中点画线法，

#include <graphics.h>
#include <stdio.h>
#include  <conio.h>

main()
{
 int gd,gm;
int x1,y1,x2,y2,x,y;
float a,b,c,d,d1,d2;
  gd=DETECT;
  initgraph(&gd,&gm,"D:\\CG\\TC20H\\$_OUTDIR\BGI");
   printf("please input two point(x1,y1,x2,y2,x1<x2):");
   scanf("%d%d%d%d",&x1,&y1,&x2,&y2);
    a=y1-y2;
b=x2-x1;
   putpixel(x1,y1,2);
getch();
 x=x1,y=y1;
d=2*a+b;
d1=2*a;
d2=2*(a+b);
   while(x<=x2)
   {
   if (d>=0)  {++x;d=d+d1;}
     else  {x++;y++;d=d+d2;}
   putpixel(x,y,2); getch();
    }

    closegraph();
}
我用的是TC,帮找下错误写谢了