What is OpenGL? Callback function

OpenGL

OpenGL is a cross-platform API of about 250 functions for rendering 2D and 3D graphics using the graphics hardware. It is required because it makes graphics programming hardware-independent, easy and fast: drawing primitives, transformations, lighting, texturing and hidden surface removal can be done with simple function calls, and the same program works on different systems.

Callback function

A callback function is a function written by the programmer that is registered with the system (GLUT) and called automatically by the library when a particular event occurs. OpenGL/GLUT programs are event driven: after registration, glutMainLoop() waits for events and calls the matching callback.

Example

#include <GL/glut.h>
#include <stdlib.h>

void display(void) {              // display callback
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1, 1, 0);
    glBegin(GL_POLYGON);
        glVertex2f(-0.5f, -0.5f); glVertex2f(0.5f, -0.5f);
        glVertex2f(0.5f, 0.5f);   glVertex2f(-0.5f, 0.5f);
    glEnd();
    glFlush();
}
void keyboard(unsigned char key, int x, int y) {   // keyboard callback
    if (key == 'q') exit(0);
}
int main(int argc, char **argv) {
    glutInit(&argc, argv);
    glutCreateWindow("Callbacks");
    glutDisplayFunc(display);      // register callbacks
    glutKeyboardFunc(keyboard);
    glutMainLoop();                // wait for events
    return 0;
}

Here GLUT calls display whenever the window needs redrawing, and keyboard when a key is pressed (the program exits on 'q').