Display Technologies. A TYPICAL GRAPHICS SYSTEM A Typical graphics system consists of Processor Memory Frame Buffer Output Devices Input Devices 9/20/2015

Display Technologies

A TYPICAL GRAPHICS SYSTEM

A Typical graphics system consists of

Processor Memory Frame Buffer Output Devices Input Devices

04/19/23 17:47 2Prepared by Narendra V G CSE MIT

A TYPICAL GRAPHICS SYSTEM

mouse

Drawing tablet

keyboard

processor

memory

Frame buffer


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 linescombination 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)


ARCHITECTURE OF A VECTOR DISPLAY

Display controller(DC)

.Move

1015

Line400300CharLuCy

Line...

JMP

Refresh buffer

Interface with host computer

(display commands) (interaction data)

Lucy


04/19/23 17:47 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.


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


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.


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


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 pixelsVideo raster devices display an image by sequentially drawing out the pixels of the scan lines that form the raster.


Resolution

Maximum number of points that can be displayed without overlap on a CRT monitor

Dependent onType of phosphor m Intensity to be displayed m Focusing and deflection systems m

REL SGI O2 monitors: 1280 x 1024


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


Aspect Ratio

Frame aspect ratio (FAR) = horizontal/vertical sizeTV 4:3HDTV 16:9Page 8.5:11 ~ 3/435mm 3:2Panavision 2.35:1 (2:1 anamorphic)Vistavision 2.35:1 (1.5 anamorphic)

Pixel aspect ratio (PAR) = FAR vres/hresNuisance in graphics if not 1


Physical Size

Physical size: Length of the screen diagonal (typically 12 to 27 inches)

REL SGI O2 monitors: 19 inches


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


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

1/60 SEC

1/30 SEC

1/60 SEC

FIELD 1 FIELD 2

FRAME

1/60 SEC


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


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

Green

Red

Blue

8

8

8

24 bits plane, 8 bits per color gun.

224 = 16,777,216


Lookup Tables Video controller often uses a lookup table to allow indirection of di

splay values in frame buffer.

Allows flexible use of colors without lots of frame-buffer memory. Allows change of display without remapping underlying data doubl

e buffering. Permits simple animation. Common sizes: 8 x 12; 8 x 24; 12 x 24.


Color Look-Up Table

Frame Buffer

CLUT

127 127

0

255

2083 00000000 00000100 00010011

to blue gun

to green gun

to red gunx

y


Pseudo Color

0

1

2

3

254

255

RED GREEN BLUE

256 colors chosen from a palette of 16,777,216.

Each entry in the color map LUT can be user defined.

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


ARCHITECTURE OF A RASTER DISPLAY

000000000000000000000000000000000000000000000000000111000000000000000000000000001100000000000000000000000000000001100000000000000011110000000000000000000000011111111110000000000000000111111111111111111000000000000111110000000011111000000000000111111111111111111000000000000111111110001111111000000000000111111110001111111000000000000111111110001111111000000000000111111111111111111000000000000000000000000000000000000000

DISPLAY CONTROLLER(DC)

VIDEO CONTROLLER

KEYBOARD

MOUSE

INTERFACE WITH HOST COMPUTER

(DIPSLAY COMMANDS) (INTERACTION DATA)

REFRESH BUFFER04/19/23 17:47 23Prepared by Narendra V G CSE MIT

RASTER SCAN AND ADVANTAGES

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

Vertical retraceHorizontal retrace

Scan line

Raster Scan

24

Prepared by Narendra V G CSE MIT

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Basic video controller refresh operations

Raster Scan generator

Xregister

Yregister

Memory address

Frame Buffer

Pixel register intensity

Horizontal and vertical deflection voltages


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

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.


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

Deflections achieved by adjusting current throughthe coils.




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.

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.


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).


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.


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.


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.



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.



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


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.



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).


Color (cont)

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.

8bits

24 bits

256 entry


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


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

beams

Raster CRT pros: 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)

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).


Frame Buffer raster CRT device

11 DACDACElectron GunRegister

Frame Buffer CRT Raster

A single-bit-plane(1 bit per pixel) Black and White frame buffer raster CRT graphics device


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.


An N bit gray level frame buffer

0

1

0

0 1 0

2N DAC

2

N=3 2N levels

Electron gun

Frame Buffer

CRT Raster

Register N

N


Simple color frame buffer

3

0

10 0

1

0

DAC

DAC

DAC

Frame Buffer

CRT RASTER


3 Bit plane frame buffer color combinations

Red Green Blue

Black 0 0 0

Red 1 0 0

Green 0 1 0

Blue 0 0 1

Yellow 1 1 0

Cyan 0 1 1

White 1 1 104/19/23 17:47 57Prepared by Narendra V G CSE MIT

A 24 Bit plane color frame buffer

0 1 0 0 1 0 1 1

1 0 1 0 1 1 0 0

0 0 0 0 1 0 1 0

3 bit DAC

3 bit DAC

3 bit DAC

registers

Blue 75

Green 172

Red 10

Color Guns

Frame Buffer

CRT Raster

8

8

8


Gray Level Frame Buffer with Look Up table

0 1 02w DAC

0

10

1 0 1 0

Frame Buffer

N=3

2 10

2N

entries

Lookup tables

W=4

Electron Gun

CRT Raster

An N Bit plane Gray Level frame buffer, with W-bit-wide lookup table


Color frame buffer(24 bit plane) with lookup tables(10 Bit wide)

W bit DAC

W bit DAC

W bit DAC

CRT Raster

W=10

W=10

W=10

N=8

2N entries04/19/23 17:47 60Prepared by Narendra V G CSE MIT

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.


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.


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


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


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.


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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Display Technologies. A TYPICAL GRAPHICS SYSTEM A Typical graphics system consists of Processor Memory Frame Buffer Output Devices Input Devices 9/20/2015