初学者指南：菱形绘制算法详解

#include <windows.h>
#include <math.h>
#include "glut.h"

int x,y;//用这两个全局变量传递当前窗口的大小。
void init(void)
{
 glClearColor(1.0,1.0,1.0,0.0);
 glMatrixMode(GL_PROJECTION);
 gluOrtho2D(-150.0,150.0,-150.0,150.0);
}

int sign(int x)
{
 if(x == 0)
 {
  return 0;
 }
 else
 {
  if(x>0)
   return 1;
  else
   return -1;
 }
}
void Doublestep(GLint x0,GLint y0,GLint x1,GLint y1)
{
 x0*=y/300;y0*=y/300;x1*=y/300;y1*=y/300;//此行代码用于保持坐标与窗口相对位置不变
 GLint d,dx,dy,current_x = x0,current_y = y0,cond;//声明有用的临时变量

 dx = abs(x1-x0);         dy = abs(y1-y0);
 int s_x = sign(x1-x0);   int  s_y = sign(y1-y0);
 cond = 2*dx;
 d = 4*dy-dx;
 bool is_changed;//标明，象限改变了，dx和dy互相颠倒过了。是改变了的。
 if(dy>dx)
 {//此时与标准情况正好相反，如果本来x坐标增加，现在变为y坐标增加
  int temp = dx;
  dx = dy;
  dy = temp;
  is_changed = true;
 }
 else
  is_changed = false;
 for(int i = 1;i<=dx;i+=2)
 {
  if(d<0)
  {  //绘制水平两点，即第一种情况,k<1/4。         
    glVertex2i(current_x,current_y);//1    
   if(is_changed)current_y+=s_y;//2            
    else                                                              
     current_x+=s_x;                                       
                                                                          
   glVertex2i(current_x,current_y);               
   if(is_changed)current_y+=s_y;//3           
    else                                                            
     current_x+=s_x;                                      
  } 

else