The term computer graphics includes almost everything on computers that is not text or sound.
every computer can do some graphics, and people have even come to expect to control their computer through icons and pictures rather than just by typing.
Here in our lab at the Program of Computer Graphics, we think of computer graphics as drawing pictures on computers, also called rendering. The pictures can be photographs, drawings, movies, or simulations -- pictures of things which do not yet exist and maybe could never exist. Or they may be pictures from places we cannot see directly, such as medical images from inside your body.
We spend much of our time improving the way computer pictures can simulate real world scenes. We want images on computers to not just look more realistic, but also to BE more realistic in their colors, the way objects and rooms are lighted, and the way different materials appear. We call this work "realistic image synthesis", and the following series of pictures will show some of our techniques in stages from very simple pictures through very realistic ones.
Following are the applications of Computer Graphics
-COMPUTER AIDED DESIGN
A major use of computer graphics is in design processes, particularly for engineering and architectural systems. For some design applications; objects are first displayed in a wireframe outline form that shows the overall sham and internal features of objects.
Software packages for CAD applications typically provide the designer with a multi-window environment. Each window can show enlarged sections or different views of objects. Standard shapes for electrical, electronic, and logic circuits are often supplied by the design package. The connections between the components have been mad automatically.
Animations are often used in CAD applications.
Real-time animations using wire frame displays are useful for testing performance of a vehicle.
Wire frame models allow the designer to see the interior parts of the vehicle during motion.
When object designs are complete, realistic lighting models and surface rendering are applied.
Manufacturing process of object can also be controlled through CAD.
Interactive graphics methods are used to layout the buildings.
Three-dimensional interior layouts and lighting also provided.
With virtual-reality systems, the designers can go for a simulated walk inside the building.
-PRESENTATION GRAPHICS
It is used to produce illustrations for reports or to generate slide for with projections.
Examples of presentation graphics are bar charts, line graphs, surface graphs, pie charts and displays showing relationships between parameters.
3-D graphics can provide more attraction to the presentation.
-COMPUTER ART
Computer graphics methods are widely used in both fine are and commercial art applications.
The artist uses a combination of 3D modeling packages, texture mapping, drawing programs and CAD software.
Pen plotter with specially designed software can create “automatic art”.
“Mathematical Art” can be produced using mathematical functions, fractal procedures.
These methods are also applied in commercial art.
Photorealistic techniques are used to render images of a product.
Animations are also used frequently in advertising, and television commercials are produced frame by frame. Film animations require 24 frames for each second in the animation sequence.
A common graphics method employed in many commercials is morphing, where one object is transformed into another.
-ENTERTAINMENT
CG methods are now commonly used in making motion pictures, music videos and television shows.
Many TV series regularly employ computer graphics method.
Graphics objects can be combined with a live action.
-EDUCATION AND TRAINING
Computer-generated models of physical, financial and economic systems are often used as educational aids.
For some training applications, special systems are designed.
Eg. Training of ship captains, aircraft pilots etc.,
Some simulators have no video screens, but most simulators provide graphics screen for visual operation. Some of them provide only the control panel.
-VISUALIZATION
The numerical and scientific data are converted to a visual form for analysis and to study the behavior called visualization.
Producing graphical representation for scientific data sets are calls scientific visualization.
And business visualization is used to represent the data sets related to commerce and industry.
The visualization can be either 2D or 3D.
Nowadays software packages provide graphics user interface (GUI) for the user to work easily.
A major component in GUI is a window.
Multiple windows can be opened at a time.
To activate any one of the window, the user needs just to check on that window.
Menus and icons are used for fast selection of processing operations.
Icons are used as shortcut to perform functions. The advantages of icons are which takes less screen space.
And some other interfaces like text box, buttons, and list are also used.
Computer Graphics Hardware
Output Devices
Display
Vector
Rastor
Hardcopy
Plotters
Printers
Input Devices
Graphics Processors
Video Display Devices
Video Monitor
CRT (cathode ray tube) was most common
Storage CRT versus Refresh CRT
Draw once and image remains on screen
Constant refreshing of screen
Refresh rates - 30 Hz, interlaced
Basic CRT design
Vacuum in a glass tube
Electron gun
Cathode with coiled wire filament
heat and it emits electrons
very high voltages
Deflection system
Phosphor coated screen
Focusing system
Acceleration system
walters@buffalo.edu 480/580 Lecture 5 Slide 1Focusing system
electrons repel each other - diverge
a) Magnetic
coil around outside of tube
very small spot size
flying spot scanners
b) Electrostatic
most common in graphics monitors
positively charged metal cylinder
(electrostatic lens)
focuses beam in middle of screen
curved screen
distance from "lens" to screen is constant
flat screen
distances increase from center
additional electronic deflection dependent focusing
Acceleration system
Speed electron towards the screen
a) Positively charged metal near screen
b) Accelerating anode
Deflection system
Control where beam hits screen
a) Magnetic deflection
2 pairs of coils mounted outside envelope
top and bottom
right and left
vary currents in coils to get correct deflection
most common
b) Electrostatic deflection
2 pairs of plates inside envelope
vary charge
walters@buffalo.edu 480/580 Lecture 5 Slide 2Screen
coated with phosphors
electrons hit screen and loose energy
heat
most goes to electrons of phosphor
jumps them to higher energy states
when fall back to lower states - give off energy as light
Phosphor characteristics
color of light - wavelength of energy
persistence
time taken to go from initial light level to 1/10th
long persistence
refresh less frequently
what happens to image of moving objects?
short persistence
refresh more frequently
less motion blur
typical 10 - 60 microseconds
Intensity
Control grid of electron gun
the more negatively charged, more electrons get repelled
the fewer make it through the control grid
Intensity distribution of spot on screen
Intensity
x (or y)
Gaussian fall off of intensity with distance
Function of the electron density in the beam
walters@buffalo.edu 480/580 Lecture 5 Slide 3Intensity
x (or y)
Resolution
Maximum number of dots that can be displayed without overlap
on CRT
(1K by 1K)
independent of screen size
OR
Maximum number of dots that can be displayed per inch or cm
(300 dpi)
How define overlap?
okay to overlap portions falling below 60% of maximum
What happens to overlap as increase intensity?
Aspect ratio
ratio of number of vertical points to horizontal points to
produce equal length lines
3/4
Rastor Scan versus Random Scan
Random scan
refresh is a function of the image complexity
Rastor scan
horizontal retrace and vertical retrace
interlaced versus noninterlaced
walters@buffalo.edu 480/580 Lecture 5 Slide 4walters@buffalo.edu 480/580 Lecture 5 Slide 5
Refresh rate
60 hz is typical
how often refresh each "dot" on screen?
once every 16667 microseconds
compare to persistence (10-60 microseconds)
"dot" is mostly dark!
flicker
when image appears to go on and off
refresh rate not high enough
CFF - critical fusion frequency
freq at which flickering display just fuses into nonflickering
what determines CFF?
for given phosphor?
Horizontal scan rate
scan lines displayed per second
approximately refresh rate times number of lines
Color CRTs
Beam penetration
Random scan
two layers of phosphor
slow beam - outer layer (say red)
faster beam - inner layer (say green)
Shadow mask
more colors
better quality
small patches of red, green and blue phosphors
perceive as one colored patch
3 electron guns
a) delta-delta
triad of patches
shadow mask
difficult to keep aligned
high resolution
b) precision in-line
easier to align, lower resolutionFlat-Panel Displays
Thinner, lighter, require less power
Examples?
Emissive versus nonemissive
Can you see it in the dark?
Could be lighted nonemisive
Emissive
Plasma panels
mixture of gases between two glass plates
vertical and horizontal conducting ribbons
apply voltage to two ribbons to make plasma glow
Thin-film electroluminescent displays
similar, but phosphor instead of gas
LED's
matrix of diodes, one per pixel
apply voltage and they produce light
Nonemissive
LCD
LC substance flow like a liquid, but have crystaline
molecular structure
Usually use nematic LC's (threadlike)
Two polarizers, two conductors, reflector
LC in normal state twists the light,
so is reflected back to viewer
apply voltage to conductors to turn off
Active Matrix LCD - transistor at each pixel (stores)
walters@buffalo.edu 480/580 Lecture 5 Slide 63D Viewing
Stereoscopic viewing
Stereo images (not full 3-d)
a) Red and green glasses
red and green images
b) Scan alternate images in alternate frames
View with goggles that shutter each lens in synch
with scanning
c) Multiple stereo images versus just two
Time Multiplexing
As rotate head, see different views
Compare to 2 view perception as move head
Projection CRT
project light from small diameter, very bright CRT onto screen
LCD Projection
flat panel display used with overhead projector
Resolution issues
Random Scan or Raster Scan?
How draw line?
What determines resolution along length of line?
What determines resolution across width of line?
Dot Matrix Printers
Print head with matrix of wire pins
Retract some pins before printing to print specified pattern
Laser Printers
Drum coated with photoelectric material (eg selenium)
Laser beam creates charge distribution on drum
Toner applied to drum and sticks according to charge
Toner transferred to paper
walters@buffalo.edu 480/580 Lecture 5 Slide 8Ink-Jet Printers
Paper rolled on drum
Boiled ink squirted onto paper through little nozzles (jets)
Charged ink stream deflected by electric field
Electrostatic Printers
Negatively charge a row at a time on paper
Apply toner
Electrothermal Printer
Heat sensitive paper
Dot matrix print head applies heat
Dye Sublimation
How get color output?
Impact device?
Non impact device?
3 pigments: cyan, magenta, yellow (sometimes black)
Will you get the same colors on a laser printer, as on a CRT?
Non Hard Copy , Non Display Output Devices
Print 3-D solid objects
Ink jet like head
Squirts fast drying thermopolymer material
Builds up 3-D models in layers
walters@buffalo.edu 480/580 Lecture 5 Slide 9Input Devices
Keyboards
Standard Keyboard
Button Boxes
Mouse
Mechanical mouse
Rotating ball
Two perpendicular padded shafts rotated by ball motion
shaft encoders output proportional to rotation
Optical mouse
Laser
Reflective Grid
Added buttons, trackball, thumbwheels
Trackball
Like upside down mouse with big ball
Fingers or hand move ball
Joystick
Movable
Measure motion
Stationary
Measure strain
Data Glove
Measures hand position and uses as input
Many degrees of freedom
(aside - data glove as output device
haptic computer interface
texture and pressure sensation via forces applied
to finger tip
)
walters@buffalo.edu 480/580 Lecture 5 Slide 10Motion Capturing Systems
Selspots
Attached to body in motion
Capture 3-D paths of points
Occlusion of visual "spots"
Full Body Scanners
How does it work?
Captures shape and color in 12 seconds
Uses?
Digitizers
Graphics Tablets
High resolution input
Trace contours
Hand held cursor
Stylus
Electromagnetic
Grid of wires
Electromagnetic pulses generated in sequence
Induces electrical signal in stylus or cursor
Acoustic
Strip microphones
Detect sound of spark on stylus tip
Time arrival at different microphones
Can be 3-D
walters@buffalo.edu 480/580 Lecture 5 Slide 9Image Scanners
Hand Held
Flat Bed
Drum
Touch Panels
Optical
Line LED's along top and side
Line of light detectors along bottom and other side
What determines Resolution?
Electrical
One plate of conducting material
One plate of resistive material
Touch pushes plates together
Measure voltage drop across resistive plate
Acoustical
Built into plasma panels
Transparent overlay on other displays
Every CAD system needs to provide hardcopy versions of circuits. Whether on paper or film, the permanent image is useful for offline consideration and for documentation. Although hardcopy graphics techniques are essentially the same as those found in display programming, there are other considerations. As a first step, it is necessary to understand the nature of these plotting devices.
Like displays, plotters fall into two broad categories: raster and calligraphic. Raster devices such as laser and ink-jet printers cover every pixel on the page and can produce arbitrarily complex images. Calligraphic devices such as pen plotters and some film recorders have a single writing head that must be moved to draw.
Unlike displays, the intensity resolution of plotters is generally low. Many hardcopy devices are strictly bilevel, able only to plot a black dot or to leave the spot white. Color printers have additional sources of ink but can still place the inks with only one intensity. Pen plotters can sometimes fake intensity by drawing over an area multiple times, but there is a limit to what the paper can endure, so these plotters still have only a few intensity levels. Essentially, the variations of layer appearance must be faked with patterned areas.
The display processor is used to convert digital information from the CPU into analog value needed by the display device. This digital-analog conversion depends on the type of display devices used and the particular graphics functions that are to be hardware implemented. For many graphics monitors, the coordinate origin is defined at the lower left screen corner.
A major task of display processor is to perform a procss called scan conversion. It is the process of separating contigous graphics objects as a collection of ellipcs, rectangles and polygons.
The display processor also designed to perform a number of additionaloperations. These functions include various line style(dashed, dotted, solid), displayinf color area and so on. Also designed to interface with interactive input devices such as mouse. Sometimes the display processor in a random graphics system is referred to as a display processing unit or a graphic controller.
Hard copy devices of Computer Graphics
Every CAD system needs to provide hardcopy versions of circuits. Whether on paper or film, the permanent image is useful for offline consideration and for documentation. Although hardcopy graphics techniques are essentially the same as those found in display programming, there are other considerations. As a first step, it is necessary to understand the nature of these plotting devices.
Like displays, plotters fall into two broad categories: raster and calligraphic. Raster devices such as laser and ink-jet printers cover every pixel on the page and can produce arbitrarily complex images. Calligraphic devices such as pen plotters and some film recorders have a single writing head that must be moved to draw.
Unlike displays, the intensity resolution of plotters is generally low. Many hardcopy devices are strictly bilevel, able only to plot a black dot or to leave the spot white. Color printers have additional sources of ink but can still place the inks with only one intensity. Pen plotters can sometimes fake intensity by drawing over an area multiple times, but there is a limit to what the paper can endure, so these plotters still have only a few intensity levels. Essentially, the variations of layer appearance must be faked with patterned areas.
