OpenGL / GLUT change callbacks inside glutMainLoop()

Question

normally a GLUT application will be structured in the following pattern:

• some initialization stuff
• glutMainLoop()

I only have access to a drawing callback function inside the main Loop. I will not be able to change the initialization part. Is there any way to change a callback function like glutKeyboardFunc() within this Loop? Or is there another way to capture keyboard inputs without access to the initialization part?

Thanks

Answer 1

Yes, you can call glut…Func anytime. For example you could have to keyboard handler functions and switch between them (contextual keys)

void keyfunc_edit(unsigned char key, int x, int y);
void keyfunc_select(unsigned char key, int x, int y);

void keyfunc_edit(unsigned char key, int x, int y)
{
    switch( key ){
    case 'x':
    case 'X':
        /* switch to select mode */
        glutKeyboardFunc(keyfunc_select);
        break;
}

void keyfunc_select(unsigned char key, int x, int y)
{
    switch( key ){
    case 'e':
    case 'E':
        /* switch to edit mode */
        glutKeyboardFunc(keyfunc_edit);
        break;
}

This works for any GLUT callback, so you can switch callbacks for display, idle and so on, at any time. For example say you want to show a load screen, then it makes perfect sense to switch the display callback to a loading screen display function, and once loading is complete change to a scene render function.