CS-460/560 Computer Graphics Spring, 2000
GRAPHICS HARDWARE 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 Resolution- 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 Persistence-- 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) Colors Selective erase (just change parts of bitmap) Bright display, good contrast (but not as good a vector scan) Disadvantages-- 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!