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HTML5 Dynamically create Canvas

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北海茫月
北海茫月 2020-12-05 03:43

Hi there I have a question about dynamically creating a canvas using javascript.

I create a canvas like this:

var canvas = document.createElement(\'c
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5条回答
  • 一整个雨季
    2020-12-05 04:14

    Alternative

    Use element .innerHTML= which is quite fast in modern browsers

    document.body.innerHTML = "<canvas width=500 height=150 id='CursorLayer'>";


// TEST

var ctx = CursorLayer.getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(100, 100, 50, 50);
    canvas { border: 1px solid black }

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  • 后悔当初
    2020-12-05 04:22

    The problem is that you do not insert your canvas element in the document body.

    Just do the following:

    document.body.appendChild(canvas);

    Example:

    var canvas = document.createElement('canvas');

canvas.id = "CursorLayer";
canvas.width = 1224;
canvas.height = 768;
canvas.style.zIndex = 8;
canvas.style.position = "absolute";
canvas.style.border = "1px solid";


var body = document.getElementsByTagName("body")[0];
body.appendChild(canvas);

cursorLayer = document.getElementById("CursorLayer");

console.log(cursorLayer);

// below is optional

var ctx = canvas.getContext("2d");
ctx.fillStyle = "rgba(255, 0, 0, 0.2)";
ctx.fillRect(100, 100, 200, 200);
ctx.fillStyle = "rgba(0, 255, 0, 0.2)";
ctx.fillRect(150, 150, 200, 200);
ctx.fillStyle = "rgba(0, 0, 255, 0.2)";
ctx.fillRect(200, 50, 200, 200);

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  • 轮回少年
    2020-12-05 04:24 
     <html>
 <head></head>
 <body>
 <canvas id="canvas" width="300" height="300"></canvas>
 <script>
  var sun = new Image();
  var moon = new Image();
  var earth = new Image();
  function init() {
  sun.src = 'https://mdn.mozillademos.org/files/1456/Canvas_sun.png';
  moon.src = 'https://mdn.mozillademos.org/files/1443/Canvas_moon.png';
  earth.src = 'https://mdn.mozillademos.org/files/1429/Canvas_earth.png';
  window.requestAnimationFrame(draw);
  }

  function draw() {
  var ctx = document.getElementById('canvas').getContext('2d');

  ctx.globalCompositeOperation = 'destination-over';
  ctx.clearRect(0, 0, 300, 300);

  ctx.fillStyle = 'rgba(0, 0, 0, 0.4)';
  ctx.strokeStyle = 'rgba(0, 153, 255, 0.4)';
  ctx.save();
  ctx.translate(150, 150);

  // Earth
  var time = new Date();
  ctx.rotate(((2 * Math.PI) / 60) * time.getSeconds() + ((2 * Math.PI) / 60000) * 
  time.getMilliseconds());
  ctx.translate(105, 0);
  ctx.fillRect(10, -19, 55, 31); 
  ctx.drawImage(earth, -12, -12);

   // Moon
  ctx.save();
  ctx.rotate(((2 * Math.PI) / 6) * time.getSeconds() + ((2 * Math.PI) / 6000) * 
  time.getMilliseconds());
  ctx.translate(0, 28.5);
  ctx.drawImage(moon, -3.5, -3.5);
  ctx.restore();

  ctx.restore();

   ctx.beginPath();
   ctx.arc(150, 150, 105, 0, Math.PI * 2, false);
   ctx.stroke();

   ctx.drawImage(sun, 0, 0, 300, 300);

   window.requestAnimationFrame(draw);
    }

   init();
   </script>
   </body>
   </html>
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  • 执念已碎
    2020-12-05 04:33

    Via Jquery: 

    $('<canvas/>', { id: 'mycanvas', height: 500, width: 200});

    http://jsfiddle.net/8DEsJ/736/

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  • 既然无缘
    2020-12-05 04:35

    It happens because you call it before DOM has loaded. Firstly, create the element and add atrributes to it, then after DOM has loaded call it. In your case it should look like that:

    var canvas = document.createElement('canvas');
canvas.id     = "CursorLayer";
canvas.width  = 1224;
canvas.height = 768;
canvas.style.zIndex   = 8;
canvas.style.position = "absolute";
canvas.style.border   = "1px solid";
window.onload = function() {
    document.getElementById("CursorLayer");
}
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