Computer Graphics System

Input & Output Technologies

Introduction to Computer Graphics and Animation

(Principle of Computer Graphics)

Rattapoom Waranusast

A Graphics System

Input devices Output devices

Processor and memory

Display Devices

• Hard Copy devices– Printers, plotters

• Transient displays– LCD Monitors, CRT

Monitors, projectors

Cathode Ray Tube

• Recently, most common is Cathode Ray Tube (CRT) monitor

• Horizontal and vertical deflectors focus an electron beam on any spot on a phosphor coated screen

Electrons hit the screen phosphor molecules and excite them

Karl Ferdinand Braun

1897 – Cathode Ray Oscilloscope

Phosphors

• Most phosphors relax back to the ground state by emitting a photon of light which is called fluorescence, which decays in under a millisecond

• Some molecules may be further excited, and emit a light call phosphorescence, which decays slower, but still rapidly (15-20 milliseconds)

• Therefore, the screen must be refreshed by redrawing the image

• They also are characterized by their persistence (time to decay of emitted light)– High persistence cheap and good for text, bad for animation– Low persistence, good for animation, but need high refresh rate

Color Systems

• Phosphors have a color. Color systems have groups of 3 different phosphors, for red, green and blue.

• 3 Electron guns used, for R G and B• Each pixel consists of 3 dots of phosphor, arranged as triangle• Combining different intensities of phosphors can generate

different colours

SONY Trinitron CRT NEC Hybrid Mask Hitachi EDPStandard Dot-trio

Shadow Masks

Shadow mask holes are arranged so that each beam can only excite it’s own color phosphor

Shadow Masks (2)

Vector Display Devices

• A.K.A. Vector Scan Displays, Random Scan Devices, Line Plotters

• The electron beam directly draws the picture

DrawLine(A, B):Turn beam off, move to A.Turn beam on, move to B.

A

B

Vector Graphics

• Although Vector Displays no longer as widely used, it is still common practice to deal with certain types of images in terms of vector graphics.

• A vector file contains a list of entries each of which describes an element of a picture.

• How a picture element is described depends on what type of element it is. e.g. a line segment can be described in terms the co-ordinates of its two end points, its thickness, and its style (solid, dotted, etc.) Also curves and shapes.

• Example: postscript files (PS/EPS)• To display on a RASTER device the graphic needs to be

rasterized

Raster Graphics

• An image made up of many small regularly placed cells called picture elements (pixels)

• Stored as an array of numerical values commonly called a pixelmap (or bitmap)

Raster Scan Devices

• Scans the screen from top to bottom in a regular pattern (common TV technology)

• A Raster is a matrix of pixels (picture elements) covering the screen

• The electron beam is turned on/off so the image is a collection of dots painted on screen one row (scan line) at a time.

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Frame Buffer

• An image is stored in a special graphics memory area called a frame buffer where each memory location corresponds to a pixel

• A display processor scans this memory and controls the electron beam at each pixel accordingly

• For a monochrome system, each pixel is either on or off, so only one bit per pixel is required, the electron beam is either on or off

• For greyscale images, 8 bits per pixel gives 256 different intensities of gray

• For a true color display there should be 8 bits/color giving a total of 24 bits (or more) per pixel.

Bits Per Pixel

• 1 Bit Per Pixel

• 2 Bits Per Pixel

• 8 Bits Per Pixel

• 24 Bits Per Pixel

1 or 0 in binary (on/off)

22 = 4 possible combinations: 00, 01, 10, 11

00000000 to 11111111. 28 = 255 possible values

000000000000000000000000 to 111111111111111111111111. i.e. 224 or 16 million possible values

8x8x1 = 64 bits 8x8x8 = 512 bits 8x8x24 = 1536 bits

Memory Usage Example

Black-and-white Greyscale True-colour

Rasterization

• Geometric and Mathematical Data structures typically in vertex coordinates not dependent on resolution

• We must convert from typical continuous representation to discrete

Anti-aliasing

Raster Scan Systems: Conclusion

• Advantages of Raster Scan systems:– Low cost– Refresh rate independent of image complexity– Handles colour and filled areas images -> high refresh– Regular repetitive => easier and cheaper to implement.

• Disadvantages– Models must be scan converted. Often this can’t be reused so

must do this every frame.– Aliasing– Requires large refresh buffers even for small or simple images.– Images bound to a certain resolution

Vector Displays

• Advantages:– High resolution and not discretized– Less Storage Space– Less Transfer Time (usually)

• Disadvantages – Limited colour capability. Problems with filled areas and shading.– Flicker occurs as complexity of image increases.– Vector data needs some processing before display– Processing required before obtaining the Vector representation– Wastage in Overlapping areas

LCD Displays• Thinner and lighter. No tube or electron beams.• Blocking/unblocking light through polarized crystals.

Crystals liquefy when excited by heat or E field.• No refresh unless the screen changes.• Color - 3 cells per pixel.• After light passes through

first layer, the LCD crystals change the plane of the light’s vibration so that it can pass through the second layer.

LCD Displays

• When an electric field is passed through the LCD layers, they align themselves with the field and untwist.

• Polarised light is let through the crystals unhindered but becomes blocked by the second polarizer layer.

• The relevant pixel then becomes darkened out

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CRT Displays

Advantages• Fast response (high

resolution possible)• Full color (large

modulation depth of E-beam)

• Saturated and natural colours

• Inexpensive, matured technology

• Wide angle, high contrast and brightness

Disadvantages• Large and heavy (typ.

70x70 cm, 15 kg)• High power consumption

(typ. 140W)• Harmful DC and AC

electric and magnetic fields

• Flickering at 50-80 Hz (no memory effect)

• Geometrical errors at edges

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LCD Displays

Advantages• Small footprint (approx 1/6 of

CRT)• Light weight (typ. 1/5 of CRT)• Low power consumption (typ. 1/4

of CRT)• Completely flat screen - no

geometrical errors• Crisp pictures - digital and

uniform colors• No electromagnetic emission• Fully digital signal processing

possible• Large screens (>20 inch) on

desktops

Disadvantages• High price (presently 3x CRT)• Poor viewing angle (typ. +/- 50

degrees)• Low contrast and luminance

(typ. 1:100)• Low luminance (typ. 200 cd/m2)

Plasma Displays

Plasma Displays

• 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

Organic LED Arrays

• Organic Light-Emitting Diode (OLED) Arrays– OLEDs function like regular semiconductor LEDs

– But they emit light• Thin-film deposition of organic, light-emitting molecules through vapor

sublimation in a vacuum.• Dope emissive layers with fluorescent molecules to create color.

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– Available for cell phones and car stereos

• OLED cons:– Not very robust, display lifetime a key issue– Currently only passive matrix displays

• Passive matrix: Pixels are illuminated in scanline order, but the lack of phospherescence causes flicker

• Active matrix: A polysilicate layer provides thin film transistors at each pixel, allowing direct pixel access and constant illum.

Logical Input Devices

• A Classification of Input Devices to enable some level of abstraction so they can be supported by various graphics systems

• Diverse variations of input devices exist• It is useful to classify object in terms of what it

does• This provides level of abstraction

– Enhances portability (device independent design of interface)

– Shields application from physical details

Classes of Logical Input Devices

• Locator/ Pick – to indicate a position or orientation (subclasses) – to select a displayed entity

• Valuator – to input a single real number

• String – To input a character string– Returns key with specific meaning– Letters, Numbers etc.

• Choice – To select from a set of possible actions or choices – Often return sensory feedback e.g. lights, clicks

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Physical Input Devices

• Keyboard: string/choice input• Gamepad: choice• Mouse: pick/locator device with relative

positioning• Tablet: pick/locator device with absolute

positioning• Joystick/Trackball: locator/valuator• Knobs (e.g. Volume control): valuator devices

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Drivers

• Nowadays days we have a lot more diverse input hardware, some of which don’t fit well into the Logical Input Device Classes – e.g.: bat, blow-suck tube, brain-computer interface, chording,

dataglove, electronic ink, eye-tracking, foot pedal, gesture, joystick, light pen, motional input device, mouthstick, OCR, paddle, pointing device, puck, stroke recognition, tongue-activated joystick, touch interactive display, touch tablet, touchpad, trackball

• Logical Input Devices were invented for the GKS standard graphics system. These days dedicated drivers for all pieces of hardware can be written that talk more directly to your Operating system.

Physical Input Devices

Other Common Input Devices

Data Gloves

Force Feedback Devices

• Combine input and some degree of output

• Useful for navigating simulated virtual environments

• Range of feedback types:– Tactile Feedback– Haptic Feedback– Force Feedback

Eye Trackers

Motion Capture