Simple Basic Examples and Skeleton Programs for OpenGL

1. Simple Basic Examples and Skeleton Programs for OpenGL

2. OpenGL Programming • OpenGL handles 2D and 3D rendering • The native windowing system handles the management of windows. These operations consist of • Opening, closing, and displaying • User interface components and widgets • Event Handling • The GLUT library provides a portable mechanism for window management with OpenGL programs • Non-portable mechanisms include GLX for X Windows implementations and WGL for Windows NT.

3. Window Management I • glutInit(int *argc, char **argv) • initializes GLUT and processes any command line arguments • glutInit() should be called before any other GLUT routine. • glutInitDisplayMode(unsigned intmode) • Specifies whether to use an RGBA or color-index color model. • You can also specify whether you want a single- or double-buffered window. (If you’re working in color-index mode, you’ll want to load certain colors into the color map; use glutSetColor() to do this.) • Finally, you can use this routine to indicate that you want the window to have an associated depth, stencil, and/or accumulation buffer. • For example, if you want a window with double buffering, the RGBA color model, and a depth buffer, you might call glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH).

4. Window Management II • glutInitWindowPosition(int x, int y) Specifies the screen location for the upper-left corner of your window. • glutInitWindowSize(intwidth, intsize) Specifies the size, in pixels, of your window. • intglutCreateWindow(char *string) • creates a window with an OpenGL context. • It returns a unique identifier for the new window. • Be warned: Until glutMainLoop() is called (see next section), the window is not yet displayed.

5. Window Management III • glutDisplayFunc(void (* func)(void)) • Is the first and most important event callback function you will see. • Whenever GLUT determines the contents of the window need to be redisplayed, the callback function registered by glutDisplayFunc() is executed.Therefore, • You should put all the routines you need to redraw the scene in the display callback function. • If your program changes the contents of the window, sometimes you will have to call glutPostRedisplay(void), which gives glutMainLoop() a nudge to call the registered display callback at its next opportunity.

6. Window Management IV • void glutMainLoop(GLvoid) – • Places the program in the GLUT event processing loop from which the program can invoke callback functions. • This must be the last function in the mainline program preceding the return(). • intglutGet( GLenum state ) • Retrieves state information about the GLUT library. • For our example, we are interested in obtaining the size of the display • glutGet(GLUT_SCREEN_WIDTH) and • glutGetGLUT_SCREEN_HEIGHT)

7. OpenGL Programming #ifdef __FLAT__ #include <windows.h> #endif #include <gl/glut.h> // The initialization function that comes after glutInit() void init(void) { // initialization code goes here // examples of initialization include: setting window size, display modes // color mode, shade model etc. }

8. OpenGL Programming // The display callback function void display(void) { // display callback called by the OpenGL event handler goes here. // This callback is called every time the main window requires redrawing } // The main function int main(int argc, char** argv) { glutInit(&argc, argv) ; // initialize the glut library init(); // perform all other initialization glutDisplayFunc(display) ; // register the display callback function glutMainLoop() ; // enter the GLUT event processing loop return 0 ; }

9. Initialization Functions • void glutInitDisplay Mode(GLenum mode) • Configure a windows display mode. Display modes define the type of buffer utilized. Examples are • GLUT_RGBA Request RGBA window (default) • GLUT_RGB An alias for GLUT-RGBA • GLUT_INDEX Request color-index window • GLUT_SINGLE Request signle-buffered window (default) • GLUT_DOUBLE Request double-buffered window • GLUT_DEPTH Request accompanying depth buffer (deafult) • GLUT_STENCEL Request accompanying stencil buffer • GLUT_ACCUM Request accompanying accumulation buffer • GLUT_ALPHA Request accompanying alpha destination buffer • GLUT_MULTISAMPLE Request a window with multi-sampling support • GLUT_STEREO Request a window with stereo support • GLUT_LUMINANCE Request a window with luminance color model

10. Initialization Functions • GLvoid glClearColor(GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha) • Sets the current clear color which is used to clear the collor buffer when glClear() is called with an argument of GL_COLOR_BUFFER_BIT • void glShadeModel(eGLenum mode) • Sets the current shading model to either GL_SMOOTH or GL_FLAT. GL_FLAT has only a single color per polygon used. GL_SMOOTH the color of a polygon is interpolated among the colors of its vertices. • GLvoid glClear(GL_bitfield mask) • Called whenever the window is to be cleared. The mask is usually set to GL_COLOR_BUFFER_BIT. This function will paint the entire window with the current clear color.

11. First Complete OpenGL Program • Creates a graphics window whose height and width are a third of the total display screen's height and width, places the upper left-hand corner of the window at the very upper left hand corner of the display, requests a double-buffered, RGBA color mode window, sets the clear color to magenta and the shade model to flat. • Displays a wire cube in the window.

12. First Complete OpenGL Program #ifdef __FLAT__ #include <windows.h> #endif #include <gl/glut.h> // The initialization function void init(void) { glutInitWindowSize( glutGet( GLUT_SCREEN_WIDTH)/3, glutGet( GLUT_SCREEN_HEIGHT)/3 ); glutInitWindowPosition( 0, 0 ); glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB); glutCreateWindow("Second Chapter - Opening an OpenGL Window") ; glClearColor(1.0, 0.0, 1.0, 0.0) ; glShadeModel(GL_FLAT) ; }

13. First Complete OpenGL Program // The display callback function void display(void) { glClear(GL_COLOR_BUFFER_BIT) ; glutWireCube(1.0); glutSwapBuffers(); } // The main function int main(int argc, char** argv) { glutInit(&argc, argv) ;   init(); glutDisplayFunc(display) ; glutMainLoop() ;   return 0 ; }

14. First Complete OpenGL Program