#include <GL/glut.h>

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

   GLfloat red = 1.0, green = 1.0, blue = 1.0;  // Initial triangle color: white.
   GLenum fillMode = GL_SMOOTH;  // Initial polygon fill: color interpolation.

   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 fillOption (GLint selectedOption)
   {
       switch (selectedOption) {
           case 1:  fillMode = GL_FLAT;    break;  //  Flat surface rendering.
           case 2:  fillMode = GL_SMOOTH;  break;  //  Gouraud rendering.
       }
       glutPostRedisplay ( );
   }

   void displayTriangle (void)
   {
       glClear (GL_COLOR_BUFFER_BIT);

       glShadeModel (fillMode);           //  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 (red, 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)
   {
       glutInit (&argc, argv);
       glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
       glutInitWindowPosition (200, 200);
       glutInitWindowSize (winWidth, winHeight);
       glutCreateWindow ("Menu Example");

       init ( );
       glutDisplayFunc (displayTriangle);

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

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

       glutReshapeFunc (reshapeFcn);

       glutMainLoop ( );
   }