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中点画圆法 理论 ： 如果我们构造函数 F(x,y)=x2+y2-R2，则对于圆上的点有 F(x,y)=0，对于圆外的 点有 F(x,y)>0，对于圆内的点 F(x,y)<0 。与中点画线法一样，构造判别式： d=F(M)=F(xp+1,yp-0.5)=(xp+1) 2+(yp-0.5) 2-R2 若 d<0，则应取 P1为下一象素，而且再下一象素的判别式为： d=F(xp+2,yp-0.5)=(xp+2) 2+(yp-0.5) 2-R2=d+2xp+3 若 d≥0，则应取 P2为下一象素，而且下一象素的判别式为 d=F(xp+2,yp-1.5)=(xp+2) 2+(yp-1.5) 2-R2=d+2(xp-yp)+5 我们这里讨论的第一个象素是（0,R），判别式 d的初始值为： d0=F(1,R-0.5)=1.25-R code： /* this is a test drawing a circle ; made by lihe 2007 12 */ #include "graphics.h" void CirclePoints(int center_x,int center_y,int x,int y,int color) { putpixel(center_x+x, center_y+y, color); putpixel(center_x+y, center_y+x, color); putpixel(center_x-x, center_y+y, color); putpixel(center_x+y, center_y-x, color); putpixel(center_x+x, center_y-y, color); putpixel(center_x-y, center_y+x, color); putpixel(center_x-x, center_y-y, color); putpixel(center_x-y, center_y-x, color); } void MidPointCircle(int center_x,int center_y,int r,int color) { int x,y; float d; x=0; y=r; d=1.25-r; CirclePoints (center_x,center_y,x,y,color); while(x<=y) { x++; if(d<0) { d+=2*x+3; } else { d+=2*(x-y)+5; y--; } CirclePoints(center_x,center_y,x,y,color); } } void initgr(void) { int gd = DETECT, gm = 0; registerbgidriver(EGAVGA_driver); initgraph(&gd, &gm, ""); } int main(void) { int x,y,r,color; initgr(); printf("please enter the coordinate x y,and the radius and the color \n " ); scanf("%d %d %d %d",&x,&y,&r,&color); MidPointCircle(x,y,r,color); getch(); closegraph(); return 0; }