1. INTRODUCTION TO OPENGL

2. INTRODUCTION TO OPENGL
What is OpenGL API
Functions
Event Driven Programming Using OpenGL

3. OpenGL OpenGL is a platform-independent API that is Easy to use
Close enough to the hardware to get excellent performance
Focus on rendering
Omitted windowing and input to avoid window system dependencies

4. OpenGL Interface Components of the OpenGL Interface
GL: core OpenGL functions
GLU: graphics utility library
(helpers for creating common objects, eg. Spheres)
GLUT: GL Utility Toolkit
(interface to windowing system)
GLX: low-level interface to X Windows

5. Basic OpenGL Syntax Function names from GL library Symbolic constants
prefixed with gl
Symbolic constants
Capital letters with underscore
OpenGL built-in data types
glBegin, glClear, glCopyPixels
GL_2D, GL_RGB, GL_POLYGON
GLbyte, GLshort, Gint, GLfloat, GLdouble, GLboolean

6. GLUT OpenGL Utility Toolkit (GLUT)
Provides functionality common to all window systems
Open a window
Get input from mouse and keyboard
Menus
Event-driven
Code is portable but GLUT lacks the functionality of a good toolkit for a specific platform
No slide bars

7. GLUT Function names from GLUT library
prefixed with glut
void glutInit (int argc, char ** argv)
Initializes GLUT and should be called before any OpenGL functions.
void glutCreateWindow (*char title)
Creates a window on the screen with the title given by the argument.

8. Event Loops & Callback Function
Events – mouse, keyboard, windows events.
Callback function – define how the program should react to specific events.
void glutDisplayFunc (void (*func) (void))
The function func() is called each time there is a display callback

9. Event Loops & Callback Function
void glutMainLoop()
Causes the program to enter an event-processing loop.
Should be the last function in main().

10. OpenGL function format
function name
dimensions
glVertex3f(x,y,z)
x,y,z are floats
belongs to GL library

11. A Simple Program #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();

12. A Simple Program

13. 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

14. USING RGB COLOR
#include <GL\glut.h> void display() { glClear (GL_COLOR_BUFFER_BIT); glColor3f(1.0, 0.0, 0.0); 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) glutInit (&argc, argv); glutCreateWindow (“Simple"); glClearColor(1.0, 1.0, 1.0, 0.0); glutDisplayFunc (display); glutMainLoop ();

15. USING RGB COLOR Set a particular color: glColor3f(r, g, b);
Set a background color:
glClearColor(r, g, b, 1);
Clear the window to background color:
glClear(GL_COLOR_BUFFER_BIT);

16. USING RGB COLOR

17. Program Structure
Most OpenGL programs have a similar structure that consists of the following functions
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

18. main
#include <GL/glut.h> int main(int argc, char** argv) { glutInit(&argc,argv); glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB); glutInitWindowSize(500,500); glutInitWindowPosition(0,0); glutCreateWindow("simple"); glutDisplayFunc(mydisplay); init(); glutMainLoop(); }

19. 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

20. 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
OpenGL also uses some internal representations that usually are not visible to the application

21. Transformations and Viewing
In OpenGL, projection is carried out by a projection matrix (transformation)
There is only one set of transformation functions so we must set the matrix mode first
glMatrixMode (GL_PROJECTION)
Transformation functions are incremental so we start with an identity matrix and alter it with a projection matrix that gives the view volume
glLoadIdentity();
glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);

22. Two- and three-dimensional viewing
In glOrtho(left, right, bottom, top, near, far) the near and far distances are measured from the camera
Two-dimensional vertex commands place all vertices in the plane z=0
If the application is in two dimensions, we can use the function
gluOrtho2D(left, right,bottom,top)
In two dimensions, the view or clipping volume becomes a clipping window

23. OpenGL Primitives GL_POINTS GL_LINE_STRIP GL_POLYGON GL_LINES
GL_LINE_LOOP
GL_TRIANGLES
GL_QUAD_STRIP
GL_TRIANGLE_STRIP
GL_TRIANGLE_FAN
[Edward Angel, Interactive computer Graphics, 2009]

24. Event Driven Programming
Mouse Event
Event that occurs when the mouse button is pressed or released.
Mouse Motion Event
Event that occurs when the mouse is moved while one of the buttons is pressed.

25. Mouse Event Contains the following information:
Button: The mouse button that is pressed – left, middle, right.
State: The state of the button – up, down.
Position: The position of the mouse when the event occurs (x, y).

26. Mouse Event Registering with MouseEvent: glutMouseFunc(myMouse);
Call-back function:
void myMouse(int button, int state, int x, int y)
Values for button:
GLUT_LEFT_BUTTON
GLUT_MIDDLE_BUTTON
GLUT_RIGHT_BUTTON
Values for state:
GLUT_UP
GLUT_DOWN
x, y:
screen coordinates of mouse position
(origin at top-left corner)

27. Mouse Motion Event Contains the following information:
Position: The current position of the mouse as the mouse is being dragged holding one of the buttons pressed. The events are continuously generated as the mouse button is pressed and dragged.

28. Mouse Motion Event Registering with MouseMotionEvent:
glutMotionFunc(myMovedMouse);
Call-back function:
void myMovedMouse(int x, int y)
x, y:
screen coordinates of mouse position
(origin at top-left corner)

29. Keyboard Event Contains the following information:
key: The ASCII value of the key pressed.
Position: The current position of the mouse when the key is pressed.

30. Keyboard Event Registering with KeyboardEvent:
glutKeyboardFunc(myKeyboard);
Call-back function:
void myKeyboard(unsigned int key, int x, int y)
Values for keys:
ASCII values for normal keys
GLUT_KEY_LEFT, GLUT_KEY_RIGHT, … (arrow keys)
x, y:
screen coordinates of mouse position
(origin at top-left corner)

31. Window Events
The Window Redraw Event occurs whenever the window needs to be redrawn. This happens when the window is first opened and when the window is exposed by moving another window off of it.
The Window Reshape Event is generated when the window is resized with the mouse.

32. Window Events Registering with WindowRedrawEvent:
glutDisplayFunc(myDisplay);
Registering with WindowReshapeEvent:
glutReshapeFunc(Reshape);