Display Technologies

1. Display Technologies

2. A TYPICAL GRAPHICS SYSTEM A Typical graphics system consists of • Processor • Memory • Frame Buffer • Output Devices • Input Devices Prepared by Narendra V G CSE MIT

3. A TYPICAL GRAPHICS SYSTEM keyboard processor Frame buffer mouse memory Drawing tablet Prepared by Narendra V G CSE MIT

4. VECTOR GRAPHICS SYSTEMS Vector (or stroke, line drawing or calligraphic) displays were developed in mid-sixties and were in common use until mid-eighties. • In these devices , everything is displayed as a combination of lines (even characters) • Typically it consists of display processor connected as an I/O peripheral to CPU, a display buffer memory and a CRT. The buffer stores the computer-produced display list or display program; it contains point, line character plotting commands (opcodes) Prepared by Narendra V G CSE MIT

5. ARCHITECTURE OF A VECTOR DISPLAY Interface with host computer . Move 10 15 Line 400 300 Char Lu Cy Line . . . JMP (display commands) (interaction data) Display controller(DC) Lucy Refresh buffer Prepared by Narendra V G CSE MIT

6. Output Technology (1/3) • Calligraphic Displays • also called vector, stroke or line drawing graphics • lines drawn directly on phosphor • display processor directs electron beam according to list of lines defined in a "display list“ • phosphors glow for only a few micro-seconds so lines must be redrawn or refreshed constantly • deflection speed limits # of lines that can be drawn without flicker. Prepared by Narendra V G CSE MIT

7. Output Technology (2/3) • Raster Display • Display primitives (lines, shaded regions, characters) stored as pixels in refresh buffer (or frame buffer) • Electron beam scans a regular pattern of horizontal raster lines connected by horizontal retraces and vertical retrace • Video controller coordinates the repeated scanning • Pixels are individual dots on a raster line Prepared by Narendra V G CSE MIT

8. Output Technology (cont) • Bitmap is the collection of pixels • Frame buffer stores the bitmap • Raster display store the display primitives (line, characters, and solid shaded or patterned area) • Frame buffers • are composed of VRAM (video RAM). • VRAM is dual-ported memory capable of • Random access • Simultaneous high-speed serial output: built-in serial shift register can output entire scanline at high rate synchronized to pixel clock. Prepared by Narendra V G CSE MIT

9. Pros and Cons • Advantages to Raster Displays • lower cost • filled regions/shaded images • Disadvantages to Raster Displays • a discrete representation, continuous primitives must be scan-converted (i.e. fill in the appropriate scan lines) • Aliasing or "jaggies" Arises due to sampling error when converting from a continuous to a discrete representation Prepared by Narendra V G CSE MIT

10. Basic Definitions • Raster: A rectangular array of points or dots. • Pixel (Pel): One dot or picture element of the raster • Scan line: A row of pixels Video raster devices display an image by sequentially drawing out the pixels of the scan lines that form the raster. Prepared by Narendra V G CSE MIT

11. Resolution • Maximum number of points that can be displayed without overlap on a CRT monitor • Dependent on • Type of phosphor m • Intensity to be displayed m • Focusing and deflection systems m • REL SGI O2 monitors: 1280 x 1024 Prepared by Narendra V G CSE MIT

12. Example • Television • NTSC 640x480x8b 1/4 MB • GA-HDTV 1920x1080x8b ~2 MB • Workstations • Bitmapped display 960x1152x1b ~1 Mb • Color workstation 1280x1024x24b 5 MB • Laserprinters • 300 dpi (8.5”x300)(11”x300) 1.05 MB • 2400 dpi (8.5”x2400)(11”x2400) ~64 MB • Film (line pairs/mm) • 35mm (diagonal) slide (ASA25~125 lp/mm) = 3000 3000 x 2000 x 3 x 12b ~27 MB Prepared by Narendra V G CSE MIT

13. Aspect Ratio Frame aspect ratio (FAR) = horizontal/vertical size TV 4:3 HDTV 16:9 Page 8.5:11 ~ 3/4 35mm 3:2 Panavision 2.35:1 (2:1 anamorphic) Vistavision 2.35:1 (1.5 anamorphic) Pixel aspect ratio (PAR) = FAR vres/hres Nuisance in graphics if not 1 Prepared by Narendra V G CSE MIT

14. Physical Size • Physical size: Length of the screen diagonal (typically 12 to 27 inches) • REL SGI O2 monitors: 19 inches Prepared by Narendra V G CSE MIT

15. Refresh Rates and Bandwidth • Frames per second (FPS) • Film (double framed) 24 FPS • TV (interlaced) 30 FPS x 1/4 = 8 MB/s • Workstation (non-interlaced) 75 FPS x 5 = 375 MB/s Prepared by Narendra V G CSE MIT

16. 1/30 SEC 1/30 SEC 1/60 SEC 1/60 SEC 1/60 SEC 1/60 SEC FIELD 1 FIELD 2 FIELD 1 FIELD 2 FRAME FRAME Interlaced Scanning • Scan frame 30 times per second • To reduce flicker, divide frame into two fields—one consisting of the even scan lines and the other of the odd scan lines. • Even and odd fields are scanned out alternately to produce an interlaced image. Prepared by Narendra V G CSE MIT

17. Frame Buffer • A frame buffer is characterized by is size, x, y, and pixel depth. • the resolution of a frame buffer is the number of pixels in the display. e.g. 1024x1024 pixels. • Bit Planes or Bit Depth is the number of bits corresponding to each pixel. This determines the color resolution of the buffer. Bilevel or monochrome displays have 1 bit/pixel (128Kbytes of RAM) 8bits/pixel ->256 simultaneous colors24bits/pixel ->16 million simultaneous colors Prepared by Narendra V G CSE MIT

18. 8 8 8 Red Blue Green Specifying Color • direct color : • each pixel directly specifies a color value • e.g., 24bit : 8bits(R) + 8bits(G) + 8 bits(B) • palette-based color : indirect specification • use palette (CLUT) • e.g., 8 bits pixel can represent 256 colors 24 bits plane, 8 bits per color gun. 224 = 16,777,216 Prepared by Narendra V G CSE MIT

19. Lookup Tables • Video controller often uses a lookup table to allow indirection of display values in frame buffer. • Allows flexible use of colors without lots of frame-buffer memory. • Allows change of display without remapping underlying data double buffering. • Permits simple animation. • Common sizes: 8 x 12; 8 x 24; 12 x 24. Prepared by Narendra V G CSE MIT

20. CLUT Frame Buffer 0 127 2083 y 00000000 00000100 00010011 to blue gun to red gun to green gun x 255 127 Color Look-Up Table Prepared by Narendra V G CSE MIT

21. Pseudo Color Prepared by Narendra V G CSE MIT

22. RASTER GRAPHICS SYSTEM One of the important achievements in graphics is the development of raster graphics in early seventies Raster displays store the display primitives (points, lines etc.) in refresh buffer in terms of their component pixels Prepared by Narendra V G CSE MIT

23. ARCHITECTURE OF A RASTER DISPLAY INTERFACE WITH HOST COMPUTER (DIPSLAY COMMANDS) (INTERACTION DATA) KEYBOARD DISPLAY CONTROLLER(DC) MOUSE 000000000000000000000000000000 000000000000000000000111000000 000000000000000000001100000000 000000000000000000000001100000 000000000011110000000000000000 000000011111111110000000000000 000111111111111111111000000000 000111110000000011111000000000 000111111111111111111000000000 000111111110001111111000000000 000111111110001111111000000000 000111111110001111111000000000 000111111111111111111000000000 000000000000000000000000000000 VIDEO CONTROLLER REFRESH BUFFER Prepared by Narendra V G CSE MIT

24. RASTER SCAN AND ADVANTAGES Scan line Vertical retrace Horizontal retrace Raster Scan • Advantages : • Lower cost ability to display solid colors and patterns • independent of texture and complexity • Disadvantages: • discrete nature of pixel representation(jagged edges) need scan conversion need raster Prepared by Narendra V G CSE MIT

25. Prepared by Narendra V G CSE MIT

26. Basic video controller refresh operations Raster Scan generator Horizontal and vertical deflection voltages X register Y register Memory address Pixel register intensity Frame Buffer Prepared by Narendra V G CSE MIT

27. Comparing Raster and Vector (1/2) • advantages of vector: • very fine detail of line drawings (sometimes curves), whereas raster suffers from jagged edge problem due to pixels (aliasing, quantization errors) • geometry objects (lines) whereas raster only handles pixels • eg. 1000 line plot: vector disply computes 2000 endpoints • raster display computes all pixels on each line

28. Comparing Raster and Vector (2/2) • advantages of raster: • cheaper • colours, textures, realism • unlimited complexity of picture: whatever you put in refresh buffer, whereas vector complexity limited by refresh rate

29. Cathode ray tube • Foremost requirement of a graphics hardware is that the screen should be dynamic. • Refresh rate for raster scan displays is usually 60 frames per second (independent of picture complexity) • Note that in vector display, refresh rate depends directly on the picture complexity. Greater the complexity, greater the refresh cycle. Prepared by Narendra V G CSE MIT

30. Cathode Ray Tubes (CRTs) • Most common display device today • Evacuated glass bottle (lastof the vacuum tubes) • Heating element (filament) • Electrons pulled towards anode focusing cylinder • Vertical and horizontal deflection plates • Beam strikes phosphor coating on front of tube

31. Deflections achieved by adjusting current through the coils. Prepared by Narendra V G CSE MIT

32. Prepared by Narendra V G CSE MIT

33. Prepared by Narendra V G CSE MIT

34. Black and white television: an oscilloscope with a fixed scan pattern: left to right, top to bottom • Paint entire screen 30 times/sec • Actually, TVs paint top-to-bottom 60 times/sec, alternating between even and odd scanlines • This is called interlacing. It’s a hack. Why do it? • To paint the screen, computer needs to synchronize with the scanning pattern of raster • Solution: special memory to buffer image with scan-out synchronous to the raster. We call this the framebuffer.

35. CRT facts • 15,000 to 20,000 volts is the voltage used to accelerate the electron beam • Control grid determines how many electrons are in the beam, thus controlling intensity. (The more negative the control-grid voltage is, the fewer the electrons that pass through the grid) • The spot is “focused” in order to cancel the divergence due to repulsion. • Spot is Gaussian distributed (no sharp edge) and is 0.005 inches in diameter. Prepared by Narendra V G CSE MIT

36. Fluorescence Vs Phosphorescence • Electron beam hits the phosphor-coated screen with a kinetic energy that is proportional to the acceleration voltage. • Phosphors are characterized by • color(usually red, green and blue) • persistence, which is the time for the emitted light to decay to 10% of the initial intensity. High persistence is good for low refresh rates, but bad for animation (“trail” is left behind with moving objects). Prepared by Narendra V G CSE MIT

37. Fluorescence Vs Phosphorescence(cont) • When electron beam hits the screen…. • After some dissipation due to heat, rest of the energy is transferred to electrons of the phosphor atoms, making them jump to higher quantum energy levels. • The excited electrons then return to their previous quantum levels by giving up extra energy in the form of light, at frequencies predicted by quantum theory. Prepared by Narendra V G CSE MIT

38. Fluorescence Vs Phosphorescence(cont) • Any given phosphor has several different quantum levels to an unexcited state. Further, electrons on some levels are less stable and return to the unexcited state more rapidly than others. • A phosphor’s Fluorescence is the light emitted as these very unstable electrons lose their excess energy while phosphor is being struck by electrons. • Phosphorescence is the light given off by the return of relatively more stable excited electrons to their unexcited state once the electron beam excitation is removed. • Typically, most of the light emitted is phosphorescence, since the excitation and the fluorescence usually just lasts a fraction of a microsecond. Prepared by Narendra V G CSE MIT

39. Flat-Panel Displays • Class of video devices that have reduced volume, weight, and power requirements compared to a CRT. They are significantly thinner. • Flat panels: i) emissive, ii) nonemissive. • Emissive displays (or emitters) are devices that convert electrical energy into light. Ex. Plasma panels, thin-film electoluminescent displays, Light-Emitting Diodes (LEDs). • (note: Flat CRTs have also been designed but not popular/successful) • Nonemissive flat-panel displays use optical effects to convert sunlight or light from some other source into graphics patterns. Ex. Liquid-crystal device. Prepared by Narendra V G CSE MIT

40. Prepared by Narendra V G CSE MIT

41. Plasma panels • Constructed by filling the region between glass plates with a mixture of gases, usually including neon. • A series of vertical conducting ribbons is placed on one glass panel, horizontal on the other. • Voltages are fired to an intersecting pair to break down a glowing plasma of electrons and ions. Refresh rate is 60 frames per sec. Prepared by Narendra V G CSE MIT

42. Prepared by Narendra V G CSE MIT

43. Display Technology: LCD • Liquid Crystal Displays (LCDs) • Liquid crystal – these compounds have a crystalline arrangement of molecules, yet they flow like a liquid • LCSs are commonly used in small systems such as laptops, calculators • LCDs: organic molecules, naturally in crystalline state, that liquify when excited by heat or E field • Crystalline state twists polarized light 90º Prepared by Narendra V G CSE MIT

44. LCD.. • Produces a picture by passing polarized light from the surroundings or from an internal light source through a liquid-crystal material that can either block or transmit the light. • The intersection of the two conductors defines a pixel position. • Polarized light is twisted as it passes through the opposite polarizer. The light is then reflected back to the viewer. • To turn off the pixel, voltage is applied to the two intersecting conductors to align the molecules so that the light is not twisted. Prepared by Narendra V G CSE MIT

45. Prepared by Narendra V G CSE MIT

46. Color • Color is achieved by having three electron guns mixing the colors red, green and blue (RGB). • White is perceived when all are illuminated and when all are off its black. • Typically each color is specified by an 8-bit value . Thus 8*3=24 bits are needed to represent a color pixel(also called true color). Prepared by Narendra V G CSE MIT

47. Color (cont) 256 entry 8bits • Storing say 24 bits of information for each pixel of a (say), 1000*1000 screen eats up 3 Megabytes of memory. Thus low end graphics workstations use a more economical approach. They use 8 bits per pixel where each 8-bit entry is an index into a 256-entry color map. Each entry in the color map is a 24-bit value containing R,G,B components of the color. This is color-Indexing. 24 bits Prepared by Narendra V G CSE MIT

48. Display Technology: Color CRTs • Color CRTs are much more complicated • Requires manufacturing very precise geometry • Uses a pattern of color phosphors on the screen: • Why red, green, and blue phosphors? Delta electron gun arrangement In-line electron gun arrangement

49. Prepared by Narendra V G CSE MIT

50. Color CRTs have • Three electron guns • A metal shadow maskto differentiate the beams