Using Graphics Libraries Lecture 3 Mon, Sep 1, 2003

Graphics Libraries Theoretically, with only a function setPixel(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 glutInitglutInit(&argc, argv). Initializes the glut library. Should be called before any other glut function. Must receive the command-line arguments. glutInitDisplayModeglutInitDisplayMode(options). Specifies color mode. Specifies single or double buffering.

Windows-Based Programming glutInitWindowSizeglutInitWindowSize(width, height). Sets the height and width of the window in pixels. glutInitWindowPositionglutInitWindowPosition(x, y). Sets the position of the upper left corner of the window. glutCreateWindowglutCreateWindow(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.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 write glutXXXFunc(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 glutDisplayFuncglutDisplayFunc(display); Called whenever the scene needs to be redrawn. Activated by calls to glutPostRedisplay(). glutReshapeFuncglutReshapeFunc(reshape); Called whenever the window is resized. Activated by resizing the window. Warning: This does not respond to iconifying the window.

OpenGL Callback Functions glutMouseFuncglutMouseFunc(mouse) Called whenever the mouse is clicked. Activated by mouse clicks. Left or right, up or down. glutKeyboardFuncglutKeyboardFunc(keyboard) Called whenever a key is pressed. Activated by keystrokes (down only) of an ASCII key (letters, digits, punctuation).

OpenGL Callback Functions glutSpecialFuncglutSpecialFunc(special) Called whenever a special key is pressed. Activated by keystrokes (down only) of a non-ASCII key (function key, arrow key, etc.). glutMotionFuncglutMotionFunc(motion) Called whenever the mouse is moved while the button is pressed.

OpenGL Callback Functions glutPassiveMotionFuncglutPassiveMotionFunc(passiveMotion) Called whenever the mouse is moved while the button is not pressed. glutIdleFuncglutIdleFunc(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 Process keyboard and mouse events glutMainLoop() display() mouse() keyboard() reshape() Library Functions

Example: Callback Functions CallbackTest.cpp

Other Initializations void init() { glClearColor(0.8, 0.8, 0.8, 0.0); // light gray glColor3f(0.0f, 0.0f, 0.0f); // black glMatrixMode(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