function writeLine(cxt,sx,sy,dx,dy,sktyle,lcap,lwidth) {
   cxt.beginPath();
   cxt.moveTo(sx,sy);
   cxt.lineTo(dx,dy);
   cxt.strokeStyle = sktyle;
   cxt.lineCap = lcap;
   cxt.lineWidth = lwidth;
   // cxt.closePath(); lineCap无效
   cxt.stroke();
}