// 交换整数 a 、b 的值
inline void swap_int(int *a, int *b) {*a ^= *b;*b ^= *a;*a ^= *b;
}// Bresenham's line algorithm
void draw_line(IMAGE *img, int x1, int y1, int x2, int y2, unsigned long c) {// 参数 c 为颜色值int dx = abs(x2 - x1),dy = abs(y2 - y1),yy = 0;if (dx < dy) {yy = 1;swap_int(&x1, &y1);swap_int(&x2, &y2);swap_int(&dx, &dy);}int ix = (x2 - x1) > 0 ? 1 : -1,iy = (y2 - y1) > 0 ? 1 : -1,cx = x1,cy = y1,n2dy = dy * 2,n2dydx = (dy - dx) * 2,d = dy * 2 - dx;if (yy) { // 如果直线与 x 轴的夹角大于 45 度while (cx != x2) {if (d < 0) {d += n2dy;} else {cy += iy;d += n2dydx;}putpixel(img, cy, cx, c);cx += ix;}} else { // 如果直线与 x 轴的夹角小于 45 度while (cx != x2) {if (d < 0) {d += n2dy;} else {cy += iy;d += n2dydx;}putpixel(img, cx, cy, c);cx += ix;}}
}

// 八对称性
inline void _draw_circle_8(IMAGE *img, int xc, int yc, int x, int y, unsigned long c) {// 参数 c 为颜色值putpixel(img, xc + x, yc + y, c);putpixel(img, xc - x, yc + y, c);putpixel(img, xc + x, yc - y, c);putpixel(img, xc - x, yc - y, c);putpixel(img, xc + y, yc + x, c);putpixel(img, xc - y, yc + x, c);putpixel(img, xc + y, yc - x, c);putpixel(img, xc - y, yc - x, c);
}//Bresenham's circle algorithm
void draw_circle(IMAGE *img, int xc, int yc, int r, int fill, unsigned long c) {// (xc, yc) 为圆心，r 为半径// fill 为是否填充// c 为颜色值// 如果圆在图片可见区域外，直接退出if (xc + r < 0 || xc - r >= img->w ||yc + r < 0 || yc - r >= img->h) return;int x = 0, y = r, yi, d;d = 3 - 2 * r;if (fill) {// 如果填充（画实心圆）while (x <= y) {for (yi = x; yi <= y; yi ++)_draw_circle_8(img, xc, yc, x, yi, c);if (d < 0) {d = d + 4 * x + 6;} else {d = d + 4 * (x - y) + 10;y --;}x++;}} else {// 如果不填充（画空心圆）while (x <= y) {_draw_circle_8(img, xc, yc, x, y, c);if (d < 0) {d = d + 4 * x + 6;} else {d = d + 4 * (x - y) + 10;y --;}x ++;}}
}

可以看到，Bresenham画圆算法（中点圆算法）的实现也非常简单。在另一个项目中，我还有一个 Python 实现的版本，不过那段代码和其余部分结合得比较紧，有点难抠，这次就不贴出来了。