# Introduction to Computer Graphics

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Introduction to Computer Graphics
Lecture 1

Objectives We explore what computer graphics is about and survey some application areas We start with a historical introduction Course Instructor: Sadia Arshid

Computer graphics deals with all aspects of creating images with a computer
Hardware Software Applications Course Instructor: Sadia Arshid

Example Where did this image come from?
What hardware/software did we need to produce it? Course Instructor: Sadia Arshid

The object is an artist’s rendition of the sun for an animation to be shown in a domed environment (planetarium) Software: Maya for modeling and rendering but Maya is built on top of OpenGL Hardware: PC with graphics card for modeling and rendering Course Instructor: Sadia Arshid

Use of Computer Designed Pictures
Art, Entertainment and publishing Movie production, Animation and special effects Computer Games Browsing on world wide web Paint System Course Instructor: Sadia Arshid

Use of Computer Designed Pictures
Monitoring a process Displaying simulations Computer aided designs Scientific analysis and visualization Course Instructor: Sadia Arshid

Computer Graphics & Image Processing
create pictures and images Synthesize them on basis of some description Model Image Processing-> Improve or alter the images Processing can remove noise or enhance contrast Enhance certain features. Course Instructor: Sadia Arshid

Computer Graphics & Image Processing
Out put Input Image description image Image Processing Pattern Recognition Description Computer Graphics Data Processing Course Instructor: Sadia Arshid

Elements of pictures Created in Computer Graphics
Basic objects are Poly line Text Filled regions Raster images Course Instructor: Sadia Arshid

Elements of pictures Created in Computer Graphics
Poly Lines Connected sequence of straight lines Line Drawings Pictures made up of poly lines If several lines in poly line Each line is called an edge Two adjacent lines meet at vertex A polyline is closed if it ends where it starts called polygon It is simple if it does not self-intersect simple polygon(Jordan polygons) Well defined two regions(inside and out side) convex polygon. Course Instructor: Sadia Arshid

Attributes of Lines & Poly Lines
Color Thickness Type( dashed, solid, dotted) Manner in which thick edges are blend Course Instructor: Sadia Arshid

Filled Region Shape filled with some color or pattern.
Boundary of filled shape is a polygon Attributes Enclosing border Pattern color Course Instructor: Sadia Arshid

Text Text can be thought of as a sequence of characters in some font.
Some Graphic devices has two modes Text Mode Graphic Mode Use for simple input & out put Characters to control operating system or edit code in program Built in character generator Capable of drawing Alphabets, Numeric & special characters Place data on pre defined grid Do not support images or tables Course Instructor: Sadia Arshid

Graphic Mode Attributes: Richer set of character shapes
Characters can be placed arbitrarily Attributes: Font Face Weight Size Style Spacing Orientation Course Instructor: Sadia Arshid

Raster Images Image is made up of many cells, in different shades.
Each cell is known as pixel. Stored in memory as 2-dimensional array of square/ rectangle cells pixel maps/bitmap Course Instructor: Sadia Arshid

How Raster Images are created
Hand Designed Images Designer figure out value of each pixel and store it in memory Paint program Computed Images Algorithm is used to render a scene. Scanned Images Digitization Course Instructor: Sadia Arshid

Representation of Shades in Raster images
Monochrome raster images Pixel can contain one of two values in a raster image(bi-level). Grey level images Pixel depth( number of bits needed to represent grey levels) n bit quantity has 2n possible values Course Instructor: Sadia Arshid

Grey Scale Quantization
Let we have original image using 8 bit per pixel Display it or alter it with some less value per bit Either display image is not capable of displaying 256 values Image takes too much memory This loss is hardly noticeable if we use 5 or 6 bits per pixel Banding Course Instructor: Sadia Arshid

Color Raster Images Commonly each pixel is Ordered triple with red, green and blue value Number of bits used to represent color of each pixel is called Color depth. Color depth of 3 allows one bit for each color. True color image- color depth of 24 bit Course Instructor: Sadia Arshid

Digitization Digitization involves Quantization Sampling
Converting set of “continues values” to discrete Decide number of bits required to save each pixel value Sampling Digitizing coordinate values Course Instructor: Sadia Arshid

Quantization Effect Original image 256 levels, 128 levels,64levels ,32levels,16levels,8levels, 4levels, 2levels Course Instructor: Sadia Arshid

Quantization Effect 8 bits 4 bits 2 bits 1 bit

Sampling Effect (a) 1024*1024, 8-bit image. (b) 512*512 image resampled into 1024*1024 pixels by row and column duplication. (c) through (f) 256*256, 128*128, 64*64, and 32*32 images resampled into 1024*1024 pixels. Course Instructor: Sadia Arshid

Resolution Number of pixels per inch(PPI) or DPI
Higher the resolution, greater number of pixels Smaller resolution, smaller digital Image Keep balance between resolution and image size. Course Instructor: Sadia Arshid

Vector graphics Geometrical primitives such as points, lines, curves, and shapes or polygon(s), Based on mathematical equations, to represent images in computer memory They can be enlarged, rotated, stretched without loss of picture resolution Course Instructor: Sadia Arshid

Vector Images vs Raster images
Vector Drawing Applications(Path Based) Create and modify technical diagrams ie houses, cars Resolution independent While zooming edges are always sharp Raster Painting Applications(Pixel Based) Create art work Zooming based on interpolation Course Instructor: Sadia Arshid

Vector images Raster images Usually have no back ground
In appropriate for photo realistic images Raster images Pixels in grid Resizing reduce quality Minimal support for transparency Course Instructor: Sadia Arshid

Graphics Display Devices

Vector Displays Early computer displays: basically an oscilloscope
Control X,Y with vertical/horizontal plate voltage Intensity or brightness of the display is sometimes called the Z axis. Course Instructor: Sadia Arshid

Vector Drawing’s Commands

Each line segment to be displayed takes very few data values
Vector displays can draw a picture very rapidly Each XY pair is used to generate deflection voltages which will sweep the beam across the face of screen When the beam strike the phosphor, visible light is produced for certain time Course Instructor: Sadia Arshid

Vector Displays: Refresh Rate
Screen refresh every 1/30th of a second assume approximately 100 microseconds per command 1/30th second = 33 milliseconds = microseconds Therefore this Vector Display can only process approximately 330 commands before flicker starts. Course Instructor: Sadia Arshid

Complex object drawing is difficult list of commands becomes very long Complex scenes visible flicker first line may have faded out before last is drawn Vector displays can not show smoothly shaded regions Scanned images Cross hatching effect Course Instructor: Sadia Arshid

Raster Displays Pen plotters Types
move the pen invisibly on the papers on the spots specify by computer Types Flatbed Plotters Move pen in two directions on stationary sheet Drum Plotters Move paper back and forth to provide one direction in motion Move pen back and forth to provide other direction. Course Instructor: Sadia Arshid

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 Course Instructor: Sadia Arshid

Examples Video Monitor Flat panel Laser printer

Dot matrix Ink jet plotter Film recorder

Line/vector display raster display
no pixels array of pixels Course Instructor: Sadia Arshid

Raster Displays Fixed scan pattern
left to right, top to bottom To paint the screen, computer needs to synchronize with the scanning pattern Framebuffer special memory to buffer image with scan-out synchronous to the raster display device. Region of memory should be large enough to hold all pixel values Graphic card House the memory require for frame buffer Course Instructor: Sadia Arshid

Scanning process RGB Frame buffer Course Instructor: Sadia Arshid

Display Technologies Cathode Ray Tubes (CRTs)
Most common display device today Evacuated glass bottle Electrons pulled towards anode focusing cylinder Vertical and horizontal deflection plates Beam strikes phosphor coating on front of tube Course Instructor: Sadia Arshid

Color CRTs have Three electron guns

Phosphers Flourescence: Phospherescence: Persistence:
Light emitted while the phospher is being struck by electrons Phospherescence: Light emitted once the electron beam is removed emits visible light for a certain period after the bombardment has ceased Persistence: Time from the removal of the excitation to the moment when phosphorescence has decayed to 10% of the initial light output Long persistence -> image smears as picture is moved on screen. Short persistence -> no smearing as image moves but more refreshing needed Course Instructor: Sadia Arshid

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, phosphor persistence, lighting... Scan controller scans quickly through the entire memory of frame buffer Sending each pixel value to its proper spot on screen surface Course Instructor: Sadia Arshid

Index Colour and lookup table
Suppose we have Color depth 6 Each pixel go through an intermediate step Before they drive CRT These bits are used as an index in to a table of 2n values Course Instructor: Sadia Arshid

Color is expensive … The more color you want, the more bits you will need for each pixel Exercise: 1024 x 1280 screen with 24 bits per pixel, how large is the frame buffer? 1024 x 1280 x 24 / 8 = 4M Byte Course Instructor: Sadia Arshid

Color Lookup Table only 3 bits per pixel
But I insist on having high quality pictures … Use Color Look Up Table (LUT) 3 bits/pixel frame buffer R G B You can still have 24 bits in each of the color table entries 1 2 3 4 5 6 7 24 bit wide Course Instructor: Sadia Arshid

Colour Index Framebuffer

LUT memory If Raster display has color depth= b bits
Each LUT entry is w bits wide then System can display 2w colors 2b at one time So it require very little memory Only 2b words of w bits each System have capability of 24 bit colors Frame Buffer 1024 x 1280 x 8 / 8 = 1M Byte LUT 768 bytes Course Instructor: Sadia Arshid

Summarizing LUT Relationship b/w bitplanes and width of LUT
System has color depth of b bits. LUT entry is w bit wide System will display: 2w colors 2b at one time Relationship b/w bitplanes and width of LUT W is multiple of 3 derive each DAC. Memory required by LUT 2b *w/8 Storage required by System b/3*rows *cols + size of LUT Course Instructor: Sadia Arshid

Advantage To reduce cost of memory
Increasing b increases amount of memory needed for the frame buffer Course Instructor: Sadia Arshid

Display Technology: LCDs
Liquid Crystal Displays (LCDs) organic molecules naturally in crystalline state that liquefy when excited by heat or E field Crystalline state twists polarized light 90º. Course Instructor: Sadia Arshid

Display Technology: LCDs
Converts electric field to visible dots Each pixel is addressed by horizontal and vertical grid wire Course Instructor: Sadia Arshid

Active matrix panels Are LCD panels
Have tiny transistors at each pixel Transistors responds to electric field adjusts the liquid crystal by an amount proportional to the field Allow the display of different level of brightness Course Instructor: Sadia Arshid

Transistors provide some “memory”
CRT uses 2 patterns Left to right Top to bottom LCD don’t use it Display whole screen at once Transistors provide some “memory” that holds the crystals in their adjusted states so that the display need not be refreshed This produce much brighter display Course Instructor: Sadia Arshid

Display Technology Plasma DMD / DLP Organic LED Arrays
Display geometry Neon bulb turn on and off by electric field DMD / DLP optical semiconductor, called a Digital Micromirror Device (DMD), uses mirrors made of aluminum to reflect light to make the picture Organic LED Arrays It consists of a series of organic layers between two electrical contacts (electrodes). Each layer is deposited on the other, creating a single unit. Course Instructor: Sadia Arshid

Hard Copy Raster Devices
Film Recorder Strip of photographic film electron beam exposes as it sweeps over it in raster pattern Have their own frame buffer or camera mounted on CRT Electronic Hard copy of Image Laser Printer Sweep a laser beam on internal defined buffer Surface becomes electronically charged causing toner to adhere to spots. Course Instructor: Sadia Arshid

InkJet Plotter Post Script Language
A tiny nozzle sweeps over the paper and squirts proper color of ink at each pixel position. Post Script Language Printer are equipped with micro processor. Generate high quality text and graphics Course Instructor: Sadia Arshid

References Computer Graphics using open GL( chapter 1) by F.S Hill
InterNET Resources Howstufff works.com/lcd.htm & A lot More Course Instructor: Sadia Arshid