perl-opengl椭圆算法

#!/usr/bin/perl -w
use strict;
use warnings;
use OpenGL qw/ :all /;
use OpenGL::Config;   

glutInit();
glutInitDisplayMode(GLUT_RGB|GLUT_SINGLE);
glutInitWindowPosition(100,100);
glutInitWindowSize(400,400);
glutCreateWindow("my  OpenGL program");
glClearColor(0,0,0,255);
glClear(GL_COLOR_BUFFER_BIT);  
glMatrixMode(GL_PROJECTION);
gluOrtho2D(-100,100,-100,100);
glutDisplayFunc(\&mydis);
glutMainLoop();
return 0;

sub mydis()
{
  my $xcenter=5;
  my $ycenter=2;
  my $Rx=16;
  my $Ry=69;
  my $Rx2=$Rx*$Rx;
  my $Ry2=$Ry*$Ry;  
  my $twoRx2=2*$Rx2;
  my $twoRy2=2*$Ry2;
  my $p;
  my $x=0;
  my $y=$Ry;
  my $px=0;
  my $py=$twoRx2*$y;
  
  glPointSize(1);
  glColor3f(255,0,255);  
  
  &showpoints($xcenter,$ycenter,$x,$y);
  $p=round($Ry2-($Rx2*$Ry)+(0.25*$Rx2));
  for (;$px<$py;$x++)
  {
     $px+=$twoRy2;
	 if ($p<0)
	 {
	    $p+=$Ry2+$px;
	 }
	 else
	 {
	    $y--;
		$py-=$twoRx2;
		$p+=$Ry2+$px-$py;		
	 }
	&showpoints($xcenter,$ycenter,$x,$y);
  }
  
  $p=round($Ry2*(($x+0.5)**2)+$Rx2*(($y-1)**2)-$Rx2*$Ry2);
  for (;$y>0;$y--)
  {
     $py-=$twoRx2;
	 if ($p>0)
	 {
	    $p+=$Rx2-$py;
	 }
	 else
	 {
	    $x++;
		$px+=$twoRy2;
		$p+=$Rx2-$py+$px;		
	 }
	&showpoints($xcenter,$ycenter,$x,$y);
  } 
  glFlush();
  glColor3f(0,255,0);
  glBegin(GL_LINES);
  glVertex2f(-100,0);
  glVertex2f(100,0); 
  glEnd();
  glBegin(GL_LINES);
  glVertex2f(0,-100);
  glVertex2f(0,100); 
  glEnd();  
  glFlush();
}

sub showpoints()
{
  my ($xcenter,$ycenter,$x,$y)=@_;
  glBegin(GL_POINTS);
  glVertex2f($xcenter+$x,$ycenter+$y);
  glVertex2f($xcenter-$x,$ycenter+$y);
  glVertex2f($xcenter+$x,$ycenter-$y);
  glVertex2f($xcenter-$x,$ycenter-$y);
  glEnd(); 
}

sub round()
{
   return  int($_[0]+0.5);
}