1 Programming With OpenGL A Technical Overview of The OpenGL Graphics API Computer Graphics.

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Programming With OpenGL

A Technical Overview of The OpenGL Graphics API

Computer Graphics

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Graphics API•OpenGL(All Platform)

•DirectX(Windows)

-DirectDraw-Direct3D

-DirectSound-DirectPlay-DirectInput-DirectSetup

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Computer Graphics

OpenGL, the standard software interface for graphics hardware, allows programmers to create interactive 2D and 3D graphics applications on a variety of systems. With OpenGL you can create high-quality color images. OpenGL makes it easy to build geometric models, change the viewing position, control the color and lighting of geometric primitives, and manipulate pixel and texture map images. This course will cover an immediately applicable subset of OpenGL, so that you can write a simple graphics program.

An Introduction

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Computer Graphics

OpenGL Advantages

• Standard & Portability - Open• OpenGL is the most logical extension of brilliant API called GL

• Supported through a larger number of vendors• 1992, SGI• SGI, IBM, DEC, Intel, Microsoft,

• Vendor and hardware independent• Better integration into the windowing system

•Win95, Win98, WinNT, Win2000,WinXP

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Objectives• Become familiar with the capabilities of OpenGL• Understand the order of operations, and the major libraries• Know how to use viewing, lighting, shading, and hidden surface removal functionality• Know how to draw images with OpenGL and process event-driven input• See how code is written and compiled


Computer Graphics

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Computer Graphics

What is OpenGL?

• A low-level graphics modeling and rendering library (includes more than 100 functions)

• State machine• A layer of abstraction between graphics

hardware and an application program• Hardware, Window System and Operating

System independent

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Computer Graphics

Where is in the Graphics SystemDeveloping &Application

OpenInventor, SoftImage 3D

Window System

Operating System

Graphics Hardware

OpenGL

X-Windows, MS Windows …

UNIX, MS Windows, OS/2...

SGI-XZ, SGI-Extreme, AGC-3D..

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Computer Graphics

What OpenGL Can Do For You

• Wireframe Models, Viewing, Transformation• Antialiasing• Flat And Smooth Shading• Shadows• Texture Mapping• Motion Blur• Haze and Fog, Depth -Of -Field Effect• Evaluator Function: 1D & 2D Bezier Curves, NUR

BS ...

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Computer Graphics

Graphics Functions• Primitive functions

• Attribute functions

• Viewing functions

• Transformation functions

• Input functions / control functions

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Computer Graphics

Overview of OpenGL Components

• OpenGL Library - gl• OpenGL Utilities - glu• OpenGL Extension to X Windows - glx• OpenGL Programming Guide Auxiliary Lib

rary - aux• Windows Graphics Library - wgl

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OpenGL Library GLProgramming With OpenGL

Computer Graphics

• Powerful But Primitive Set Of Rendering Commands• All high level drawing is done in terms of these commands• You can write your own toolkit based on these functions

• The GL Library Includes• Functions supporting drawing primitives

• Lines, Points, Circles, Polygons, Characters, ...• Clipping, Viewing, Projections, Transformation• Shading, Lighting• Direct colors• 3D rendering functions such as

• Antialiasing, Multi buffering, Texture, ...• Pixels manipulation (read, write, copy)

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Computer Graphics

OpenGL Utility Library GLU• Utility library is a set of commonly used graphics routines

• built on top of OpenGL• Uses lower level OpenGL commands for

• Setting up matrixes for viewing and projection• Performing polygon tessellation

- concave polygons, polygons with holes, self- intersecting polygons, ..• Rendering surfaces (spheres, cylinders, disks, ..)• Mipmapping• NURBS (Non-Uniform Rational B-Spline) curves• Error handling

• This library is part of the OpenGL implementation

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OpenGL Auxiliary Library• This is a simple, platform-dependent library• Makes programming examples simpler & more complete

• Managing windows• Handling input events• Drawing classic 3-D objects• Managing a background process• Running a program

• Do not use them in a production application


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OpenGL Command Syntax• OpenGL commands use :

– the prefix gl– initial capital letters for each word making up the command name

• OpenGL define constants– begin with GL– use all capital letters– use underscore to separate words

• OpenGL uses command extensions for clarification– whenever the same command exists to use different arguments

• Examples :– glClearColor()– GL_COLOR_BUFFER_BIT– glColor3fv()


Computer Graphics

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Computer Graphics

OpenGL Command Syntax• � glVertex3fv v indicates vector format, if present data type: f float d double float s signed short integer i signed integer number of components (2, 3, or 4)• Other data types in OpenGL commands - b character - ub unsigned character - us unsigned short integer - ui unsinged integer

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Computer Graphics

Drawing Geometry• Vertices can define a variety of geometric objects, and

different numbers of vertices are required depending on the object.

• We can group as many vertices as desired, using the functions glBegin() and glEnd()

• Types of primitives glBegin (GLenum primitiveType)• Example: glBegin (GL_POLYGON); glVertex3f(-. 5f, -. 5f, -. 5f); glVertex3f(. 5f, -. 5f, -. 5f); glVertex3f(-. 5f, .5f, -. 5f); glVertex3f(. 5f, .5f, -. 5f); glEnd();

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Computer Graphics

Primitive Types

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Attributes• An attribute is any property that determines how a geometric primitive is to be rendered

• The present values of attribute are part of the state of the graphics system• Each geometric type has a set of attributes


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Attributes• Vertex attributes: glPointSize() point size glColor*()/ glIndex*() current vertex color glNormal*() current vertex normal (lighting) glMaterial*() current material property (lighting) glTexCoord*() current texture coordinate glEdgeFlag*() edge status (surface primitives)• RGB colors: glColor3f(1.0,0.0,0.0) glClearColor(1.0,1.0,1.0,1.0)• Indexed color: glIndexi(element) Alpha channel

- opacity or transparency

Computer Graphics


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Computer Graphics

Drawing Geometry• Example: Drawing a green, flat triangle strip: glBegin (GL_ TRIANGLE_ STRIP); glColor3f( 0. f, 1. f, 0. f); glNormal3f( 0. f, 0. f, 1. f); glVertex3f(-. 5f, -. 5f, -. 5f); glVertex3f(. 5f, -. 5f, -. 5f); glVertex3f(-. 5f, .5f, -. 5f); glVertex3f(. 5f, .5f, -. 5f); glEnd();

• glBegin - glEnd paradigm allows great flexibility in describing primitives.

• Any combination of color, normal, texture, material etc. information can be bound with any given vertex.

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OpenGL The Statemachine• OpenGL knows about various different states or modes• These states remain in effect until you change them• Examples of states variables are:

– the current color– current viewing and transformation matrix– line and polygon stippling patterns– polygon drawing modes– shading modes– pixel packing conventions– position and characteristics of light sources– material properties

• Each state variable has a default value• You can query the current variable value


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States

• glEnable (GLenum capability)

• glDisable (GLenum capability)

• GLboolean glIsEnabled (GLenum cap)

- turn on and off OpenGL states

- capability can be one of (partial list): GL_ BLEND (alpha blending)

GL_ DEPTH_ TEST (depth buffer)

GL_ FOG

GL_ LIGHTING

GL_ LINE_ SMOOTH (line antialiasing)

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Computer Graphics

Structure of a Typical Program

main: create window initialize GL states (e. g., viewing, color, lighting) initialize display lists check for events (and process them) if window event (window moved, exposed, etc.) - modify viewport, if needed - redraw else if mouse or keyboard - do something, e. g., change states & redraw

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Initialization• void auxInitWindow(Glbyte *titleString)

• void auxInitDisplayMode(Glbitfield mask)

• void auxInitPosition(Glint x,Glint y, Glsizei width,Glsizei height)

Mask: AUX_RGBA or AUX_INDEX AUX_SINGLE or AUX_DOUBLE AUX_DEPTH AUX_STENCIL AUX_ACCUM

Computer Graphics


- Win32 API: CreateWindow(…)

- Win32 API: MoveWindow(…)

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Control Functions

• void auxMainLoop(void(*displayFunc)void))

-Windows message: WM_PAINT• void auxReshapeFunc(void(*function) (GLsizei,Glsizei))

-Windows message: WM_SIZE

• void auxMouseFunc(Glint button,Glint mode, void(*function)(AUX_EVENTREC*))

- Windows Message: WM_MOUSEDOWN

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Redrawredraw: clear screen (to background color) change state( s), if needed render some graphics change more states render some more graphics ....

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Homework: The Sierpinski Gasket1. Pick an initial point at random inside the

triangle.

2. Select one of the three vertices at random.

3. Find the point halfway between the initial point and the randomly selected vertex.

4. Display this new point by putting some sort of marker, such as a small circle, at its location.

5. Replace the initial point with this new point.

6. Return to step 2.


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Computer Graphics

1 Programming With OpenGL A Technical Overview of The OpenGL Graphics API Computer Graphics.

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