x*x + y*y = r*r
2*x + 2*y*(dy/dx) = 0
dy = -(x/y)*dx


x=0; y=100 // radius
dx = 1; //pixel
a = y;

while(y > x) {
  x += dx; a += x;
  if(a > y) {a -= y; y--}
  plot (x,y); plot (y,x); // one quadrant. repeat for other quadrants
}