void picoDrawCircleRaw(int mx, int my, int r,int color) {
/*
  A first point of the circle is x=r,y=0.
  For a closed circle boundary line, the next possible point is either
  the left neighbour (x-1,y) or the lower neighbour (x,y+1).
  We insert x and y into the circles defining equation x*x+y*y=r*r to
  calculate radius error. Whichever gives less error is the better point
  for the circle. From this we calculate again the best neighbour. For 
  symmetry reasons, we only need do do it for an eighth of the circle
  and mirror it. 
*/
int x=r,
    y=0,	
    nrx,    // new sqared radius if we dec x
    nry,    // new sqared radius if we inc y
    erx,    // error if we dec x
    ery,    // error if we inc y
   r2=r*r,  // square of r	    
   ar2=r2;  // radius of a circle that goes through last drawn point
   do{
      picoDrawPointRaw(mx+x, my+y, color);
      picoDrawPointRaw(mx+x, my-y, color);
      picoDrawPointRaw(mx-x, my+y, color);
      picoDrawPointRaw(mx-x, my-y, color); 
      
      picoDrawPointRaw(mx+y, my+x, color);
      picoDrawPointRaw(mx+y, my-x, color);
      picoDrawPointRaw(mx-y, my+x, color);
      picoDrawPointRaw(mx-y, my-x, color); 
    nry=ar2+2*y+1;	// if we inc y nyr=(y+1)^2+x^2 = y^2+2y+1+x^2 = r^2+2y+1
    nrx=ar2-2*x+1;	// if we dec x nxr=y^2+(x-1)^2 = y^2+x^2-2x-1 = r^2-2x+1
    ery=r2-nry; if (ery<0) ery=-ery;
    erx=r2-nrx; if (erx<0) erx=-erx;
    if (ery<erx) {
          y++; 
	  ar2=nry; 
	  } else {
	  x--; 
	  ar2=nrx;
	  }
         } while (x>=y);      
}