Draw Circle according to Horn's Method

drawCircle

// https://de.wikipedia.org/wiki/Rasterung_von_Kreisen#Methode_von_Horn

void drawCircle(int xPos, int yPos, int r) {
  int d = -r;
  int x = r;
  int y = 0;
  while (y <= x) {
    point(xPos+x,yPos+y); // might be executed in parallel
    point(xPos-x,yPos+y);
    point(xPos+x,yPos-y);
    point(xPos-x,yPos-y);
    point(yPos+y,xPos+x);
    point(yPos-y,xPos+x);
    point(yPos+y,xPos-x);
    point(yPos-y,xPos-x);
    d += 2*y + 1; // d += (y << 1) + 1; 
    y++;
    if (d > 0 ) {
      d -= 2*x + 2; // d -= (x << 1) + 2;
      x--;
    }
  }
}

void setup() {
  size(400,400);
  point(200,200);
  for (int r=5; r<=200; r+=10) 
    drawCircle(200,200,r);
}