HTML5 Canvas

Question

我还是画布元素的新手，我试图在canvas元素的随机位置绘制随机多边形形状（三角形）。但是我无法理解它。

到目前为止，我有这个，很好地画了一个ploygon，但是如何添加随机性和定位完全没有我

var c = document.getElementById('c');
if (c.getContext) {
  c2 = c.getContext('2d');
  var width = c2.width;
  var height = c2.height;
  var maxAmount = 20;

  for (i = 0; i < maxAmount; i++) {
    var polySize = 50;
    var posx = (Math.random() * (width - polySize)).toFixed();
    var posy = (Math.random() * (height - polySize)).toFixed();

    c2.fillStyle = '#f00';
    c2.beginPath();
    c2.moveTo(posx, posy);
    c2.lineTo(100, 50);
    c2.lineTo(50, 100);
    c2.lineTo(0, 90);
    c2.closePath();
    c2.fill();
  }
}

<canvas id="c" \>

Answer 1

您正在尝试获取画布的Context2D属性的宽度和高度属性，这两个属性都返回undefined。
您需要的是canvas元素的宽度和高度属性。

现在，由于您对其他答案的评论，如果您需要移动整个形状，只需使用您在posx和posy变量中保存的第一个点，然后调整其他点位置：

＆＃13;

＆＃13;

var c = document.getElementById('c');
c.width =500;
c.height= 500;
if (c.getContext) {
  var c2 = c.getContext('2d');
  var width = c.width;
  var height = c.height;
  var maxAmount = 20;

  for (var i = 0; i < maxAmount; i++) {
    var polySize = 50;
    var posx = (Math.random() * (width - polySize));
    var posy = (Math.random() * (height - polySize));

    c2.fillStyle = '#f00';
    c2.beginPath();
    c2.moveTo(posx, posy);
    c2.lineTo(posx+100, posy+50);
    c2.lineTo(posx+50,posy+100);
    c2.lineTo(posx+0, posy+90);
    c2.closePath();
    c2.stroke();
    c2.fill();

  }
}

＆＃13;

<canvas id="c"><\canvas>

＆＃13;

＆＃13;

＆＃13;

Answer 2

此处未设置画布宽度和高度属性。所以当在代码行之后调用代码时，它返回NaN

var width = c2.width;
var height = c2.height;

要获得随机位置，请尝试以下代码库

<canvas id="c" width="250" height="250" \>

var c = document.getElementById('c');
if (c.getContext) 
{
  c2 = c.getContext('2d');
  var width = c.width;
  var height = c.height;
  var maxAmount = 5;

  for (i = 0; i < maxAmount; i++) {
    var polySize = 50;
    var posx = (Math.random() * (width - polySize)).toFixed();
    var posy = (Math.random() * (height - polySize)).toFixed();

    c2.fillStyle = '#f00';
    c2.beginPath();
    c2.moveTo(posx, posy);
    c2.lineTo(100, 50);
    c2.lineTo(50, 100);
    c2.lineTo(0, 90);
    c2.closePath();
    c2.fill();
  }
}