1. Display Technologies

2. A TYPICAL GRAPHICS SYSTEM
A Typical graphics system consists of
Processor
Memory
Frame Buffer
Output Devices
Input Devices
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3. A TYPICAL GRAPHICS SYSTEM
keyboard
processor
Frame buffer
mouse
memory
Drawing tablet
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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)
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5. ARCHITECTURE OF A VECTOR DISPLAY
Interface with host computer .
Move 10 15
Line
400
300
Char Lu Cy
JMP
(display commands) (interaction data)
Display controller(DC)
Lucy
Refresh buffer
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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.
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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
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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.
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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
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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.
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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
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Example
Television
NTSC x480x8b 1/4 MB
GA-HDTV x1080x8b ~2 MB
Workstations
Bitmapped display 960x1152x1b ~1 Mb
Color workstation 1280x1024x24b 5 MB
Laserprinters
300 dpi (8.5”x300)(11”x300) 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
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Aspect Ratio
Frame aspect ratio (FAR) = horizontal/vertical size
TV :3
HDTV :9
Page :11 ~ 3/4
35mm :2
Panavision :1 (2:1 anamorphic)
Vistavision :1 (1.5 anamorphic)
Pixel aspect ratio (PAR) = FAR vres/hres
Nuisance in graphics if not 1
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Physical Size
Physical size: Length of the screen diagonal (typically 12 to 27 inches)
REL SGI O2 monitors: 19 inches
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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
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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.
1/30 SEC
1/60 SEC
FIELD 1
FIELD 2
FRAME
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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 colors 24bits/pixel -> 16 million simultaneous colors
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Specifying Color
Green
Red
Blue 8
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
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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.
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Color Look-Up Table
Frame Buffer
CLUT
127
255
2083
to blue
gun
to green
to red x y
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Pseudo Color
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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
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23. ARCHITECTURE OF A RASTER DISPLAY
INTERFACE WITH HOST COMPUTER
(DIPSLAY COMMANDS) (INTERACTION DATA)
DISPLAY CONTROLLER(DC)
KEYBOARD
MOUSE
VIDEO CONTROLLER
REFRESH BUFFER
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24. RASTER SCAN AND ADVANTAGES
Scan line
Horizontal retrace
Vertical 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
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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
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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 line plot: vector disply computes 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

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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.
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30. Cathode Ray Tubes (CRTs)
Most common display device today
Evacuated glass bottle (last of 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

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Deflections achieved by adjusting current through
the coils.
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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.

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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 inches in diameter.
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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).
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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.
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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.
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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.
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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.
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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
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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.
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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).
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Color (cont)
256 entry
8bits
24 bits
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.
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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

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50. Color CRTs have Three electron guns
A metal shadow mask to differentiate the beams

51. Raster CRT pros: Cons: Allows solids, not just wireframes
Leverages low-cost CRT technology (i.e., TVs)
Bright! Display emits light
Cons:
Requires screen-size memory array
Discreet sampling (pixels)
Practical limit on size (call it 40 inches)
Bulky
Finicky (convergence, warp, etc)

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Frame Buffer
A frame buffer is a large contiguous piece of computer memory.
At a minimum, there is one memory bit for each pixel (picture element) in the raster; this amount of memory is called bit plane
A 1024 * 1024 element square raster requires
2 20 or 1,048,576 ( 210*210) memory bits in a single bit plane. Each bit has 2 states (monochrome display).
Conversion from digital to analog is done by DAC (digital-to-analog converter).
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53. Frame Buffer raster CRT device1
DAC
Register
Electron Gun
Frame Buffer
CRT Raster
A single-bit-plane(1 bit per pixel) Black and White frame buffer raster CRT graphics device
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Color and Gray levels
Color or gray levels are incorporated into a frame buffer by adding additional bit planes.
The binary value from each of the N bit planes is loaded into corresponding positions into a register. The resulting binary number is interpreted as an intensity level between 0 (dark) and 2N-1(full intensity)
A Raster with 3 bit planes generates 8 (23) intensity levels. In this case, the frame buffer should have 3,145,728 ( 3 * 1024 * 1024) memory bits.
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55. An N bit gray level frame buffer
Register N N 1 2
2N DAC
Electron gun
N=3 2N levels
Frame Buffer
CRT Raster
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56. Simple color frame buffer
DAC 3 1 1
DAC
DAC
Frame Buffer
CRT RASTER
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57. 3 Bit plane frame buffer color combinations
Red
Green
Blue
Black 1
Yellow
Cyan
White
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58. A 24 Bit plane color frame buffer
registers 8
Color Guns
3 bit DAC
Blue 75 8
3 bit DAC
Green 172 8
3 bit DAC
Red 10
CRT Raster
Frame Buffer
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59. Gray Level Frame Buffer with Look Up table1 10
Electron Gun 2 1 1
2w DAC
2N entries
N=3
Lookup tables
W=4
Frame Buffer
An N Bit plane Gray Level frame buffer, with W-bit-wide lookup table
CRT Raster
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60. Color frame buffer(24 bit plane) with lookup tables(10 Bit wide)
W bit DAC
W bit DAC
CRT Raster
N=8
2N entries
W bit DAC
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W=10

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Resolution
Resolution
The Maximum number of points that are displayed without overlap.
This is usually given as the number of horizontal points versus the number of vertical points. These points are called pixels or picture elements.
The maximum resolution may be determined by the characteristics of the monitor for a random scan system or by a combination of monitor and graphics card memory for a raster scan system.
Typical resolution on high-quality systems is 1280 by 1024, higher also available.
Physical size of the graphics monitor is measured as length of the screen diagonal which generally varies from 12 in. to 27in.
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Aspect Ratio
Aspect Ratio
The aspect ratio is the ratio of horizontal dimension/vertical dimension.
Example
If the monitor dimensions are 8 inches by 6 inches, the aspect ratio is 8/6 which is equal 1.33.
If the resolution of the screen is 640 by 480, the length of the pixel is 640/8 equal to 80 pixels per inch. Similarly height is 480/6 equal to 80 pixels per inch. Thus the pixel is a square.
If the horizontal size of a pixel is not equal to the vertical size, then it must be corrected for image display else the image will appear distorted.
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63. Image resolutions in practice
WORKSTATIONS
Bitmapped display 960 * 1152* 1b approx 1MB
Color Display 1280* 1024*24b approx 5MB
TELEVISION
NTSC 640*480*8b approx ¼ MB
HDTV 1980*1080*8b approx 2 MB
LASER PRINTERS
300 dpi (8.5*300)(11*300) approx 1.05 MB
2400 dpi (8.5*2400)(11*2400) approx 64MB
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64. Speed requirements and scanning rates
Speed requirements for memory access
1024*768*8 = 768 Kbytes= 786,432 bytes
Read 786*103 bytes in 1600*10-5 secs (inverse of 60) for 60 HZ.
Rough estimation of scanning rates.
Frequency X number of vertical lines (note scan always means a full horizontal scan)
Example: for an IBM VGA 60*480 = 30 HZ
For 1024 * 768 = 46 Khz
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65. Dot size and Addressability
The image quality achievable with display devices depends on both the addressability and the dot size of the device.
Dot (spot) size is the diameter of the single dot created on the device.
Addressability is the number of individual dots per inch that can be created; it may differ in horizontal and vertical directions.
Addressability in x is the reciprocal of the distance between the centers of dots at addresses (x,y) and (x+1,y). Similarly the other direction is calculated.
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Interdot distance
Interdot distance is the reciprocal of addressability
It is usually desirable that the dot size be somewhat greater than the interdot distance, so that smooth shapes can be created.
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