310482: Graphics Programming

04/21/23

310482: Graphics Programming

Seree Chinodom

[email protected]://www.compsci.buu.ac.th/~seree/lecture/310482

04/21/23

Display Technologies

Cathode Ray Tubes (CRTs)– Most common display device today– Evacuated glass bottle– Extremely high voltage– 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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Electron Gun

Contains a filament that, when heated, emits a stream of electrons

Electrons are focused with an electromagnet into a sharp beam and directed to a specific point of the face of the picture tube

The front surface of the picture tube is coated with small phospher dots

When the beam hits a phospher dot it glows with a brightness proportional to the strength of the beam and how often it is excited by the beam

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Display Technologies: CRTs

Vector Displays– Early computer displays: basically an oscilloscope– Control X,Y with vertical/horizontal plate voltage – Often used intensity as Z

Name two disadvantages Just does wireframe

Complex scenes visible flicker

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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

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Raster Displays– Black and white television: an oscilloscope with a

fixed scan pattern: left to right, top to bottom– 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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Phosphers– Flourescence: Light emitted while the phospher is

being struck by electrons– Phospherescence: Light emitted once the

electron beam is removed– Persistence: The time from the removal of the

excitation to the moment when phospherescence has decayed to 10% of the initial light output

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Raster Displays– 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,

lighting...

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Raster Displays– Interlaced Scanning– Assume can only scan 30 times / second– To reduce flicker, divide frame into two “fields” of

odd and even lines

1/30 Sec 1/30 Sec1/60 Sec 1/60 Sec 1/60 Sec 1/60 SecField 1 Field 1Field 2 Field 2

Frame Frame

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Raster Displays– 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 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

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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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Display Technology: Color CRTs

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

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Display Technology: Raster

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)

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Display Technology: LCDs

Liquid Crystal Displays (LCDs)– LCDs: organic molecules, naturally in crystalline

state, that liquefy when excited by heat or E field– Crystalline state twists polarized light 90º

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Display Technology: LCDs

Transmissive & reflective LCDs:– LCDs act as light valves, not light emitters, and

thus rely on an external light source.– Laptop screen: backlit, transmissive display– Palm Pilot/Game Boy: reflective display

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Display Technology: Plasma

Plasma display panels– Similar in principle to

fluorescent light tubes– Small gas-filled capsules

are excited by electric field,emits UV light

– UV excites phosphor– Phosphor relaxes, emits

some other color

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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

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Display Technology: DMDs

Digital Micromirror Devices (projectors)– Microelectromechanical (MEM) devices,

fabricated with VLSI techniques

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Display Technology: DMDs

DMDs are truly digital pixels Vary grey levels by modulating pulse length Color: multiple chips, or color-wheel Great resolution Very bright Flicker problems

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Display Technologies: Organic LED Arrays Organic Light-Emitting Diode (OLED) Arrays

– The display of the future? Many think so.– OLEDs function like regular semiconductor LEDs– But with thin-film polymer construction:

Thin-film deposition or vacuum deposition process…not grown like a crystal, no high-temperature doping

Thus, easier to create large-area OLED sheet

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Display Technologies: Organic LED Arrays OLED pros:

– Transparent– Flexible– Light-emitting, and quite bright (daylight visible)– Large viewing angle– Fast (< 1 microsecond off-on-off)– Can be made large or small

OLED cons:– Not quite there yet (96x64 displays…)– Not very robust, display lifetime a key issue

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Raster Scan Displays

Beam of electrons deflected onto a phosphor coated screen– Phosphors emit light when excited by the electro

ns– Phosphor brightness decays -- need to refresh th

e display– Phosphors make up screen elements called pixel

s

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Basic Definitions

Raster: A rectangular array of points or dots.Pixel (Pel): One dot or picture element of the rasterScan 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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Frame Buffers

A frame buffer may be thought of as computer memory organized as a two-dimensional array with each (x,y) addressable location corresponding to one pixel.

Bit Planes or Bit Depth is the number of bits corresponding to each pixel.

A typical frame buffer resolution might be

640 x 480 x 81280 x 1024 x 81280 x 1024 x 24

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1-Bit Memory, Monochrome Display (Bitmap Display)

Electron Gun

1 bit 2 levels

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3-Bit Color Display

3

red

green

blue

COLOR: black red green blue yellow cyan magenta white

R G B

0 0 0

1 0 0

0 1 0

0 0 1

1 1 0

0 1 1

1 0 1

1 1 1

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True Color Display

Green

Red

Blue

8

8

8

24 bitplanes, 8 bits per color gun.224 = 16,777,216

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Color Map Look-Up Tables

Extends the number of colors that can be displayed by a given number of bit-planes.

67

100110100001

0

67

255

10011010 0001

R G B

RED

GREEN

BLUE

Pixel displayedat x', y'

Pixel inbit mapat x', y'

0 x0

y

xmax

maxy

Frame buffer Look-up table Display

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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.

28 x 24 Color Map LUT

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Drawing

Most computer generated images made up of geometric primitives (polygons, lines, points)

Application draws them by setting bits in the

framebuffer Most graphics applications involve scene dat

abase management

SceneDatabase

GraphicApplication

DeviceDriverFramebuffer

Disp

lay

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A DDA Line Drawing Function

Line(int x1, int y1, int x2, int y2) {

int dx = x1 - x2, dy =y2 -y1;int n = max(abs(dx),abs(dy));float dt = n, dxdt = dx/dt, dydt = dy/dt;float x = x1, y = y1;while (n--){

DrawPoint( round(x), round(y) );x +=dxdt;y +=dydt;

}}

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We Can Do Better Than That...

Get rid of all those nasty floating point operations The idea: find the next pixel from the current one

So which of the green pixels is next?

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The Key

We’re only ever going to go right one pixel, or up and right one pixel (if the slope of the line is between 0 and 1). Call these two choices “E” and “NE”

Let’s think of pixels as “lattice points” on a grid Given an X coordinate, we only need to choose betw

een y and y+1 (y is the Y coordinate of the last pixel)

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The Midpoint Test

Look at the vertical grid line that our line intersects

On which side of the midpoint (y+1/2) does the intersection lie?

If it’s above the midpoint, go NE, otherwise go E

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Our Example

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Implicit Functions

Normally, a line is defined as y= mx +b Instead, define ,F(x,y) = ax +by + c and let th

e line be everywhere where F(x,y) = 0 Now, if F(x,y) > 0 , we’re “above” the line, an

d if F(x,y) < 0 , we’re “below” the line

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Who Cares?

We can evaluate the implicit line function at the

midpoint to figure out where to draw next! In fact, we can use the last function evaluati

on to find the next one cheaply! For ANY x, y:

F(x + 1,y) - F(x,y) = a

F(x + 1,y + 1) - F(x,y) = a - b

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Midpoint Algorithm

Line(int x1, int y1, int x2, int y2){

int dx = x2 - x1, dy =y2 -y1;int e = 2*dy - dx;int incrE = 2*dy, incrNE = 2*(dy-dx);int x = x1, y = y1;DrawPoint( x, y );while (x < x2) {

x++;if ( e <= 0 ) { e += incrE; }else { y++; e += incrNE; }DrawPoint( x, y );

}}

“e” holds the implicit function evaluation at each x value (actually, it’s multiplied by 2, but all we care about is the sign).

Easy extension for lines with arbitrary slopes

310482: Graphics Programming

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