CS 450: COMPUTER GRAPHICS REVIEW: INTRODUCTION TO COMPUTER GRAPHICS – PART 1 SPRING 2015 DR. MICHAEL J. REALE

WHAT IS COMPUTER GRAPHICS? Computer graphics – generating and/or displaying imagery using computers Example applications: Graphics, Charts, and Data Visualization Challenges: large data sets, best way to display data CAD/CADD/CAM Challenges: need pixel-perfect rendering Computer Art / Movies Virtual-Reality Environments / Training Simulations Games

CHARACTERISTICS OF COMPUTER GRAPHICS Depending on your application, your focus and goals will be different: Real-time vs. Non-real-time Virtual Entities / Environments vs. Visualization / Representation Developing Tools / Algorithms vs. Content Creation

CG DEFINITIONS Real-time 15 fps = BARE MINIMUM but still skips 24 fps = minimum without skips Computer-graphics application programming interfaces (CG API) Interface between programming language and hardware Common CG APIs: GL, OpenGL, DirectX, VRML, Java 2D, Java 3D, etc.

DISPLAY DEFINITIONS Refresh rate = frequency that picture is redrawn Usually expressed in Hertz (e.g., 60 Hz) Persistence = how long phosphors emit light after being hit by electrons in CRT Low persistence  need higher refresh rates LCD monitors have an analogous concept  response time

MORE DISPLAY DEFINITIONS Pixels = “picture element”; single point on screen Resolution = (number of pixels in width) x (number of pixels in height) Aspect ratio = resolution width / height (Although sometimes vice versa) Refresh buffer (or frame buffer) = contains picture of screen you want to draw Each row = scan line

VIDEO DISPLAY DEVICES CRT Plasma LCD/LED 3D

CRT Cathode-Ray Tube Primary video display mechanism for years  now replaced by LCD monitors/TVs Shoots electrons at phosphor screen  keeps redrawing (refreshing) Two types: Vector displays Electron gun draws primitives directly Advantages: draws non-aliased lines Disadvantages: not very flexible; cannot draw shaded polygons Mostly abandoned in favor of raster-scan displays Raster-scan displays Electron gun sweeps across screen, one row at a time, from top to bottom, to draw refresh buffer Advantages: flexible Disadvantages: lines, edge, etc. can look jagged (i.e., aliased)

CRT Interlacing = first draw even-numbered scan lines, then do odd- numbered lines Effectively doubles your refresh rate Also used to save data in TV transmission Color CRTs Beam-penetration Have red and green layer of phosphors  speed of electrons determines which turn on Inexpensive, but limited in number of colors Shadow-mask Three electron guns and three phosphor dots (one for red, one for green, and one for blue) Shadow mask makes sure 3 guns hit the 3 dots

PLASMA DISPLAYS Fire voltage through gas to make glowing plasma For color  use three subpixels (red, green, and blue) Advantages: very thin display; pixels very bright  good at any viewing angle Disadvantages: expensive

LCD DISPLAYS LCD = Liquid Crystal Displays Liquid crystal = maintain a certain structure, but can move around like liquid Structure is twisted, but applying electrical current straightens it out Basic idea: Two polarized light filters (one vertical, one horizontal) Light passes through first filter  polarized light in vertical direction “ON STATE”  no current  crystal twisted  causes light to be reoriented so it passes through horizontal filter “OFF STATE”  current  crystal straightens out  light does NOT pass through

LCD DISPLAYS Light Sources Mirror in back of display Fluorescent light in center of display LED lights Two types: Passive-matrix LCDs  use grid that sends charge to pixels through transparent conductive materials Simple Slow response time Imprecise voltage control  leakage to nearby pixels Active-matrix LCDs  use transistor at each pixel location using thin-film transistor technology Transistors control voltage at each pixel location  prevent leakage to other pixels Control voltage  get 256 shades of gray Color  have 3 subpixels (one red, one green, and one blue)

3D DISPLAYS Humans see depth because of binocular vision  each eye sees different view Older approaches: Anaglyph 3D – red/blue glasses + showing red/blue images Poor color quality View Master toys – shows photograph from different angle in each eye

3D DISPLAYS Newer approaches: Active 3D Special “shutter” glasses that sync up with monitor TV Showing only one image at time (but REALLY fast)  glasses close opposite eye Advantages: can use monitor/TV with high enough refresh rate; full screen resolution Disadvantages: out of sync  flickering; image looks darker; glasses can be cumbersome Passive 3D Polarized light glasses + TV shows two images at once on alternating lines of resolution Advantages: lightweight (cheap) glasses; image brighter than active 3D Disadvantages: need special TV; only seeing HALF the vertical resolution Virtual reality displays (e.g., Oculus Rift) Separate screens for each eye Advantages: no drop in resolution or brightness Disadvantages: heavy headset