4 - Intro to Computer Graphics

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Introduc)ontoComputerGraphics

IT137–Mul)media

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Outline

•  Computer Graphics –  Sources

 –  Copyright Issues

 –  Types

•  Vector –  Characteristics

 –  File formats –  Authoring software

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Outline

•  Bitmap –  Characteristics

 –  Processes

•  Images –  Concept

 –  Eyes vs. the Camera

•  Digital Camera –  CCD and CMOS

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Outline

•  ImageDataTypes

 –  Color – Grayscale – Monochrome/Binary

•  ImageDigi)za)on

 –  ImageResolu)on –  PixelAccuracy –  DitheringandBanding –  ImageCompression

•  ImageFileTypes

 –  JPEG – GIF –  PNG –  TIFF – GraphicAnima)onFiles –  Postscripts

•  Metafiles

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ComputerGraphics

•  refers to the use of computer to display andmanipulate information in graphical or pictorialform, either on a visual‐display unit or via a

printer or plotter. It can either be 2D or 3D.

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ComputerGraphics

•  Scanned Images - scanners

•  Still Images and Video Images – from

digital cameras and video cameras•  Photo CD

•  Stock Photography

•  Clip Art

•  Original Artwork – from digital tablet

Sources of Graphics

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ComputerGraphics

•  One can alter or manipulate artwork only if:

 –  Permission to use is granted by owner, artist orpublisher

 –  It is created by you

 –  It is clearly public domain

 –  It is used strictly for instructional purposes orfalls within the restrictions of fair use.

Copyright Issues

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ComputerGraphics

•  We have 2 major2D graphic

types:•  Vector

 –  Draw-type

•  Bitmap –  Raster

Types of Graphics

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Vector/Draw-TypeGraphics

•  Vector graphics are made up of manyindividual, scalable objects defined bymathematical equations rather thanpixels, so they always render at the

highest quality.•  Objects may consist of lines, curves, and

shapes with editable attributes such ascolor, fill, and outline.

•  Rasterizing is the conversion of vectorimages to bitmap images. Resultingimages are often called raster graphics. 

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Vector/Draw-TypeGraphics

•  Changing the attributes of a vector objectdoes not affect the object itself.

•  An object can also be modified by shapingand transforming it using nodes and control 

handles. –  Nodes - points through which a path passes,

preferably where it changes direction.

 –  Control handles - used to alter the shape andangle of the adjacent curve segment

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Vector/Draw-TypeGraphics

•  Scalable

•  Resolution independent

•  No background

•  Cartoon‐like

•  Inappropriate for photo‐realistic

images

Characteristics

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Vector/Draw-TypeGraphics

•  AI (Adobe Illustrator)

•  CDR (CorelDRAW)

•  CMX (Corel Exchange)

•  CGM (Computer Graphics Metafile)

•  DXF (AutoCAD)

•  WMF (Windows Metafile) 

File Formats

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Vector/Draw-TypeGraphics

•  Adobe Illustrator

•  CorelDRAW

•  Serif DrawPlus

•  Inkscape 

Authoring Software

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Bitmap/RasterGraphics

•  Stores the graphic/image data in thesame manner that the computer monitorcontents are stored in video memory.

•  The term digital image is usually refersto bitmap images.

•  Has a finite collection of discrete

samples, called pixels (pictureelements) in 2D, of any observableobject

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Bitmap/RasterGraphics

•  Pixels - point locations with associated samplevalues, usually of light intensities/colors,transparency, and other control information.

•  They are not little circles or little squares. 

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Bitmap/RasterGraphics

•  Interpolation is a process of estimatingcolor values for new pixels (based onsurrounding pixels) when the imagessize is increased.

•  Resampling to reduce the size meansthrowing away some pixels, therebyreducing image quality.

•  Scaling has no effect on the number of pixels in an image.

•  All scanned images and images fromdigital cameras are bitmaps.

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Bitmap/RasterGraphics

•  Pixels in a grid

•  Resolution dependent

•  Resizing reduces quality

•  Easily converted

•  Restricted to rectangle

•  Minimal support for transparency

Characteristics

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Image

•  Electromagnetic radiation that entersthe human visual system through eyesand is incident upon the cells of the

retina.

•  Wavelength of light is between 4,000and 7,700 Angstrom (violet and red),

i.e. between 4 x 10−7 and 7.7 x 10−

7m

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TheHumanEye

•  Light enters through pupil (the circular opening inthe center of iris), passes through the crystallinelens and vitreous humor to be converged to a nodalpoint focused on the retina.

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Eyevs.theCamera

•  Retina = Film

•  Lens = To adjust focus

•  Aperture = Iris

 –  Controls theamount of light thatreaches the camerasensor

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TheDigitalCamera

•  Two types of sensor:

 – Charge Coupled Devices (CCD)

 – Complementary Metal Oxide

Semiconductor (CMOS)•  Both are N x M rectangular grid photosensor

(sensitive to light intensity)

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IMAGEDATATYPES

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ImageDataTypes

 Eachpixelisstoredasasinglebit(0or

1 Forsimplegraphicsortextonly.

Monochrome/BinaryImages

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ImageDataTypes

•  Eachpixelhasagrayvaluebetween

0and255.•  A640x480grayscaleimage

requiresover300KBofstorage.

GrayscaleImages

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ImageDataTypes

 Eachpixelisrepresentedbythreebytes(e.g.,RGB

 24-bitimagessupport256x256x256possiblecombinedcolors(16,777,216

 8-bitimagesexistbutundergoesbandingbecauseoflimitedcolors.

ColorImages

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ImageDataTypes

•  Togetthespecificcolor,aColorLook-UpTable(LUTsisused.

•  LUTsarealsoknownasthepalee.•  Most24-bitimagesare32-bitimages,the

extrabyteofdataforeachpixelisusedto

storeanalphavalue(α)represen)ngspecial

effectinforma)on.

ColorImages

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ImageDigi)za)on

•  Themainparametersofthedigi)za)onare: – Imageresolu)on–thenumberofsamplesinthegrid

 – Pixelaccuracy–howmanybitsareusedpersample.

 – ImageCompression–algorithmstoproducesmallerfilesizes

•  Theseparametershaveadirecteffectontheimagequalitybutalsotothestoragesizeoftheimage

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ImageDigi)za)on

•  Example: – Imageresolu)on=640x480

 – Screenresolu)on=640x480

•  Howwilltheimageappearonscreen?

ImageResolu)on

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ImageDigi)za)on

•  Resolu)ondiffersonscreenandonprint.•  Displayingonscreen–72to100dpi•  Prin)ng–300dpi

ImageResolu)on

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ImageDigi)za)on

•  DigitalimageconsistsofNxMpixels,each

representedbykbits.

•  Apixelcan,thus,have2kdifferentvalues•  Thepixelvaluesareconsideredasintegersvaryingfrom0(blackpixelto2k-1(white

pixel.

•  MayalsobetermedasColorDepth.

PixelAccuracy

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ImageDigi)za)on

•  Thesameimagesize256x256pixels,

differentinnumberofgraylevels:

•  32(5bits• 16(4bits

•  8(3bits•  4(2bits

PixelAccuracy

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ImageDigi)za)on

•  Banding–reducescolorswithoutdithering,resul)ngtoareasofsolidcolors.

•  Dithering–theprocessofposi)oningdifferentcoloredpixelsside-by-sidetocreate

theillusionthatnocolorsaremissing.

•  Ditheringisatechniqueusedfordisplayingmonochromeimages

Ditheringvs.Banding

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ImageFileTypes

•  GIF(GraphicsInterchangeFormat•  JPEG(JointPhotographicExpertsGroup• PNG(PortableNetworkGraphics

•  TIFF(TaggedImageFileFormat•  GraphicsAnima)onFiles•  Postscript/EncapsulatedPostscript

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ImageFileTypes–GIF

•  GraphicsInterchangeFormat•  CreatedbyUNISYSCorp.andCompuserve,ini)allyfortransminggraphicalimagesoverphonelinesviamodems.

•  Limitedtoonly8-bit(256colorimages,suitableforimageswithfewdis)nc)vecolors(e.g.graphicsdrawing=smallfilesize

•  Types: – GIF87a–transparentGIF – GIF89a–animatedGIF

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ImageFileTypes-JPEG

•  JointPhotographicExpertsGroup•  Supportsmillionsofcolors(24-bit• Bestforcon)nuous-toneimagessuchasphotographs.

•  Lossycompressionwhichallowsusertosetthedesiredlevelofquality/compression.

•  Doesnotsupporttransparency.

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ImageFileTypes-JPEG

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ImageFileTypes

•  Supportsfrom8-bitto32-bitcolordepths•  Usescompressionforsmallfilesizes•  Supportstransparencyandinterlacing•  Capableofgammacorrec8on

 – Note:somebrowsersrequirepluginsforsupport

PortableNetworkGraphics

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ImageFileTypes

•  storesmanydifferenttypesofimages(e.g.,monochrome,grayscale,8-bit&24-bitRGB,

etc.

•  DevelopedbytheAldusCorp.inthe1980'sandlaterbyAdobeSystems

•  TIFFisalosslessformat(whennotu)lizingthenewJPEGtagwhichallowsforJPEG

compression

TaggedImageFileFormat