Opengl (1)

Introduction to OpenGL

TA: Mani Thomas
CISC 440/640
mani@udel.edu

Acknowledgements
 Most of the material for the slides were
adapted from
• E. Angel, “Interactive Computer Graphics”, 4th edition
 Some of the slides were taken from
• CISC 440/640 Computer Graphics (Spring 2005)
 Some of the images were taken from
• F.S.Hill, “Computer Graphics using OpenGL”
 Other resources
• http://www.lighthouse3d.com/opengl/glut/
• Jackie Neider, Tom Davis, and Mason Woo, “The
OpenGL Programming Guide” (The Red Book)

The Programmer’s Interface
 Programmer sees the graphics system
through a software interface: the
Application Programmer Interface (API)

API Contents
 Functions that specify what we need to form
an image
• Objects
• Viewer
• Light Source(s)
• Materials
 Other information
• Input from devices such as mouse and keyboard
• Capabilities of system

History of OpenGL
 SiliconGraphics (SGI) revolutionized the
graphics workstation by implementing
the pipeline in hardware (1982)
 To access the system, application
programmers used a library called GL
 With GL, it was relatively simple to
program three dimensional interactive
applications

OpenGL: What is It?
 Thesuccess of GL lead to OpenGL
(1992), a platform-independent API that
was
• Easy to use
• Close enough to the hardware to get excellent
performance
• Focus on rendering
• Omitted windowing and input to avoid window
system dependencies

OpenGL Evolution
 Controlled
by an Architectural Review
Board (ARB)
• Members include SGI, Microsoft, Nvidia, HP,
3DLabs, IBM,…….
• Relatively stable (present version 2.0)
• Evolution reflects new hardware capabilities
• 3D texture mapping and texture objects
• Vertex programs
• Allows for platform specific features through
extensions

OpenGL Libraries
 GL (Graphics Library): Library of 2-D, 3-D
drawing primitives and operations
• API for 3-D hardware acceleration
 GLU (GL Utilities): Miscellaneous functions
dealing with camera set-up and higher-level
shape descriptions
 GLUT (GL Utility Toolkit): Window-system
independent toolkit with numerous utility
functions, mostly dealing with user interface

Software Organization

application program

OpenGL Motif
widget or similar GLUT
GLX, AGL
or WGL GLU

X, Win32, Mac O/S GL

software and/or hardware

Lack of Object Orientation
 OpenGL is not object oriented so that
there are multiple functions for a given
logical function
•glVertex3f
•glVertex2i
•glVertex3dv
 Underlying storage mode is the same
 Easy to create overloaded functions in
C++ but issue is efficiency

OpenGL function format
function name
dimensions

glVertex3f(x,y,z)

x,y,z are floats
belongs to GL library

glVertex3fv(p)

p is a pointer to an array

simple.c
#include <GL/glut.h>
void mydisplay(){
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_POLYGON);
glVertex2f(-0.5, -0.5);
glVertex2f(-0.5, 0.5);
glVertex2f(0.5, 0.5);
glVertex2f(0.5, -0.5);
glEnd();
glFlush();
}
int main(int argc, char** argv){
glutCreateWindow("simple");
glutDisplayFunc(mydisplay);
glutMainLoop();
}

Event Loop
 Note that the program defines a display
callback function named mydisplay
• Every glut program must have a display
callback
• The display callback is executed whenever
OpenGL decides the display must be
refreshed, for example when the window is
opened
• The main function ends with the program
entering an event loop

Default parameters
 simple.c is too simple
 Makes heavy use of state variable
default values for
• Viewing
• Colors
• Window parameters

OpenGL Camera
 Right-handed system
 From point of view of
camera looking out into
scene:
• OpenGL places a camera at
the origin in object space
pointing in the negative z
direction
 Positive rotations are
counterclockwise around
axis of rotation

Coordinate Systems
 The units in glVertex are
determined by the application and
are called object or problem
coordinates
 The viewing specifications are
also in object coordinates and it is
the size of the viewing volume
that determines what will appear
in the image
 Internally, OpenGL will convert to
camera (eye) coordinates and
later to screen coordinates

Transformations in OpenGl
 Modeling transformation
• Refer to the transformation of models (i.e., the
scenes, or objects)
 Viewing transformation
• Refer to the transformation on the camera
 Projection transformation
• Refer to the transformation from scene to
image

Model/View Transformations
 Model-view transformations are usually
visualized as a single entity
• Before applying modeling or viewing transformations,
need to set glMatrixMode(GL_MODELVIEW)
• Modeling transforms the object
• Translation: glTranslate(x,y,z)
• Scale: glScale(sx,sy,sz)
• Rotation: glRotate(theta, x,y,z)
• Viewing transfers the object into camera coordinates
• gluLookAt (eyeX, eyeY, eyeZ, centerX, centerY,
centerZ, upX, upY, upZ)

Model/View transformation

Courtesy: Neider, Davis and Woo, “The OpenGL Programming Guide”

Projection Transformation
 Transformation
of the 3D scene into the
2D rendered image plane
• Before applying projection transformations,
need to set glMatrixMode(GL_PROJECTION)
• Orthographic projection
• glOrtho(left, right, bottom, top, near, far)
• Perspective projection
• glFrustum (left, right, bottom, top, near, far)

Projection Transformation

Orthographic projection

Perspective projection

F.S.Hill, “Computer Graphics using OpenGL”

Program Structure
 Most OpenGL programs have the following
structure
• main():
• defines the callback functions
• opens one or more windows with the required properties
• enters event loop (last executable statement)
• init(): sets the state variables
• Viewing
• Attributes
• callbacks
• Display function
• Input and window functions

simple.c revisited
#include <GL/glut.h> includes gl.h
int main(int argc, char** argv)
{
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(500,500);
glutInitWindowPosition(0,0);
glutCreateWindow("simple"); define window properties
glutDisplayFunc(mydisplay);

init(); display callback
glutMainLoop(); set OpenGL state
}
enter event loop

GLUT functions
 glutInit allows application to get command line
arguments and initializes system
 gluInitDisplayMode requests properties for the
window (the rendering context)
• RGB color
• Single buffering
• Properties logically ORed together
 glutWindowSize in pixels
 glutWindowPosition from top-left corner of display
 glutCreateWindow create window with title “simple”
 glutDisplayFunc display callback
 glutMainLoop enter infinite event loop

Window Initialization
black clear color
void init()
{ opaque window
glClearColor (0.0, 0.0, 0.0, 1.0);
glColor3f(1.0, 1.0, 1.0); fill/draw with white
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
}

viewing volume

Display callback function
void mydisplay()
{
glClear(GL_COLOR_BUFFER_BIT);

glBegin(GL_POLYGON);
glVertex2f(-0.5, -0.5);
glVertex2f(-0.5, 0.5);
glVertex2f(0.5, 0.5);
glVertex2f(0.5, -0.5);
glEnd();

glFlush();
}

Input and Interaction
 Multiple input devices, each of which can send a
trigger to the operating system at an arbitrary time by a
user
• Button on mouse
• Pressing or releasing a key
 Each trigger generates an event whose measure is put
in an event queue which can be examined by the user
program

Callbacks
 Programming interface for event-driven
input
 Define a callback function for each type
of event the graphics system recognizes
 This user-supplied function is executed
when the event occurs
mouse callback function
• GLUT example: glutMouseFunc(mymouse)

GLUT event loop
 Last line in main.c for a program using GLUT is the
infinite event loop
glutMainLoop();
 In each pass through the event loop, GLUT
• looks at the events in the queue
• for each event in the queue, GLUT executes the
appropriate callback function if one is defined
• if no callback is defined for the event, the event is ignored
 In main.c
• glutDisplayFunc(mydisplay) identifies the function to
be executed
• Every GLUT program must have a display callback

Posting redisplays
 Many events may invoke the display callback
function
• Can lead to multiple executions of the display callback on a
single pass through the event loop
 We can avoid this problem by instead using
glutPostRedisplay();
which sets a flag.
 GLUT checks to see if the flag is set at the end of the
event loop
• If set then the display callback function is executed

Double Buffering
 Instead of one color buffer, we use two
• Front Buffer: one that is displayed but not written to
• Back Buffer: one that is written to but not displayed
 Program then requests a double buffer in main.c
• glutInitDisplayMode(GL_RGB | GL_DOUBLE)
• At the end of the display callback buffers are swapped
void mydisplay()
{
glClear(GL_COLOR_BUFFER_BIT|….)
.
/* draw graphics here */
.
glutSwapBuffers()
}

Using the idle callback
 The idle callback is executed whenever there are no events in the event queue
• glutIdleFunc(myidle)
• Useful for animations
void myidle() {
/* change something */
t += dt
glutPostRedisplay();
}

Void mydisplay() {
glClear();
/* draw something that depends on t */
glutSwapBuffers();
}

Using globals
 The form of all GLUT callbacks is fixed
• void mydisplay()
• void mymouse(GLint button, GLint state, GLint
x, GLint y)
 Must use globals to pass information to callbacks

float t; /*global */

void mydisplay()
{
/* draw something that depends on t
}

Other important functions
 glPushMatrix() / glPopMatrix()
• Pushes/pops the transformation matrix onto the matrix
stack
 glLoadIdentity(), glLoadMatrix(), glMultMatrix()
• Pushes the matrix onto the matrix stack
 Chapter 3 of the “Red Book” gives a detailed
explanation of transformations
• Jackie Neider, Tom Davis, and Mason Woo, “The
OpenGL Programming Guide” (The Red Book)

Assignment policy
 How to submit
 What to submit

 On late submission

How to submit
 Submit as a tar/zip file
• Unix:
> tar -cf username_projectNum_(440|640).tar
projectDir
> gzip username_projectNum_(440|640).tar
• Windows:
• Use a zip utility
 Naming convention
• username_projectNum_(440|640).(tar.gz|zip)
 Submit the tar/zip file through the course web (More
details will be announced later)

What to submit
 Must contain
• Readme
• Makefile
• Source codes
• Output figures (if any)
 Must NOT contain
• obj intermediate files
• obj data files

What to submit: Readme
% My name
% My email: myemail@udel.edu
% Project Num

% Part 1: description of this project
This project is to apply xxx algorithm to plot xxx, …

% Part 2: what I did and what I didn't do
I completed all/most/some functionalities required in this project.
The system is robust and the rendering is fairly efficient, …

I didn't do …. The reason is ….

% Part 3: What files contained

% Part 4: How to compile and how to run
The project is developed in windows system and tested in stimpy (strauss) unix system

On late submission
 N * 10 percent of the points you got will be
deducted if there are N (<=5) late days (not
counting weekends).
 No acceptance for the submission more than
5-day late
 Each student has three free (i.e. without any
penalty) late days for entire semester.
• You should notify the TA the use of free late days
ahead

OpenGL: Setup in Unix
Steps to compile the code on Strauss
1. run following command
2. setenv LD_LIBRARY_PATH /home/base/usrb/chandrak/640/OpenGL/Mesa-
2.6/lib:/usr/openwin/lib:/opt/gcc/lib (This is present as a comment in the Makefile)
3. download Makefile and hello.c
4. compile and run hello.c:
strauss> gmake -f Makefile_composor
5. run your code (Use ./hello if path not set properly)
strauss> hello

Steps to compile the code on stimpy
1. run following command
2. setenv LD_LIBRARY_PATH /usr/local/mesa/lib:/usr/openwin/lib
3. download Makefile_stimpy and hello.c
4. compile and run hello.c:
stimpy> gmake -f Makefile_stimpy
5. run your code (Use ./hello if path not set properly)
stimpy> hello

OpenGL: Setup in Windows
 Go to the GLUT webpage
• http://www.opengl.org/resources/libraries/glut.html
 From the bottom of the page, download the
following
• Pre-compiled Win32 for Intel GLUT 3.7 DLLs for
Windows 95 & NT
 Follow the instructions in
• http://www.lighthouse3d.com/opengl/glut/
 When creating the Visual C/C++ project, use
the console based setup

Office Hours
 Tuesday 5:30 – 7:30 pm
 Pearson Hall 115B

 Webpage

• vims.cis.udel.edu/~mani/TA%20Courses/Fall05/grap
 Email - mani@udel.edu

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