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问题:

I would like to know how it is possible to set the background image of a canvas to a .png file. I do not want to add the image in the back of the canvas and make the canvas transparent.

I want the user to be able to actually draw on that canvas with the background being the .png image so that I can extract it later as a .png with the drawings that the user made.

回答1:

As shown in this example, you can apply a background to a canvas element through CSS and this background will not be considered part the image, e.g. when fetching the contents through toDataURL().

Here are the contents of the example, for Stack Overflow posterity:

<!DOCTYPE HTML>
<html><head>
  <meta charset="utf-8">
  <title>Canvas Background through CSS</title>
  <style type="text/css" media="screen">
    canvas, img { display:block; margin:1em auto; border:1px solid black; }
    canvas { background:url(lotsalasers.jpg) }
  </style>
</head><body>
<canvas width="800" height="300"></canvas>
<img>
<script type="text/javascript" charset="utf-8">
  var can = document.getElementsByTagName('canvas')[0];
  var ctx = can.getContext('2d');
  ctx.strokeStyle = '#f00';
  ctx.lineWidth   = 6;
  ctx.lineJoin    = 'round';
  ctx.strokeRect(140,60,40,40);
  var img = document.getElementsByTagName('img')[0];
  img.src = can.toDataURL();
</script>
</body></html>

回答2:

You can give the background image in css :

#canvas { background:url(example.jpg) }

it will show you canvas back ground image

回答3:

You can draw the image on the canvas and let the user draw on top of that.

The drawImage() function will help you with that, see https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Canvas_tutorial/Using_images

回答4:

You can use this plugin, but for printing purpose i have added some code like <button onclick="window.print();">Print</button> and for saving image <button onclick="savePhoto();">Save Picture</button>

     function savePhoto() {
     var canvas = document.getElementById("canvas");
     var img    = canvas.toDataURL("image/png");
     window.location = img;}

checkout this plugin http://www.williammalone.com/articles/create-html5-canvas-javascript-drawing-app