用Ｃ语言如何实现　（用中点算法和Bresenham算法）　画圆．　

#include<stdio.h>
#include<graphics.h>
void MidPointCircle(int r,int color);
void CirclePoints(int x,int y,int color);
void main()
{
int gdriver=DETECT, gmode;

initgraph(&gdriver, &gmode," ");
setbkcolor(BLACK);
MidPointCircle(100,BLUE); /*下次画圆半径大点，你给的半径真夸张*/
getch();
closegraph();
}

void MidPointCircle(int r,int color)
{
int x,y;
float d;

x=0;
y=r;
d=1.25-r;
CirclePoints(x,y,color);
while(x<y)
{
if(d<0)
d+=2*x+3;
else
{
d+=2*(x-y)+5;
y--;
}
x++;
CirclePoints(x,y,color); /*重点是这里*/
}
}

void CirclePoints(int x,int y,int color) /*圆心为（320，240）*/
{
putpixel(320+x,240+y,color);
putpixel(320+y,240+x,color);
putpixel(320-x,240+y,color);
putpixel(320+y,240-x,color);
putpixel(320+x,240-y,color);
putpixel(320-y,240+x,color);
putpixel(320-x,240-y,color);
putpixel(320-y,240-x,color);
}

中点算法:
void MidpointLine(int x0,int y0,int x1,int y1,int color)
{
int x,y,mid,dx,dy,d_up,d_down;
if(x0>x1)
{
x=x0;
x0=x1;
x1=x;
y=y0;
y0=y1;
y1=y;
}
dy=y1-y0;dx=x1-x0;
mid=dx-2*dy;
d_up=2*dx-2*dy;d_down=-2*dy;
x=x0;y=y0;
while(x<=x1)
{
putpixel(x,y,color);
x++;
if(mid<0)
{
y++;
mid=mid+d_up;
}
else
mid=mid+d_down;
}
return ;
}

数值微分法:
int DDALine (int x0,int y0,int x1,int y1,int color)
{
int coordinate;
float dx=x1-x0/*X方向的增量*/,
dy=y1-y0/*Y方向的增量*/;
float eXY;
float coorXY;
if(dx==0) /*X方向的增量为0*/
{
coorXY=x0;
if(dy>=0) /*Y方向的增量为正方向*/
{
for(coordinate=y0;coordinate<=y1;coordinate++)
putpixel(coorXY,coordinate,color);
}
else /*Y方向的增量为负方向*/
{
for(coordinate=y0;coordinate>=y1;coordinate--)
putpixel(coorXY,coordinate,color);
}
}
else if(dy/dx<=1 && dy/dx>=-1) /*|dy/dx|<=1*/
{
/*X方向的增量为+1/-1,Y方向随之增量为+eXY/-eXY*/

coorXY=y0; /*Y方向的初始值为y0*/
if(dx>=0)
{
eXY=dy/dx; /*当dy>=0时,eXY=dy/dx>=0,
当dy<0 时,eXY=dy/dx<0*/
for(coordinate=x0;coordinate<=x1;coordinate++)
{
putpixel(coordinate,(int)(coorXY),color);
coorXY+=eXY; /*Y方向增量为eXY*/
}
}
else
{
eXY=-dy/dx; /*当dy>=0时,eXY=-dy/dx>=0,
当dy<0 时,eXY=-dy/dx<0*/
for(coordinate=x0;coordinate>=x1;coordinate--)
{
putpixel(coordinate,(int)(coorXY),color);
coorXY+=eXY; /*Y方向增量为eXY*/
}
}
}
else /*|dy/dx|>1*/
{
coorXY=x0; /*X方向的初始值为y0*/
if(dy>=0)
{
eXY=dx/dy; /*当dx>=0时,eXY=dx/dy>=0,
当dx< 0时,eXY=dx/dy<0*/
for(coordinate=y0;coordinate<=y1;coordinate++)
{
putpixel((int)(coorXY),coordinate,color);
coorXY+=eXY; /*X方向增量为eXY*/
}
}
else
{
eXY=-dx/dy; /*当dx>=0时,eXY=-dx/dy>=0,
当dx< 0时,eXY=-dx/dy<0*/
for(coordinate=y0;coordinate>=y1;coordinate--)
{
putpixel((int)(coorXY),coordinate,color);
coorXY+=eXY; /*X方向增量为eXY*/
}
}
}
return 0; /*返回值为0*/
}