lecture02-1 - Introduction to Computer Graphics CS 445 /...

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Unformatted text preview: Introduction to Computer Graphics CS 445 / 645 Lecture 2 General Graphics Systems Daniel Rozin, Wooden Mirror (1999) Announcement Announcement • Class web site is up Overview – Read Chapter 2 • Display devices • Graphics hardware • Input devices • Graphics Software Display technologies Cathode Ray Tubes (CRTs) • Most common display device today • Evacuated glass bottle • Extremely high voltage CRT details • Heating element Heating (filament) (filament) • Electrons pulled Electrons towards anode focusing cylinder cylinder • Vertical and Vertical horizontal deflection plates plates • Beam strikes Beam phosphor coating on front of tube front Electron gun Contains a filament that, when heated, emits a Contains stream of electrons stream Electrons are focused with an electromagnet into a Electrons sharp beam and directed to a specific point of the face of the picture tube face The front surface of the picture tube is coated with The small phospher dots small When the beam hits a phospher dot it glows with a When brightness proportional to the strength of the beam and how long it is hit beam CRT characteristics What’s the largest (diagonal) CRT you’ve seen? • Why is that the largest? – Evacuated tube == massive glass – Symmetrical electron paths (corners vs. center) How might one measure CRT capabilities? • Size of tube • Brightness of phosphers vs. darkness of tube • Speed of electron gun • Width of electron beam Width • Pixels? Display technologies: CRTs Vector Displays • Anybody remember Battlezone? Tempest? Anybody Battlezone Tempest Display technologies: CRTs Vector Displays • Early computer displays: basically an oscilloscope • Control X,Y with vertical/horizontal plate voltage Control • Often used intensity as Z (close things were brighter) Name two disadvantages Just does wireframe Just Complex scenes cause visible flicker Complex Display technologies: CRTs Raster Displays • Raster: A rectangular array of points or dots • Pixel: One dot or picture element of the raster • Scan line: A row of pixels Display technologies: CRTs Raster Displays • Black and white television: an oscilloscope with a fixed scan Black pattern: left to right, top to bottom pattern: – As beam sweeps across entire face of CRT, beam intensity As changes to reflect brightness changes • Analog signal vs. digital display Display technologies: CRT Can a computer display work like a black and white TV? • Must synchronize – Your program makes decisions about the intensity signal at the pace Your of the CPU… of – The screen is “painted” at the pace of the electron gun scanning the The raster raster • Solution: special memory to buffer image with scan-out synchronous to Solution: the raster. We call this the framebuffer. framebuffer • Digital description to analog signal to digital display Display Technologies: CRTs Phosphers • Flourescence: Light emitted while the phospher is being struck by electrons being • Phospherescence: Light emitted once the electron beam is removed beam • Persistence: The time from the removal of the excitation to the moment when phospherescence has decayed to 10% of the initial light output decayed Display Technologies: CRTs Refresh • Frame must be “refreshed” to draw new images • As new pixels are struck by electron beam, others are decaying • Electron beam must hit all pixels frequently to eliminate flicker • Critical fusion frequency – Typically 60 times/sec – Varies with intensity, individuals, phospher persistence, Varies lighting... lighting... Display Technologies: CRTs Interlaced Scanning • Assume we can only scan all pixels of entire screen Assume 30 times / second 30 • To reduce flicker, divide frame into two “fields” of odd To and even lines and 1/30 Sec 1/60 Sec 1/60 Sec Field 1 Field 2 Frame 1/30 Sec 1/60 Sec 1/60 Sec Field 2 Field 1 Frame Display Technologies: CRTs CRT timing • Scanning (left to right, top to bottom) – Vertical Sync Pulse: Signals the start of the next field – Vertical Retrace: Time needed to get from the bottom of the current field to the top of the next field current – Horizontal Sync Pulse: Signals the start of the new scan line – Horizontal Retrace: The time needed to get from the end of the current scan line to the start of the next scan line the What is a pixel? Wood chips Wood Chrome spheres Trash Daniel Rozin – NYU: (movies) http://fargo.itp.tsoa.nyu.edu/~danny/art.html Display Technology: Color CRTs Color CRTs are much more complicated Color much • Requires manufacturing very precise geometry • Uses a pattern of color phosphors on the screen: Delta electron gun arrangement In-line electron gun arrangement • Why red, green, and blue phosphors? Display Technology: Color CRTs Color CRTs have • Three electron guns • A metal shadow mask to differentiate the beams metal shadow to Display Technology: Raster Raster CRT pros: • Allows solids, not just wireframes • Leverages low-cost CRT technology (i.e., TVs) • Bright! Display emits light Bright! emits Cons: • Requires screen-size memory array • Discreet sampling (pixels) • Practical limit on size (call it 40 inches) • Bulky • Finicky (convergence, warp, etc) CRTs – A Review • CRT technology hasn’t changed much in 50 years • Early television technology – high resolution high – requires synchronization between video signal and requires electron beam vertical sync pulse electron • Early computer displays – avoided synchronization using ‘vector’ algorithm – flicker and refresh were problematic CRTs – A Review • Raster Displays (early 70s) – like television, scan all pixels in regular pattern – use frame buffer (video RAM) to eliminate sync problems • RAM – ¼ MB (256 KB) cost $2 million in 1971 – Do some math… - 1280 x 1024 screen resolution = 1,310,720 pixels - Monochrome color (binary) requires 160 KB - High resolution color requires 5.2 MB Movie Theaters U.S. film projectors play film at 24 fps • Projectors have a shutter to block light during frame advance • To reduce flicker, shutter opens twice for each frame – resulting in 48 To fps flashing fps • 48 fps is perceptually acceptable European film projectors play film at 25 fps • American films are played ‘as is’ in Europe, resulting in everything American moving 4% faster moving • Faster movements and increased audio pitch are considered Faster perceptually acceptable perceptually Viewing Movies at Home Film to DVD transfer • Problem: 24 film fps must be converted to Problem: – NTSC U.S. television interlaced 29.97 fps 768x494 NTSC – PAL Europe television 25 fps 752x582 Use 3:2 Pulldown • First frame of movie is broken into first three fields (odd, even, odd) • Next frame of movie is broken into next two fields (even, odd) • Next frame of movie is broken into next three fields (even, odd, even)… Display Technology: LCDs Liquid Crystal Displays (LCDs) • LCDs: organic molecules, naturally in crystalline LCDs: state, that liquefy when excited by heat or E field state, • Crystalline state twists polarized light 90º. Crystalline Display Technology: LCDs Liquid Crystal Displays (LCDs) • LCDs: organic molecules, naturally in crystalline LCDs: state, that liquefy when excited by heat or E field state, • Crystalline state twists polarized light 90º Display Technology: LCDs Transmissive & reflective LCDs: • LCDs act as light valves, not light emitters, and thus rely on an LCDs external light source. external • Laptop screen – backlit – transmissive display • Palm Pilot/Game Boy – reflective display Display Technology: Plasma Plasma display panels Plasma • Similar in principle to Similar fluorescent light tubes fluorescent • Small gas-filled capsules Small are excited by electric field, are emits UV light • UV excites phosphor • Phosphor relaxes, emits Phosphor some other color some Display Technology Plasma Display Panel Pros • Large viewing angle • Good for large-format displays • Fairly bright Cons • Expensive • Large pixels (~1 mm versus ~0.2 mm) • Phosphors gradually deplete • Less bright than CRTs, using more power Review Read Chapter 2 • Details about display devices Implement OpenGL • Section 2.9 is good introduction to OpenGL ...
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This note was uploaded on 01/23/2012 for the course CS 445 taught by Professor Bloomfield,a during the Spring '08 term at UVA.

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