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Raster Scan And Random Scan

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Raster Scan And Random Scan

  1. 1. Raster Scan and Random Scan Systems
  2. 2. Raster Scan <ul><li>A Raster scan CRT works much like a television set. </li></ul><ul><li>Light occurs when an electron beam stimulates a phosphor. </li></ul>
  3. 3. Principle of Raster Scan Systems <ul><li>In Raster scan, the electron beam from electron gun is swept horizontally across the phosphor. </li></ul><ul><li>After each horizontal sweep the beam is moved. </li></ul><ul><li>After the bottom line is swept, the beam returns to the top and the sweep process begins again. </li></ul>
  4. 4. DIAGRAM SCANLINE HORIZONTAL RETRACE VERTICAL RETRACE
  5. 5. <ul><li>The lines numbered as 0,1,2,3,4,5..10 represent lines that the electrons beam is “painting” on the screen from left to right. </li></ul><ul><li>When the beam reaches the right side of the bottom line, it has to move back to the upper left corner of the screen. </li></ul>
  6. 6. <ul><li>Horizontal retrace is used to refer to the beam moving back to the left at the end of each line. </li></ul><ul><li>Vertical retrace refers to the movement from bottom to top. </li></ul>VERTICAL RETRACE HORIZONTAL RETRACE
  7. 7. INTERLACING <ul><li>The screen is painted 60 times per second but only half of the lines are painted per frame. </li></ul><ul><li>For example : Very odd-numbered lines, alternating back for forth between even-numbered and odd-numbered lines on each pass. </li></ul><ul><li>The entire screen, in two passes, is painted 30 times every second. </li></ul>
  8. 8. DIAGRAM
  9. 9. PROGRESSIVE SCANNING <ul><li>The alternative to interlacing is called progressive scanning, which paints every line on the screen 60 times per second. </li></ul><ul><li>Most computer monitor use progressive scanning because it significantly reduce flicker. </li></ul>
  10. 10. Random Scan <ul><li>In a Random scan system, also called vector, stoke writing, or calligraphic the electron beam directly draws the picture. </li></ul>A B C D E F
  11. 11. WORKING <ul><li>Here the electron gun of a CRT illuminates points and / or straight lines in any order. </li></ul><ul><li>The display processor repeatedly reads a variable ‘display file’ defining a sequence of X,Y coordinates the electron gun the whole display area is updated many times a second from image data held in raster memory. </li></ul>
  12. 12. Advantages of random scan: <ul><li>Very high resolution, limited only by monitor. </li></ul><ul><li>Easy animation, just draw at different position. </li></ul><ul><li>Requires little memory. </li></ul>
  13. 13. Disadvantages: <ul><li>Requires “intelligent electron beam, i.e., processor controlled. </li></ul><ul><li>Limited screen density before have flicker, can’t draw a complex image. </li></ul><ul><li>Limited color capability </li></ul>
  14. 14. THANK YOU………

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