CS488 Introduction to Computer Graphics Luc R ENAMBOT 1 What is - - PowerPoint PPT Presentation

CS488 Introduction to Computer Graphics

Luc RENAMBOT

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What is Computer Graphics ?

• Mathematics + computer science + art =

computer graphics

• Rendering of images on a device
• Rendering: Creating images from models
• Models: objects constructed from geometric

primitives (points, lines, polygons) specified by their vertices

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Models

• Models exist in n-dimensional

'mathematically pure' space

• n typically 2 or 3
• n can be > 3 with scientific data

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Rendering

• Rendered version typically created on

physical 2D media (e.g. a video screen)

• Rendered version can be simple or complex

(lighting, shadows, colors, texture)

• Rendering a single image can take
• from a small fraction of a second (game)
• to hours or days (movie)

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Common Uses

• Scientific/medical visualization
• CAD/CAM
• Multimedia
• Computer interfaces (Windows, X, Aqua)
• Virtual reality
• Special effects
• Artistic expression
• Way cool video games

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Scientific Visualization

Brief History: 60s

• 1960s

Vector display devices

• 1963 Ivan Sutherland's

Sketchpad

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"A display connected to a digital computer gives us a chance to gain familiarity with concepts not realizable in the physical world. It is a looking glass into a mathematical wonderland."

Brief History: 70s

• 1970s Raster display

devices

• 1972 Pong
• 1976 Star Wars

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1976 Star Wars

• 'Pilots Briefing'
• CG done here at UIC

1976 Star Wars

• 'Pilots Briefing'
• CG done here at UIC

Brief History: 80s

• 1980s Raytracing, Realism,

Multimedia

• 1982 Tron, Star Trek II
• 1985 Last Starfighter
• 1989 Abyss, The

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Brief History: 90s

• 1990s

Virtual reality, Interactivity, 'Cheaper' graphics horsepower

• 1991 Terminator 2
• 1993 Jurassic Park
• 1995 Toy Story
• 1996 Titanic, Twister
• 1999 Star Wars - the Phantom

Menace

• Lots of commercials & TV

series using CG

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Brief History: 2000s

• 2000s CG all over the

place

• 2001 Final Fantasy
• 2001 Lord of the Rings:

2001/2002/2003

• 2005 King Kong

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Software

• Many application programs available to

produce computer graphics, either as 2D images, 3D models, or animated sequences (Corel Draw, Photoshop, AutoCAD, Maya, SoftImage, etc.)

• We will deal with the lower level routines

which do the work of converting models into a displayable form on the display device.

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Various Environments

• Graphics languages / libaries / APIs

(Application Programming Interfaces)

• GKS
• DirectX
• QuickDraw and X (as in X11 server)
• Postscript, PDF
• OpenGL

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For the class

• We will be using OpenGL in this course
• Linux machines in the CS Computer Graphics

lab to give a common grading platform

• OpenGL is available for all the major platforms
• Accelerated on almost all current graphics

cards

• Mesa (www.mesa3d.org): open source

implementation running on the processor

• Usually in C/C++ but various bindings available

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Hardware

• In this class we are going to concern
• urselves with producing images on video

screens rather than onto printers, or plotters since that allows a much greater amount of interactivity

• Convenient to think of models in

mathematical terms but hardware brings CG back to reality

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Video Display Hardware

• Vector (calligraphic) displays
• Raster displays

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Vector displays

• Lines drawn directly, no predefined

grid

• Commands tell the electron gun

where to move and when to turn

• n/off
• + Lines are smooth
• + Close to the 'pure mathematics' of

the model

• - Slower with more elements to be

drawn, can start to flicker

• - Only lines possible, no filled

polygons, or bitmaps

• - Monochrome for each electron gun

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Raster displays

• Image represented by a rectangular

grid of pixels (picture elements)

• Image stored in a frame buffer
• Electron gun(s) continually scanning

in a regular pattern (line by line across entire screen)

• + Constant time to redraw any

number of elements

• + No flicker
• - Jagged edges from conversion to

pixels

• - Discretized version of the model

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Vector vs. Raster

• You need to keep redrawing

the image on the screen to keep it from fading away

• Vector displays redraw as

quickly as possible given the number of objects on the screen

• CRT based raster displays

redraw the image (or refresh the screen) at a fixed rate (e.g. 60 times per second) no matter how complex the scene.

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Vector Games

• Asteroids, Battlezone, ...
• Monochrome, color filters,

RGB electron guns

• Vectrex console in the 80s

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Buffers in Raster Displays

• Pretty much all CG done

using raster displays

• The screen is

represented by a 2D array of elements

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Frame Buffer

• Array of computer memory used to store

an image to be displayed

• The user manipulates the values in the frame

buffer: 60 times a second (or at some other fixed rate) the frame buffer is copied onto the display device.

• If video screen is 512 pixels wide by 512

pixels tall the frame buffer must be able to store 512 X 512 elements ... one element for each pixel on the screen

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Monochrome display

• 512 x 512 x 1bit (bit is

either 0=off, or 1=on.)

• Each of the 512 x 512

pixels can be either on

• r off (white or black)

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Grayscale display

• 512 x 512 x 8bit (each

pixel is 8 bits deep so values 0-255 are possible)

• Each of the 512 x 512

pixels can be one of 256 shades of grey (from black to white)

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Color display: 24-bit

• 512 x 512 x 24bit (each pixel has 8 bits for red, 8 bits

for green, and 8 bits for blue)

• Each pixel can be black->bright red (0-255) combined

with black->bright green (0-255) combined with black- >bright blue (0-255)

• Each of the 512 x 512 pixels can be one of 16 million

colors

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Color display: 8-bit with colormap

• Want benefits of 24 bit color with only 8 bit display
• 512 x 512 x 8-bit (each pixel is 8 bits deep so values

0-255 are possible)

• Each of the 512 x 512 pixels can be one of 256 index

values into a video lookup table

• Video lookup table has 256 24-bit RGB values where

each value has 8 bits for red, 8 bits for green, and 8 bits for blue

• 16 million colors are possible, but only 256 of them can

be displayed at the same time

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Color display: 8-bit with colormap

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Frame Buffer

• Depth of frame buffer

(e.g. 8 bit) determines number of simultaneous colors possible

• Width of color map (e.g.

24 bit) determines number of colors that can be chosen from

Screen size Monoch rome 8-bit 24-bit 512x512 32K 256K 768K 640x480 38K 300K 900K 1280x1024 160K 1.3M 3.8M 1920x1200 280K 2.2M 6.6M

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Hardware affects on computer animation

• A program may take several minutes, hours,
• r days to render a single image. These

images can then be stored and played back

• r recorded to create animation
• A more interesting situation is when the

animation is live with the computer generated image changing while the user watches, or as the user interacts with the computer

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Animation

• For animation, a SECOND frame buffer is

needed

• This is similar to how a motion picture

works at 24 frames per second:

• 1. A new frame is moved in front of the lens
• 2. The shutter is opened to display the frame
• 3. The shutter is closed

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Computer Graphics

• In the case of a film, each of the frames has already

been generated and its just a question of showing them

• In interactive computer graphics the frames are being

drawn one at a time. The frame buffer is regularly sent to the display device (eg 60 times per second) whether the frame buffer has been completely updated or not

• We don't want the user to see the screen being cleared

and the new image being drawn on the screen each time the display refreshes. We only want the user to see a succession of completely drawn images

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Another buffer needed

• Solution is to have two frame buffers
• User draws into one buffer while the
• ther is being displayed
• When drawing is complete the buffers are

swapped

• Only swap buffers in between screen

refreshes

• when electron gun of the CRT is off and

moving from bottom of screen to the top

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Drawing speed

• If the screen refreshes 60 times a second you can have a

new image on the screen:

• 1/60th of a second ... 60 frames per second if all frames

take equal time

• 1/30th of a second ... 30 frames per second if all frames

take equal time

• 1/20th of a second ... 20 frames per second if all frames

take equal time

• 1/10th of a second ... 10 frames per second if all frames

take equal time

• Very small change in the time it takes to draw a scene

into the frame buffer can have a major impact on the speed of the application

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How to run at 45fps ?

• Can your program run at 45 frames per

second (fps)?

• Yes:
• if 30 frames take 1/60th each
• the next 15 frames take 1/30th each you

will display 45 frames per second

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Human Perception

• For smooth motion you want at least 10

frames per second

• Preferably more than 15 frames per second
• Movie (theater): 24fps
• TV: 30fps (60 updates or fields)
• 60 frames is good
• More is better up to screen rate

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Next time...

• 2D graphics: “how we convert from

geometry to pixels”

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Bonus Video

• Larry Cuba’s StarWars video
• EVL (then called the Circle Graphics

Habitat) at the University of Illinois at Chicago Circle

• Dept. of Chemistry computer