1. Computer Graphics System Input & Output Technologies
Introduction to Computer Graphics and Animation
(Principle of Computer Graphics)
Rattapoom Waranusast

2. A Graphics System
Output devices
Input devices
Processor and memory

3. Display Devices Hard Copy devices Transient displays
Printers, plotters
Transient displays
LCD Monitors, CRT Monitors, projectors

4. 1897 – Cathode Ray Oscilloscope
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

5. 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

6. 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
Standard Dot-trio
SONY Trinitron CRT
NEC Hybrid Mask
Hitachi EDP

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

8. Shadow Masks (2)

9. Vector Display DevicesB
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.

10. 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

11. 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)

12. 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.

14. 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.

15. Bits Per Pixel 1 Bit Per Pixel
1 or 0 in binary (on/off)
22 = 4 possible combinations: 00, 01, 10, 11
to = 255 possible values
to i.e or 16 million possible values

16. Memory Usage Example Black-and-white Greyscale True-colour
8x8x1 = 64 bits
8x8x8 = 512 bits
8x8x24 = 1536 bits

17. Rasterization
Geometric and Mathematical Data structures typically in vertex coordinates not dependent on resolution
We must convert from typical continuous representation to discrete

18. Anti-aliasing

19. 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

20. Vector Displays Advantages: Disadvantages
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

21. 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.

22. 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

23. CRT Displays Disadvantages 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 Hz (no memory effect)
Geometrical errors at edges

24. LCD Displays Advantages Disadvantages
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)

25. Plasma Displays

26. Plasma Displays Plasma Display Panel Pros Cons 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

27. 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.

28. 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.

29. 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

30. 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

31. 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

32. 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.

33. Physical Input Devices

34. Other Common Input Devices

35. Data Gloves

36. 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

37. Eye Trackers

38. Motion Capture