code: function iterBox(bbox, wid, fn) {
  		var b = bbox,
  			i0 = b.y * wid + b.x,
  			i1 = (b.y + b.h - 1) * wid + (b.x + b.w - 1),
  			cnt = 0, incr = wid - b.w + 1, i = i0;
  
  		do {
  			fn.call(this, i);
  			i += (++cnt % b.w == 0) ? incr : 1;
  		} while (i <= i1);
  	}
docs: iterates @bbox within a parent rect of width @wid; calls @fn, passing index within parent