphics system </li></ul>
  3. 3. Raster scan systems <ul><li>Frame buffer can be anywhere in the system memory & video controller access the frame buffer to refresh the screen </li></ul><ul><li>Other processors such as coprocessors & accelerators implement various graphics operations </li></ul>
  4. 4. Fig : Architecture of a raster system with a fixed portion of the system reserved for frame buffer
  5. 5. Video controller <ul><li>Screen positions in frame buffer locations are referenced in Cartesian coordinates </li></ul><ul><li>Fig : Origin of coordinate system for identifying screen positions is usually specified in the lower left corner </li></ul>
  6. 6. Basic refresh operations <ul><li>2 registers are used to store the coordinates of the screen pixels (xreg,yreg) </li></ul><ul><li>Initially xreg is set to zero & yreg is set ymax </li></ul><ul><li>Value from frame buffer for this pixel position is retrieved & used to set the intensity value of the CRT beam </li></ul><ul><li>xreg is incremented by 1 & process repeated for each pixel along the scan line </li></ul><ul><li>Then xreg is reset to zero & yreg is decremented by one </li></ul><ul><li>The process repeated till y=0, x=xmax </li></ul>
  7. 7. video controller <ul><li>Video controller resets the register to the first pixel position on top scan line & the refresh process starts over </li></ul>
  8. 8. Basic video controller refresh operation <ul><li>Refreshing is 60 frames per second </li></ul><ul><li>Cycle is too slow </li></ul><ul><li>To speed up pixel processing, video controllers can retrieve multiple pixel values from refresh buffer on each pass </li></ul><ul><li>Multiple pixel intensities (control beam intensity for a group of adjacent pixels) are then stored in pixel register </li></ul><ul><li>In high quality systems, 2 frame buffers are used </li></ul><ul><ul><li>1 – refreshing </li></ul></ul><ul><ul><li>2- filling intensity values </li></ul></ul><ul><ul><li>Fast mechanism for generating real-time animations </li></ul></ul>
  9. 9. Fig : Architecture of Raster scan graphics system with a display processor
  10. 10. Raster scan display processor <ul><li>Display processor or graphics controller or display coprocessor </li></ul><ul><li>Display processor – free the CPU from the graphics chores </li></ul><ul><li>In addition to the system memory, a separate display processor memory area can be provided </li></ul><ul><li>Display processor digitize a picture definition into a set of pixel intensity values for storage in frame buffer </li></ul><ul><li>This digitization process is called scan conversion </li></ul>
  11. 11. <ul><li>Character defined with rectangular grids of pixel positions (rectangular grid pattern – frame buffer) </li></ul><ul><li>Character defined as a curve outline (curve outlines, character shapes – frame buffer) </li></ul>
  12. 12. Display processor operation <ul><li>Generating various line styles (dashed, dotted or solid) </li></ul><ul><li>display color area </li></ul><ul><li>performing certain transformations </li></ul><ul><li>manipulations on displayed objects </li></ul><ul><li>To reduce memory </li></ul><ul><ul><li>Frame buffer is organized as linked list </li></ul></ul><ul><ul><li>Encoding intensity information </li></ul></ul>
  13. 13. 1. Run-length encoding <ul><li>Store each scan line as a set of integer pairs </li></ul><ul><li>First number of each pair indicates intensity value </li></ul><ul><li>Second number specifies the number of adjacent pixels on the scan line with that intensity value </li></ul><ul><li>Advantage – save storage space if a picture is to be constructed with long runs of single color </li></ul>
  14. 14. 2.Cell encoding <ul><li>Encode the raster as set of rectangular areas </li></ul><ul><li>Disadvantage </li></ul><ul><ul><li>Intensity changes are difficult to make </li></ul></ul><ul><ul><li>Storage requirements actually increase as length of runs decrease </li></ul></ul>
  15. 15. Random scan systems (Vector systems) <ul><li>Fig : Architecture of random scan system </li></ul>
  16. 16. Random scan systems <ul><li>Application program is input & stored in system memory along with a graphics package </li></ul><ul><li>Graphics commands in application program are translated to display file by graphics package & stored in system memory </li></ul><ul><li>Display processor access display file to refresh the screen </li></ul><ul><li>At each refresh cycle, display processor cycles through each command in display file </li></ul>
  17. 17. Random scan systems <ul><li>Display processor in random scan system is also called as display processing unit or a graphics controller </li></ul><ul><li>Graphics patterns are drawn by directing e- beam along the component lines of the picture </li></ul><ul><li>Lines are defined by values for their coordinate endpoints & input coordinate values are converted to x& y deflection voltages </li></ul><ul><li>A scene is then drawn one line at a time by positioning the beam to fill in the line between specified endpoints </li></ul>