javascript - HTML-Canvas - JavaScript-动画/绘图

Question

我有这个:

http://jsfiddle.net/geduardcatalin/6Lg6ymt3/

如何使第二条线从中心开始并停在左下角，就像第一条线停在我想要的位置(中心)？

var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");

var canvasWidth = canvas.width;
var canvasHeight = canvas.height;

var i = 0;
var j = 0;
var k = 0;
var l = 0;

function drawLine1() {
  i = i + 1;
  j = j + 1;
  x1 = i;
  y1 = j;

  if (i >= 149) {
    i = 149;
  }

  if (j >= 149) {
    j = 149;
  }

  ctx.beginPath();
  ctx.strokeStyle = "#000000";
  ctx.lineWidth = 1;
  ctx.moveTo(0, 0);
  ctx.lineTo(x1, y1);
  ctx.stroke();
}

function drawLine2() {
  k = k - 1;
  l = l + 1;
  x2 = k;
  y2 = l;

  if (k >= 299) {
    k = 299;
  }

  if (l >= 299) {
    l = 299;
  }

  ctx.beginPath();
  ctx.strokeStyle = "#000000";
  ctx.lineWidth = 1;
  ctx.moveTo(150, 150);
  ctx.lineTo(x2, y2);
  ctx.stroke();
}

function drawLines() {
  ctx.clearRect(0, 0, canvasWidth, canvasHeight);
  drawLine1();
  drawLine2();
  //setTimeout(drawLine2, 0000);
}

setInterval(drawLines, 40);

* {
  margin: 0;
  padding: 0;
}
html {
  background-color: #555555;
}
#wrapper {
  width: 300px;
  height: 300px;
  margin: 70px auto;
  border: 1px solid red;
}

<div id="wrapper">
  <canvas id="myCanvas" height="300px" width="300px"></canvas>
  <p id="demo"></p>
</div>

Answer 1

Line2 中的变量 k 和 l 都应从 150 开始，并将 draw Line2() 更改为:

这是更新后的JSFiddle .

function drawLine2(){
    k = k - 1;
    l = l + 1;
    x2 = k;
    y2 = l;

    if ( k <= 0 ){
        k = 0;
    }
    if ( l >= 299 ){
        l = 299;
    }

    ctx.beginPath();
    ctx.strokeStyle = "#000000";
    ctx.lineWidth = 1;
    ctx.moveTo(150, 150);
    ctx.lineTo(x2, y2);
    ctx.stroke();
}