Lecture 3Lecture 3

Principles of Computer Graphics

Lecture 3 1

Why we learn computer graphics?

Appreciate what we see The knowledge can applied when we want The knowledge can applied when we want to develop specific engineering program that requires computer graphicsq p g pSome of theories computer graphics has engineering applicationsTrend: Engineering Computer Graphic Group under Faculty of ME in UT, Austin

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Definition of Computer Graphics

R f t t d i Refers to any computer device or program that makes a computer capable of displaying and capable of displaying and manipulating pictures. (Webopedia)Computer graphics generally means Computer graphics generally means creation, storage and manipulation of models and images models and images

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Computer Graphics

i. Interactiveii. Clippingiii. Projectioniv. Hidden line or surface removalv. Surface detail and texturev. Surface detail and texture

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i. Interactive Computer Graphics

First interactive graphics system, Sketchpad, by Ivan Sutherland

Note: using CRT monitor, light pen and function-key panel

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Interactive Computer Graphics

Touch Screen on PDA

HP I-PaqTM Wii by Nintendo

Virtual Sphere

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Desktop

Semi ImmersiveSe e s e

Fully Immersivee.G CAVE

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Menu and Icon: Interactive

Microsoft word with all the menu

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UI FactC t d i h h d ’tComputer speed increases however human doesn’t.

Human Computer Interface Research Center

•Human Interface Technology Lab (University of Washington)u a te ace ec o ogy ab (U e s ty o as gto )•Group for User Interface Research (UC Berkeley)•Human Computer Interaction Institute (Carnegie Mellon)•Graphics, Visualization, and Usability Center (Georgia Tech)

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Cool fact on UI

90/10 rule10% of the features using 90% of the time consumedconsumed

Xerox-First developed mouse-First developed personnel workstationFi t d l d l i lti i d-First developed overlapping multiwindows

1985, famous lawsuit, Apple vs. Microsoft over Windows use of icons, pointers, etc., p ,

Microsoft Windows is considered as a Mac imitation with minimal improvements

Apple lost all claims

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Apple lost all claims

Computer Graphics

T t li i i li ti To create realism in visualization. i. Interactive ii. Clippingiii. Projectioniv. Hidden line or surface removalv. Surface detail and texturev. Surface detail and texture

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ii. Clipping

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Clipping con’t

Example: G l th d i i Example: Google earth, rendering is carried out when we view on certain areaarea

Why? Save memory allocation when only viewport is rendered or displayedo y e po t s e de ed o d sp ayed

Problem: display actual shape and display shapep y p

Technique: display the visible part and discard the invisible part

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p

Clipping Technique

F t h i C h d S h l dFamous technique: Cohen and Sutherland

Principle: Edges of the screen is extended to form nine regions. The central region will be the entities to be displayed.p y

Polygon: extra line is createdCurves: breaking the curves into segments of straight lines

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2D CLIPPING

Th d ill i th ti f

2D CLIPPING

010100011001

The procedure will examine the vertices of the lines using four bits code

b3 b2 b1 b0

0100

DC

If TRUE bits = 1

b3 = ( x < xmin) vertex to the left of ADDisplay0000

01001000

A B

b2 = ( x > xmax) vertex to the right of BC

b1 = (y < ymin) vertex below AB

b0 = (y > ymax) vertex above CD

01100010

1010A B

(y y )

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3D ClippingFront plane Center plane Rear plane

010000010001

011010011001

010010

011000

Front plane

000000000001

001010001001

000010

001000

Center plane

100000100001

101010101001

100010

101000

Rear plane

010000010001 010010

010110010100010101

000000000001 000010

000110000100000101

100000100001 100010

100110100100100101

27 R i27 Regions

6 bit outcode records results of four bounds tests: First bit: outside back plane, behind back planeS d bit t id f t l i f t f f t l Second bit: outside front plane, in front of front plane Third bit: outside top plane, above top planeFourth bit: outside bottom plane, below bottom planeFifth bit: outside right plane, to right of right planeSi th bit t id l ft l t l ft f l ft l

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Sixth bit: outside left plane, to left of left plane

Computer Graphics

T t li i i li ti To create realism in visualization. i. Interactive ii. Clippingiii. Projectioniv. Hidden line or surface removalv. Surface detail and texturev. Surface detail and texture

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iii. Type of projectionP ll l P j ti P ti P j tiParallel Projection:Used by engineer for measurement

Perspective Projection:Imitates eyes or camera to look more natural

SCREEN

SCREEN

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Projection in games

Isometric projection has been used Isometric projection has been used in games since it first used in 1982

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Computer Graphics

T t li i i li ti To create realism in visualization. i. Interactive ii. Clippingiii. Projectioniv. Hidden line or surface removalv. Surface detail and texturev. Surface detail and texture

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iv. Hidden line & Surface RemovalBack-Face Removal: faces that are facing camera are visibleBack-Face Removal: faces that are facing camera are visible

invisibleback face

invisibleback face

visible

visible

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iii. Hidden line & Surface Removal-cont

Z d thZ-depthsorting all the faces (polygons) in the

di t th l t scene according to the largest z coordinate value of each.

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iii. Hidden line & Surface Removal-cont

Ray Tracing MethodRay Tracing Method

0Ryx0y)f(x,

222 =−+

=Eq of the circle

0Ryx +

)6183()68()13( tttQtdPQ

+=+=+=

Let’s say eye point P = (-3,1) with Vector direction of (0.8, -0.6)

)6.1,8.3()6,8(.)1,3( tttQ −+−=−+−=

Therefore 0960)(

2 =+−

=

ttQf

Then, solve the quadratic equations.Then, solve the quadratic equations.imaginary answer: no intersectionOne answer: tangent to the circle, visible pointTwo answer: two points,

i ibl hidd

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one visible, one hidden

Computer Graphics

T t li i i li ti To create realism in visualization. i. Interactive ii. Clippingiii. Projectioniv. Hidden line or surface removalv. Surface detail and texturev. Surface detail and texture

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v. Surface detail and texture

Type of lightType of lightPoint light source: provide shine on surfaceAmbient light: a light with uniform g gbrightness

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IlluminationSurfaces comprises of small flat Surfaces comprises of small flat faces called polygon, each patch has its own normal and center point. pThe basic principle of illumination is light rays will strike on each patch and the li h ill b b b d fl d

specular

light will be absorbed, reflected or scattered. Realism can be achieved by the combination of spot light and

diffuse

ambientcombination of spot light and ambient light

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Shading

Basic principle: each vertex on the polygon has its own color. The color p ygwill be interpolated as they move toward center of the polygon

Flat shading: same color across the polygonGourand shading: lights are computed at its vertices and interpolation across the polygon is carried outinterpolation across the polygon is carried outPhong lighting model: linearly interpolate a normal vector N across the polygon on from the normal on each vertex and it is done for each pixel in the polygon. Then color the pixel accordingly

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accordingly.

Texture mappingA texture map is applied (mapped) to polygon 2D A texture map is applied (mapped) to polygon. 2D mapping is similar to process of applying patterned paper to the object. 3D texture mapping is analogous to craving process. g pVisual affect or illusion is used to create realism such sense of depth.

iClone

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