OpenGLA Brief Overview

What is OpenGL? It is NOT a programming language. It is a Graphics Rendering API consisting of

a set of function with a well defined interface.

Whenever we say that a program is OpenGL-based or OpenGL applications, we mean that it is written in some programming language (such as C/C++ or Java) that makes calls to one or more of OpenGL libraries.

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Useful Websites and Books Official Site

http://www.opengl.org Non official sites

http://nehe.gamedev.net/http://google.com/

BOOKS OpenGL Red Book & OpenGL Blue Book

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Three Views of OpenGL Programmer’s view

Specify a set of objects to render Describe the properties of these objects Define how these objects should be viewed

State machine States determine how the inputs are processed. Change a state (such as color) by using state

changing functions OpenGL uses Rendering Pipeline Model

Models -> Transformer -> Clipper -> Projector -> Rasterizer -> Image

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OpenGL API Functions OpenGL contains over 200 functions

Primitive functions: define the elements (e.g. points, lines, polygons, etc.)

Attribute functions: control the appearance of primitives (e.g. colors, line types, light source, textures, etc.)

Viewing functions: determine the properties of camera. Handle transformations.

Windowing functions: not part of core OpenGL Other functions

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Libraries and HeadersLibrary Name Library File Header File Note

OpenGL opengl32.lib (Win)

-lgl (UNIX)

gl.h “core” library

Auxiliary library

glu32.lib (Win)

-lglu (UNIX)

glu.h handles a variety of accessory functions

Utility toolkit glut32.lib (Win)

-lglut (UNIX)

glut.h

glaux.h

window management & others

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Function Naming Conventions

glColor3f(…) library name, command name, # of arguments, argument type

gl: OpenGL

glu: GLU

glut: GLUT

f: the argument is float type

i: the argument is integer type

v: the argument requires a vector

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A Sample Programvoid main (int argc, char **argv)

{

glutInit (&argc, argv);

glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);

glutInitWindowSize (500, 500);

glutCreateWindow (“My First Program");

myinit ();

glutDisplayFunc ( display );

glutReshapeFunc ( resize );

glutKeyboardFunc ( key );

glutMainLoop ();

}

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1. Initialize & Create Window

void main (int argc, char **argv)

{

glutInit (&argc, argv); // GLUT initialization

glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB); // display model

glutInitWindowSize (500, 500); // set window size

glutCreateWindow (“My First Program"); // create window

……

}

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2. Initialize OpenGL State

void myinit(void)

{

glClearColor(1.0, 1.0, 1.0, 1.0); // background color

glColor3f(1.0, 0.0, 0.0); // line color

glMatrixMode(GL_PROJECTION); // set up viewing:

glLoadIdentity(); // load identity matrix

gluOrtho2D(0.0, 500.0, 0.0, 500.0); // specify Orthographic view

glMatrixMode(GL_MODELVIEW); // go back to MV matrix

}

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3. Register Callback Functions

void main (int argc, char **argv)

{

……

glutDisplayFunc ( display ); // display callback

glutReshapeFunc ( resize ); // window resize callback

glutKeyboardFunc ( key ); // keyboard callback

……

}

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(side note) GLUT Callback Functions• Contents of window need to be refreshed

glutDisplayFunc() • Window is resized or moved

glutReshapeFunc()• Key action

glutKeyboardFunc()• Mouse button action

glutMouseFunc()• Mouse moves while a button is pressed

glutMotionFunc()• Mouse moves regardless of mouse button state glutPassiveMouseFunc()• Called when nothing else is going on

glutIdleFunc()

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3.1 Rendering Callback

void display( void )

{

int k;

glClear(GL_COLOR_BUFFER_BIT);

for( k=0; k<5000; k++)

……

}

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3.2 Window Resize Callback

void resize(int w, int h)

{

……

display();

}

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3.3 Keyboard Input Callback

void key( char mkey, int x, int y ){ switch( mkey ) { case ‘q’ : exit( EXIT_SUCCESS ); break; …… }}

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4. Event Process Loop This is where your application receives

events, and schedules when callback functions are called

void main (int argc, char **argv)

{

……

glutMainLoop();

}

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2D Geometric Primitives Primitives – fundamental entities such

as point and polygons Basic types of geometric primitives

Points Line segments Polygons

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GL_POINTS GL_LINES GL_LINE_STRIP GL_LINE_LOOP

GL_POLYGON GL_QUADS GL_TRIANGLES GL_TRIANGLE_FAN

All geometric primitives are specified by vertices

2D Geometric Primitives

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glBegin( type ); glVertex*(…); …… glVertex*(…);glEnd();

type determines how vertices are combined

Specifying Geometric Primitives

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Example

void drawSquare (GLfloat *color)

{

glColor3fv( color ); // sets the color of the square

glBegin(GL_POLYGON);

glVertex2f ( 0.0, 0.0 );

glVertex2f ( 1.0, 0.0 );

glVertex2f ( 1.1, 1.1 );

glVertex2f ( 0.0, 1.0 );

glEnd();

glFlush() // force the renderer to output the results

}

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2D Viewing• Where do we draw the 2D object?

– Assume that we draw objects on an infinite sheet of paper

– Then we need to specify clipping region and project these 2D objects to the screen

void gluOrtho2D(GLdouble left, GLdouble right, GLdouble bottom, GLdouble top);

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Coordinate Systems and Transformation• Identify which matrix we wish to alter• Set the matrix to an identity matrix• Alter the matrix to form the desired matrix

glMatrixMode(GL_PROJECTION);

glLoadIdentity();

gluOrtho2D(-1.0, 1.0, -1.0, 1.0);

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Coordinate Systems and TransformationglMatrixMode(GLenum mode);

– Identify which matrix we wish to alter – The mode is usually GL_MODELVIEW, GL_PROJECTION

glLoadIdentity();– Set the current matrix to an identity matrix

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Enabling GL Features• Features – lighting, hidden-surface

removal, texture mapping, etc…• Each feature will slow down the

rendering process.• We can enable/disable each feature

individuallyvoid glEnable(GLenum feature)

void glDisable(GLenum feature)

E.g. glEnable(GL_LIGHTING);

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Enabling GL FeaturesGL_ALPHA_TEST

If enabled, do alpha testing. GL_AUTO_NORMAL

If enabled, generate normal vectors when either GL_MAP2_VERTEX_3 or GL_MAP2_VERTEX_4 is used to generate vertices. See glMap2.

GL_BLEND If enabled, blend the incoming RGBA color values with the values in the color buffers..

GL_CLIP_PLANE i If enabled, clip geometry against user-defined clipping plane i..

GL_COLOR_LOGIC_OP If enabled, apply the currently selected logical operation to the incoming RGBA color and color buffer values.

GL_COLOR_MATERIAL If enabled, have one or more material parameters track the current color.

GL_COLOR_TABLE If enabled, preform a color table lookup on the incoming RGBA color values.

GL_CONVOLUTION_1D If enabled, perform a 1D convolution operation on incoming RGBA color values.

GL_CONVOLUTION_2D If enabled, perform a 2D convolution operation on incoming RGBA color values.

……

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Saving the State• State changing functions – overwrites the

state variables• We can store previous state values for later

use– Matrix stacks

void glPushMatrix()void glPopMatrix()

– Attribute stacksvoid glPishAttrib(GLbitfield mask)void glPopAttrib()

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Saving the StateglMatrix(GL_PROJECTION)// set projection matrix// draw sceneglPushMatrix();// change the project matrix//draw sceneglPopMatrix();

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Double Buffering• Use two buffers: front buffer and back buffer

to guarantee the displaying of a fully redrawn butter image

glutSwapBuffers();– Replace glFlush() by glutSwapBuffer() in

the display callback if using double buffering

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Transformations and Camera Analogy• Modeling transformation

– Positioning and moving the model.• Viewing transformation

– Positioning and aiming camera in the world.• Projection transformation

– Adjusting the lens of the camera.• Viewport transformation

– Enlarging or reducing the physical photograph.

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Transformations in OpenGL

• Transformations are specified by matrix operations. Desired transformation can be obtained by a sequence of simple transformations that can be concatenated together.

• Transformation matrix is usually represented by 4x4 matrix (homogeneous coordinates).

• Provides matrix stacks for each type of supported matrix to store matrices.

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Specifying Operations (1)• Translation

glTranslate {fd} (TYPE x, TYPE y, TYPE z)

Multiplies the current matrix by a matrix that translates an object by the given x, y, z.

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Specifying Operations (2)• Scaling

glScale {fd} (TYPE x, TYPE y, TYPE z) Multiplies the current matrix by a

matrix that scales an object by the given x, y, z.

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Specifying Operations (3)• Rotation

glRotate {fd} (TYPE angle, TYPE x, TYPE y,

TYPE z) Multiplies the current matrix by a matrix

that rotates an object in a counterclockwise direction about the ray from origin through the point by the given x, y, z. The angle parameter specifies the angle of rotation in degrees.

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Order of Transformations

• The transformation matrices appear in reverse order to that in which the transformations are applied.

• In OpenGL, the transformation specified most recently is the one applied first.

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Viewing-Modeling Transformation

• If given an object, and we want to render it from a viewpoint, what information do we have to have?

– Viewing position– Which way I am looking at– Which way is “up”…..

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Default Viewing

+X

+Z

+YBy default, the camera is at the origin, looking down the negative z-axis

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Where are we and what are we looking at?

x

y

z

x

y

zEyepoint

(eyex, eyey, eyez)

Model

View-up vector

(upx, upy, upz)

Loot at

(atx, aty, atz)

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Viewing in OpenGL• Look-At Function

gluLookAt (eyex, eyey, eyez, atx, aty, atz, upx, upy, upz )

Defines a viewing matrix and multiplies the current matrix by it. Alters the ModelView matrix.

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Projection Transformation

• Projection & Viewing Volume• Projection Transformation• Viewpoint Transformation

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Perspective Projection Volume

Far-plane: zFarNear-plane: zNear

Viewing volume

hw

aspect ratio = w/hy

z

x

fovy

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Perspective Projection CommandsglFrustum( left, right, bottom, top, zNear, zFar

)

Creates a matrix for a perspective viewing frustum and multiplies the current matrix by it. Alters the Projection matrix.

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gluPerspective( fovy, aspect, zNear, zFar )

Alternative to glFrustum(..). Creates a matrix for an perspective viewing frustum and multiplies the current matrix by it. Alters the Projection matrix.

Note: fovy is the field of view (fov) angle between the top and bottom planes of the clipping volume. aspect is the aspect ratio

Perspective Projection Commands

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Viewport• Viewport

– The region within the window that will be used for drawing the clipping area

– By default, it is set to the entire rectangle of the window that is opened

– Measured in the window coordinates, which reflect the position of pixels on the screen related to the lower-left corner of the window

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Viewport Transformation

A viewpoint is defined as the same size as the window

A viewpoint is defined as half the size of the window

h

w h

w

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Viewport Commands

• glViewport( x, y, width, height )

Defines an area of the window into which the final image is mapped

(x, y) specifies the lower-left corner of the viewport

(width, height) specifies the size of the viewport rectangle

Advanced Topics Lighting Texture Mapping Plotting Implicit Functions Shadows Fog Picking (object selection) GUI (glut pop-up menus, glui library)

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Reference links given on slide 3