Using Graphics Libraries

Lecture 3Mon, Sep 1, 2003

Graphics Libraries

Theoretically, with only a functionsetPixel(x, y, red, green, blue),

we could create any graphics image.However, it would be quite tedious.A graphics library provides an abundance of useful functions to simplify the task of creating graphics.

Device Independence

A library is device-independent if it provides a common API, regardless of the hardware on which it is used.The OpenGL API for Windows is identical to the OpenGL API for the Macintosh.Of course, the library must be compiled separately for each hardware system.

Windows-Based Programming

OpenGL consists of three libraries gl – graphics library

Basic functions. glu – graphics library utility

Composites of basic GL functions. glut – graphics library utility toolkit

Functions that interact with the windowing system.

Window-based Programming

int main(int argc, char* argv[]){

glutInit(&argc, argv);glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);glutInitWindowSize(screenWidth, screenHeight);glutInitWindowPosition(100, 150);glutCreateWindow(“My Window Title");

glutDisplayFunc(display);glutReshapeFunc(reshape);glutKeyboardFunc(keyboard);glutMouseFunc(mouse);

init();glutMainLoop();return 0;

}

The glut functions are used in main() to create and open a graphics window.Functions used to create a graphics window. glutInit(&argc, argv). glutInitDisplayWindow(options). glutInitWindowSize(width, height). glutInitWindowPosition(x, y). glutCreateWindow(name).

Window-Based Programming

Windows-Based Programming

glutInit(&argc, argv). Initializes the glut library. Should be called before any other glut

function. Must receive the command-line

arguments.

glutInitDisplayMode(options). Specifies color mode. Specifies single or double buffering.

Windows-Based Programming

glutInitWindowSize(width, height). Sets the height and width of the

window in pixels.

glutInitWindowPosition(x, y). Sets the position of the upper left

corner of the window.

glutCreateWindow(name). Creates, but does not display, the

window.

Callback Functions

A callback function is a user-specified function that the library will call whenever necessary.Each callback function must be registered with glut.glut provides for over 20 callbacks.

Callback Functions

The glut library contains functions with names of the form

glutXXXFunc(parameter),where XXX stands for some form of windows interaction (mouse, keyboard, etc.).The parameter is a user-defined function xxx().

Callback Functions

In the main function we writeglutXXXFunc(xxx).

Then when XXX is activated by the user (mouse click, keystroke, etc.), the function xxx() is called to handle the event.

OpenGL Callback Functions

glutDisplayFunc(display); Called whenever the scene needs to be

redrawn. Activated by calls to glutPostRedisplay().

glutReshapeFunc(reshape); Called whenever the window is resized. Activated by resizing the window. Warning: This does not respond to

iconifying the window.

OpenGL Callback Functions

glutMouseFunc(mouse) Called whenever the mouse is clicked. Activated by mouse clicks. Left or right, up or down.

glutKeyboardFunc(keyboard) Called whenever a key is pressed. Activated by keystrokes (down only)

of an ASCII key (letters, digits, punctuation).

OpenGL Callback Functions

glutSpecialFunc(special) Called whenever a special key is

pressed. Activated by keystrokes (down only) of

a non-ASCII key (function key, arrow key, etc.).

glutMotionFunc(motion) Called whenever the mouse is moved

while the button is pressed.

OpenGL Callback Functions

glutPassiveMotionFunc(passiveMotion) Called whenever the mouse is moved

while the button is not pressed.

glutIdleFunc(idle) Called whenever nothing else is

happening.

The Main Loop

Typically main() ends by calling glutMainLoop()

This function runs forever.It calls the callback functions as necessary.It handles all drawing commands as they are generated.

The Main Loop

main

display

reshape

keyboard

mouse

Processkeyboard

and mouseevents

glutMainLoop()

display()

mouse()

keyboard()

reshape()

LibraryFunctions

Example: Callback Functions

CallbackTest.cpp

Other Initializations

void init(){

glClearColor(0.8, 0.8, 0.8, 0.0); // light grayglColor3f(0.0f, 0.0f, 0.0f); // blackglMatrixMode(GL_PROJECTION);glLoadIdentity();gluOrtho2D(xmin, xmax, ymin, ymax);glViewport(0, 0, screenWidth, screenHeight);

}

World Coordinates

The call to gluOrtho2D() establishes the world coordinates of the window.gluOrtho2D(xmin, xmax, ymin, ymax)The x-coordinates go from xmin to xmax from left to right.The y-coordinates go from ymin to ymax from bottom to top.

Example: Draw a 2D Object

DrawTeapot.cpp