var paint, r;    //设置画笔和半径
var width, height; //获得canvas的长度和宽度
var count = 0;   //设置正铉曲线的便宜量
var colors = ["#66ccff", "#ff0000"];

/*程序入口*/
function main() {
  var canvas1 = document.getElementById("canvas1");
  paint = canvas1.getContext("2d");
  width = paint.canvas.width;
  height = paint.canvas.height;
  r = width / 2;
  start();
  setInterval(start, 200);
}

/*开始进行清除和绘制*/
function start() {
  paint.clearRect(0, 0, width, height);
  drawXY();
  drawSin();
  paint.restore();
}

/*绘制一个坐标系*/
function drawXY() {
  paint.save();
  paint.translate(r, r);
  paint.beginPath();
  paint.lineWidth = 1;
  paint.moveTo(0, -r);
  paint.lineTo(0, r);
  paint.stroke();

  paint.lineWidth = 1;
  paint.moveTo(-r, 0);
  paint.lineTo(r, 0);
  paint.stroke();
}

/*正弦曲线可表示为y=Asin(ωx+φ)+k，定义为函数y=Asin(ωx+φ)+k在直角坐标系上的图象，*/

/*绘制sin正弦图像*/
function drawSin() {
  paint.save();
  count++;//设置每一刻的偏移量
  var i, j, rad, y;
  for (j = 0; j < 2; j++) {
    for (i = -1000; i < 1000; i++) {
      paint.beginPath();
      paint.strokeStyle = colors[j];
      rad = i * Math.PI / 180;
      y = -20 * Math.sin(rad - (count + j));
      paint.arc(i, y, 0.3, 0, 2 * Math.PI, false);
      paint.stroke();
    }
  }

  paint.restore();
}