先stroke后closePath就不会有连接
cav_round1.beginPath();
cav_round1.moveTo(0, -round1_R[0]);cav_round1.lineTo(0, -round1_R[1]);
cav_round1.arc(0, 0, round1_R[1], -90*Math.PI/180, -30*Math.PI/180);
cav_round1.stroke();
cav_round1.closePath();
先stroke后closePath就不会有连接
cav_round1.beginPath();
cav_round1.moveTo(0, -round1_R[0]);