#include <GL/glut.h>

   GLsizei winWidth = 400, winHeight = 400;  // Initial Display-window size.

   GLfloat red = 1.0, green = 1.0, blue = 1.0;    //  Initial color values.
   GLenum renderingMode = GL_SMOOTH;               //  Initial fill method.

   void init (void)
   {
       glClearColor (0.6, 0.6, 0.6, 1.0);  // Set display-window color to gray.

       glMatrixMode (GL_PROJECTION);
       gluOrtho2D (0.0, 300.0, 0.0, 300.0);
   }

   void mainMenu (GLint renderingOption)
   {
       switch (renderingOption) {
           case 1:  renderingMode = GL_FLAT;    break;
           case 2:  renderingMode = GL_SMOOTH;  break;
       }
       glutPostRedisplay ( );
   }

   /*  Set color values according to the submenu option selected.  */
   void colorSubMenu (GLint colorOption)
   {
       switch (colorOption) {
           case 1:
               red = 0.0;  green = 0.0;  blue = 1.0;
               break;
           case 2:
               red = 0.0;  green = 1.0;  blue = 0.0;
               break;
           case 3:
               red = 1.0;  green = 1.0;  blue = 1.0;
       }
       glutPostRedisplay ( );
   }

   void displayTriangle (void)
   {
       glClear (GL_COLOR_BUFFER_BIT);

       glShadeModel (renderingMode);     //  Set fill method for triangle.
       glColor3f (red, green, blue); //  Set color for first two vertices.

       glBegin (GL_TRIANGLES);
           glVertex2i (280, 20); 
           glVertex2i (160, 280);
           glColor3f (1.0, 0.0, 0.0);  // Set color of last vertex to red.
           glVertex2i (20, 100);
       glEnd ( );

       glFlush ( );
   }

   void reshapeFcn (GLint newWidth, GLint newHeight)
   {
       glViewport (0, 0, newWidth, newHeight);

       glMatrixMode (GL_PROJECTION);
       glLoadIdentity ( );
       gluOrtho2D (0.0, GLfloat (newWidth), 0.0, GLfloat (newHeight));

       displayTriangle ( );
       glFlush ( );
   }

   void main (int argc, char **argv)
   {
       GLint subMenu;                       //  Identifier for submenu.

       glutInit (&argc, argv);
       glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
       glutInitWindowPosition (200, 200);
       glutInitWindowSize (winWidth, winHeight);
       glutCreateWindow ("Submenu Example");

       init ( );
       glutDisplayFunc (displayTriangle);

       subMenu = glutCreateMenu (colorSubMenu);
           glutAddMenuEntry ("Blue", 1);
           glutAddMenuEntry ("Green", 2);
           glutAddMenuEntry ("White", 3);

       glutCreateMenu (mainMenu);      // Create main pop-up menu.
           glutAddMenuEntry ("Solid-Color Fill", 1);
           glutAddMenuEntry ("Color-Interpolation Fill", 2);
           glutAddSubMenu ("Color", subMenu);


       /*  Select menu option using right mouse button.  */
       glutAttachMenu (GLUT_RIGHT_BUTTON);

       glutReshapeFunc (reshapeFcn);

       glutMainLoop ( );
   }