Computer Graphics
Spring, 2000

Display Devices
   Vector Scan
   Raster Scan

Both based on CRT (TV)
   Electron beam accelerated toward screen, focused
   Beam deflected by magnetic or electric fields
   Strikes phosphorescent material on screen
   -->"point" that glows
   Pixel--visible point where electron beam hits screen
        Screen phosphors glow & fade
        Have a finite size

   Max number of pixels that can be plotted without overlap
   Expressed in # horizontal X # vertical pixels
   Depends on:
     phosphor used
     focusing system (how small a point)
     deflection system (speed of moving an turning on/off beam)
     size of video memory (raster scan)--as we'll see

Aspect Ratio--
   Ratio of # of columns of pixels to # of rows of pixels
   e.g., SVGA VESA mode 100h: Res = 640 X 400, A.R. = 1.6
         Standard Windows: Res = 640 X480. A.R. = 1.33 

Pixel Ratio (many authors call this Aspect Ratio)--
   Ratio of height of a pixel to its width
   i.e., Ratio of # of horizontal pixels to vertical pixels needed
      to produce equal length lines
   For a square screen, A.R. = P.R.
   If P.R. != 1, figures are distorted (squares --> rectangles)
Dot Pitch--
   Distance between centers of adjacent pixels of same color
   Should be less than 0.28 mm for sharp graphics images
   For fixed size screen, Inc Resolution ==> Dec dot pitch

   After beam leaves a phosphor, it fades
   Time for intensity to be reduced by 10% of initial level
   Value depends on type of phosphor (10 - 100 msec.)
   Finite persistence ==> screen must be redrawn (refresh)
   Refresh rate determined by persistence
   If persistence = 20 msec, & we're painting entire screen,
      first pixel on screen will be invisible after that time ==>
      screen must be refreshed at least once every 20 msec ==>
      Refresh rate must be > 50 Hz.

Typical Graphics Hardware System--

   CPU--Runs user program that determines what is to be drawn
   CRT--does the actual display
   Display Controller--Provides analog voltages needed to move
      beam and vary its intensity
   DPU (display processing unit)--Dedicated processor that
      generates the signals needed to drive display controller;
      (Offloads task of video control to separate processor)
   VRAM (Video memory)--Stores info needed to draw the picture;
      Dual-ported (written to by CPU, read from by DPU);
      Very fast (640X480, 50 Hz ==> 65 nsec access time!)
      Also called Refresh Buffer or Frame Buffer
   I/O devices (mouse, keyboard, scanner, printer, etc.)--
      interface CPU with user)

   VRAM, DPU, & Display Controller often on a single video card.

Vector Scan systems-
   Also called random, stroke, calligraphic displays
   Images drawn as line segments (vectors)
   Beam can be moved to any position on screen
   Refresh Buffer stores plotting commands
      ==> Refresh Buffer called "Display File"
   These provide DPU with needed endpoint coordinates
   Pixel size independent of frame buffer
      ==> very high resolution

Advantages of Vector Scan--
   High resolution (good for detailed line drawings)
   crisp lines (no "jaggies")
   high contrast (beam can dwell on a pixel==> very intense)
   selective erase (remove commands from display file)
   animation (change line endpoints slightly after each refresh)

Disadvantages of Vector Scan--
   Complex drawings can have flicker--
      Many lines ==> time to draw > refresh time ==> flicker
   High cost--very fast deflection system
   Hard to get colors
   No area fill ==> difficult to use for realistic (shaded) images
   1960s Technology, only used for special purpose stuff today

Raster Scan Systems (TV Technology)--
   Beam continually traces back & forth, up & down raster pattern
   Intensity adjusted as raster scan takes place
      ==> brightness of pixels varies in time
   Each pixel on screen visited during each scan
   Scan rate should be at least 30 Hz to avoid flicker
   Refresh buffer stores intensity values for each pixel on screen
      ==> Resolution determined by size of Frame buffer
   Simplest system: a bit per pixel; frame buffer called a bit map
   Gray Scale--n bits per pixel ==> 2^n intensity values possible
   Memory intensive-- 1000 X 1000 X 256 shades of gray ==> 8 Mbits

Scan Conversion--
   Process of determining which pixels need to be turned on in the
   frame buffer to draw a given graphics primitive.

Advantages of Raster Scan Systems--
   Low cost (TV technology)
   Area fill (entire screen is painted with each scan)
   Selective erase (just change parts of bitmap)
   Bright display, good contrast (but not as good a vector scan)

   Large memory requirement for high resolution (but cost of VRAM
      has decreased enormously!)
   Aliasing (due to finite size of pixels) -->
      Jagged lines (staircase effect)
      Scintillation, "creep", Moire patterns, esp in animations

Raster scan is the principal "now" technology for graphics displays!