openGL: Animation using Idle Function

#include <GL/glut.h>  // GLUT, include glu.h and gl.h

// Global variable
GLfloat angle = 0.0f;  // Current rotational angle of the shapes

/* Initialize OpenGL Graphics */
void initGL() {
   // Set "clearing" or background color
   glClearColor(0.0, 0.0, 0.0, 1.0); // Black and opaque
 glLoadIdentity(); // Reset the model-view matrix
 glMatrixMode(GL_PROJECTION);     // To operate on Model-View matrix
glOrtho(-1.0,1.0,-1.0,1.0,-1.0,1.0);

}

/* Called back when there is no other event to be handled */
void idle() {
   glutPostRedisplay();   // Post a re-paint request to activate display()
}


/* Handler for window-repaint event. Call back when the window first appears and
   whenever the window needs to be re-painted. */
void display() {
   glClear(GL_COLOR_BUFFER_BIT);   // Clear the color buffer
               
      glPushMatrix();        // Save model-view matrix setting
   glTranslatef(-0.5, 0.4, 0.00);    // Translate
   glRotatef(angle, 0.0, 0.0, 1.0); // rotate by angle in degrees
   glBegin(GL_QUADS);                  // Each set of 4 vertices form a quad
      glColor3f(1.0, 0.0, 0.0);     // Red
      glVertex2f(-0.3, -0.3);
      glVertex2f( 0.3, -0.3);
      glVertex2f( 0.3,  0.3);
      glVertex2f(-0.3,  0.3);
   glEnd();
   glPopMatrix();              // Restore the model-view matrix

         glPushMatrix();                   // Save model-view matrix setting
   glTranslatef(-0.4, -0.3, 0.0);   // Translate
   glRotatef(angle, 0.0, 0.0, 1.0); // rotate by angle in degrees
   glBegin(GL_QUADS);
      glColor3f(0.0, 1.0, 0.0); // Green
      glVertex2f(-0.3, -0.3);
      glVertex2f( 0.3, -0.3);
      glVertex2f( 0.3,  0.3);
      glVertex2f(-0.3,  0.3);
   glEnd();
   glPopMatrix();                    // Restore the model-view matrix

       glPushMatrix();                  // Save model-view matrix setting
   glTranslatef(-0.7, -0.5, 0.0);   // Translate
   glRotatef(angle, 0.0, 0.0, 1.0); // rotate by angle in degrees
   glBegin(GL_QUADS);
      glColor3f(0.2, 0.2, 0.2); // Dark Gray
      glVertex2f(-0.2, -0.2);
      glColor3f(1.0, 1.0, 1.0); // White
      glVertex2f( 0.2, -0.2);
      glColor3f(0.2, 0.2, 0.2); // Dark Gray
      glVertex2f( 0.2,  0.2);
      glColor3f(1.0, 1.0, 1.0); // White
      glVertex2f(-0.2,  0.2);
   glEnd();
     glPopMatrix();                 // Restore the model-view matrix

        glPushMatrix();                // Save model-view matrix setting
   glTranslatef(0.4, -0.3, 0.0);    // Translate
   glRotatef(angle, 0.0, 0.0, 1.0); // rotate by angle in degrees
   glBegin(GL_TRIANGLES);
      glColor3f(0.0, 0.0, 1.0); // Blue
      glVertex2f(-0.3, -0.2);
      glVertex2f( 0.3, -0.2);
      glVertex2f( 0.0,  0.3);
   glEnd();
     glPopMatrix();                   // Restore the model-view matrix

          glPushMatrix();              // Save model-view matrix setting
   glTranslatef(0.6, -0.6, 0.0);    // Translate
   glRotatef(180.0 + angle, 0.0, 0.0, 1.0); // Rotate 180+angle degree
   glBegin(GL_TRIANGLES);
      glColor3f(1.0, 0.0, 0.0); // Red
      glVertex2f(-0.3, -0.2);
      glColor3f(0.0, 1.0, 0.0); // Green
      glVertex2f( 0.3, -0.2);
      glColor3f(0.0, 0.0, 1.0); // Blue
      glVertex2f( 0.0,  0.3);
   glEnd();
     glPopMatrix();               // Restore the model-view matrix

          glPushMatrix();               // Save model-view matrix setting
   glTranslatef(0.5, 0.4, 0.0);     // Translate
   glRotatef(angle, 0.0, 0.0, 1.0); // rotate by angle in degrees
   glBegin(GL_POLYGON);
      glColor3f(1.0, 1.0, 0.0); // Yellow
      glVertex2f(-0.1, -0.2);
      glVertex2f( 0.1, -0.2);
      glVertex2f( 0.2,  0.0);
      glVertex2f( 0.1,  0.2);
      glVertex2f(-0.1,  0.2);
      glVertex2f(-0.2,  0.0);
   glEnd();
   glPopMatrix();                  // Restore the model-view matrix



   glutSwapBuffers();   // Double buffered - swap the front and back buffers

   // Change the rotational angle after each display()
   angle += 0.1;
}

/* Handler for window re-size event. Called back when the window first appears and
   whenever the window is re-sized with its new width and height */


void reshape(int width, int height) {  // GLsizei for non-negative integer
   // Compute aspect ratio of the new window
   if (height == 0) height = 1;                // To prevent divide by 0
   GLfloat aspect = (GLfloat)width / (GLfloat)height;

   // Set the viewport to cover the new window
   glViewport(0, 0, width, height);

   // Set the aspect ratio of the clipping area to match the viewport
   glMatrixMode(GL_PROJECTION);  // To operate on the Projection matrix
   glLoadIdentity();
   if (width >= height) {
     // aspect >= 1, set the height from -1 to 1, with larger width
      gluOrtho2D(-1.0 * aspect, 1.0 * aspect, -1.0, 1.0);
   } else {
      // aspect < 1, set the width to -1 to 1, with larger height
     gluOrtho2D(-1.0, 1.0, -1.0 / aspect, 1.0 / aspect);
   }
}



/* Main function: GLUT runs as a console application starting at main() */
int main(int argc, char** argv) {
   glutInit(&argc, argv);          // Initialize GLUT
   glutInitDisplayMode(GLUT_DOUBLE);  // Enable double buffered mode
   glutInitWindowSize(640, 480);   // Set the window's initial width & height - non-square
   glutInitWindowPosition(50, 50); // Position the window's initial top-left corner
   glutCreateWindow("Animation via Idle Function");  // Create window with the given title
   glutDisplayFunc(display);       // Register callback handler for window re-paint event
  glutReshapeFunc(reshape);       // Register callback handler for window re-size event
   glutIdleFunc(idle);             // Register callback handler if no other event
   initGL();                       // Our own OpenGL initialization
   glutMainLoop();                 // Enter the infinite event-processing loop
   return 0;
}