#include <GL/glut.h>

   class scrPt {
      public:
         GLint x, y;
   };

   void setPixel (GLint x, GLint y)
   {
      glBegin (GL_POINTS);
         glVertex2i (x, y);
      glEnd ( );
   }

   void circleMidpoint (scrPt circCtr, GLint radius)
   {
      scrPt circPt;

      GLint p = 1 - radius;         // Initial value of midpoint parameter.

      circPt.x = 0;             // Set coordinates for top point of circle.
      circPt.y = radius;

      void circlePlotPoints (scrPt, scrPt);

      /* Plot the initial point in each circle quadrant. */
      circlePlotPoints (circCtr, circPt);

      /* Calculate next points and plot in each octant. */
      while (circPt.x < circPt.y) {
         circPt.x++;
         if (p < 0)
            p += 2 * circPt.x + 1;
         else {
            circPt.y--;
            p += 2 * (circPt.x - circPt.y) + 1;
         }
         circlePlotPoints (circCtr, circPt);
      }
   }

   void circlePlotPoints (scrPt circCtr, scrPt circPt);
   {
      setPixel (circCtr.x + circPt.x, circCtr.y + circPt.y);
      setPixel (circCtr.x - circPt.x, circCtr.y + circPt.y);
      setPixel (circCtr.x + circPt.x, circCtr.y - circPt.y);
      setPixel (circCtr.x - circPt.x, circCtr.y - circPt.y);
      setPixel (circCtr.x + circPt.y, circCtr.y + circPt.x);
      setPixel (circCtr.x - circPt.y, circCtr.y + circPt.x);
      setPixel (circCtr.x + circPt.y, circCtr.y - circPt.x);
      setPixel (circCtr.x - circPt.y, circCtr.y - circPt.x);
   }