void render_cylinder()
{
   float dx,dy,predx,predy;
   
   for(int i=0;i<=360;i+=30)
   {
      dx=cos((float)i*(pi/180))*.5;
      dy=sin((float)i*(pi/180))*.5;

      glColor4f(dx+.5,dy+.5,.5,1);

      if(i>0)
      {
         glBegin(GL_TRIANGLES);

         glVertex3f(dx,dy,-.5);
         glVertex3f(predx,predy,-.5);
         glVertex3f(0,0,-.5);

         glVertex3f(predx,predy,.5);
         glVertex3f(dx,dy,.5);
         glVertex3f(0,0,.5);

         glEnd();

         glBegin(GL_QUADS);

         glVertex3f(predx,predy,-.5);
         glVertex3f(dx,dy,-.5);
         glVertex3f(dx,dy,.5);
         glVertex3f(predx,predy,.5);

         glEnd();

      }

      predx=dx;
      predy=dy;
   }
}